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Synthesis of Cobalt Based Nanoferrites and
Study of Their Structural and Conduction
Properties
By
Muhammad Akram
CIIT/FA08-PPH-015/ISB
PhD Thesis
In
Physics
COMSATS Institute of Information Technology
Islamabad-Pakistan
Fall, 2014
ii
COMSATS Institute of Information Technology
Synthesis of Cobalt Based Nanoferrites and Study of
Their Structural and Conduction Properties
A Thesis Presented to
COMSATS Institute of Information Technology, Islamabad
In partial fulfillment
of the requirement for the degree of
PhD Physics
By
Muhammad Akram
CIIT/FA08-PPH-015/ISB
Fall, 2014
iii
Synthesis of Cobalt Based Nanoferrites and Study of
Their Structural and Conduction Properties
A Post Graduate Thesis submitted to the Department of Physics as partial
fulfillment of the requirement for the award of Degree of PhD (Physics).
Name Registration No.
Muhammad Akram CIIT/FA08-PPH-015/ISB
Supervisor
_________________________
Dr. Muhammad Anis-ur-Rehman
Associate Professor, Department of Physics,
COMSATS Institute of Information Technology (CIIT),
Islamabad.
February, 2015
iv
Final Approval
This thesis titled
Synthesis of Cobalt Based Nanoferrites and Study of
Their Structural and Conduction Properties
By
Muhammad Akram
CIIT/FA08-PPH-015/ISB
has been approved
For the COMSATS Institute of Information Technology, Islamabad
External Examiner:_____________________________________________________
Dr. Muhammad Munib Asim, CSO, KRL, Rawalpindi.
External Examiner:_____________________________________________________
Dr. Muhammad Maqsood, Director, NINVAST, NCP, Islamabad
Supervisor:___________________________________________________________
Dr. M. Anis-ur-Rehman, Department of Physics/ Islamabad.
Head of the Department: __________________________________________
Dr. Sadia Manzoor, Department of Physics/ Islamabad.
Chairman of the Department: _______________________________________
Prof. Sajid Qamar, Department of Physics/ Islamabad.
Dean, Faculty of Science: _________________________________________
Prof. Arshad Saleem Bhatti
v
Declaration
I Mr. Muhammad Akram, Reg. # CIIT/FA08-PPH-015/ISB, hereby declare that I
have produced the work presented in this thesis, during the scheduled period of study.
I also declare that I have not taken any material from any source except referred to
wherever due that amount of plagiarism is within acceptable range. If a violation of
HEC rules on research has occurred in this thesis, I shall be liable to punishable action
under the plagiarism rules of the HEC.
Date: _________________
___________________________
Muhammad Akram
CIIT/FA08-PPH-015/ISB
vi
Certificate
It is certified that Mr. Muhammad Akram, CIIT/FA08-PPH-015/ISB has carried out
all the work related to this thesis under my supervision at the Department of Physics,
COMSATS Institute of Information Technology, Islamabad and the work fulfills the
requirement for award of PhD degree.
Date: _________________
Supervisor:
_________________________
Dr. Muhammad Anis-ur-Rehman,
Associate Professor
Head of the Department:
_______________________
Dr. Sadia Manzoor
Department of Physics
vii
I dedicate this thesis to my
parents and teachers
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Acknowledgements
All praise to Almighty Allah, the most gracious and the most merciful, whose
blessings are unlimited, and who blessed me with opportunity to pay my contribution
in efforts to explore some facts of HIS created striking and outstanding universe.
Without the help and blessing of my Allah, I was unable to complete my project.
Countless prayers for HIS holy prophet Muhammad (P.B.U.H), who is forever, a
light of guidance and wisdom for all humanity.
Special gratitude to my supervisor Dr. Muhammad Anis-ur-Rehman, for all kinds
of support he has provided me. I am sincerely obliged to him for providing the full
guidance and freedom with respect to the research activities. It was his
encouragement, that the project has been completed. He remains in my favor in all
hard times. I have really no words to express my thoughts for him. God bless him all
the time in every walk of life.
I would like to express my sincere thanks to my supervisory committee members Prof.
Arshad Saleem Bhatti (Dean of Sciences) and Prof. Ishaq Ahmad for their noble
cooperation and guidance. I am also grateful to Prof. Sajid Qammar, Chairman of
the Department of Physics and Dr. Sadia Manzoor Head of the Department of
Physics for providing support. I would especially like to thank Dr. Javed Akhtar from
PINSTECH, for sparing his time for discussion from his busy time schedule during
my visits to PINSTECH.
I am very thankful to all my teachers especially for their cooperation and healthy
suggestions during my study and research work at COMSATS Institute of
Information Technology Islamabad, Pakistan. Higher Education Commission
(HEC), Pakistan is highly acknowledged for providing financial support through
“National Research Program for Universities (NRPU)” and “Indigenous 5000
Scholarship Program”. This scholarship policy made it possible for me to do this
work.
ix
My special thanks goes to my lab fellows Ghulam Hasnain, Ali Abdullah, Ghulam
Asghar, Said Nasir Khisro, Muhammad Mubeen, Anwar-ul-Haq, Yasin Shami, Ms.
Zeb-un-Nisa and Ms. Mariam Ansari for their help in research work. I would also like
to thanks Ms. Fatima-tuz-Zahra, Arsalan Munir and Rafiq-ur-Rehman for helping me
when I felt stuck. I would also like to express my thanks to Muhammad Farooq Nasir,
Mohsin Rafiq and M. Hafeez. I would also like to thanks Mr. Tanveer Baig, Program
coordinator and his colleagues in the Physics Office for their nice cooperation
My deep gratitude goes to my parents and my family members. Without their
continuous encouragement and support it would have been impossible for me to
accomplish all of my study and research work. I am deeply grateful to my son
Muhammad Hassan and daughter Hadia Noor who missed me very much whenever
they wanted me with them.
Muhammad Akram
CIIT/FA08-PPH-015/ISB
x
ABSTRACT
Synthesis of Cobalt Based Nanoferrites and Study of Their
Structural and Conduction Properties
Crystal structure and cation distribution at particular sites in crystal lattice play the
primary role in determining the properties of nanocrystalline transition metal oxide
materials. Spinel ferrites are a class of compounds with general formula MFe2O4 (M =
Mn, Co, Ni, Zn, Mg, etc.). The main focus of this research work was the synthesis of
nanocrystalline CoFe2O4 ferrite with different dopant elements like Zn, Mn and Cd
with different ratio by weight. The study of cation distribution in doped Co
nanocrystalline ferrites and dependence of structural and electrical properties on
dopant cations and distribution of these cations in certain interstitial sites in entire
crystal structure, was made. All compositions were synthesized and characterized
under same conditions so that a comparative analysis could be done.
In the first experiment nanocrystalline ferrite particles of Co1-xZnxFe2O4 (x= 0.0 to 1.0
with step of 0.2) were synthesized by co-precipitation synthesis technique. In the case
of CoZnFe2O4 relative concentration of Co and Zn with their particular site occupancy
plays a crucial role in deciding the ultimate material properties. Samples synthesized
at the reaction temperature of 70°C were sintered for 3 hours at 600°C. For the
selection of sintering temperature to have maximum crystallinity, thermal analysis
was done through differential scanning calorimetry (DSC) and thermogravimetry
analysis (TGA) techniques. The FCC spinel structure of the synthesized particles was
confirmed by X-ray diffraction (XRD) patterns. Lattice constants obtained were in the
range 8.36(1) to 8.44(1) Å. The crystallite sizes calculated from the most intense peak
(311) via the Scherrer equation, were found in the range of 10 nm to 35 nm. XANES
spectroscopy was used at Fe, Co and Zn K-edges to examine the cation distribution in
the crystal structure. Dependence of electrical transport properties on shift in crystal
structure due to the successive replacement of Co by Zn in CoFe2O4 was examined.
The dc electrical conduction measurements were taken as a function of temperature
ranging from 313 K to 700 K. Activation energy values (0.518-0.537 eV) indicated the
polaron hopping conduction mechanism. The ac electrical transport properties were
xi
studied by measuring the dielectric constant, dielectric loss tangent (tan δ) and ac
conductivity as a function of frequency. A regular shift in electrical properties is
observed depending upon the cation distribution. Jonscher power law and Maxwell-
Wagner two layer models were employed to investigate the conduction phenomenon.
Manganese substituted cobalt ferrites are promising materials for stress and torsion
sensor applications. Effects of Mn doping on the crystal structure and change in
electrical transport properties with the shift of cation distribution in CoFe2O4were
studied. Co1-xMnxFe2O4(x= 0.0 to 1.0 with step of 0.2) nanocrystallite particles with
stoichiometric proportion were synthesized via co-precipitation method at 70°C of
reaction temperature. The crystalline phase of FCC spinel structure, with lattice
constants in the range 8.36(1) to 8.46(8) Å, were confirmed by XRD patterns. Space
group was found to be Fd3m. The crystallite sizes were found to be in the range from
16 nm to 35 nm. X-ray absorption fine structure (XAFS) spectrometry is an elemental
specific technique and is sensitive to the local crystal structure. X-ray absorption near-
edge structure (XANES) spectroscopy is a prevailing tool for the structural study of
metal oxide materials. XANES spectroscopy is used at Fe K-edges to investigate the
cation distribution in the crystal structure. DC electrical resistivity measurements were
done at different temperatures by means of two-probe method from 370 K to 700 K.
AC electrical properties were also analyzed. Results are explained in terms of polaron
hopping model under the effects of cation distribution. Nyquist plots were done to
determine the equivalent circuits for grains and grain boundary conduction
mechanism.
Co contents were also replaced by Cd successively in CoFe2O4 nanocrystalline
particles. FCC spinel structure with space group Fd3m of Co1-xCdxFe2O4(x= 0.0 to 1.0
with step of 0.2) crystal was confirmed by XRD patterns. XANES spectroscopy was
used at Fe and Co K- edges to find the distribution of cations. The lattice constants
were in the range from 8.36(1) Å to 8.60(1) Å. The variation in dielectric properties
such as dielectric constant, dielectric loss tangent (tan δ) and ac conductivity (σac)
have been observed as a function of composition and frequency. Conduction
mechanism was correlated with cation type and hopping lengths for charge carriers. The
xii
results were discussed in terms of the polaron hopping model under the effects of
cation distribution.
A systematic replacement of Co, in CoFe2O4 ferrites by Zn, Mn and Cd is done.
Structural and electrical properties are correlated and explained. Deep structural
investigation by XAFS is rarely reported. Electrical properties could be controlled by
structure (cation distribution and site occupancy by specific element) for the desired
useful applications. Mn substituted Co ferrites are useful for relatively low resistance
demanding applications while Cd substituted Co ferrites are suggested for high
resistance requiring applications.
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Table of Contents
1 Introduction ........................................................................................................ 1
1.1 Materials at nano length scale ...................................................................... 2
1.2 Ferrites ........................................................................................................ 4
1.2.1 Soft ferrites .......................................................................................... 5
1.2.2 Hard ferrites ......................................................................................... 5
1.3 Types of ferrites .......................................................................................... 5
1.4 Spinel ferrites .............................................................................................. 5
1.5 Crystal structure of the spinel ferrites .......................................................... 6
1.5.1 Tetrahedral A sites ............................................................................... 7
1.5.2 Octahedral B sites ................................................................................ 9
1.6 Classification in spinel ferrites .................................................................... 9
1.6.1 Normal spinel ferrites ........................................................................... 9
1.6.2 Inverse spinel ferrites ........................................................................... 9
1.6.3 Intermediate spinel ferrites .................................................................. 10
1.7 Importance and applications of spinel ferrites ............................................. 10
1.8 Literature review/Background .................................................................... 11
1.9 Aims and objectives ................................................................................... 11
1.10 Thesis Synopsis .......................................................................................... 14
2 Synthesis of ferrite nanoparticles ....................................................................... 19
xiv
2.1 Synthesis of nanocrystalline ferrites ........................................................... 20
2.2 Solid state reaction method......................................................................... 20
2.3 Chemical methods ...................................................................................... 21
2.3.1 Sol-gel method .................................................................................... 22
2.3.2 Spray Pyrolysis ................................................................................... 22
2.3.3 Freeze-drying method ......................................................................... 22
2.3.4 Micro emulsion method ...................................................................... 22
2.3.5 Combustion flame synthesis ................................................................ 23
2.3.6 Hydrothermal method ......................................................................... 23
2.3.7 Chemical co-precipitation method ....................................................... 23
2.4 Influence of different parameters on synthesis by co-precipitation method . 25
2.5 Synthesis of Zn, Mn and Cd doped CoFe2O4nanocrystalline ferrites particles
……………………………………………………………………………...27
Chapter 3.................................................................................................................. 30
3 Characterization techniques ............................................................................... 30
3.1 Thermal analysis ........................................................................................ 31
3.1.1 Differential scanning calorimetry (DSC) ............................................. 31
3.1.2 Thermogravimetry analysis (TGA) ...................................................... 32
3.2 Structural analysis ...................................................................................... 33
3.2.1 X-ray diffraction (XRD) analysis ........................................................ 33
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3.2.1.1 Structure determination ................................................................ 35
3.2.2 X-ray absorption fine structure (XAFS) spectroscopy.......................... 36
3.3 Electrical properties ................................................................................... 40
3.3.1 DC electrical properties ....................................................................... 40
3.3.2 Frequency dependent alternating current (ac) measurements ............... 43
4 Thermal, structural and electrical properties of nanocrystalline Co1-xZnxFe2O4
(x = 0.0 to 1.0) ......................................................................................................... 50
4.1 Thermal analysis ........................................................................................ 51
4.2 Structural properties ................................................................................... 54
4.2.1 X-ray diffraction (XRD) studies .......................................................... 54
4.2.2 X-Ray absorption fine structure (XAFS) studies .................................. 57
4.3 Electrical measurements ............................................................................. 59
4.3.1 DC electrical properties ....................................................................... 59
4.3.2 AC electrical properties ....................................................................... 64
4.3.2.1 Dielectric constant ....................................................................... 64
4.3.2.2 Dielectric loss (tan δ) ................................................................... 66
4.3.2.3 AC conductivity ........................................................................... 67
4.4 Summary.................................................................................................... 73
5 Thermal, structural and electrical properties of nanocrystalline Co1-xMnxFe2O4
(x = 0.0 to 1.0) ......................................................................................................... 75
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5.1 Thermal analysis ........................................................................................ 76
5.2 Structural analysis ...................................................................................... 78
5.2.1 The X-ray diffraction (XRD) studies ................................................... 78
5.2.2 X-ray absorption fine structure (XAFS) measurements ........................ 80
5.3 Electrical measurements ............................................................................. 82
5.3.1 DC electrical properties ....................................................................... 82
5.3.2 AC electrical properties ....................................................................... 86
5.3.2.1 Dielectric Constant ....................................................................... 86
5.3.2.2 Dielectric loss (tan δ) ................................................................... 88
5.3.2.3 AC conductivity ........................................................................... 89
6 Thermal, structural and electrical properties of nanocrystalline Co1-xCdxFe2O4
(x = 0.0 to 1.0) ......................................................................................................... 95
6.1 Thermal analysis ........................................................................................ 96
6.2 Structural analysis ...................................................................................... 97
6.2.1 X-ray diffraction (XRD) studies .......................................................... 97
6.2.2 X-ray Absorption fine structure (XAFS) measurements..................... 100
6.3 Electrical properties ................................................................................. 102
6.3.1 DC electrical properties. .................................................................... 102
7 Comparison of doping effects .......................................................................... 117
7.1 Thermal studies ........................................................................................ 118
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7.2 Structural changes .................................................................................... 120
7.2.1 Crystal structure ................................................................................ 120
7.2.2 Lattice constant ................................................................................. 120
7.2.3 Hopping lengths ................................................................................ 121
7.3 Occupancy of interstitial sites ................................................................... 122
7.4 DC electrical properties ............................................................................ 123
7.4.1 DC electrical resistivity ..................................................................... 123
7.4.2 Activation energies ........................................................................... 126
7.5 AC electrical properties ............................................................................ 127
7.5.1 Dielectric constant ............................................................................ 127
7.5.2 Dielectric loss (tan δ) ........................................................................ 129
7.5.3 AC conductivity (ac) ........................................................................ 131
8 Conclusions ……………………………………………….…………………….…134
8.1 Summary /Conclusions…………………….………………………….….135
8.2 Future work and recommendations……………………………….......136
9 References……………………………………………………………….......137
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List of Figures
Figure 1.1: Typical materials with dimension in nano length scale [2]……................2
Figure 1.2: Calculated surfaces to bulk atomic ratio (for Fe) [6]……………...........3
Figure 1.3: A spinel unit cell structure showing oxygen atoms, tetrahedral A sites, and
octahedral B [21]. ...................................................................................................... 7
Figure 1.4: (a) Tetrahedral A sites (b) Octahedral B site (c) A complete cubic spinel
unit cell (d) Two alternate sub cells [23] ...................................................................... 8
Figure 2.1: Flow chart of co-precipitation method for the synthesis of Co1-xZnxFe2O4 .
……………………………………………………………………………………….29
Figure 3.1: A model DSC scan ................................................................................ 31
Figure 3.2: Schematic diagram of processes involving DSC .................................... 32
Figure 3.3: Α schematic thermo balance .................................................................. 33
Figure 3.4: Diffraction of scattered X-ray beam from atoms in a crystal .................. 34
Figure 3.5: (a) Energy levels of an electron in the crystal lattice for excitation
energies corresponding the single (EXAFS) and multiple scattering for (XANES) (b)
Corresponding to a free electron outgoing wave and scattered wave interference (A)
X-ray photon absorbing atom, (B) neighboring atom (c) dependence of energy of
absorption coefficient μ (left) in the absence of neighboring atoms (right)In the
presence of scattering from the neighboring atom [98]..……………………………….37
Figure 3.6: A typical XAFS spectrum ...................................................................... 38
Figure 3.7: Schematic of the XAFS experiment ....................................................... 39
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Figure 3.8: (a) Two-probe (b) Four-probe resistance measurement circuitry ........... 41
Figure 3.9: Circuitry of the apparatus used for dc electrical resistivity
measurements…. ..................................................................................................... 42
Figure 3.10: (a) Relaxation time (τ) of masses after turning off the applied electric
field. (b) Impedance plane of real capacitor (c) Response of real ε′ and imaginary ε′′
parts of permittivity ............................................................................................... 43
Figure 3.11: General polarization mechanisms ........................................................ 44
Figure 3.12: ε′ and ε″ as a function of frequency and different polarization
mechanisms ............................................................................................................. 46
Figure 3.13: A model complex plane plot for parallel RC equivalent
circuit………………………………………………………………………………..……….49
Figure 4.1(a): The Differential scanning calorimetriy (DSC) of CoFe2O4 ................ 51
Figure 4.1(b): The Differential scanning calorimetriy (DSC) of ZnFe2O4 ................ 52
Figure 4.2(a): The Thermogravimetric Analysis (TGA) of CoFe2O4 ........................ 52
Figure 4.2(b): The Thermogravimetric Analysis (TGA) of ZnFe2O4 ........................ 53
Figure 4.3: X-ray diffraction pattern of Co1-xZnxFe2O4(x = 0.0, 0.2, 0.4, 0.6, 0.8,
1.0)…………………………………………………………………………………...54
Figure 4.4: Change in lattice constants and crystallite size with Zn concentration in
Co1-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0)…………………...…………………...55
Figure 4.5: Change in hopping length between consecutive tetrahedral A sites (LA)
and consecutive octahedral B sites (LB) with Zn concentration (x) in Co1-xZnxFe2O4 (x
= 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)……………..……………………………………………56
Figure 4.6: XANES spectra after normalization at K-edge of Fe (a), K-edge of Co (b),
and K-edge of Zn (c) for Cox Zn1−x Fe2O4. ‘A’ indicates the pre edge peak…...…..58
xx
Figure 4.7: Variation of resistivity ρ (Ω-cm) with change in temperature for
Co1-xZnxFe2O4 .......................................................................................................... 60
Figure 4.8: Variation in dc electrical resistivity at different temperatures of
composition Co1-xZnxFe2O4 (x=0.0,0.2, 0.4, 0.6, 0.8, 1.0) ........................................ 61
Figure 4.9: Variation of resistivity ρ (Ω-cm) with change in temperature for
Co1-xZnxFe2O4 with linear fit ..................................................................................... 61
Figure 4.10: Variation in activation energy (E) with change of Zn concentration (x)
in Co1-xZnxFe2O4. .................................................................................................... 63
Figure 4.11: Variation of drift mobility µd (cm2/Vs) with change in temperature for
Co1-xZnxFe2O4. ........................................................................................................ 63
Figure 4.12: Effect of Zn concentration and frequency of the applied electric field on
dielectric constants for Co1-xZnxFe2O4. .................................................................... 64
Figure 4.13: Effect of Zn concentration on dielectric constant at different frequencies
for Co1-xZnxFe2O4.................................................................................................... 66
Figure 4.14: Effect of Zn concentration and frequency of the applied electric field on
tan δ for Co1-xZnxFe2O4 ........................................................................................... 66
Figure 4.15: Change in tangent loss for all compositions at different
frequencies……..…………………………………………………………………….67
Figure 4.16: Effect of Zn concentration and frequency of the applied electric field on
ac conductivity for Co1-xZnxFe2O4 .......................................................................... 68
Figure 4.17: Variation in ac conductivity at different frequencies of composition
Co1-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0)…….…………………………………..69
Figure 4.18: Variation in ac conductivity with respect to power law with variation in
concentration of composition Co1-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0) .............. 69
xxi
Figure 4.19: Nyquist plots for all the samples of Co1-xZnxFe2O4
(x=0.0 -1.0)………….........................................................................................................71
Figure 5.1: The differential scanning calorimetriy (DSC) of CoFe2O4 and
MnFe2O4……………………………………………………………………………..76
Figure 5.2: The thermogravimetric analysis (TGA) of CoFe2O4 and MnFe2O4 ......... 77
Figure 5.3: XRD patterns of Co1-xMnxFe2O4(x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0)............... 78
Figure 5.4: Change in hopping length between tetrahedral A sites (LA) and octahedral
B sites (LB) with Mn concentration (x) in Co1-xMnxFe2O4 ........................................ 79
Figure 5.5: Change in theoretical density (ρx ) and measured density (ρm ) with Mn
concentration (x) in Co1-xMnxFe2O4 ......................................................................... 79
Figure 5.6: Normalized XANES spectra at Fe K-edge for the different composition of
Co1−x MnxFe2O4. Inset is the pre edge peak. ............................................................ 81
Figure 5.7: Variation of dc resistivity ρ (Ω-cm) with change in temperature for
Co1-xMnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0) ........................................................... 82
Figure 5.8: Variation of dc resistivity ρ (Ω-cm) at diffferent temperature for
Co1-xMnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0) ........................................................... 83
Figure 5.9: Variation of resistivity ρ (Ω-cm) with change in temperature for
Co1-xMnxFe2O4. Lines are the Arrhenius relation fit. ................................................... 83
Figure 5.10: Variation in activation energy (E) with change of Mn concentration (x)
in Co1-xMnxFe2O4 .................................................................................................... 85
Figure 5.11: Variation of drift mobility µd (cm2/Vs) with change in temperature for
Co1-xMnxFe2O4. ........................................................................................................ 85
Figure 5.12: Change in dielectric constant with ln(f) in Co1-xMnxFe2O4.................. .86
Figure 5.13: Change in dielectric constant at different frequencies in Co1-xMnxFe2O4 ..
……………………………………………………………………………………..87
xxii
Figure 5.14: Effect of Mn concentration and frequency of the applied electric field on
tan δ for Co1-xMnxFe2O4. ......................................................................................... 88
Figure 5.15: Variation in dilectric loss (tan δ) at different frequencies of the applied
electric field for Co1-xMnxFe2O4. ............................................................................. 89
Figure 5.16: Effect of Mn concentration and frequency of the applied electric field on
ac conductivity for Co1-xMnxFe2O4. ........................................................................ 89
Figure 5.17: Change in ac conductivity at different frequencies for
Co1-xMnxFe2O4.……………………………………………………………………....90
Figure 5.18: Variation in ac conductivity with respect to power law with variation in
concentration of composition Co1-xMnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0) ............. 91
Figure 5.19: Nyquist plots for all the compositions for Co1-xMnxFe2O4 (x=0.0, 0.2,
0.4, 0.6 0.8, 1.0) ...................................................................................................... 92
Figure 6.1: The differential scanning calorimetric (DSC) thermogram of CoFe2O4 and
CdFe2O4 .................................................................................................................. 96
Figure 6.2 The thermogravimetric analysis (TGA) thermogram of CoFe2O4 and
CdFe2O4 .................................................................................................................. 97
Figure 6.3: X-ray diffraction pattern of Co1-xCdxFe2O4 ............................................ 98
Figure 6.4: Change in lattice constants (a) and crystallite size with Cd concentration
in Co1-xCdxFe2O4 .................................................................................................... 99
Figure 6.5: Change in hopping length between tetrahedral A sites (LA) and octahedral
B sites (LB) with Cd concentration (x) in Co1-xCdxFe2O4 .......................................... 99
Figure 6.6: Normalized XANES spectra at Fe K-edge for the different compositions
of Co1−x Cdx Fe2O4. Inset shows the pre edge peak heights in enlarged view ........... 101
xxiii
Figure 6.7: Normalized XANES spectra at Cd K-edge for the different composition of
Co1−x Cdx Fe2 O4. Inset shows the pre edge peak heights in enlarged view.. ............ 101
Figure 6.8: Change in dc resistivity with Cd concentration (x) and temperature in
Co1-xCdxFe2O4. ...................................................................................................... 102
Figure 6.9: Change in dc resistivity with Cd concentration (x) at different
temperatures in Co1-xCdxFe2O4. ............................................................................. 103
Figure 6.10: Change in dc resistivity with Cd concentration (x) and temperature in
Co1-xCdxFe2O4. Lines are the Arrhenius relation fit..………………………………103
Figure 6.11: Effect of Cd concentration on activation energyin Co1-xCdxFe2O4 ...... 104
Figure 6.12: Effect of Cd concentration and temperature on drift mobility of
Co1-xCdxFe2O4 ....................................................................................................... 105
Figure 6.13: Effects of Cd concentration and frequency of the applied electric field on
dielectric constants for Co1-xCdxFe2O4 .................................................................... 106
Figure 6.14: Effects of Cd concentration at different frequencies of the applied
electric field on dielectric constants for Co1-xCdxFe2O4........................................... 106
Figure 6.15: Effects of Cd concentration (x) and frequency of the applied electric
field on dielectric loss (tan δ) for Co1-xCdxFe2O4 ................................................... 108
Figure 6.16: Effects of Cd concentration (x) and frequency of the applied electric
field on dielectric loss factor () for Co1-xCdxFe2O4............................................... 109
Figure 6.17: Effects of Cd concentration (x), at different frequency of the applied
electric field on dielectric loss (tan δ) for Co1-xCdxFe2O4 ........................................ 110
Figure 6.18: Effects of Cd concentration (x) and frequency of the applied electric
field on ac conductivity (ac) for Co1-xCdxFe2O4 ..................................................... 111
xxiv
Figure 6.19: Effects of Cd concentration (x) at different frequencies of the applied
electric field on ac conductivity (ac) for Co1-xCdxFe2O4 ........................................ 112
Figure 6.20: Variation in ac conductivity with respect to power law with variation in
concentration of composition Co1-xCdxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0) ............ 113
Figure 6.21: Nyquist plots for Co1-xCdxFe2O4 ....................................................... 114
Figure 7.1: The differential scanning calorimetric (DSC) thermogram of AFe2O4
(A= Co, Zn, Mn, Cd) ............................................................................................. 119
Figure 7.2: The thermogravimetric Analysis (TGA) thermogram of AFe2O4
(A= Co, Zn, Mn, Cd) ............................................................................................. 120
Figure 7.3: Variation in dc resistivity with composition at 423K as a function of
concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). .............................................. 125
Figure 7.4: Variation in dc resistivity with composition at 473K as a function of
concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). .............................................. 125
Figure 7.5: Variation in dc resistivity with composition at 573K as a function of
concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). .............................................. 126
Figure 7.6: Variation in dc resistivity with composition at 673K as a function of
concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). .............................................. 126
Figure 7.7: Variation in activation energy with composition as a function of
concentration x in Co1-xAxFe2O4 (A= Zn, Mn, Cd). ............................................... 127
Figure 7.8: Variation in dielectric constant with composition at 100 kHz frequency as
a function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). ......................... 128
Figure 7.9: Variation in dielectric constant with composition at 1.2 MHz frequency
as a function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). ..................... 129
xxv
Figure 7.10: Variation in dielectric constant with composition at 3MHz frequency as
a function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). ......................... 129
Figure 7.11: Variation in dielectric loss with composition at 100 kHz frequency as a
function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). ............................ 130
Figure 7.12: Variation in dielectric loss with composition at 1.2 MHz frequency as a
function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). ............................ 131
Figure 7.13: Variation in dielectric loss with composition at 3 MHz frequency as a
function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). ............................ 131
Figure 7.14: Variation in ac conductivity with composition at 100 kHz frequency as a
function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). ............................ 132
Figure 7.15: Variation in ac conductivity with composition at 1.2 MHz frequency as a
function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). ............................ 133
Figure 7.16: Variation in ac conductivity with composition at 3 MHz frequency as a
function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd). ............................ 133
Figure 7.17: Variation in ac conductivity with respect to power law with variation in
concentration of composition Co1-xAxFe2O4 (A=Zn, Mn, Cd) (x=0.0, 0.2, 0.4, 0.6, 0.8,
1.0)………………………………………………………………….….…………….134
xxvi
List of Tables
Table 4.1 Crystallite size d(311), lattice constant (a), Cell volume (V), hopping lengths
LA and LB, X-ray density (ρx), measured density (ρm), porosity (P) drift mobility (µd) at
different temperatures and activation energy E for different values of ‘x’ in
Co1-xZnxFe2O4 .......................................................................................................... 72
Table 5.1: Crystallite size d(311), lattice constant (a), hopping lengths LA (Å) and LB (Å)
Cell volume (V), X-ray density(ρx), measured density (ρm), porosity (P) for different
values of ‘x’ in Co1-xMnxFe2O4................................................................................. 93
Table 6.1: Crystallite size d(311), lattice constant (a), Cell volume (V), popping lengths
LA (Å) and LB (Å) X-ray, density (ρx), measured density (ρm), and porosity (P) for
different values of ‘x’ in Co1-xCdxFe2O4 ................................................................. 115
Table 7.1: Lattice constants “a” in Angstroms (Å) for all compositions
Co 1-xMx Fe2O4 (M = Zn, Mn, Cd & x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)......................... 122
Table 7.2: Hopping Lengths between cations at tetrahedral A sites (LA) in Angstroms
(Å) and between octahedral B sites (LB) in Angstroms (Å) for all sample compositions
Co1-xAx Fe2O4 (A= Zn, Mn, Cd & x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) ........................... .123
xxvii
LIST OF ABBREVIATIONS
XRD X-ray diffraction
DSC Differential Scanning Calorimetry
TGA Thermogravimetric Analysis
XAFS X-ray Absorption Fine Structure
XANES X-ray Absorption Near Edge Fine Structure
EXAFS Extended X-ray Absorption Fine Structure
FWHM Full-width at half-maximum
ρx Theoretical density
ρm Measured density
ρ Resistivity as a function of temperature
Å Angstrom
Ω Ohm
μ Micro
θ Theta
α Alpha
β Beta
ε/ Dielectric constant
γ Gamma
ω Omega
π Pi
δ Delta
σac ac conductivity
tanδ Loss tangent
xxviii
List of Publications
1. Dependence of Site Occupancy and Structural and Electrical Properties on
Successive Replacement of Co by Zn in CoFe2O4
M. Akram and M. Anis-ur-Rehman
Journal of Electronic Materials: 43, 2 (2014), 485-492
2. Structural and Magnetic Properties of Nanocrystalline Mg-Co Ferrites
M. Anis-ur-Rehman, Muhammad Ali Malik, M. Akram, M. Kamran, Kishwar Khan
and Asghari Maqsood
J. Supercond Nov Magn 25, 8 (2012), 2691-2696
3. Structural, Dielectric, and Magnetic Characterization of Nanocrystalline Ni–
Co Ferrites
Kishwar Khan, Asghari Maqsood, M. Anis-ur-Rehman, Muhammad Ali Malik, M.
Akram
J. Supercond. Nov. Magn. 25, 8 (2012), 2707-2711
4. Proficient magnesium nanoferrites: synthesis and characterization
M. Anis-ur-Rehman, Muhammad Ali Malik, M Akram, Kishwar Khan and Asghari
Maqsood
Phys. Scr. 83 (2011) 015602- 015607
5. Microstructures, Electrical and Magnetic Properties of Zn Doped Co
Nanoferrites
M. Akram, M. Anis-ur-Rehman, M. Mubeen and M. Ali
Key Engineering Materials Vols. 510-511 (2012) 221-226
6. Role of Synthesis Parameters on Structural and Electrical Properties of
M-type Hexa-Ferrites
G. Asghar, S. Nasir, M.S. Awan, G.H. Tariq, M. Akram and M. Anis-ur-Rehman
Key Engineering Materials Vols. 510-511 (2012) 201-205
7. Proficient Ni-Zn-Cr Ferrites: Synthesis and Characterization
S. Nasir, M.A. Malik, G. Asghar, G.H. Tariq, M. Akram and M. Anis-ur-Rehman
Key Engineering Materials Vols. 510-511 (2012) 343-347
8. Correlation of Microstructures with Electrical Transport Properties in Mn
Doped Co Nanoferrite Particles
M. Akram, M. Anis-ur-Rehman, S. Nasir and G. Asghar
Key Engineering Materials Vols. 510-511 (2012) 487-492
1
Chapter 1
1 Introduction
2
1.1 Materials at nano length scale
In the technology and science; the prefix ‘nano’ symbolizes a billionth part of a unit.
One billionth (10−9) of a meter indicates a nanometer. An idea about the length of
some typical objects is given in Figure 1.1. Nanoscale is usually referred to the range
beginning from the atomic size to 100 nanometers, and the materials with smallest
dimension below 100 nanometers are called nanomaterial [1]. Study of the materials
at nano length scale is the natural evolution of science exploring the characteristics of
matter with such a small dimension.
Figure 1.1 Typical materials with dimension in nano length scale [2]
One of the basic questions is “what is the size of a material where it begins to act
more like an atom or molecule?” Progression in instrumentation, particularly in
spectroscopy, has allowed examining the matter and phenomena with resolution of
angstrom-level [3]. It is leading towards the profound understanding of nano
structured materials.
Nanoscience probes the objects having size in nano length scale and playing an
imperative role for coming technological era. Nanotechnology is considered as the
technology of designing, manufacturing and application of nanostructures and
devices. Materials with nano length scale reveal quite unusual characteristics as
compared to their bulk counterpart for two reasons [4, 5]. First the large surface to
volume ratio which in turn produces a large fraction of coordinately unsaturated sites
at the surface and second, the quantum effects. The ratio of low coordinated atoms at
the surface increase largely when the size falls less than 35 nanometers as shown in
Figure 1.2 [6]. A nanocrystalline has crystalline order in along with nanoscale length
3
size. A significant feature of nanotechnology is that by having control over chemical
composition, material size and crystal structure, at the nanoscale one can tailor the
material for desired properties [7]. Material properties depend on chemistry and
pattern of crystal structure. Characteristic dimensions where material properties turn
into size receptive are between 1nm and 100 nm. Research on nano structured
materials is intensively studied area nowadays [8, 9].
As per theory of quantum confinement, holes of valence band and conduction band
electrons are restrained spatially by surface potential barrier. Regarding electronic and
optical characteristics, nano solids show the subsequent properties different from their
bulk counterpart. Spontaneous contraction occurs in chemical bonds in surface atoms
which causes the increase in binding energy. Properties such as dielectrics get
changed with the reduction of particle size because when the band gap expands the
energy levels of the core bands shift towards higher binding energy [10]. For the
materials with band gap lying in the visible spectrum, a modification in band gap with
change in size causes the variation in color. Magnetic properties of materials such as
Ni, Fe, Co, and Fe3O4, etc are size dependent [11]. The ‘coercive force’ or magnetic
Figure 1.2 Calculated surfaces to bulk atomic ratio (for Fe) [6]
4
memories have size dependence. In recent years nanocrystalline ferrite materials are
gaining interest for their diverse technological applications.
1.2 Ferrites
Ferromagnetic materials which are primarily composed of ferric oxide (Fe2O3) are
known as “ferrite materials.” The ferrites history began centuries prior to the Christ
birth with the finding of stones having attraction to iron. The vast deposits of these
stones were found at Magnesia in Asia Minor. These were used initially as pigments
in paints at some stage in the Paleolithic. Later in China these were used as the
magnetic compass. Ferrites have the general chemical formula MeFe2O4, owning the
structure of the mineral spinel, MgAl2O4. Here Me stands for a divalent metal ion
having an ionic radius ranges from 0.6 to 1Å. For simple ferrites, Me may be the
Co2+, Mn2+, Cu2+, Zn2+, Fe2+, Ni2+, Mg2+ or Cd2+. In case of mixed spinel ferrites the
combination of these transition metals is also possible. This substitutional potential
has led to the broad technological utilization of ferrites [12].
Ferrite crystal structure can be considered as the interconnected arrangement of
divalent oxygen anions and metal cations [13]. Even though the majority of ferrites
include iron oxide however there exist some ferrites based on Cr, Mn and some other
elements. Although Cr and Mn did not belong to ferromagnetic family of elements but
by the grouping with elements such as oxygen and with different metal ions, they can
perform as magnetic ions. Ferrites are examined broadly due to owing high electrical
resistivity as compared to the metals. Ferrites have high values of resistivity ranging
from~102-cm to 1011-cm at room temperature depending upon their chemical
composition [14]. Another important factor, which is of considerable importance in
ferrites and is completely insignificant in metals, is porosity. Because of possessing
small eddy current losses with high electric resistivity these materials have better
properties as compared to the other magnetic materials. Ferrites also show very useful
dielectric properties. It grants them an advantage over the nickel, iron and other
transition metals. Depending upon the magnetic properties, ferrites are divided as
"soft" and "hard" which refers to their low or high coercivity.
5
1.2.1 Soft ferrites
Soft ferrites have low coercivity (Hc< 10 A/Cm) and reveal ferrimagnetic behavior
[15]. There is a net magnetic moment which is caused by two sets of unpaired inner
electron spin moments in opposite directions which do not terminate each other.
Ferrites containing manganese, nickel or zinc compounds are used in transformer or
electromagnetic cores. These materials are widely used in inductors, RF transformers
and in the Switched-Mode Power Supply (SMPS) as a core because of having
relatively low losses even at high frequencies [16].
1.2.2 Hard ferrites
In contrast to soft ferrites hard ferrites have high coercivity (Hc> 300 A/Cm) [15].
These materials are composed of barium, iron and strontium oxides. After
magnetization hard ferrites retain high remanence. They have large magnetic
permeability and can conduct magnetic flux very well after magnetic saturation. This
is why ceramic magnets can store strong magnetic field. In radios these magnets are
used most commonly [16].
1.3 Types of ferrites
Ferrites are generally divided in three groups for their crystal structure foundations
Spinel ferrites (Cubic ferrites)
Garnets (Ortho ferrites)
Hexa ferrites (Hexagonal ferrites)
1.4 Spinel ferrites
The word spinel originates from Italian “spinella”, diminutive of spine. These are also
called cubic ferrites and most of the spinel compounds have space group Fd3m.
Spinel ferrites make a great group of oxides which acquires the structure of the natural
spinel MgAl2O4[17]. Several of the technologically important spinels are synthetic,
however one of the most significant and perhaps the oldest with useful applications
the magnetite Fe3O4, is a natural oxide. Their presence in abundance indicates the
6
stability of spinel crystal structure. Spinels are mostly ionic. The meticulous site
occupancy by cations is influenced by many factors such as covalent bonding effects
and crystal field stabilization energies of transition-metal cations. Several different
types of cation grouping may exist in a spinel structure with a net charge eight to
equalize the anion’s net charge. Spinel ferrites make the most widely useable ferrites
group [18].
1.5 Crystal structure of the spinel ferrites
Bragg and Nishikawa determined spinel structure for the first time in 1915 [19, 20].
The spinel unit cell is composed with two kinds of alternately repeating sub cells in
the three dimensional arrays. Eight such sub cells form a complete repeating unit cell.
Spinel ferrites have a cubic close-packed arrangement of oxygen anions with divalent
metal cations and trivalent Fe3+ at two different kinds of crystallographic sites called
tetrahedral sites (A sites) and octahedral sites (B sites) respectively as given in Figure
1.3 [21]. The formula can be written as M8Fe16O32. Total 96 interstices are formed by
oxygen anions in the unit cell, 64 A sites and 32 B sites.
The cubic unit cell with spinel crystal structure ferrites is shaped by 56 atoms, 32 of
oxygen atoms spread in a cubic close packed configuration (CCP) whereas 24 metal
cations are distributed among 8 A sites and 16 B sites. Total available A sites are 64
but only 8 are occupied while available B sites are 32 from which only 16 are
occupied for charge neutrality [22]. Depending upon the distribution of cations the
structural relation for a spinel structure could be (Me2+δ Fe3+
1-δ) A [Me2+1-δ Fe3+
1+δ] B O4
where Me stands for a divalent metal cation and δ for inversion parameter. The ideal
molar ratio of trivalent to divalent ions is 2:1.With reference to cation allocation
among these A sites and B sites, spinel structure can be normal, inverse, or partially
inverse. For complete normal spinel δ =1, for complete inverse δ = 0 and for mixed
ferrite δ ranges between these two extreme values [21].
7
1.5.1 Tetrahedral A sites
All interstitial sites in the spinel ferrites are not same; sites which are called A sites
are coordinated by 4 nearest neighboring oxygen ions. Hence A sites are called
tetrahedral sites. There are 3 atoms in a plane touching each other and the fourth atom
sits on top with symmetrical position to generate a tetrahedral site. Only 8 out of 64 A
sites are occupied to maintain the charge neutrality in the crystal system (Figure 1.4)
[23].
Figure 1.3 A spinel unit cell structure showing oxygen atoms, tetrahedral A sites,
and octahedral B sites [21].
8
Figure 1.4 (a) Tetrahedral A sites (b) Octahedral B site (c) A complete cubic spinel unit
cell (d) Two alternate sub cells [23]
9
1.5.2 Octahedral B sites
The interstitial sites formed by the coordination of 6 nearest neighboring oxygen ions
in cubic spinel crystal structure are called octahedral B sites. The oxygen ions around
the octahedral B site occupy the corners of an octahedron (Figure 1.3 b). Four atoms
out of 6 lies in a plane whereas each one of other 2 lies above and below of the plane
in the symmetrical position. Out of 32 octahedral B sites only16 are filled to maintain
the charge neutrality with in a spinel structure (Figure 1.4) [23].
1.6 Classification in spinel ferrites
On the basis of cation allocation at interstices the spinel ferrites are placed in three
groups.
Normal spinel ferrites
Inverse spinel ferrites
Intermediate spinel ferrites
1.6.1 Normal spinel ferrites
In normal spinel ferrites, B sites are filled by only one type of cations. In these ferrites
the A sites are occupied by the divalent cations while the trivalent cations lie at B
sites. In order to represent the ionic allocation; square brackets are used to indicate the
B sites. ZnFe2O4 is a typical example of normal spinel ferrites [22].
1.6.2 Inverse spinel ferrites
In these ferrites A site is occupied by half of the trivalent metal ions and other half of
trivalent metal ions lies at B sites. All the divalent metal ions lie on octahedral B sites.
The formula (Me3+)A [M+2Me3+]BO4 represents these ferrites. Fe3O4 has a typical
pattern of inverse spinel ferrites where divalent cations of Fe occupy the B sites [22].
10
1.6.3 Intermediate spinel ferrites
When the distribution of metal cations is intermediate between normal and inverse
conditions, the ferrites are considered as mixed spinel. Generally this condition can be
expressed as (Mδ2+Me1-δ
3+)A [M1-δ2+Me1+δ
3+]B O4. Where δ stands for inversion
parameter. Quantity δ depends on the synthesis method and nature of the constituents
of the ferrites. For complete normal spinel δ =1, for complete inverse δ = 0 and for
mixed ferrite, this δ ranges between 0 and 1. If there is unequal number of each kind
of cations on octahedral sites, the spinel is called mixed spinel. Typical examples of
mixed spinel ferrites are MgFe2O4 and MnFe2O4 [22].
1.7 Importance and applications of spinel ferrites
Ferrites make a large group of oxide materials with amazing combination of electric
and magnetic properties. They have a vast field of applications covering a notable
range from circuitry of millimeter wave integrated to handling power. Ferrites
comprise two important aspects, magnetic phenomena along with ceramic
microstructures [24]. Applications of ferrites materials are based on the very
fundamental properties of ferrites, a considerable saturation magnetization and high
values of electrical resistivity, low eddy current electrical losses, and high chemical
stability. The synthesis of ferrites can be done by several methods. The possibility to
produce a vast number of solid solutions opens the path for tailoring their properties
for a number of applications. In several applications, ferromagnetic metals cannot
substitute ferrites and for many other applications often ferrites compete metals for
cost-effective reasons [25, 26]. The opportunity of ferrites synthesis in the form of
nanoparticles has unlocked a new research field, with innovatory applications not only
in the electronics industry but also in biotechnology. Here are some important features
making ferrites most widely used material [27].
Useful materials for high frequencies applications
Microwave frequency applications
Mechanical strength
Durability (time and temperature stability)
Wide range of material selection
11
Useful dielectric properties
Low cost materials and also low cost of synthesis
On the surface, the uncompensated spins produce a phenomenon known as “exchange
bias” this property is used in sensors for magnetic field. There is also a great
possibility for magnetic nanostructures that electric field can affect their magnetic
order. It is called “current-induced magnetization dynamics”. Cobalt is well known
because of its ability to reduce magnetic losses at high frequencies. Among spinel
ferrites, cobalt ferrite CoFe2O4 is especially interesting because of its applications in
biotechnology [28], magnetic storage appliances [29] and in high frequency devices
[30]. CoFe2O4 has high cubic magneto crystalline anisotropy, high coercivity and
moderate saturation magnetization. Cobalt ferrite nanocrystalline particles as a photo-
magnetic material show interesting light-induced coercivity change [31]. Many
biotechnological applications have been developed depending upon biogenic and
synthetic nanoparticles [32]. In magnetic resonance imaging (MRI), super
paramagnetic particles are selectively associated with healthy tissues because these
particles change the rate of proton decay from the excited to the ground state.
Consequently, a disparate and darker contrast is obtained from these healthy regions
of tissue [33, 34]. Thermal energy obtained from hysteresis loss of ferrites is used in
heating of specific tissues for treatment of cancer (hyperthermia) [35]. Grain
dimensions, chemical composition, crystallinity, and cations allocation play an
influential role in defining the properties of nano ferrite particles. Furthermore, the
structural, magnetic and electrical properties of ferrites are very much sensitive to
synthesis conditions.
1.8 Literature review/Background
An active research area in physics, materials engineering, chemistry and in
biomedical engineering is the exploitation of nanocrystalline materials. A striking
significance is the tunability of the ultimate properties by miniaturizing the materials.
Due to the accessibility of new and precise techniques for preparation and
characterization of nano ferrite particles, the research has got a boost in present years
[36-38]. The nanocrystalline spinel ferrite materials have unusual characteristics as
12
compared to bulk due to large fraction of atoms which lies at the surface. Surface has
a large number of defects as all the atoms present at the surface are under-
coordinated.
Different features of processing, effects due to additives and properties and
applications of soft ferrites have been explored by several researchers [39-41]. To
explore the useful properties of spinel ferrites materials for different applications
many researchers have done a large quantity of work. A number of synthesis methods
have been employed up till now to produce nanocrystalline materials, like as
mechanical alloying [42], co-precipitation [43, 44], citrate precursor [45],
hydrothermal [46], sonochemical [47], sol-gel [48], shock wave [49], forced
hydrolysis in a polyol [50], reverse micelle [51], and even by the use of egg white for
aqueous medium [52].
In spinel ferrites trivalent and bivalent cations are located at octahedral sites and
tetrahedral sites which exist in close packing crystal structure of oxygen anions.
Properties like catalytic, electric and magnetic could be influenced significantly by the
variation in distribution of cations between these interstitial sites 52]. Several probes
such as Mossbauer spectroscopy, X-ray and neutron diffraction have been employed
to collect the information about cation distribution [53]. Nevertheless, the XRD has
limited effectiveness due to the analogous scattering factors for Fe and Co and also
due to the peak broadness for nano dimensions of the particles [54]. Nano crystals of
ferromagnetic zinc ferrites have been synthesized by thermal decomposition of metal
surfactant complexes at ambient temperature [55]. It has been shown that properties
of ZnFe2O4 depends upon particle size while the electrical conductivity is temperature
dependent.
Electrical transport properties of Mn-Zn ferrites have been examined by Ravinder et
al. [56]. Co1-xMnxFe2O4 was synthesized by Lee et al. as a bulk phase in air[57]. With
increasing contents of Mn lattice constant increases which is directly related to the
Mn2+ cation doping. The electrical conduction properties of Ni ferrite nanoparticles
with Zn substitution have also been considered by Islam et al. [58]. The magnetic
performance of ZnFe2O4 has drawn a lot of interest and remains a subject of serious
studies. ZnFe2O4 synthesized by co-precipitation route has been investigated and has
13
been verified that ZnFe2O4 could be used as gas-sensing material which has a high
value of sensitivity and better selectivity for C2H5OH. Lu et al. has reported recently
the magnetic and electrical transport properties of ZnxFe3-xO4 (x= 0.0 to 1.0) nano
crystalline ferrites synthesized via sol-gel method [59]. Saturation magnetization was
found to increase first until x=0.2 then diminish with further increase in x i.e. Zn
concentration. The dependence of the Curie temperature (TC) upon the x was also
investigated. Nissato and Yamamoto [60] studied the physical and magnetic
properties of Co spinel ferrite particles with NiO substitution doping. They have
reported that Co-Ni spinel ferrite fine particles with single-phase were achieved
without post annealing by co-precipitation method. Nanoparticles of cobalt ferrites
with lanthanide ions as dopant element CoLn0.12Fe1.88O4 were synthesized by Khan
and Zhang using oil in water micelle method [61]. Magnetic properties were
modulated due to lanthanide ions doping. A considerable change in coercivity and
blocking temperature was observed. Nanoparticles of cobalt ferrite were synthesized
by Li et al. in water and oil by using microemulsions reverse micelles by changing
composition of cations [62]. Particle size was estimated by electron microscopy and
was found to be in the range of 12 - 18 nm. XRD studies revealed that at small
Co2+:Fe2+precursor ratio 1.10 and 1.5 in the precursor, the formed particles maintain
the structure γ - Fe2O3. But with the further increase in the Co2+ contents CoFe2O4 is
formed.
Wei et al. synthesized NiMnxFe2-xO4, (x = 0.0 to 1) spinel ferrite by ceramic method
[63]. The allocation of cations was analytically calculated for the first time by XRD
with computer computation. It was shown that both Mn3+ and Ni2+ ions take up
octahedral sites predominantly with rise in manganese ion. It was due to their
preference for large site energy in octahedral coordination. Han et al. synthesized
cubic spinel with single phase Cu0.5-xNi0.5ZnxFe2O4 (x = 0.0 to 0.5) by
co-precipitation method [64]. By increasing Zn contents saturation magnetization was
found to increase. A quantum mechanical method, magnetic moments fitting, was
adopted to estimate the cation distribution. For XRD patterns the Rietveld fitting was
made by using the cation distribution thus obtained by quantum mechanical method
and was found reasonable. Mohan et al. studied cation distribution in CoFe2O4 by
Raman spectroscopy and Mossbauer method [61]. Particle size was in range 6 nm to
14
500 nm. Studies were done at room temperature and also at higher temperature. The
dependence of cation distribution on temperature and size was confirmed
conclusively. Akhtar et al. synthesized perovskite based SrFe0.5Nb0.5O3 and SrFeO3
materials by solid state reaction method. Investigations for structural studies were
done by XAFS and XRD techniques [65]. It was demonstrated by XAFS data that
SrFeO3 has valence state “4” but it converted in to “3” with the substitution of 50%
Nb5+ at Fe site. Yao-Jen Tu et al. used XANES to study the oxidation state of As(III).
The XANES analysis of K-edge proved that the As (V) adsorbed on ferrite was not
converted to toxic As (III) by Fe2+ in entire ferrite structure [66]. Kravtsova et al.
studied the electronic and atomic structure of nanoclusters of free niobium on the
basis of XANES [67]. Trail et al. investigated the potential of zircon for continuous
recording and the study of conditions evolving in magmatic redox was done by
quantifying Ce valence in synthetic crystals and in natural by XANES [68]. Bilovol et
al examined the Electronic structure of firstly quenched ribbons of composition
NdyFe(86-y-x)B14Tix (x = 0, 2, 4; y= 7,8 at %) by X –ray absorption (Fe and Ti K-edge)
was studied by X-ray absorption (Fe and Ti K-edge) and X-ray photoelectron
spectroscopy [69].
Exact determination of site occupancy in these complicated systems is currently the
topic of interest. Determination of precise structural information is the key to the
controlled properties particularly the electrical response, which is the main aim of the
current research.
1.9 Aims and objectives
Among ferrites, spinel ferrites have the most exciting crystal system. These materials
are especially interesting for addressing the basic correlation between material
properties and crystal structure. Crystal chemistry of any material reveals how we can
link together the chemical composition, internal crystal structure, microstructures and
ultimate physical properties of that material [70, 71]. The spinel ferrites have
advantage over other electromagnetic materials due to their inherent high resistivity
which results in low losses over wide frequency range and high thermal stability for
wide temperature range.
15
CoFe2O4 having spinel crystal structure is a distinguishing ferrimagnetic oxide
material. Being a magnetic material, with a Curie temperature (Tc) about 793 K,
CoFe2O4 is well known to have large magnetic anisotropy. It has good chemical
stability, mechanical hardness and moderate saturation magnetization. For several
years, CoFe2O4 has been investigated for fundamental research and potential
applications as well. As a potential predominant magnetic and electrically resistive
material, it has been applied in high-density magnetic recording media, high
performance electromagnetic and spintronic devices. Cobalt fine powders and films
have attracted considerable attention for their wide range of technological applications
such as transformer cores, recording heads, antenna rods, memory, ferrofluids,
biomedical application and sensors. Cobalt ferrites have a strong crystal field. The
useful properties of the spinel ferrites are firmly connected to the allocation of cations
at interstitial sites (A and B sites) in the spinel crystal system, internal crystal structure
and synthesis method. Also when materials are synthesized at nano length scale the
properties are governed by the size [72]. Crystal structure of spinel ferrites offer
opportunity to tailor their properties by specific cation doping at particular interstitial
crystal sites. Control over the cation distribution and crystal structure provides a way
to tailor the properties. So we selected cobalt ferrites as a base material and then it
was doped with Zn, Mn and Cd. Zn, Mn and Cd has considerably larger ionic radius
as compared to Co. A certain variation in the crystal system and crystal field was
expected to occur. So we were aiming to synthesize the many compositions of doped
nanocrystalline cobalt ferrites with certain dopant elements like Zn, Mn and Cd.
Therefore a range of compositions of Co ferrites with these dopant elements was
synthesized by co-precipitation method and explored for variation in crystal structure
and electrical conduction properties. A number of synthesis methods have been
explored over years. But co-precipitation method is one of the mostly used synthesis
route.
CoFe2O4 is considered to have inverse spinel structure whereas ZnFe2O4 and CdFe2O4
are known to have normal spinel structure [22]. MnFe2O4 is known for having partial
inverse spinel structure. So we replace Co from CoFe2O4 gradually with Zn, Mn and
Cd to observe the change in crystal structure for the whole range of transformation
from inverse spinel to normal spinel structure. Mostly these materials have been
16
studied for their magnetic properties but sequential studies for electrical properties of
these materials are rare in literature. We were aiming to investigate the dependence of
structural and electrical properties of our synthesized materials on many aspects
including dopant type, dopant concentration and cation distribution in particular
crystal system of spinel ferrites.
In reality the electrical properties of these materials are more complicated than an
ideal situation because cationic distribution over A and B sites vary with chemical
composition of compounds. A precise study of the cationic distribution in the entire
crystal structure is of great importance in understanding the material properties. There
are reports in the literature about the cation occupancy depending on mathematical
calculations based on Mossbauer spectroscopy and XRD [70]. In finding the local
surroundings of Fe ions Mossbauer spectroscopy is efficient but it could not give
information for Co cations. It is also not much effective in case of dilute samples [73,
74]. For the manipulation and understanding of material properties, it requires a
precise study of site occupancy and cation distribution. Therefore, we make use of
XAFS spectroscopy because it has revealed itself the most capable technique for
examining the allocation of cation sites in nanocrystalline particles of doped cobalt
ferrites. In the XANES region the absorption modulations are powerful because of
manifold scattering [75-78]. The shift in edge position can elaborate the oxidation
state [79]. The electrical properties of these materials strictly depend on the entire
crystal structure and cation allocation within the crystal structure. Therefore a precise
study for cation distribution in the crystal structure is carried out. Finally the DC and
AC electrical conduction properties of the prepared materials have been investigated
in this research work. It helped us in finding the dependence of electrical conduction
properties on the cation allocation and the entire crystal structure.
1.10 Thesis Synopsis
Thesis consists on an introduction to nanotechnology, synthesis techniques,
characterization techniques, characterizations and discussion of results. Chapter wise
detail is given as under.
17
Chapter 1:
The introduction to the nanoscience, a brief history of ferrites, crystal structure of
spinels and objectives of this research work are discussed. Very inserting crystal
structure and wide range of applications motivates for this research work.
Chapter 2:
Synthesis method plays a very important role in deciding the final material properties.
The fundamental requirement from the synthesis route is to have phase pure material
with desired properties. Synthesis methods can be categorized on the bases of
technique followed. For synthesis of materials at nano length scale there are two main
approaches i.e. bottom-up and top-down approach. Wet chemical methods provide
better options for synthesis of nano ferrites materials. In this chapter different
synthesis techniques have been discussed. Detailed procedure of preparation of
ferrites sample material by co-precipitation method for this research work is discussed
in this chapter.
Chapter 3:
The progress in scientific research based upon the characterizing tools. Every material
has its own specific physical and chemical properties. There are many experimental
techniques to investigate different properties of cobalt based nanocrystalline ferrite
materials. In this chapter the brief introduction to experimental tools and techniques
is given. Also the theories and formulae used in evaluating the results are given.
Chapter 4:
In this chapter the Zn doped CoFe2O4 has been discussed. Zinc is recognized for
playing a vital role in deciding the final properties when it is used as a dopant element
in ferrites [41].By using co-precipitation method nanocrystalline ferrite particles of
Co1-xZnxFe2O4 with x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 were synthesized. Synthesized ferrite
particles were investigated very precisely for their thermal, structural and electrical
properties. XANES spectroscopy was used to study the site occupancy and cation
distribution.
18
Chapter 5:
Manganese substituted cobalt ferrites are promising materials for stress and torsion
sensor applications. The study the cobalt based nanoferrite particles with Mn as a
dopant element with different ratio by weight is presented in chapter 5. Synthesized
samples were characterized to study the effects of successive replacement of Co with
Mn on cation distribution, structural and conduction (electrical) properties. A very
precise technique (XANES) was applied to study the cation shift.
Chapter 6:
CdFe2O4 is generally understood as normal spinel in which all Fe3+ ions occupy B-
sites and all Cd2+ ions occupies A sites. The gradual replacement of nonmagnetic Cd
ions in cobalt ferrite is expected to create considerable distortion in (CoCd)Fe2O4
system. In chapter 6 the study of thermal, structural and electrical properties of
Co1-xCdxFe2O4(x = 0.0 to 1.0) is presented.
Chapter 7:
We have replaced Co from the CoFe2O4 gradually by three different metal cation ions.
The variations in the different properties on account of dopant type and dopant
concentration were observed. A comparison of results thus obtained is given here in
this chapter
Chapter 8:
Conclusions about the results obtained from this research work are given in this
chapter.
Chapter 9:
References and citations are given here in the chapter
19
Chapter 2
2 Synthesis of ferrite nanoparticles
20
2.1 Synthesis of nanocrystalline ferrites
An overlook of the scientific literature plainly signifies that the synthesis is the critical
step in the preparation of a material with desired properties. Components of the
modern age devices need increasing control over the characteristics of materials for
their particular applications. The ultimate material properties deeply depend upon
preparation method. Also the microstructures such as crystallite size distribution,
aggregates, porosity, vacancies, morphology, stresses and defects etc. plays a vital
role in deciding the final performance. These microstructures are mostly affected by
the synthesis route and post synthesis treatments. Moreover, the synthesis technique is
of fundamental importance for the addition of specific chemical elements in the
conventional compounds to modify the properties as per need. There are two main
approaches to synthesize materials at nano length scale [76]:
Top-down technique
Bottom-up technique
Top-down technique involves reduction of size by mechanical process like milling
and grinding. It is a traditional way to manufacture nano materials and have been
employed to produce nanoparticles of minerals for example coal, clay, and metals.
Bottom-up synthesis technique initiates at the molecular stage to generate
nanoparticles. Nano ferrite particles may be produced by a range of methods using
solid, liquid and gas phase processes. These techniques may be classified as
i. Solid state reaction method (Dry method)
ii. Chemical methods (Wet methods)
2.2 Solid state reaction method
Solid state reaction method is a conventional route for the synthesis of ferrites
particles [68]. However, this method is often slow and difficult to carry out to
completion unless performed at high temperatures typically above than 1100 °C, so
that reacting atoms can diffuse through solid materials to the reaction front. For this,
the precursors of metal oxides or carbonates in their stoichiometric proportions are
21
mixed and ground for having homogeneous mixture. This powder is kept at elevated
temperatures for long time. The benefits of solid state reaction method is low cost of
synthesis at industrial scale but short comings of this synthesis route includes the
deprived chemical homogeneity, large grain sizes and broad size division and often
undesirable secondary phases. To surmount these confines, milling and calcinations
are done repeatedly. However, these repeated cycles of milling introduce considerable
quantity of impurities. Therefore, ultimate properties are compromised. Hence,
chemical synthesis routes are now gaining attention.
2.3 Chemical methods
Presently, particular focus has been given to unconventional synthesis routes [80, 81].
Especially wet chemical preparation methods have some certain advantages for
producing fine ferrite nanoparticles. Wet chemical methods have potential to attain
better chemical homogeneity at the molecular level. It is particularly, significant for
electro-ceramic materials because their properties are usually modified by addition of
a precise amount of particular dopant elements. Moreover, several chemical synthesis
routes have ability to produce powder with nano sized grains which could be sintered
at lower temperatures [82, 83]. A basic knowledge of thermodynamics, crystal
chemistry, reaction kinetics and phase equilibrium is vital for obtaining the advantage
of various benefits of chemical synthesis to form new desired materials. There are
different chemical methods for synthesis of ferrites among them a few are described
below.
i. Sol-gel method
ii. Spray pyrolysis
iii. Freeze-drying method
iv. Micro emulsion method
v. Combustion flame synthesis
vi. Hydrothermal method
vii. Chemical co-precipitation method
22
2.3.1 Sol-gel method
It is also known as chemical solution deposition method [80]. In sol-gel technique,
metal oxides are prepared via hydrolysis of reacting precursors then their hydroxides
are produced. The removal of water condensation of hydroxide molecules causes the
development of network of metal hydroxide. By linking of hydroxide species in a
network structure, a dense porous gel is obtained. Finally solvents are removed by
heating which gives a fine powder of the metal oxide [73]. Microstructure like
porosity and crystallinity depends upon the pH, chemical composition and the rate of
removal of solvent. Templates or surfactants are not required in this method [84].
2.3.2 Spray pyrolysis
In this process droplets of aerosol are produced by atomization of the starting
suspension or solution. Evaporation of these droplets occurs by the condensation of
solute within these droplets. Drying of the precipitates with heat treatment gives
porous particles and then by sintering we can achieve dense material. It is a useful
synthesis route for having homogenized pure nanoparticles. However, a large quantity
of uses of solvent increases the cost.
2.3.3 Freeze-drying method
In synthesis by this technique, fine droplets are produced from concentrated solution
and then freezing of these droplets is done by keeping them in a bath at very low
temperature like as ice-acetone or liquid nitrogen. These particles are dried by ice
sublimation in vacuum.
2.3.4 Micro emulsion method
This is one of the famous techniques for the synthesis of nanoparticles [85]. Water
and oil remains in two separate phases on their mixing together. Energy is required
for the combination of these two phases because of interfacial tension between the
water and oil (30-50 dynes/cm). To overcome this, surface active molecules are used
which are called surfactants. It is a simple technique as compared to the normal
emulsion because higher shear conditions are not needed here. Lipophilic (oil-loving)
23
and hydrophilic (water loving) moieties both are present in surfactants. These
surfactant molecules form an interface linking the oil and water by reducing the
interfacial tension [77]. The continuity of phase depends upon the nature of surfactant.
Emulsions are stable kinetically, but are unstable thermodynamically.
2.3.5 Combustion flame synthesis
In this route of synthesis for precursor decomposition the energy is supplied with
burning of precursor by air-fuel mixture. The suitable precursor (gas/vapor) is mixed
with a fuel like methane or acetylene and oxygen. By keeping the temperature
constant this mixture is burnt in a flame. Temperature must be kept constant because
variation in temperature can cause the growth of particles with different size. Nano
particles are formed with the decomposition of precursors.
2.3.6 Hydrothermal method
Hydrothermal is a useful synthesis technique to produce nanostructures [86, 87]. In
this method the synthesis is carried out at high pressure and temperature. Synthesis is
done within a closed container having water as a solvent. Precursors in form of
chlorides or nitrates are typically dissolved in a solvent and placed in an autoclave.
High pressure and temperature is produced inside the autoclave by external heating.
Heating is done by a conventional oven or microwave. The reaction occurring in such
conditions of high pressure and temperature is recognized as a hydrothermal reaction.
A liquid having temperature above of its critical point is known as supercritical and
can acts like both gas and liquid. At this stage the surface tension becomes very low
and it can dissolve the compound efficiently. To increase the reaction rate we can use
electric field, ultrasonic or microwaves. This method has some disadvantages such as
large distribution of particle size, not appropriate for every material and non-
expectable morphology of the material thus produced.
2.3.7 Chemical co-precipitation method
A famous technique for the preparation of fine ferrite nanoparticles is precipitation of
solid particles in a solution. Common process involves simultaneous occurrence of
24
reactions, nucleation, growth and agglomeration process [88]. Heterogeneous or
homogeneous nucleation causes the formation of precipitates. Nuclei formation
generally proceeds by diffusion. Growth rate of the particles is defined by the
concentration of reactants and reaction temperature. For having unagglomerated nano
ferrite particles along with a very small size distribution, nucleation must start at the
same time. And without any particle agglomeration, subsequent growth must occur. In
general the reaction kinetics can affect the physical properties like crystallinity,
microstructures, particle size and size distribution [89]. Furthermore, the pH and rate
of mixing of the solutions are also important.
From synthesis methods discussed above, we employed the chemical co-precipitation
method successfully for obtaining nanocrystalline ferrite particles with different
chemical compositions. This synthesis route is an economical technique to make out
ultrafine particles [86]. Co-precipitation is broadly used to synthesize ferrites
nanoparticles. Co-precipitation synthesis is comparatively simple to scale up and very
high processing temperature or complicated procedures are not required in this
method. Co-precipitation has a superior feature in comparison to other synthesis
routes that it discards most of the impurities to the solution during the formation of
solid product. We used co-precipitation method to synthesize nanocrystalline ferrites
such as Co-Zn ferrites, Co-Mn ferrites and Co-Cd ferrites. The primary molar
proportion (M2+:Fe3+) is always taken1:2 as the stoichiometry. Where M2+ stands for
Fe2+, Co2+, Zn2+, Mn2+, and Cd2+.
There are two steps which are involved in co-precipitation method [90, 91]:
Co-precipitation step
In the initial phase highly dispersed solid hydroxides of metals in the shape of
colloidal particles are acquired by simultaneous precipitation of metal cations in
alkaline medium. The reaction for the synthesis of Co-Zn mix ferrites take place as
25
Fe(NO3)3.9H2O (1-x) + Co (NO3)2.6H2O + xZn (NO3)2.6H2O +8NaOH
↓
(1-x) Co (OH)2.xZn(OH)2.2Fe(OH)3
Ferritization step
In ferritization step, the product obtained from the precipitation step is heated in
alkaline solution to transform metal hydroxides solid solution in to Co-Zn ferrites,
(1-x)Co(OH)2.xZn(OH)2.2Fe(OH)3
↓
Co(1-x)ZnxFe2O4.nH2O + (4-n)H2O
Here ‘n’ is an integer. A particular aspect of the "co-precipitation method" is that even
after many hours of heating in alkaline solution a certain amount of connected water
molecules remains there.
2.4 Influence of different parameters on synthesis by
co-precipitation method
Some important parameters which can influence the synthesis and the ultimate
properties of prepared nano crystalline ferrites particles are given here.
Role of anions
Anions used in the chemical reaction affect the ultimate properties of acquired
particles. However, typically in the synthesis of Co based nanocrystalline ferrite
particles salts are used in the form of chloride or nitrates. Hence, to achieve more
same conditions for all three metals, we used salts in the form of their nitrates [91].
26
Mixing rate of reagents
In deciding the size of the produced particles, mixing rate of reagents plays an
important role. Particle formation in co-precipitation involves two consecutive
processes which are nucleation and succeeding particle growth. Particle size and size
distribution depends on the relative rates of these two processes. We can have small
size distribution by creation of new nuclei and growth of the earlier created particles
simultaneously. Small sized particles can be obtained by fast nucleation formation and
low growth rate. This condition is achieved by quick addition with dynamic mixing of
reagents. On the other hand, slow mixing rate leads to the large size distribution.
Therefore, to have small size with thin size dispersion and chemically homogeneous
particles, we need the fast mixing of reagents.
Influence of temperature
For a chemical reaction the required activation energy is different for different
reagents. Activation energy computed from the reaction kinetics for three diverse
ferrites increases with the sequence as given below in the temperature range 20°C -
100°C. EA (Ni ferrite) > EA(Co ferrite) > EA(Zn ferrite). The increase in temperature
in this range increases the formation rate of ferrites particles.
Role of pH in the reaction
In synthesis of the ferrites by co-precipitation method the yield amount grows with
increase of pH from 6.8 to 8.6. Further raise of pH by 8.6 to 10 results in to only
minor increase in yield but further raise in pH from 12.5 to 14 gives a considerable
increase in the yield. Also, at higher pH values the particle size remains smaller
because of the simultaneous creation of nucleation sites.
Concentration of reagents
For better mixing of the reacting solution, 0.2-0.4 molar solutions are generally taken
for ferrite particles synthesis. Low-viscose suspension is essential for higher mixing
rate. Whereas, with concentration higher than 0.5 molar extensive mixing by stirrer
becomes difficult because of higher viscosity.
27
Role of co-precipitation base
The choice of co-precipitation base is very significant in the synthesis processes for
the preparation of nanocrystalline ferrite particles. Typically NaOH is employed for
co-precipitating. Effort with NH4OH did not proved to be useful [92, 93].
2.5 Synthesis of Zn, Mn and Cd doped CoFe2O4 nanocrystalline
ferrites particles
Co1-xMxFe2O4 (M=Zn, Mn, Cd ) nanocrystalline spinel fine particles were synthesized
by co-precipitating route with x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0. The chemicals reagents
used in our work were ferric nitrate (hydrated) Fe(NO3)3.9H2O, zinc nitrate (hydrated)
Zn(NO3)2.6H2O, cobalt nitrate (hydrated) Co(NO3)2.6H2O, Cadmium nitrate
(hydrated) Cd(NO3)2.6H2O and Manganese nitrate (hydrated) Mn(NO3)2.6H2O. All
reagents used were of analytical grade and used without further purification. To have
Co1-xZnxFe2O4,aqueous solutions (0.4 M) of the chemicals Fe(NO3)3.9H2O,
Zn(NO3)2.6H2O and Co(NO3)2.6H2O, were mixed with steady stirring by a magnetic
stirrer, until a clear solution was obtained. 1.5 M aqueous solution of (NaOH) as a
precipitating reagent was added quickly in metal solutions, kept in a beaker, at room
temperature with steady stirring.
The equation,
(2.1)
was used to measure the chemicals for their stoichiometric quantities. Similar method
was adopted for other compositions like Co1-xMnxFe2O4 and Co1-xCdxFe2O4for all
values of x. To have maximum homogeneity, solution was stirred initially for
30minutes at room temperature and then temperature was raised up to70°C under
steady stirring. To convert hydroxides into ferrites the solution was kept for 45
minutes at 70°C. Rearrangement of atoms and dehydration involves in obtaining
ferrites from intermediate hydroxide phase. By the management of the
co-precipitation steps we can control the particle size during synthesis processes. The
pH value during the reaction was maintained between 12.5 and 13. To get rid of
28
contaminations deionized water was used in washing of product thoroughly. The
particles thus obtained kept at 105°C temperature for overnight in an electric oven to
eliminate water contents. Homogeneous mix of dried powder was achieved by
grinding. A hydraulic press was used to for making pellets of grinded powder. The 50
lb/in2load was applied on each pellet of for 5 minutes. Sintering of these pellets was
done at 600 °C for 3h. The samples were cooled by slow cooling to room temperature
and were used for further purification. The flow chart for synthesis processes for
Co1-xZnxFe2O4is given in Figure 2.1. Similar method was adopted for other
composition like Co1-xMnxFe2O4 and Co1-xCdxFe2O4 for all values of x (x=0.0, 0.2,
0.4, 0.6, 0.8, 1.0).
29
Co-precipitation method
Figure 2.1 Flow chart of co-precipitation method for the synthesis of Co1-xZnxFe2O4
Fe(NO3)
3.9H
2O Co(NO
3)
2.6H
2O Zn(NO
3)
2.6H
2O
Aq. sol NaOH
pH = 12.5
0.1
+ + Aqueous solution
Aqueous solution
Aqueous solution
All well stirred solutions are mixed together
Washing
& Drying
Sintering at
600±5oC for
3hrs
Pellets
Characterizations
Stirred at 70 0C
30
Chapter 3
3 Characterization techniques
31
Each material has its specific physical and chemical properties. There are many
experimental techniques to investigate different properties of cobalt based
nanocrystalline ferrite materials. To investigate the structural, thermal and electrical
properties of the synthesized materials in the present research work following
techniques were used.
3.1 Thermal analysis
To know the specific transition temperatures and information about the suitable
sintering temperature for having pure phase crystalline material, we use Differential
Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) techniques. We
employed DSC and TGA techniques on, as prepared powder samples before any
further heat treatment. This analysis was done from room temperature to 1000°C.
3.1.1 Differential scanning calorimetry (DSC)
DSC is applied to examine the loss of solvents, glass transition crystallization
temperature, melting temperature and other processes concerning energy changes
[93]. We measured the energy produced (exothermic) or absorbed (endothermic) as a
function of temperature and time. A model DSC scan is shown here in Figure 3.1.
Figure 3.1 A model DSC scan
32
Simultaneous heat flow to the sample and a reference at the same temperature is
recorded. A reference material was used for this purpose. The temperature of the
sample material and reference are raised at a constant rate. Because the DSC analysis
is carried out at a same pressure the heat flow is same to enthalpy. The differential
rate of heat flow dH/dt between sample and reference may be either positive or
negative consistent with exothermic or endothermic reaction. In a thermogram the
area below the peak is directly proportional to the heat evolved or absorbed in the
reaction as shown in the Figure 3.2. The curve height is directly proportional to the
reaction rate. We analyzed our samples from room temperature to 1000°C with
calorimetric accuracy of ±2%.
Figure 3.2 Schematic diagram of processes involving DSC
3.1.2 Thermogravimetry analysis (TGA)
TGA is an analytical technique used for finding the materials thermal stability and
fraction of its portion of volatile material. It is done by observing the weight change
which arises when the sample is heated. Usually this measurement is taken in air or in
33
an inert gas like Argon or Helium [94]. The weight is recorded as a function of
increasing temperature (Figure 3.3). The weight changes may occur due to absorption,
adsorption, desorption decomposition, oxidation or dehydration of a specimen as a
function of temperature or time. These specific characteristics are associated with the
chemical crystal structure of the material. The thermogravimeteric analyses of
synthesized sample materials were done from room temperature to 1000°C with
accuracy of ±2%.
Figure 3.3 Α schematic thermo balance
3.2 Structural analysis
3.2.1 X-ray diffraction (XRD) analysis
XRD is a versatile technique which is non-destructive and can explore many details
regarding the crystallographic structure of the sample material. Atoms allocated in a
regular order in the space forms a crystal lattice. These atoms are in a pattern to form
a sequence of parallel planes apart from each other by a specific distance (inter-planer
distance) d which depends on the nature of the material. These planes of atoms in a
crystal lattice can be associated to the unit cell [95].
34
While X-ray shines at a powder specimen, the atomic layers in the crystal perform
like mirrors which reflect the beam of X-ray. The electric field associated with X-ray
gets scattered after interacting with that of charged entities like electrons in an atom
with amplitude given as 𝐹(𝑞) = ∫ 𝑓(𝑟) 𝑒𝑖𝑞−𝑟𝑑𝑟; here ‘f(r)’ is directly proportional to
electron density and ‘q’ represents the variation in the incident and scattered wave
vectors. Constructive or destructive interference may occur between the scattered
waves. Diffraction peaks are obtained for constructive interference. In this way the
obtained interplanar distances are used for identification of the crystal structure. The
width of the line and peak shape depends upon the crystal structure of the sample
material. The schematic illustration of the Bragg’s Law is given in Figure 3.4.
2dsinθ = nλ
Where (n = 1, 2, 3…)
Figure 3.4 Diffraction of scattered X-ray beam from atoms in a crystal
In this research work, the XRD machine (PANalytical X'pertpro MPD) with radiation
Cu-Kα was used. Data obtained from XRD was used to have information about the
crystal chemistry, phase identification, phase purity and crystallite size. The X-ray
35
used were CuKα (λ=1.5406 Å) while the tube was operated at 30mA and 40kV. The
X-ray patterns were recorded by varying 2θ from 20ο to 80ο. The accuracy in θ
measurements is better than 0.01degree.
3.2.1.1 Structure determination
The crystallite sizes were calculated through the Scherrer equation [96, 97] by using
the full-width at half-maximum (FWHM) value of the most intense peak, which is
(311).
0.89
cost
(3.1)
In this formula, β is the FWHM of the intensity versus 2θ profile, “λ” is the
wavelength of the Cu Kα (incident radiation X-ray, λ=1.5406 Å), and θ is the Bragg
diffraction angle. Scherrer equation assumes approximation and give the crystallite
size if the particle size distribution is narrow and strain induced effects are quite
negligible.
The lattice constant ‘a’ was determined by the check cell software using (h k l) values
obtained from XRD data. For the calculation of the theoretical density for the
synthesized samples we use the relation.
3x
nM
Na
(3.2)
Here M is the molecular weight of the sample, n is the number of molecules per unit
cell, 'a’ is the lattice parameter, and N is the Avogadro's number. The measured
density, ρm was determined by using the relation
2m
m
r h
(3.3)
36
Here r represents the radius, m is the mass, and h is the thickness of the sample pellet.
For the determination of porosity (P) of the synthesized samples for all compositions
we use the relation.
1 m
x
P
(3.4)
Where ρm is measured density and ρx is theoretical density.
Using lattice constant the distances between cation ions i.e. hopping length in
tetrahedral A sites (LA) and in octahedral sites B sites (LB) were calculated by the
following relations [88]
𝐿𝐴 =𝑎√3
4 (3.5)
𝐿𝐵 =𝑎√2
4 (3.6)
3.2.2 X-ray absorption fine structure (XAFS) spectroscopy
When an assembly of atoms is exposed to X-ray, at certain energy, a sharp rise in the
absorption is observed. This sharp rise is called the absorption edge. The binding
energy of a core level determines the absorption edge energy. There is a range of
information in the X-ray absorption edges about the local structure and the chemical
state of the absorbing atom. Extended X-ray absorption fine structure (EXAFS) and
X-ray absorption near-edge structure (XANES) spectroscopy are powerful tools for
the structural study of metal oxide materials [93]. These techniques are element
specific and sensitive to the local structure.
In X ray absorption the primary process is the photoionization of an atom. Excitation
of inner or outer electron is achieved which is defined by the photon energy ħω. In
XAFS electrons are made ejected from K-shell (inner-shell). It requires a minimum of
energy (ħω = EB, here EB represents the binding energy of the K-shell electrons). K
edge absorption is observed in the XAFS spectrum at the first excitation threshold
when photon energy equals to the K shell binding energy. Now it has been established
37
that XANES region extends by 50-100 eV ahead of the absorption edge [94]. It is
defined by the local vacant states density in an absorbing atom and also due to
multiple scattering. XANES spectrum is highly responsive to coordination chemistry
(tetrahedral coordination and octahedral coordination) and to oxidation state of the
absorbing atom. In the presence of the neighboring atom, the photo electron will
scatter by the electrons of the neighboring atom. In this way scattered photo electron
will return back to absorbing atom. In Figure 3.5 the photons absorption by an atom in
a solid is illustrated [94].
Figure 3.5 (a) Energy levels of an electron in the crystal lattice for excitation energies
corresponding the single (EXAFS) and multiple scattering for (XANES) (b)
Corresponding to a free electron outgoing wave and scattered wave interference (A)
X-ray photon absorbing atom, (B) neighboring atom (c) dependence of energy of
absorption coefficient μ (left) in the absence of neighboring atoms (right) In the
presence of scattering from the neighboring atom [98].
38
Because of the dependence of absorption coefficient on an available electronic state
the existence of photo-electron scattered back from the neighboring atom will change
the absorption coefficient. Thus X-ray absorption fine structure (XAFS) spectrum can
be divided into two different parts XANES and EXAFS depending on the energy
range of the X-ray beam compared to the absorption edge. A typical XAFS spectrum
is shown in Figure 3.6 [99].
Figure 3.6 A typical XAFS spectrum
To have X-ray absorption fine structure (XAFS) measurements pellets were prepared
by pressing the mix of 100 mg boron nitride powder and 5to10 mg of sample powder
(nanometric). The EXAFS measurements were taken in transmission mode at room
temperature for Fe K-edge, at the XAFS beam line 11.1 in ELETTRA synchrotron
light source with a double crystal Si (111) as a monochromator. The K-edge and pre-
edge regions were scanned at uniform energy steps ΔE of 0.2 and 5 eV, respectively,
while the scanning of EXAFS region was done at uniform photoelectron wave
number steps of Δk = 0.03 Å−1. The time for data acquisition was 2 s/point. For each
stoichiometric sample two spectra were collected for every edge in order to get better
results with least statistical noise.
For data acquisition the most commonly used method is transmission mode. The
schematic of the XAFS experiment is given in Figure 3.7.
39
Figure3.7 Schematic of the XAFS experiment
The beam intensity is measured before and after interaction with sample. The process
has accordance with the Bear-Lambert’s law of dilute solutions.
I = Ioexp(-µd) (3.7)
Io represents the photons intensity before their interaction with the sample of thickness
d, and I1, I2 are the intensities after interaction with the samples 1 and 2 respectively.
Ionization chambers filled by a noble gas or N2 are usually used to measure the X-ray
intensity. The sample is placed amid two chambers, which are linked with a normal
current measuring device. The current achieved is proportional to X-ray intensity. In
such a way the obtained physical quantity is the total inelastic photo absorption cross
section. Energy calibration was done by locating the first inflection point of the
absorption edge of metallic Fe to 7112 eV. For the purpose of energy-scale
calibration, a metallic Fe foil was placed after the second ionization chamber. The
edge of metallic Fe foil spectra was aligned within 0.01 eV. The relation for EXAFS
signals is χ(k) = (μ − μ0)/μ0, where μ represents the absorption coefficient
(experimental) and μ0 is a smooth spline symbolizing the fixed-atom absorption
background [92].
40
To infer the results from the experimental data the normalization procedure was done
by Xanda VIPER (Visual Processing in EXAFS Research) program. For the
normalization procedure for XANES spectra the smooth pre-edge absorption was
subtracted from the experimental spectra and taking unity as edge jump height. In the
XANES spectrum the transitions from 1s to 4p gives main peak and observed small
pre-edge peak is due to the transition from 1s to 3d [100, 101]. Both the quadrupole
transitions from 1s to 3d and dipole transitions from 1s to 4p are allowed for the Fe K-
edge, although the quadrupole transition intensity remains very low generally [102].
The rise in the pre-edge peak intensity is attributed to the local mixing of 3d and 4p
orbitals, which is permitted in case of tetrahedral symmetry while it is not allowed in
case of octahedral symmetry, because of the presence of inversion symmetry. The
peak height of this pre-edge peak gives the information about the site occupancy of
cations. So with the help of XANES study we can estimate the degree of inversion in
spinel ferrites.
3.3 Electrical properties
3.3.1 DC electrical properties
One of the most significant properties of the soft ferrites is their resistivity. Electrical
properties of ferrites depend upon chemical composition, synthesis method and
various heat treatments during the preparation. While doing resistivity measurements,
temperature was also factor to consider. The dc electrical resistivity is given by
𝜌 =𝑅𝐴
𝐿 (3.8)
Where A = Cross section area of the sample pellet
R = Material resistance
L = Thickness of the sample pellet
Resistance occurs due to the following reasons
i. In case of metals with increase in temperature, vibrations of lattice ions cause
the increase in resistance. But in the materials such as semiconductor where
41
charge carriers movement increases with increase in temperature, the
resistance decreases.
ii. Imperfections and dislocation in a crystal lattice is also a cause of resistivity
which is lower than that due to lattice vibrations.
Electrical properties of ferrites are influenced by their method of preparation and their
composition. The most common methods to determine the electrical resistivity are the
following [103],
i. Two probe method
ii. Four probe method
Four probe method is used for the samples with low resistance. Two probe method is
used most commonly for the measurement of samples with high resistivity. For
semiconductors and insulators when the resistivity of the samples itself is very high,
the contact resistance becomes negligible and two probe method is applicable. To
measure the dc resistivity of our samples, we used two probe method. The schematic
diagram for both the two probe and four probe method is given in Figure 3.8.
Figure 3.8 (a) Two-probe (b) Four-probe resistance measurement circuitry
In two probe method the contact resistance is not included because it is negligible as
compared to the sample resistance. The circuit diagram of the system used to measure
the dc resistivity is shown in Figure 3.9. The sample was placed between two
42
electrodes in pressure contacts. The electrodes with sample specimen kept in the tube
furnace. The temperature in the furnace was controlled by a power regulator. The
pressure contacts are made between electrodes and pellet. A dc source is employed
across the sample.
Figure 3.9 Circuitry of the apparatus used for dc electrical resistivity measurements
The change in current flowing through the sample due to known value of applied
voltage is measured as a function of temperature at step size of 5C. The accuracy in
resistance measurements was 0.1 %. The data acquired in such a way was used to
evaluate the temperature dependent dc resistivity and activation energy of the
synthesized samples. Arrhenius relation was used to find the activation energy [100].
𝜌 = 𝜌0 𝑒𝑥𝑝 (∆𝐸
𝑘𝐵𝑇) (3.9)
where ‘ΔE’, ‘kB’ ‘ρ’,‘T’ and ‘ρ0’, are the activation energy, Boltzmann’s constant,
resistivity at temperature T, temperature, and resistivity respectively.
For all the samples drift mobility (µd) was determined using the following relation
1d
ne
(3.10)
43
Here ρ is the electrical resistivity at a particular temperature, e is the charge of an
electron and n is the concentration of charge carrier, which can be calculated from
the relation,
a m FeN Pn
M
(3.11)
Where Na is the Avogadro's number, M is the molecular weight, ρm the measured
density of sample and PFe is the number of iron atom in the chemical formula unit of
the ferrites.
3.3.2 Frequency dependent alternating current (ac) measurements
Precision component analyzer (6400B) was used to measure ac electrical properties. It
offers both two probe and four probe methods to measure different ac quantities in 20
Hz to 3 MHz frequency range. The derive level of this apparatus to measure ac
quantities ranges from 1mV to 10V rms. It measures real as well as the imaginary
component of the impedance vector. Which is a significant parameter to characterize.
The impedance vector can be written as Z=R+jX, where R is the real part and X is the
imaginary part. The response of the real and imaginary factors is shown in Figure
3.10.
Figure 3.10 (a) Relaxation time (τ) of masses after turning off the applied electric
field. (b) Impedance plane of real capacitor (c) Response of real ε′ and imaginary ε′′
parts of permittivity
44
Admittance is the reciprocal of the impedance. The reactance purity is the quality
factor Q. For capacitors the dissipation factor (D) is generally spoken in terms of tan
δ. Tan δ is the ratio of the energy dissipated by a component to the energy stored in
that component. Accuracy in capacitance measurements was 0.2 % whereas the
accuracy in measurements of dissipation factor was 0.02 %.
The dielectric characteristics of ferrites normally emerge from hopping of charge
carriers (electrons or polarons) between cationic sites and across the grain boundaries.
Charge polarization owing to applied external electric field is the source of dielectric
behavior. Polarization occurs by the following four basic mechanisms. These
processes are shown in the Figure 3.11.
Figure 3.11 General polarization mechanisms
45
A. Interfacial polarization or space charge polarization
Placement of charges at grain boundaries, inter phase boundaries and at the
surfaces skin contribute to the dielectric response of the material when
external field is applied.
B. Dipolar or orientation polarization
Some materials are naturally dipolar such as water. The orientation of
dipoles of these materials against the applied field also contributes in
polarization.
C. Ionic polarization
Ionic crystals have incorporated dipoles which just revoke one another. The
slight displacement from their mean position due to external applied fields
can make a net polarization.
D. Electronic or atomic polarization
Usually all materials have this kind of polarization. In general an atom has
spherical symmetry with its electrons. An applied electric displace the
electrons slightly at s side from the nucleus and as a result dipole moment is
formed.
All of the four mechanisms of polarization take part towards the dielectric constant at
lower frequencies for an applied electric field.
46
Figure 3.12 ε′ and ε″ as a function of frequency and different polarization mechanisms
Overall volume polarization is the vector sum of all above mentioned dipole moments
per unit volume. The movement of charges in polarization mechanisms cannot follow
the higher frequencies and their contribution vanishes one by one. The dielectric
constant decreases with rise in frequency and at each relaxation frequency, there is a
peak of dissipation factor as shown in the Figure 3.12 [101].
The complex dielectric constant can be expressed as
휀∗ = 휀′ − 𝑗휀′′ (3.12)
where ‘ ’ ‘ ’ ‘* ’, are imaginary part, real part and complex part of the dielectric
constant respectively.
The dielectric behavior can be interpreted on the foundation of Koop’s theory [102],
which stands on the Maxwell-Wagner’s double layer model [103]. As said by
Maxwell-Wagner’s model the dielectric structure of the material is composed by two
layers. Well conducting grains make first layer which are separated by poor
conducting grain boundaries making the second thin layer. At lower frequencies,
47
grain boundaries are found to be more effective and relatively low resistive grains are
more effective on higher frequencies. One can derive complex dielectric constant by
using Maxwell-Wagner two layer model, which is given by,
휀∗ = 휀∞ +휀𝑠 − 휀∞
1 + 𝑗𝜔𝜏− 𝑗
𝜎𝑑𝑐
𝜔휀0 (3.13)
where‘0’ ‘’ ‘dc’, ‘’,and ‘s’ are dielectric constant of free space, dielectric
constant for electronic polarization, conductivity, relaxation time , dielectric constant
at dc field and =2f.
From the above equation the real part of dielectric constant is given by
휀 ′ = 휀∞ +휀𝑠 − 휀∞
1 + 𝜔2𝜏2 (3.14)
The dielectric constant () was calculated from capacitance (C) values found from
precision component analyzer in all frequency range using equation [102]
휀′ =𝐶𝑑
𝐴𝜀0 (3.15)
Here ‘d’, ‘0’ and ‘A’ are the thickness, permittivity of free space and face area of the
pellet respectively. The dielectric loss factor (ε'') and alternating current conductivity
(ac) was also calculated by using the following appropriate relations [103]
= 𝑡𝑎𝑛 𝛿 (3.16)
𝑎𝑐 = 𝜔휀0휀′𝑡𝑎𝑛𝛿 (3.17)
where ω=2πf.
AC conductivity in nanocrystalline material is frequency dependent. In the low
frequency region the change in conductivity is due to the interfacial polarization
effects. As the frequency reduces the interfacial charge accumulation occurs and
ultimately conductivity drops. At the higher frequency values the conductivity
increases. The dependence of conductivity is simply related by the Jonscher’s law as
48
ac = 0 + A 𝜔n (3.18)
Here pre-exponential constant is represented by A and 𝜔 = 2πf represents the
angular frequency, n stands for the exponent of the power law and its value lies
between 0 and 1. We employed the Jonscher’s power law to find the conductivity
mechanism. ac represents the ac conductivity and 0 stands for the limiting
conductivity at zero frequency.
An impedance analysis is an important way to explore the electrical characteristics of
nanocrystalline ferrites materials. We can separate the resistance of grains from the
other resistance sources like grain boundaries. The Nyquist plots give us useful
information about the reactive “imaginary part” and resistive “real part” components
of the synthesized material. To measure the dispersion, an alternating field with
varying frequency is applied on material and response is observed as a function of
frequency. Complex plane graphs (Nyquist plots) can be drawn in form of any one of
the four achievable complex relations, the impedance (Z), the permittivity (ε), the
admittance (Y), the electric modulus (M) and dielectric loss (tan δ). These are related
to each other [102] as
𝑡𝑎𝑛 𝛿 =
=
Z
Z=
Y
Y=
M
M (3.19)
For differentiating between the contribution of grains and grain boundaries the
complex plane plots has been drawn in the frequency range of 20 Hz to 3M Hz at
room temperature. Nyquist plots were attained by plotting the () against ().
Usually, two semicircles can be observed in these plots; first semicircle for low
frequency region signifies the resistance of the grain boundary whereas the second
semicircle achieved at high values of frequencies indicates the resistance of grains.
Two semicircles trend, which is attributed to the resistance of the grain and grain
boundaries can be modeled approximately with an ideal equivalent circuit consisting
on two parallel R–C elements in series. A model complex plane plot for parallel RC
equivalent circuit is shown in Figure 3.13. Errors in measurements are indicated in
graphs, usually these are within the plot symbols.
49
Figure 3.13 A model complex plane plot for parallel RC equivalent circuit
50
Chapter 4
4 Thermal, structural and electrical properties
of nanocrystalline Co1-xZnxFe2O4
(x = 0.0 to 1.0)
51
Crystal structure and cation distribution at particular sites in crystal lattice plays
primary role in deciding the properties of nanocrystalline transition metal oxide
materials. Zinc is well known for playing a vital role in influencing the ferrite
properties [104]; thus the composition was varied by successive replacement of Co by
Zn in CoFe2O4. Nanocrystalline ferrite particles of Co1-xZnxFe2O4 with (x = 0.0, 0.2,
0.4, 0.6, 0.8, 1.0) were synthesized by co-precipitation method as explained in
synthesis method in section 2.5. Synthesized ferrite particles were investigated very
precisely for their thermal, structural and electrical properties. Site occupancy and
cation distribution is determined by x-ray absorption near edge spectroscopy
(XANES).
4.1 Thermal Analysis
Specific transition temperatures were investigated by differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA). DSC analysis was employed on, as
prepared powder samples before any further heat treatment. Thermograms for DSC
and TGA of CoFe2O4 and ZnFe2O4 from ambient temperature to 1000°C are shown
in Figures 4.1 and 4.2 respectively.
Figure 4.1 (a) The differential scanning calorimetry (DSC) of CoFe2O4
52
Figure 4.1 (b) The differential scanning calorimetry (DSC) of ZnFe2O4
Figure 4.2(a) The thermogravimetric analysis (TGA) of CoFe2O4
53
Figure 4.2(b) The thermogravimetric analysis (TGA) of ZnFe2O4
In DSC graphs for both samples; a wide exothermic peak without any sharp peak for
any kind of transition indicates the temperature range for crystal recovery
temperature. In TGA plot all major weight loss processes in both compositions has
been completed before 600°C. Weight loss processes begins from room temperature.
Initially from room temperature to about 100°C the weight loss is due to dehydration
of the material. At the next step, the major weight loss occurs from 100°C to 300°C.
This weight loss can be attributed to the decomposition of nitrates, removal of OH
ions and some organic residues that were left in the sample. Also at this stage the
crystallization of sample materials is started. After 300°C the weight loss is due to the
removal of structural water. After 600°C the samples becomes stable by weight loss
[105]. The composition ZnFe2O4 is more thermally stable than CoFe2O4, keeping in
view the lesser weight loss. Considering the DSC and TGA graphs of CoFe2O4 and
ZnFe2O4, temperature of 600°C was selected for sintering of samples to obtain the
maximum crystalline phase.
54
4.2 Structural properties
4.2.1 X-ray diffraction (XRD) studies
The X-ray diffraction (XRD) patterns confirmed the FCC spinel structure for all the
samples of Co1-xZnxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) nanocrystalline particles with
‘x’ changing from 0.0 to 1.0 as shown in Figure 4.3.
Figure 4.3 X-ray diffraction pattern of Co1-xZnxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)
The crystallite sizes were calculated by the data obtained from the XRD. The
crystallite sizes were in the range from 102 nm to 354 nm evaluated by Scherrer
equation through the full-width at half-maximum (FWHM) value of the most intense
peak which is (311). The lattice constant 'a' is found to increase with increase of Zn
concentration. It is due to the fact that the radius of Zn2+ ions (0.82 Å) is larger than
that of the Co2+ ions (0.78Å). The addition of Zn at the expense of Co in the
composition is expected to increase the lattice constant. The linear increase in lattice
20 30 40 50 60 70 80
(3 3 1)
(5 3 3)(5 1 1)
(4 2 2)(4 0 0)
(3 1 1)
(2 2 0)
x = 0.0
x = 1.0
x = 0.8
x = 0.6
x = 0.4
x = 0.2
Inte
nsit
y (
a.
u)
2Degree)
(4 4 0)
55
constant with increase in zinc contents indicate that lattice expands without upsetting
the symmetry of lattice. It is in accordance with Vegard’s Law [106]. The values of
lattice constants and crystallite sizes with possible errors are given in table 4.1 and are
also shown in Figure 4.4.
Figure 4.4 Change in lattice constants and crystallite size with Zn concentration in
Co1-xZnxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)
Lattice constant values are in good agreement with the reported values [107].
Increases in crystallite sizes come about due to coalescence. During sintering two or
more particles seems to fuse together by melting of their surfaces. In nano sized
particles, surfaces melts below the melting point of their bulk counter part because of
large fraction of low coordinated atoms that lies at surface skin. The values of
theoretical density (ρx), measured density (ρm), and porosity (P), are given in table 4.1.
It was observed that due to increase of Zn concentration, no significant change
occurred in theoretical density. Because with increase of Zn concentration lattice
constant increased, this decreased the density but at the same time the Zn atoms which
56
replace Co atoms, have greater atomic mass as compared to that of cobalt atoms and
compensate the decrease in density.
The distance between cations at two consecutive tetrahedral A sites (LA) and between
two consecutive octahedral B sites (LB) plays an important role in the electrical
conduction phenomenon. These are determined by using the equations 3.5 and 3.6.
For the conduction mechanism, electrons hop between ions of the same element
existing in different valence states on equivalent lattice sites. The change in hopping
lengths LA and LB is shown in Figure. 4.5 and are given in table 4.1 with possible
errors. Both LA and LB increases with increase in Zn content. This is due to the
increase in lattice constant in the crystal geometry of the prepared samples.
Figure 4.5 Change in hopping length between consecutive tetrahedral A sites (LA) and
consecutive octahedral B sites (LB) with Zn concentration (x) in Co1-xZnxFe2O4
(x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)
0.0 0.2 0.4 0.6 0.8 1.0
3.60
3.61
3.62
3.63
3.64
3.65
3.66
LA
LB
x ( Zn Concentration )
LA
(Ho
pp
ing
len
gth
) Å
2.95
2.96
2.97
2.98
2.99
LB
(Ho
pp
ing
Len
gth
)Å
57
4.2.2 X-Ray absorption fine structure (XAFS) studies
We are concerned primarily to absorption coefficient µ in X-ray absorption
spectroscopy. µ gives the probability that according to the Beer’s law the X-ray may
be absorbed. When X-ray having energy equal to the core level electron’s binding
energy shines on that atoms, we get a sharp increase in absorption, which is called
absorption edge and the core level promoted to continuum. Measurements of XAFS is
actually a measure of energy dependence of µ above and at the binding energy of core
level. The XAFS measurements were taken at room temperature in transmission mode
at Fe, Zn, and Co K-edges, at the XAFS beam line of the ELETTRA synchrotron light
source. After normalization the absorption spectra of the near-edge region (XANES)
for Fe, Zn, and Co K-edges for all compositions of CoxZn1−xFe2O4 are given in Figure
4.6. XANES region of the XAFS spectra provides information for the oxidation state
of the absorbing atom, its local atomic surroundings, especially on the site symmetry.
Typically the oxidation state changes by +2 to +3 when the edge position gets rises
about 3 eV [108]. There are some characteristic features in XANES spectrum about
the transition metal oxides. A rise in the valence of a metal atom causes a shift to
higher energies. Absorption edge shoulders and the main peak correspond to the sharp
resonance of the environment of the metal atom and excitation to 4p states. The d-
states splitting which is dissimilar for octahedral and tetrahedral site symmetry also
influence the shape of peak for pre-edge. The broadening in main peak for a certain
environment occurs due to multiple scattering by surrounding atom shells. With
decrease in centrosymmetry degree of the environment the intensity of the pre-edge
peak increases.
There is an eminent behavior of the pre-edge peak for sites with octahedral and
tetrahedral symmetry and these are present in the spinel crystal structure. The pre-
edge peak height is more intense for the tetrahedral symmetry and less intense for
octahedral symmetry. This is mainly because of highly non centrosymmetric
tetrahedral symmetry. This enables the transition from p to d state and it contributes to
height of the pre-edge peak. The pre-edge peak becomes the addition of these
contributions, when both octahedral and tetrahedral sites are occupied; the peak
intensity increases directly with increase in proportion of tetrahedral sites. In the Fe
58
K-edge data, the intensity of the pre-edge peak increases relatively with increasing Co
content. This shows that an increasing fraction of Fe occupies the tetrahedral sites as a
function of Co. Investigation of the XAFS spectrum showed that end member
CoFe2O4 in the series CoxZn1−xFe2O4 has an inverse spinel structure while ZnFe2O4
has a normal spinel structure depending upon the cation distribution. For K-edge of Fe
the intensity of the pre-edge peak increases with increasing Co contents (Figure 4.6)
[109]. It shows that rising Fe fraction goes to tetrahedral sites as a function of Co. In
the intermediate compositions of CoxZn1−xFe2O4 when 0.2 ≤x ≤ 0.8 Co and Zn atoms
occupy always the octahedral and tetrahedral sites, respectively.
Figure 4.6 XANES spectra after normalization at K-edge of Fe (a), K-edge of Co (b),
and K-edge of Zn (c) for Cox Zn1−x Fe2 O4.‘A’ indicates the pre-edge peak
59
These results clearly suggest that we can change the degree of inversion successively
from a complete normal ferrite (ZnFe2O4) to an inverse ferrite (CoFe2O4) by changing
the stoichiometries. Hence the material properties can be controlled by selecting a
certain composition.
4.3 Electrical measurements
4.3.1 DC electrical properties
Hopping of electrons between ions of the same element existing in different valence
state at equivalent lattice sites causes the conduction in ferrites occurs [110]. For all
these samples the dc electrical resistivity was measured by two probe method from
313 K to 700 K temperature range. It was observed that the dc electrical resistivity
rises notably with increase in Zn content as shown in Figure 4.7. Accuracy in
measurements has been mentioned in section 3.3.1. The change in resistivity at
different temperature values for all the compositions is shown in Figure 4.8. At all the
temperature values the resistivity increases with increase in Zn concentration. The
resistivity is maximum at lowest temperature. The variation in resistivity can be
elucidated on the basis of actual location of cations in the spinel structure and also by
microstructures of the material. The spinel cubic structure of ferrites consists of two
interpenetrating sub-lattice shaping two kinds of interstitial sites where metal ions are
placed, the A-sites and B-sites. Now it is an established fact and also suggested by
Hosseinpour et al. (2007) that the general distribution of cations in the mix spinel
ferrites is [110]
(M2+1-(δ+γ) Fe3+
(δ+γ)) 8[M2+
δ Fe2+γFe3+
2-(δ+γ)] O2-
32
There are two possible electron hopping processes, the first process is
Fe2+ + Fe3+ ↔ Fe3+ + Fe2+
By this process, an electron can jump from Fe2+ the divalent ferrous ion at an
octahedral B site to the Fe3+ a trivalent ferric ion at similar site. By this hopping
mechanism the ionic state and charge neutrality of the crystal stay unaffected as
under.
60
(Fe3+ ) [Fe2+ + Fe3+] O2-4 = (Fe3+ ) [Fe3+ + Fe2+] O2-
4
The second possible reaction is
M2+ + Fe3+ ↔ M3+ + Fe2+
But if the electron hopping occurs in this way, the initial and final charge state of the
crystal will become changed. So the hopping of electrons between Fe2+ and Fe3+
existing at the equivalent lattice sites is the most favorable mode of conduction
mechanism. Therefore in CoZnFe2O4 conduction mechanism is considered as the
electron hopping between Fe2+and Fe3+in (B) sites and mobility of holes between Co2+
and Co3+ ions [111]. The tetrahedral A site is strongly preferred site of occupancy for
Zn in Zn Fe2O4 as shown in the XANES analyses above.
Figure 4.7 Variation of resistivity ρ (Ω-cm) with change in temperature for
Co1-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0)
300 400 500 600 700
0.0
2.0x107
4.0x107
6.0x107
8.0x107
1.0x108
-cm
)
Temperature (K)
x=0.0
x=0.2
x=0.4
x=0.6
x=0.8
x=1.0
61
Figure 4.8 Variation in dc electrical resistivity at different temperatures of
composition Co1-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0)
Figure 4.9 Variation of resistivity ρ (Ω-cm) with change in temperature for
Co1-xZnxFe2O4 with linear fit
0.0 0.2 0.4 0.6 0.8 1.0
-5.0x105
0.0
5.0x105
1.0x106
1.5x106
2.0x106
2.5x106
3.0x106
3.5x106
4.0x106
4.5x106
cm
)
Concentration x
373 K
473 K
0.0
2.0x103
4.0x103
6.0x103
8.0x103
1.0x104
1.2x104
1.4x104
573 K
673 K
15 20 25 30 35 40
4
6
8
10
12
14
16
18
20
ln
(
-cm
)
1/kBT (eV)
-1
X = 1.0
X = 0.8
X = 0.6
X = 0.4
X = 0.2
X = 0.0
62
The addition of Zn ions at the expense of Co leads to increase of Fe3+ and
corresponding decrease of Fe2+ ions in octahedral site and a simultaneous decrease of
Co2+ ions. By this the available Fe2+ and Fe3+ pairs at octahedral B-sites decreases and
hence conduction phenomenon decreases. In Figure 4.8 the variation in dc electrical
resistivity at different temperatures for all the compositions is shown.
With increase of Zn concentration the lattice expands and jump length for hopping of
electrons increases as given in table 4.1 and shown in Figure 4.5 which increase the
required activation energy, therefore dc resistivity increases with increase of Zn
concentration. At higher temperature at about 400 K conductivity is attributed to the
hopping of holes between Co2+ and Co3+ ions. So the placement of zinc at the expense
of cobalt will consequently decrease the process and as a result conductivity decreases
[110]. The structure of the prepared material consists of conducting grains separated
by highly resistive thin layers (grain boundaries). Decrease in resistivity in the
temperature range from 313 K to 700 K, with increase of temperature showed the
semiconducting nature of the material. The values of activation energies (E)
evaluated through the slopes of the linear plots of dc electrical resistivity are shown in
Figure 4.10.
Activation energies increase with increase of Zn content. It can be attributed to
increase in hopping length and lattice expansion with increase of Zn concentration.
E ranges from 0.518 eV to 0.537 eV clearly shows that the conduction is occurring
here due to polaron hopping [110]. The hopping depends upon the activation energy,
which is associated with the electrical energy barrier experienced by the electrons
during hopping. The plots for drift mobility are shown in Figure 4.11. Drift mobility
is decreased with increase in Zn concentration which increases resistivity. Drift
mobility is low at low temperatures and increases sharply with increase in
temperature.
63
Figure 4.10 Variation in activation energy (E) with change of Zn concentration (x)
in Co1-xZnxFe2O4
Figure 4.11Variation of drift mobility µd (cm2/Vs) with change in temperature for
Co1-xZnxFe2O4
0.0 0.2 0.4 0.6 0.8 1.00.515
0.520
0.525
0.530
0.535
0.540
(
eV)
x ( Zn concentration)
15 20 25 30 35 40
-2.0x10-6
0.0
2.0x10-6
4.0x10-6
6.0x10-6
8.0x10-6
1.0x10-5
1.2x10-5
1.4x10-5
1.6x10-5
1.8x10-5
2.0x10-5
d
(cm
2/
Vs)
1/kBT(eV)
-1
X = 0.0
X = 0.2
X = 0.4
X = 0.6
X = 0.8
X = 1.0
64
4.3.2 AC electrical properties
4.3.2.1 Dielectric constant
The dielectric constants () of zinc substituted cobalt ferrites have been measured at
room temperature in the frequency range from 100 Hz to 3 MHz. The plots of
dielectric constant versus frequency show normal dielectric behavior of the spinel
ferrites. The variations of dielectric constant as a function of frequency for mixed Co-
Zn ferrites with different compositions are shown in Figure 4.12. Accuracy in
measurements has been mentioned in section 3.3.2. The characteristic features of the
dielectric properties have been known to arise from distribution and type of the
cations among the tetrahedral A-site and octahedral B-sites in the spinel structure.
2 4 6 8 10 12 14 16
0
10000
20000
30000
40000
50000
60000
Die
lectr
ic c
on
sta
nt (
)
ln (f)
X = 0.0
X = 0.2
X = 0.4
X = 0.6
X = 0.8
X = 1.0
Figure 4.12 Effect of Zn concentration and frequency of the applied electric field on
dielectric constants () for Co1-xZnxFe2O4
The graphs shown in Figure 4.12 show that the dielectric constant is maximum for
CoFe2O4 and it decreases with an increase in zinc content at lower frequencies but is
65
higher at higher frequencies for x = 0.4. The dielectric constant decreases with
increase of frequency. All the samples show the frequency dependent behavior. As the
frequency of the externally applied electric field increases gradually, the dielectric
constant decreases. This reduction occurs because beyond a certain frequency of the
externally applied electric field the movements of charges cannot follow the
frequency of the external applied electric field and as a result dielectric constant ()
decreases. The dielectric distribution curves can be explained by Koop’s theory which
is based on the Maxwell Wagner model considering the homogeneous double layer
structure [103]. There are conducting grains and insulating grain boundary regions
inside the medium. These regions form capacitors and resistors parallel circuits.
According to Rabkin and Novikova [112], the polarization phenomenon in the ferrites
occurs by a mechanism comparable to the conduction process. Polarization
phenomena occur due to local displacement of electrons, which determines the
dielectric constant in ferrites. The compositional dependence of the dielectric
constants of mixed Co-Zn ferrites can be explained by considering the site
occupation. The number of ferrous ions on the octahedral sites which take part in the
electron exchange interaction between Fe2+ and Fe3+ and are responsible for the
polarization is maximum in the case of cobalt ferrite (CoFe2O4), therefore a high
value of the dielectric constant is expected and is observed. It has been already
observed for spinel structure that the electron hopping between Fe3+ and Fe2+ leads to
the large dielectric constant () and dielectric relaxation [113]. As the zinc content in
the mixed Co-Zn ferrites was gradually increased at the expense of cobalt, the number
of ferrous ions on the octahedral sites decreases resulting in a continuous decrease in
the dielectric constant (). The variation in dielectric constant at different frequencies
for Co1-xZnxFe2O4 is given in Figure 4.13. From the Figure 4.13 it is clear that the
dielectric constant gradually decreases with increase in frequency. The dielectric constant
has the maximum value at x = 0.4 that is for Co0.6Zn0.4Fe2O4.
66
0.0 0.2 0.4 0.6 0.8 1.0
-400
-200
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Die
lect
ric
con
sta
nt (
)
Concentration (X)
Freq 100k
Freq 500k
Freq 1.2 M
Freq 3 M
Figure 4.13 Effect of Zn concentration on dielectric constant () at different
frequencies for Co1-xZnxFe2O4
4.3.2.2 Dielectric loss (tan δ)
The dissipation factor (D) is typically expressed in terms of tan δ which may be
defined as the ratio of the energy dissipated to the energy stored in a component
[114]. The change of dielectric loss (tan δ) as a function of frequency for all
compositions at room temperature is shown in Figure 4.14. The possible errors in
measurements are given in section 3.3.2.
Figure 4.14 Effect of Zn concentration and frequency of the applied electric field on
tan δ for Co1-xZnxFe2O4
2 4 6 8 10 12 14 16
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
tan
ln (f)
X = 0.0
X = 0.2
X = 0.4
X = 0.6
X = 0.8
X = 1.0
67
In plots for (tan δ) the peaks emerges when hopping frequency of localized electron
matches with the externally applied alternating electric field [115]. For different
polarization mechanisms, resonance occurs at different frequencies. Here the first
peak at lower frequencies corresponds to the interfacial polarization and the second
peak corresponds to the dipole polarization. The shifting of these peaks towards
higher frequency region is attributed to the increase of the hopping rate of the charge
carriers. The tangent loss is observed to be higher for higher frequencies.
0.0 0.2 0.4 0.6 0.8 1.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
tan
Concentration (X)
Freq 100k
Freq 500k
Freq 1.2 M
Freq 3 M
Figure 4.15 Change in tangent loss with Zn concentration for all compositions at
different frequencies
4.3.2.3 AC conductivity
Generally, conduction phenomenon is due to electron hopping among the ions of the
same element existing in the different valence states [116]. As explained earlier in the
cubic close packed crystal lattice of spinel ferrites the most favorable electron
hopping process at B-B sites. Jump length calculations shown in a previous section
68
demonstrate that the hopping length between two metal ions on (B) sites is less than
that of the hopping length between two metals ions at (A) site. Hence the hopping
probably between A to A site is very small. Also the hopping conduction among A-A
sites is not possible because preferred site occupancy for iron ions is B-site [117]. As
shown in the Figure 4.16, the ac conductivity increases [113].
Figure 4.16 Effect of Zn concentration and frequency of the applied electric field on
ac conductivity for Co1-xZnxFe2O4
At higher frequencies the value of ac conductivity is maximum it is due to higher
pumping force provided to charge carriers by high frequency and also the density of
material plays a role [118]. In Figure 4.17 variation in ac conductivity at different
frequencies for Co1-xZnxFe2O4 is given. AC conductivity is higher for higher frequencies
of applied electric field. Also for the composition x=0.4 the ac conductivity has highest
value. The n (frequency exponent) is related with the correlation degree among
moving ions. For ion conducting material in dispersive region of frequency the non-
zero vale of n is for the virtue of energy stored in the collective motion of ions. A
higher value of n implies that large energy is stored in such collective motion of ions.
The particular values of ‘n’ between ‘0’ and ‘1’show the hopping mechanism of ac
2 4 6 8 10 12 14 16
0.000
0.005
0.010
0.015
0.020
0.025
0.030
ac c
on
du
ctivity (
-cm
)-1
ln (f)
X = 0.0
X = 0.2
X = 0.4
X = 0.6
X = 0.8
X = 1.0
69
conduction. The graphs for change in ac conductivity with respect to the Jonscher’s
power law with variation in concentration of composition Co1-xZnxFe2O4 (x=0.0, 0.2,
0.4, 0.6 0.8, 1.0) are shown in Figure 4.18. The ac conductivity is observed to increase
with increase in cobalt contents.
0.0 0.2 0.4 0.6 0.8 1.0
0.000
0.005
0.010
0.015
0.020
0.025
0.030
a
c (
S-c
m-1)
Concentration x
1 kHz
10 kHz
100 kHz
1 MHz
Figure 4.17 Variation in ac conductivity at different frequencies of composition
Co1-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0)
0.0 0.2 0.4 0.6 0.8 1.0
0.2
0.3
0.4
0.5
0.6
0.7
0.8
po
we
r n
Concentration x
Figure 4.18 Variation in ac conductivity with respect to power law with variation in
concentration of composition Co1-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0)
70
Nyquist plots for all composition are shown in Figure 4.19. An important way to
examine the electrical features of ferrite materials is the impedance analysis. In this
way we can separate the resistance of grains from the other resistive sources such as
grain boundaries. The Nyquist plots provide very useful information about the
reactive and real components of the material. For dispersion measurements, an
alternating electrical field with changing frequency is applied on sample material and
the response is observed as a function of frequency. To differentiate the contribution
of grains and grain boundaries the complex plane plots has been drawn in the
frequency range of 100 Hz to 3 MHz at room temperature. Nyquist plots were
attained by plotting the () against (). Two semicircles trend, which are attributed
to the resistance of the grain and grain boundaries can be modeled approximately with
an ideal equivalent circuit consisting on two parallel R–C elements in series. The plots
shown in Figure 4.19 indicate that the resistance of the grain boundaries is higher than
entire grains. Nyquist plots for all compositions are shown in Figure 4.19
71
0 10000 20000 30000 40000 50000
0
20000
40000
60000
80000
100000
"
'
x=0.2
-5000 0 5000 10000 15000 20000 25000 30000 35000 40000
-5000
0
5000
10000
15000
20000
25000
30000
35000
40000
"
'
x=0.4
0 5000 10000 15000 20000 25000 30000
-5000
0
5000
10000
15000
20000
25000
30000
35000
"
'
x=0.6
0 2000 4000 6000 8000 10000 12000 14000
-2000
0
2000
4000
6000
8000
10000
12000
14000
16000
"
'
x=0.8
0 100 200 300 400 500
0
100
200
300
400
500
600
"
'
x=1
Figure 4.19. Nyquist plots for all the samples of Co1-xZnxFe2O4 (x=0.0 – 1.0)
0 50 100 150 200 250 300 350 400
0
100
200
300
400
500
600
Die
lectr
ic L
oss F
acto
r (
'')
Dielectric Constant (')
x = 0.0
72
Table 4.1 Crystallite size d(311), lattice constant(a), Cell volume (V), hopping lengths
LA and LB, X-ray density(ρx), measured density (ρm), porosity (P) drift mobility (µd) at
different temperatures and activation energy E for different values of ‘x’ in
Co1-xZnxFe2O4
Parameter x = 0.0 x = 0.2 x = 0.4 x = 0.6 x = 0.8 x = 1.0
d(3 11)(nm) 35(4) 10(2) 15(2) 19(2) 26(3) 30(3)
a(Å) 8.362(1) 8.368 (5) 8.393(1) 8.411(3) 8.425(1) 8.443(1)
V(Å3) 584 586 590 595 605 601
LA(Å) 3.619(1) 3.624(5) 3.632(1) 3.641(3) 3.645(1) 3.654(1)
LB(Å) 2.955(1) 2.959(5) 2.966(1) 2.973(3) 2.976(1) 2.983(1)
ρx(g/cm3) 5.34 5.37 5.38 5.37 5.26 5.33
ρm(g/cm3) 3.57 2.83 2.89 2.86 2.95 3.10
P (fraction) 0.33 0.47 0.46 0.44 0.43 0.43
µd(cm2/Vs) at
313K
3.55x10-10
2.15x10-10
1.21x10-10
8.06x10-11
4.58x-11
3.27x10-11
µd(cm2/Vs) at
500K 3.79x10-07 3.07x10-07 1.61 x10-07 9.84x10-08 6.60 x10-08 5.00 x10-08
µd(cm2/Vs) at
700K 1.52 x10-05 1.04 x10-05 6.16 x10-06 3.79 x10-06 2.73 x10-06 2.59 x10-06
E(eV) 0.518 0.521 0.523 0.525 0.531 0.537
73
4.4 Summary
It is important to acquire powder of high density, small grain size and narrow size
distribution with controlled stoichiometry, to have material with required properties.
Co1-xZnxFe2O4 nanocrystalline particles (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared
by chemical co-precipitation method. Specific thermal transition temperatures were
studied by differential scanning calorimetry (DSC) and thermogravimetric analysis
(TGA) to estimate the sintering temperature. After analysis of DSC and TGA plots the
600°C temperature was chosen for sintering. Sintering at 600°C gave us maximum
crystalline spinel phase. FCC spinel crystal structure was confirmed by X-ray
diffraction analysis. Lattice constant calculations show that lattice constant increases
with increase in Zn concentration. Cation distribution was found by XAFS analysis
which is a powerful technique for determination of local crystal structure with
elemental specification. XANES studies showed that CoFe2O4 has an inverse spinel
structure while ZnFe2O4 has normal spinel structure. In the midway CoxZn1−xFe2O4
compounds when 0.2 ≤x ≤ 0.8, Co and Zn atoms always occupy the octahedral sites
and tetrahedral sites, respectively whereas crystallite site sizes were in range 10 nm to
35nm. DC electrical resistivity was measured by two probe method. It was found that
dc electrical resistivity decreases with increase in temperature. With increase in
dopant concentration (Zn), dc electrical resistivity increased. Activation energies
calculated by linear fitting of resistivity measurements for all the samples ensured that
the conduction in Zn doped Co nanoferrites is due to polaron hopping. The drift
mobility of charge carriers increased with increase in temperature but decreased with
increase in Zn concentration. In ac studies the dielectric constant decreased with
increase of frequency. Depending upon the composition the dielectric constant
increased up to x = 0.4 (40 % of Zn) and then decreased with further increase in Zn
concentration. The dielectric losses were found to be lower for higher frequencies. AC
conductivity increases with increase in frequencies. Depending upon the composition
the ac conductivity is maximum for x = 0.4 (40 % of Zn). In graphs for change in ac
conductivity with respect to the Jonscher’s power law the values of ‘n’ between 0 and
1 indicates the polaron hopping mechanism of ac conduction. This study ascertains a
fact that properties of Co1-xZnxFe2O4 ferrites with spinel structure strongly depend
upon the elemental composition and site occupancy by cation. The conversion of
74
normal spinel structure of ZnFe2O4 into inverse spinel structure of CoFe2O4 pursues a
regular pattern with gradual replacement of Co by Zn even in nanocrystalline
materials as proved by XANES studies.
75
Chapter 5
5 Thermal, structural and electrical properties
of nanocrystalline Co1-xMnxFe2O4
(x = 0.0 to 1.0)
76
Manganese substituted cobalt ferrites are promising materials for stress and torsion
sensor applications. In the present study cobalt based nanoferrite particles with Mn as
a dopant element with different ratio by weight are synthesized. Mn has larger ionic
radius as compared to the cobalt. The replacement of Co by Mn was expected to
modify the structural parameter which could affect the ultimate material properties.
Synthesized samples were characterized to study the effects of successive replacement
of Co with Mn on cation distribution, structural and conduction (electrical) properties.
A very precise technique (XANES) was applied to study the cation shift.
5.1 Thermal analysis
Specific transition temperatures were studied by differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA). We employed DSC analysis on, as
prepared powder samples before any further heat treatment. Thermograms for (DSC)
and (TGA) of CoFe2O4 and MnFe2O4 from room temperature to 1000°C are shown in
Figures 5.1-5.2 respectively.
0 200 400 600 800 1000
-0.02
-0.01
0.00
0.01
He
at F
low
(W
/g)
Temperature (oC)
CoFe2O
4
-0.02
-0.01
0.00
0.01
0.02
MnFe2O
4
Figure 5.1 The differential scanning calorimetry (DSC) of CoFe2O4 and MnFe2O4
77
0 200 400 600 800 1000
60
65
70
75
80
85
90
95
100
We
igh
t %
Temperature (oC)
CoFe2O
4
MnFe2O
4
Figure 5.2 The thermogravimetric analysis (TGA) of CoFe2O4and MnFe2O4
In DSC graph a wide exothermic peak without any sharp peak for any kind of
transition indicates the temperature range for crystal recovery temperature for both
compositions. In TGA plot all major weight loss processes have been completed
before 600°C for CoFe2O4 whereas for MnFe2O4, the processes of weight loss
proceeds up to about 900°C. Initial weight loss up to about 100°C is due to the
dehydration of the samples. From 100 to about 400 °C weight loss can be associated
to the decomposition of nitrates and removal of OH ions. At the next step the weight
loss occurs due to the removal of structural water. After the gradual weight loss up to
800°C, the rate of weight loss increased in case of MnFe2O4, this weight loss may be
due to dehydration, and some organic residues left in the sample [119]. In DSC graph
a wide exothermic peak without any sharp peak for any kind of transition indicated
the temperature range for crystal recovery temperature. Considering the DSC and
TGA graphs, 600°C was the selected temperature for sintering to obtain the maximum
crystalline phase.
78
5.2 Structural analysis
5.2.1 The X-ray diffraction (XRD) studies
The X-ray diffraction (XRD) patterns confirmed FCC spinel structure for all
synthesized samples of Co1-xMnxFe2O4 nanocrystalline particles with ‘x’ changing
from 0.0 to 1.0 and are shown in Figure 5.3. The XRD patterns were indexed using
the JCPDS card numbers 22-1086 and 74-2403. The crystallite sizes were calculated
by Scherrer equation (3.1) using the full width at half maximum (FWHM) value of
the most intense (311) peak. The data obtained from XRD was used for this purpose.
The crystallite sizes were in the range from 16nm to 35nm.
Figure 5.3 XRD patterns of Co1-xMnxFe2O4(x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0)
The crystal structure is found to be cubic spinel with space group Fd3m and the lattice
constant ranges from 8.362(1) Å to 8.465(8) Å. Lattice constant increases with
increase in Mn concentration within error. Increase in lattice constant occurs because
the Mn has larger ionic radius as compared to the Co which was replaced by the Mn
79
ion. MnFe2O4 is a mixed spinel with 20% degree of inversion. When Mn goes on A-
sites, lattice constant increases. The distance between cations at tetrahedral A sites
(LA) and between octahedral B sites (LB) was calculated using the relations 3.5 and
3.6, are shown in Figure 5.4.
Figure 5.4 Change in hopping length between tetrahedral A sites (LA) and octahedral
B sites (LB) with Mn concentration (x) in Co1-xMnxFe2O4
Figure 5.5 Change in theoretical density (x ) and measured density(m ) with Mn
concentration (x) in Co1-xMnxFe2O4
0.0 0.2 0.4 0.6 0.8 1.0
3.45
3.50
3.55
3.60
3.65
3.70
3.75
LA
LB
X (concentration)
LA(H
opp
ing L
en
gth
) Å
2.80
2.85
2.90
2.95
3.00
3.05
3.10
3.15
3.20
LB(H
op
pin
g L
en
gth
)Å
0.0 0.2 0.4 0.6 0.8 1.0
4.8
4.9
5.0
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
6.0
6.1
6.2
rx
rm
Concentration (x)
r x (
g /cm
3)
0.0
0.8
1.6
2.4
3.2
4.0
4.8
5.6
rm (g
/cm
3)
80
Values are also given in table 5.1 with possible errors. For conduction mechanism
electrons have to hop between ions of the same element existing in different valence
state on equivalent lattice sites. The change in theoretical density (x) and measured
density (m) with Mn concentration (x) in Co1-xMnxFe2O4 is given in Figure 5.5. The
values of theoretical density (ρx), measured density (ρm), and porosity (P), are given in
table 5.1. It was observed that due to increase of Mn concentration both of the
theoretical density (ρx), and measured density (ρm) decrease. X-ray densities were
higher than corresponding physical measured densities. The lower values of measured
densities can be attributed to the porosity of the samples.
5.2.2 X-ray absorption fine structure (XAFS) measurements
CoFe2O4, ZnFe2O4, MnFe2O4, CdFe2O4 and their mix, they all are having spinel
crystal structure, but entirely they all differ from each other depending upon the
allocation of cations. To investigate the cationic distribution in CoMnFe2O4 crystal
system the XAFS spectroscopy was employed. The normalized absorption spectra in
the near-edge region (XANES) at Fe, K-edges for different compositions of
Co1−x Mnx Fe2 O4 are shown in Figure 5.6.
The X-ray absorption fine structure (XAFS) measurements for all compositions of
Co1−x Mnx Fe2O4 was taken in transmission mode at Fe K-edges, at the XAFS beam
line of the ELETTRA synchrotron light source using a double crystal Si (111)
monochromator. To have XAFS measurements sample pellets were prepared by
pressing 5-10 mg of sample powder (nanometric) with 100 mg of boron nitride
powder.
Normalization procedure was done by Xanda VIPER (Visual Processing in EXAFS
Research) Program. The normalized XANES spectra were obtained by subtracting the
smooth pre-edge absorption from the experimental spectra and taking the edge jump
height as unity. After normalization the absorption spectra of the near-edge region
XANES are shown in Figure 5.6. A small pre-edge peak shown in inset of the graph
(Figure 5.6) is observed which is due to the 1s to 3d transition, while the main peak is
attributed to 1s to 4p transitions [109]. As explained in the previous chapter an
eminent behavior of the pre-edge peak is for sites with octahedral and tetrahedral
81
symmetry which are present in the spinel crystal structure. This peak height is higher
for the tetrahedral symmetry and has low intensity for octahedral symmetry. Both 1s
to 4p dipole transitions and 1s to 3d quadrupole transitions and are allowed for the Fe
K-edge, although quadrupole transition intensity is generally very low [120].
Figure 5.6 Normalized XANES spectra at Fe K-edge for the different composition of
Co1−x Mnx Fe2 O4. Inset is the pre edge peak.
For Fe K-edge data, the intensity of the pre edge peak increases relatively with
increasing Co content. It shows that an increasing fraction of Fe goes to tetrahedral
sites as a function of Co and Co occupies the octahedral B sites. Know it is well
known that spinel structure of MnFe2O4 is a partially inverted (Inversion degree is
low) because manganese ions predominantly occupies tetrahedral sites. The degree of
inversion in MnFe2O4 is 0.20 because only 20% Mn goes to octahedral sites.
Therefore in the Co Mn Fe2O4 mix system the degree of inversion depends upon the
relative concentration of Co and Mn ions. These results clearly showed that the degree
of inversion can successively be changed the stoichiometries. Since the ultimate
electric and magnetic properties of spinels depend upon the particular cations at
82
particular sites in the crystal structure. Hence, the material properties can be
controlled by selecting an appropriate composition.
5.3 Electrical measurements
5.3.1 DC electrical properties
The dc electrical resistivity for all these samples were measured by two probe method
from 370 K to 690 K as shown in Figure 5.7. The change in resistivity at different
temperature values for all the compositions is shown in Figure 5.8 and also with
Arrhenius linear fit in figure 5.9.
300 400 500 600 700
0.0
2.0x107
4.0x107
6.0x107
8.0x107
1.0x108
(
-cm
)
Temperature (K)
x=0.0
x=0.2
x=0.4
x=0.6
x=0.8
x=1.0
Figure 5.7 Variation of dc resistivity ρ (Ω-cm) with change in temperature for
Co1-xMnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0)
83
0.0 0.2 0.4 0.6 0.8 1.0
-5.0x105
0.0
5.0x105
1.0x106
1.5x106
2.0x106
2.5x106
3.0x106
3.5x106
(
-cm
)
(
-cm
)
Concentration x
373 K
473 K
573 K
673 K
0.0
5.0x103
1.0x104
Figure 5.8 Variation of dc resistivity ρ (Ω-cm) at different temperature for
Co1-xMnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0)
Figure 5.9 Variation of resistivity ρ (Ω-cm) with change in temperature for
Co1-xMnxFe2O4. Lines are the Arrhenius relation fit.
15 20 25 30 35 40
4
6
8
10
12
14
16
18
20
ln
(cm
)
1/kBT (eV)
-1
X = 0.0
X = 0.2
X = 0.4
X = 0.6
X = 0.8
X = 1.0
84
The resistivity plots in Figure 5.7 show that with decrease in temperature resistivity
increases. At all the temperature values the resistivity increases with increase in Mn
concentration. The dc electrical resistivity of the ferrite nanoparticles was found to
vary from 2.26×106 Ω cm to 7.74×106 Ω cm at 373K with changing Mn concentration
from x = 0.0 to1.0 in Co1-xMnxFe2O4. It is observed that dc electrical resistivity
increases significantly with increase of Mn concentration. With increase in
temperature resistivity decreased nonlinearly. It is also observed that resistivity
increases with decrease in crystallite size. The spinel structure of ferrites consists of
two interpenetrating sub-lattice forming two types of sites where metal ions are
located, the tetrahedral (A) and octahedral (B) sites. The jump length between B-B
sites is less as compared to A-A or A-B sites. In spinel structure the most favorable
sites for hopping of charge carriers are from B to B sites [121].
The increase in resistivity does not strictly mean that the number of charge carriers is
decreasing but it is the rate of hopping of charges which does affect. The variation of
resistivity can be explained on the basis of actual location of cations in the spinel
structure and also by microstructures of the material. In CoMnFe2O4 conduction
mechanism is considered as the electron hopping between Fe2+and Fe3+in (B) sites
and mobility of holes between Co2+ and Co3+ ions. Addition of Mn+2 ions causes the
increase in Fe2+ ions on octahedral site and decrease in Co2+ ions. Above 400K
conduction decreases with increase in Mn concentration, because in this range
conductivity is due to the jumping of holes between Co2+ and Co3+ions. So
replacement of Mn at the expense of Co will consequently decrease the conductivity.
From collected data it is observed that conductivity increases with increase in
temperature because at high temperature hopping of electrons between Fe2+ and Fe3+
and holes is maximized. Therefore with increase in temperature conductivity
increases and resistivity decreases.
The values of activation energies (E) evaluated from the slopes of the linear plots of
dc electrical resistivity are shown in Figure 5.10. Activation energies increase with
increase of Mn content and it may be attributed to increase in hopping length and
lattice expansion with increase of Mn concentration. E ranges from 0.516 eV to
0.557 eV clearly suggests that the conduction is due to polaron hopping [111].
85
Figure 5.10 Variation in activation energy (E) with change of Mn concentration (x)
in Co1-xMnxFe2O4
The hopping depends upon the activation energy, which is associated with the
electrical energy barrier experienced by the electrons during hopping. The plots for
drift mobility are shown in Figure 5.11.
Figure 5.11 Variation of drift mobility µd (cm2/Vs) with change in temperature for
Co1-xMnxFe2O4
0.0 0.2 0.4 0.6 0.8 1.0
0.46
0.48
0.50
0.52
0.54
0.56
0.58
0.60
E
(e
V)
x ( Mn Concentration)
15 20 25 30 35 40
0.0
4.0x10-6
8.0x10-6
1.2x10-5
1.6x10-5
2.0x10-5
Dri
ft m
obili
ty
d(c
m2
V-1
s-1)
1/kBT (eV) -1
X = 0.0
X = 0.2
X = 0.4
X = 0.6
X = 0.8
X = 1.0
86
Drift mobility decreased with increase in Mn concentration which in turn increases
resistivity. Drift mobility is low at low temperature and increases sharply with
increase in temperature due to ease in hopping process.
5.3.2 AC electrical properties
5.3.2.1 Dielectric Constant
The dielectric properties of manganese substituted cobalt ferrites have been measured
at room temperature in the frequency range from 100 Hz to 3 MHz. The graphs of
dielectric constant versus frequency in Figure 5.12 show a normal dielectric behavior
for the synthesized materials. The change in the values of dielectric constant as a
function of frequency for mixed Co-Mn ferrites with different compositions was
observed.
Figure 5.12 Change in dielectric constant with ln(f) in Co1-xMnxFe2O4.
The characteristic features of the dielectric behavior have been known to arise from
distribution and type of the cations among the tetrahedral A-sites and octahedral B-
sites in the structure [110]. The fall in dielectric constant is speedy at low frequencies
and becomes slow for higher frequencies and then approach to frequency independent
2 4 6 8 10 12 14 16
0
10000
20000
30000
40000
50000
60000
Die
lectr
ic c
on
sta
nt (
)
ln (f)
X = 0.0
X = 0.2
X = 0.4
X = 0.6
X = 0.8
X = 1.0
87
behavior. The dielectric distribution curves can be explained by Koop’s theory.
Koop’s theory based on the Maxwell Wagner model considering the homogeneous
double layer structure [103]. There are conducting grains and insulating grain
boundary regions inside the medium. These regions form capacitors and resistors.
Hence, equivalent capacitor resistor parallel circuits made up. According to Rabkin
and Novikova [28], the polarization phenomenon in the ferrites occurs by a
mechanism comparable to the conduction process. The local displacement of
electrons gives the phenomena of polarization, which determines the dielectric
constant of ferrites. The fall in polarization which means reduction in dielectric
constant as a function of frequency takes place when hopping frequency of charge
carriers cannot follow the externally applied alternating electric field [112].
The variations in dielectric constant at different frequencies for all compositions of
Co1-xMnxFe2O4 are given in Figure 5.13. It is clear from plots that the dielectric constant
gradually decreases with increase in frequency and has decreasing trend with increase in
Mn concentration.
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
200
250
300
350
D
iele
ctr
ic c
on
sta
nt (
)
Concentration (X)
Freq 100k
Freq 500k
Freq 1.2 M
Freq 3 M
Figure 5.13 Change in dielectric constant at different frequencies in Co1-xMnxFe2O4
88
The compositional dependence of the dielectric constants of mixed Co-Mn ferrites
can be explained by site occupancy. In the case of CoFe2O4 the ferrous ions content is
higher than in other mixed Co-Mn ferrites. As a consequence, it is possible for these
ions to be polarized to the maximum possible extent and give rise to a higher value of
dielectric constant. As the Mn content in the mixed Co-Mn ferrites gradually
increased at the expense of cobalt the ferrous and cobalt ions decreased. Also the
jump length increased as explained earlier. Therefore, the dielectric constant
decreases with increase in Mn content.
5.3.2.2 Dielectric loss (tan δ)
The change of dielectric loss (tan δ) as a function of frequency for all compositions at
room temperature is shown in Figure 5.14. In Figure 5.15 the variation in tangent loss
at different frequencies as a function of composition (x) for Co1-xMnxFe2O4 is given.
The dissipation factor (D) is typically expressed in terms of tan δ which may be
defined as the ratio of the energy dissipated to the energy stored in a component. In
plots for tan δ the peaks emerge when hopping frequency of localized electron
matches with the externally applied alternating electric field. The shifting of these
peaks towards higher frequency region is attributed to the increase of the hopping rate
of the charge carriers [122].
Figure 5.14 Effect of Mn concentration and frequency of the applied electric field on
tan δ for Co1-xMnxFe2O4.
2 4 6 8 10 12 14 16
0
2
4
6
8
10
12
14
16
tan
ln (f)
X= 0.0
X= 0.2
X= 0.4
X= 0.6
X= 0.8
X= 1.0
89
0.0 0.2 0.4 0.6 0.8 1.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
tan
Concentration (X)
Freq 100k
Freq 500k
Freq 1.2 M
Freq 3 M
Figure 5.15 Variation in dielectric loss (tan δ) at different frequencies of the applied
electric field for Co1-xMnxFe2O4.
5.3.2.3 AC conductivity
The variation in ac conductivity is shown in Figure 5.16. In Figure 5.17 variation in ac
conductivity at different frequencies as a function of composition for Co1-xMnxFe2O4
is given. AC conductivity is higher for higher frequencies of applied electric field.
Figure 5.16 Effect of Mn concentration and frequency of the applied electric field
on ac conductivity for Co1-xMnxFe2O4
2 4 6 8 10 12 14 16
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
ac c
on
du
ctivity (
-cm
)-1
ln (f)
X= 0.0
X= 0.2
X= 0.4
X= 0.6
X= 0.8
X= 1.0
90
Generally, conduction phenomenon is due to electron hopping among the ions of the
same element existing in the different valence state. As explained earlier in the cubic
close packed crystal lattice of spinel ferrites the most favorable electron hopping
process at B-B sites. Jump length calculations shown in a previous section
demonstrate that the hopping length between two metal ions on (B) sites is less than
that of the hopping length between two metals ions at (A) site.
Hence, the hopping probably between A to A site is very small. Also the hopping
conduction among A-A sites is not possible because preferred site occupancy for iron
ions is B-site [39]. A significant increase in magnitude with increase in cobalt
contents was observed. The variation in composition dependent behavior also arises
due to the variation in material density. At higher frequencies the value of ac
conductivity is maximum it is due to the pumping force of the externally applied
frequency which helps in pushing the charge carriers for hopping in between localized
cation sites and also in making free the electrons trapped in various trapping centers in
the material.
0.0 0.2 0.4 0.6 0.8 1.0
-0.006
-0.003
0.000
0.003
0.006
0.009
a
c (S
-cm
-1)
Concentration x
1 kHz
10 kHz
100 kHz
1 MHz
Figure 5.17 Change in ac conductivity at different frequencies for Co1-xMnxFe2O4
91
The graphs for change in ac conductivity with respect to the Jonscher’s power law are
shown in Figure 5.18. The particular values of ‘n’ between ‘0’ and ‘1’show the
hopping mechanism of ac conduction.
For all the composition Nyquist plots are shown in Figure 5.19. The Nyquist plots
provide very useful information about the reactive and real components of the
material. To differentiate the contribution of grains and grain boundaries the complex
plane plots has been drawn in the frequency range of 20 Hz to 3 MHz at room
temperature. Nyquist plots were attained by plotting the () against (). Two
semicircles trend, which are attributed to the resistance of the grain and grain
boundaries can be modeled approximately with an ideal equivalent circuit consisting
on two parallel R-C elements in series. The plots indicate that the resistance of the
grain boundaries is higher than entire grains
0.0 0.2 0.4 0.6 0.8 1.0
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
Po
we
r n
Concentration x
Figure 5.18 Variation in ac conductivity with respect to power law with variation in
concentration of composition Co1-xMnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0)
92
0 5000 10000 15000 20000 25000 30000
-5000
0
5000
10000
15000
20000
25000
30000
35000
40000
"
'
x=0.2
-2000 0 2000 4000 6000 8000 10000 12000 14000
0
10000
20000
30000
40000
50000
"
'
x=0.4
0 500 1000 1500 2000 2500
0
2000
4000
6000
8000
10000
"
'
x=0.6
-500 0 500 1000 1500 2000 2500 3000 3500 4000
0
5000
10000
15000
20000
25000
30000
"
'
x=0.8
0 200 400 600 800 1000
0
2000
4000
6000
8000
10000
12000
14000
"
'
x=1.0
Figure 5.19 Nyquist plots for all the compositions for Co1-xMnxFe2O4 (x=0.0, 0.2,
0.4, 0.6 0.8, 1.0)
0 50 100 150 200 250 300 350 400
0
100
200
300
400
500
600
Die
lectr
ic L
oss F
acto
r (
'')
Dielectric Constant (')
x = 0.0
93
Table 5.1 Crystallite size d(311), lattice constant(a), hopping lengths LA(Å) and LB(Å)
Cell volume (V), X-ray density(ρx), measured density (ρm), resistivity (ρ) and porosity
(P) for different values of ‘x’ in Co1-xMnxFe2O4
Parameter x = 0.0 x = 0.2 x = 0.4 x = 0.6 x = 0.8 x = 1.0
d(3 11)(nm) 35(4) 16(2) 28(3) 18(2) 35(4) 27(3)
a(Å) 8.362(1) 8.364(1) 8.373(2) 8.384(8) 8.465(8) 8.408(1)
V(Å3) 584 584 586 588 605 592
LA(Å) 3.619(1) 3.619(1) 3.624(2) 3.629(8) 3.663(8) 3.637(1)
LB(Å) 2.955(1) 2.955(1) 2.959(2) 2.963(8) 2.991(8) 2.970(1)
ρx(g/cm3) 5.336 5.318 5.281 5.244 5.079 5.171
ρm(g/cm3) 3.570 2.703 3.095 2.981 2.125 2,544
ρ (Ω-cm) 2.74×106 2.88×106 2.9×106 3.4×106 3.68×106 7.74×106
P 0.331 0.492 0.414 0.431 0.582 0.508
94
Summary
Co1-xMnxFe2O4 nanocrystallite particles for x = 0.0, 0.2, 0.3, 0.4, 0.6, 0.8 and 1.0, were
prepared successfully by co-precipitation method. To examine the specific thermal
transition temperatures, differential scanning calorimetry and thermogravimetric
analysis were done. The temperature of 600°C was selected for sintering. The crystal
structure is found to be an inverse cubic spinel with a space group of Fd3m and the
lattice constant ranges from 8.362(1) Å to 8.465(8) Å. Lattice constant increases with
increase of Mn concentration in CoFe2O4. XANES shows that an increasing fraction of
Fe goes to tetrahedral sites as a function of Co and Co occupies the octahedral B sites.
It is observed that dc electrical resistivity increases significantly with increase of Mn
concentration. The maximum value was found to be 7.74×106 Ω cm at 373K.
Activation energies increase with increase of Mn content and it may be attributed to
increase in hopping lengths. E ranges from 0.516eV to 0.557eV which clearly
suggest that the conduction is due to polaron hopping. Drift mobility decreases with
increasing Mn concentration. The dielectric constant decreases with the increase of
frequency. At higher frequencies the value of ac conductivity was found to be
maximum. The ac conductivity has decreasing trend with decrease in cobalt content
although microstructures are also affecting this trend. The dielectric losses decreases
with increase in frequency. In plots for variation in ac conductivity with respect to the
Jonscher’s power law the values of ‘n’ between 0 and 1 indicates the polaron hopping
mechanism of ac conduction. This study reveals a fact that properties of
Co1-xMnxFe2O4 ferrites with spinel structure strongly depend upon the elemental
composition and site occupancy by cations. The site occupancy can be controlled by
controlling the elemental composition.
95
Chapter 6
6 Thermal, structural and electrical properties
of nanocrystalline Co1-xCdxFe2O4
(x = 0.0 to 1.0)
96
CdFe2O4 is generally understood as normal spinel in which all Fe3+ ions occupies B-
sites and all Cd2+ ions occupies A-sites [112]. CoFe2O4 has an inverse spinel structure.
We replace Co from the CoFe2O4 to investigate the whole transformation from inverse
to normal spinel. Also the Cd has quite larger ionic radius as compared that of cobalt.
The gradual replacement of nonmagnetic Cd ions in cobalt ferrite is expected to create
considerable distortion in CoCdFe2O4 system. A little work was found on mixed Co-
Cd ferrites [117]. CoFe2O4 nanocrystalline particles were prepared by co-precipitation
method and Co contents were replaced by Cd successively. A study of thermal,
structural and electrical properties of Co1-xCdxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) is
presented here.
6.1 Thermal Analysis
Particular transition temperatures were studied by differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA). We employed DSC analysis for as
synthesized powder before any further heat treatment. Plots of thermal studies (DSC
and TGA) for CoFe2O4and CdFe2O4 from room temperature to 1000°C are shown in
Figure 6.1 and Figure 6.2 respectively.
Figure 6.1The differential scanning calorimetric (DSC) thermogram of
CoFe2O4and CdFe2O4
0 200 400 600 800 1000
-0.02
-0.01
0.00
0.01
0.02
Exo Up
CdFe2O4
Heat
Flo
w (
W/g
)
Temprature (°C)
CoFe2O
4
97
In DSC graphs a wide exothermic peak without any sharp peak for any kind of
transition indicates the temperature range for crystal recovery temperature. In TGA
graphs all major weight losing phenomena have been completed before 600°C. After
dehydration, removal of nitrates or any other organic residual (if present) in the
sample occurs. At the next step weight loss can be attributed to the removal of
structural water for both end compositions in Co1-xCdxFe2O4 (x = 0.0 to 1.0). Taking
into account these facts, 600°C was the chosen temperature for sintering to obtain the
maximum crystalline phase of Cd doped CoFe2O4.
6.2 Structural Analysis
6.2.1 X-ray diffraction (XRD) studies
The X-ray diffraction (XRD) patterns confirmed FCC spinel structure for all the
prepared samples of Co1-xCdxFe2O4 nanocrystalline particles with ‘x’ changing from
0.0 to 1.0 as shown in Figure 6.3.
Figure 6.2 The thermogravimetric analysis (TGA) thermogram of CoFe2O4 and CdFe2O4
0 200 400 600 800 1000
80
82
84
86
88
90
92
94
96
98
100
CoFe2O
4
CdFe2O
4
We
igh
t (
)
Temprature (°C)
98
The crystallite sizes of the samples were in the range from 162 nm to 354 nm. It is
difficult to relate the crystallite size distribution with any single parameter. The
formation of nucleation sites at different times, coalescence and lattice constants may
affect the crystallite size. During sintering, two or more particles seem to fuse
together by melting of their surfaces. In our prepared samples the crystallite size
decreases up to x = 0.4 and then increased with further increase of Cd content which
may be due to increase in lattice constants. The lattice constants were in range from
8.362(1) Å to 8.602(2) Å. The lattice constant 'a' was found to increase with an
increase of Cd concentration at the expense of Co. It is due to the fact that the radius
of Cd2+ ion (0.97 Å) is larger than that of the Co2+ ions (0.78Å). A linear increase in
the lattice constant with an increase in Cd contents indicates that the lattice expands
without upsetting the symmetry of the lattice. It is in accordance with Vegard’s Law
[106]. The values of lattice constants and grain sizes are given in the table 6.1 and are
shown in
Figure 6.3 X-ray diffraction pattern of Co1-xCdxFe2O4
20 30 40 50 60 70 80
(3 3 1)(5 3 3)(4 4 0)(5 5 1)
(4 0 0)
(3 1 1)(2 2 0)
X = 1.0
X = 0.8
X = 0.6
X = 0.4
X = 0.2
X = 0.0
Inte
nsity (
a.
u)
2 (Degree)
99
Figure 6.4 Change in lattice constants (a) and crystallite size with Cd concentration
in Co1-xCdxFe2O4
Figure 6.5 Change in hopping length between tetrahedral A sites (LA) and
octahedral B sites (LB) with Cd concentration (x) in Co1-xCdxFe2O4
0.0 0.2 0.4 0.6 0.8 1.0
8.35
8.40
8.45
8.50
8.55
8.60
Lattice Constant
Crystallite Size
X (Concentration)
La
ttic
e C
on
sta
nt (Å
)
16
18
20
22
24
26
28
30
32
34
36
38
40
Cry
sta
llite S
ize
(nm
)
0.0 0.2 0.4 0.6 0.8 1.0
3.62
3.64
3.66
3.68
3.70
3.72
3.74
LA
LB
X (Concentration)
LA
(H
op
pin
g le
ng
th)Å
2.94
2.96
2.98
3.00
3.02
3.04
LB
(Ho
pp
ing
len
th) Å
100
Figure 6.4. The distance between cation ions at tetrahedral A-sites (LA) and between
octahedral B-sites (LB) was calculated using the relations (3.5) and (3.6) respectively.
For conduction mechanism, electrons hop between ions of the same element existing
in different valence states on equivalent lattice sites. The change in hopping lengths
LA and LB is shown in Figure 6.5, values are also given in table 6.1 with possible
errors. Both LA and LB increase with an increase of Cd content. It is due to the increase
in lattice constant with increasing Cd content in the crystal geometry of the samples.
6.2.2 X-ray Absorption fine structure (XAFS) measurements
For Co1−x Cdx Fe2 O4 series the XAFS spectroscopy was also done at XAFS beam line
of the ELETTRA synchrotron light source. The XAFS measurements were taken at
room temperature in transmission mode at Fe, and Co K-edges, Cd has well known
site occupancy of tetrahedral symmetry. The normalized absorption spectra in the
near-edge region (XANES) at Fe K-edges and Co K-edges for different compositions
of Co1−xCdxFe2O4 are shown in Figure 6.6 and in Figure 6.7 respectively.
Normalization procedure was done by Xanda VIPER (Visual Processing in EXAFS
Research) Program. The normalized XANES spectra were obtained by subtracting the
smooth pre-edge absorption from the experimental spectra and taking the edge jump
height as unity. A small pre-edge peak shown in inset of the graph (Figure 6.6 and
Figure 6.7) is observed which is due to the 1s to 3d transition, while the main peak is
attributed to 1s to 4p transitions [107]. For the Fe K-edge, both 1s to 3d quadrupole
transitions and 1s to 4p dipole transitions are allowed [108], although the intensity of
the quadrupole transition is generally very low. The pre-edge peak becomes the
addition of these contributions, when both octahedral and tetrahedral sites are
occupied, the peak intensity increases directly with increase the proportion of
tetrahedral sites. For K-edge data of Fe, the intensity of the pre-edge peak increases
with rising Co content. It shows that a rising fraction of Fe goes to tetrahedral sites as
a function of Co and Co occupies the octahedral B sites. Cd has well known
occupancy of octahedral B site. These results clearly suggested that we can change the
101
.
Figure 6.7 Normalized XANES spectra at Cd K-edge for the different composition of
Co1−x Cdx Fe2 O4. Inset shows the pre edge peak heights in enlarged view.
Figure 6.6 Normalized XANES spectra at Co K-edge for the different compositions of
Co1−x Cdx Fe2 O4. Inset shows the pre edge peak heights in enlarged view
7000 7200 7400 7600
0.0
0.2
0.4
0.6
0.8
1.0
No
rma
lize
d a
bso
rptio
n
Energy (eV)
X = 0.0
X = 0.2
X = 0.4
X = 0.6
X = 0.8
X = 1.0
7110 7111 7112 7113 7114 7115 7116 71170.00
0.02
0.04
0.06
0.08
0.10
X = 0.0
7700 7800 7900 8000 8100 8200 8300 8400 8500
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Norm
aliz
ed a
bsorp
tion
Energy (eV)
X = 0.0
X = 0.2
X = 0.4
X = 0.6
X = 0.8
7705 7710 7715 7720
0.00
0.05
X = 0.0
102
degree of inversion successively by changing the stoichiometry. Hence we can control
the material properties by selecting an appropriate composition
6.3 Electrical properties
6.3.1 DC electrical properties.
Conduction in ferrites occurs as a result of electron hopping between ions of the same
element existing in different valence state at equivalent lattice sites [108]. The dc
electrical resistivity for all these samples was measured by two probe method from
313 K to 700 K temperature range. It is observed that dc electrical resistivity increases
significantly with increase in Cd concentration as shown in the Figure 6.8. The change
in resistivity at different temperature values for all the compositions is shown in Figure
6.9 and 6.10. The resistivity plots in Figure 6.8 show that with decrease in temperature
resistivity increases. At all the temperature values the resistivity increases with increase in
Cd concentration. The rate of resistivity after x = 0.6 is higher which can be attributed to
the large expansion in lattice and increase in hopping lengths.
400 450 500 550 600 650 700 750
-2.0x109
0.0
2.0x109
4.0x109
6.0x109
8.0x109
1.0x1010
1.2x1010
1.4x1010
1.6x1010
(
-cm
)
Temperature (K)
x=0.0
x=0.2
x=0.4
x=0.6
x=0.8
x=1.0
Figure 6.8 Change in dc resistivity with Cd concentration (x) and temperature in
Co1-xCdxFe2O4
103
In Figure 6.9 the variation in dc electrical resistivity at different temperatures for all
the composition is shown.
0.0 0.2 0.4 0.6 0.8 1.0
0.0
3.0x109
6.0x109
9.0x109
1.2x1010
1.5x1010
(
-cm
)
(
-cm
)
Concentration x
423K
473K
0.0
5.0x106
1.0x107
1.5x107
2.0x107
2.5x107
573K
673K
Figure 6.9 Change in dc resistivity with Cd concentration (x) at different temperatures
in Co1-xCdxFe2O4
Figure 6.10 Change in dc resistivity with Cd concentration (x) and temperature in
Co1-xCdxFe2O4. Lines are the Arrhenius relation fit.
16 18 20 22 24 26 28
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
ln
(cm
)
(1/kBT (eV)
-1
X = 0.0
X = 0.2
X = 0.4
X = 0.6
X = 0.8
X = 1.0
104
Conduction mechanism in CoCdFe2O4 is considered as electron hopping between
Fe2+and Fe3+in (B) sites. The tetrahedral A site is strongly preferred site of occupancy
for Cd. The addition of Cd ions at the expense of Co leads to increase of Fe3+ and a
corresponding decrease of Fe2+ ions in the octahedral B site and a simultaneous
decrease in Co2+ ions. Due to this mechanism, the available Fe2+ and Fe3+ pairs at
octahedral B sites decrease and hence conduction phenomenon decreases. Also with
an increase of Cd concentration the lattice expands and hopping length of electrons
increased as given in the table 6.1 and shown in Figure 6.5. It increases the required
activation energy; therefore dc resistivity increases with increase of Cd concentration
(Figure 6.10). The values of activation energies (E) evaluated from the slopes of the
linear plots of dc electrical resistivity are shown in Figure 6.11.
Activation energies increase with an increase of Cd content and it is attributed to
increase in the hopping length and lattice expansion. E ranges from 0.54 eV to
0.98 eV clearly suggested that the conduction is due to polaron hopping [111]. The
hopping depends upon the activation energy, which is associated with the electrical
energy barrier experienced by the electrons during hopping. The plots for drift
mobility are shown in Figure 6.12. Drift mobility decreases with increase in Cd
Figure 6.11 Effect of Cd concentration on activation energy in Co1-xCdxFe2O4
0.0 0.2 0.4 0.6 0.8 1.00.5
0.6
0.7
0.8
0.9
1.0
E
(e
V)
x ( Cd Concentration)
105
concentration which correspondingly increases resistivity. Drift mobility varies
directly with temperature. With increase of temperature hopping processes of charge
carriers increases which consequently increases drift mobility of charge carrier.
6.3.2 AC electrical properties
6.3.2.1 The dielectric constant (ε')
The dielectric characteristics of ferrites normally arise from hopping of charge
carriers between cationic sites and across the grain boundaries. The Variation in
dielectric constant at different frequencies for Co1-xCdxFe2O4 is given in Figure 6.13.
From the Figure 6.13 it is clear that the dielectric constant gradually decreases with
increase in frequency. At all temperatures the dielectric constant has higher values for x =
0.6. Dielectric constant initially increases up to x = 0.6 and then decreases with further
increase in Cd concentration. The variation in dielectric constant for all samples with
change in frequency and composition is shown in Figure 6.14.
16 18 20 22 24 26 28
0.0
4.0x10-6
8.0x10-6
1.2x10-5
1.6x10-5
2.0x10-5
Dri
ft m
obili
ty
d(c
m2
V-1
s-1
)
1/kBT (eV) -1
X = 0.0
X = 0.2
X = 0.4
X = 0.6
X = 0.8
X = 1.0
Figure 6.12 Effect of Cd concentration and temperature on drift mobility
of Co1-xCdxFe2O4
106
1 2 3 4 5 6 7
-50000
0
50000
100000
150000
200000
250000
Die
lectr
ic C
onsta
nt (
')
log (f)
x = 1.0
x = 0.8
x = 0.6
x = 0.4
x = 0.2
x = 0.0
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
200
250
300
D
iele
ctr
ic C
on
sta
nt (
')
Concentration (X)
Freq 100k
Freq 500k
Freq 1.2 M
Freq 3 M
Figure 6.14 Effects of Cd concentration at different frequencies of the applied electric
field on dielectric constants for Co1-xCdxFe2O4
At low frequency region, the dielectric constant has high values. In the high-
frequency region, values of () decrease to very small values. The decline in
Figure 6.13 Effects of Cd concentration and frequency of the applied
electric field on dielectric constants for Co1-xCdxFe2O4
107
dielectric constant is fast at low frequencies and turn into slow decline at higher
frequencies approaching to frequency independent behavior. When an alternating
electric field is applied on dielectric material, dipoles orientation changes as the
direction of the applied field. For lower frequencies, the orientation of dipoles can
easily follow the applied field. At comparatively higher frequencies, the direction of
dipoles begins lagging the applied field caused by inertial effects and the dielectric
constant turns to a complex quantity. Grain boundaries are found to be more effective
for lower frequencies whereas the relatively low resistive grains are more effective on
higher frequencies [123]. In ferrites the exchange of electrons between Fe2+ and Fe3+
determines polarization. The high value of dielectric constant at lower frequency is
because of a large amount of species such as Fe2+ ions, grain surface defects and
oxygen vacancies [113]. Polarization phenomenon in ferrite material occurs by a
mechanism similar to conduction process. Considering composition the dielectric
constant increases up to x = 0.4 and then decreases for further increase in Cd+2
content. It has been reported that Cd ions have preference for tetrahedral A-sites
occupy [107] while Fe and Co ions occupy both of sites tetrahedral (A) and octahedral
(B) sites partially [112]. According to this behavior addition of Cd ion at the expense
of Co ion causes the migration of Fe3+ from tetrahedral (A) sites to octahedral (B)
sites. With increasing in Fe3+ at B-sites the hopping rate between Fe3+and Fe2+
increases from one B-site to another neighboring B-site. This hopping mechanism
leads to increase dielectric dispersion. Because the hopping length increases with an
increase in Cd2+ contents. So with further addition of Cd2+ ions, the increase in jump
length between two nearest B-sites can go up to such extent that may cause the
decrease in hopping phenomenon. Therefore, dielectric constant decreases with
further increase in Cd2+ ions.
6.3.2.2 Dielectric losses (tan δ) and dielectric loss factor (ε'')
The change in tan δ and loss factor as a function of frequency for all compositions
at room temperature is shown in Figures 6.15 and 6.16 respectively. In plots for tan δ
the peaks emerge when hopping frequency of localized electron matches with the
externally applied alternating electric field. The shifting of these peaks towards higher
108
frequency region is attributed to the increase of the hopping rate of the charge
carriers.
Figure 6.15 Effects of Cd concentration (x) and frequency of the applied electric field
on dielectric loss (tanδ) for Co1-xCdxFe2O4
The peak behavior of tan δ can be clearly explained according to Rezlescuu model
[38]. According to this model
= 1 (6.1)
Where =2fmax and is the relaxation time in hopping process. The relaxation
time of hopping process is inversely proportional to the jumping probability per unit
time (P). According to this relation
= 1/P (6.2)
It is obvious from relations (6.1) and (6.2) that ( fmax ) and (P) are proportional to
each other. The peaks for loss tangent are found to shift towards the higher
frequencies up to x = 0.4 and then with further increase in Cd concentration peaks
shift towards low frequency region. This behavior is in accordance with dielectric
constant. In crystal system for Co1-xCdxFe2O4 the existence of Co3+ and Co2+ ions
furnishes p-type carriers. The local dislocation of p-type carriers along the externally
1 2 3 4 5 6 7
0
2
4
6
tan
log (f)
x = 0.0
x = 0.2
x = 0.4
x = 0.6
x = 0.8
x = 1.0
109
applied electric field also contributes for net polarization along with n-type carriers.
But this contribution of p-type carriers’ remains smaller as compared to the electron
hopping between Fe3+ and Fe2+.
Figure 6.16 Effects of Cd concentration (x) and frequency of the applied electric field
on dielectric loss factor () for Co1-xCdxFe2O4
In the present crystal system the conduction mechanism is attributed to the hopping of
electrons between Fe3+ and Fe2+ and hopping of holes among Co3+ and Co2+. Initially
the addition of Cd2+ contents at the cost of cobalt pushes some of the Fe3+ ions from
A-sites to B-sites. The increase in Fe3+ ions at octahedral sites raises the electron
hopping. With further increase in cadmium the increase of jumping length makes
hopping of charge carriers difficult. Consequently the hopping phenomenon begins
lagging the applied field.
() also shows dispersion as a function of frequency exactly in the similar way as
that of (). The values of () decreases faster than () and becomes closer in the
high-frequency region.
In Figure 6.17 the variation in tangent loss at different frequencies for Co1-xCdxFe2O4
is shown. Plots in the figure indicates that tangent loss is higher for x = 0.6. It initially
increases with increasing Cd contents and then decreases.
1 2 3 4 5 6 7
-50000
0
50000
100000
150000
200000
250000
300000
350000
400000
Die
lectr
ic L
oss F
acto
r (
'')
log (f)
0.0
0.2
0.4
0.6
0.8
1.0
110
0.0 0.2 0.4 0.6 0.8 1.00.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
tan
Concentration (x)
100 kHz
500 kHz
1.2 MHz
3 MHz
Figure 6.17 Effects of Cd concentration (x), at different frequency of the applied
electric field on dielectric loss (tanδ) for Co1-xCdxFe2O4
6.3.2.3 AC conductivity (ac)
As explained earlier in the cubic close packed crystal lattice of spinel ferrites the most
favorable electron hopping process at B-B sites. Jump length calculations shown in a
previous section demonstrate that the hopping length between two metal ions on (B)
sites is less than the hopping length between two metals ions at (A) site. Hence the
hopping probably between A to A site is very small. Also the hopping conduction
among A-A sites is not possible because preferred site occupancy for iron ions is B-
site [110]. Effects of Cd concentration (x) and frequency of the applied electric field
on ac conductivity (ac) for Co1-xCdxFe2O4 are shown in Figure 6.18.
111
Figure 6.18 Effects of Cd concentration (x) and frequency of the applied electric field
on ac conductivity (ac) for Co1-xCdxFe2O4
It has been reported in literature (ac) is an increases with frequency in case of
electron hopping mechanism and decreases with increasing frequency for band
conduction. The increase in (ac) with externally applied electric field frequency is
due to the pumping force of the externally applied frequency which helps in pushing
the charge carriers in between localized sites and also liberating the electrons trapped
in various trapping centers. Plots in Figure 6.18 show that ac approximately remains
constant at lower frequencies and then starts increasing for higher frequencies.
Alternating current conductivity (ac ) increases with increase in Cd contents up to
x=0.4 and then decrease for further addition of Cd concentration. It is the same trend
as explained in the case of () and (tan δ).
1 2 3 4 5 6 7
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
ac c
on
du
ctivity (
-m)-1
log (f)
x = 0.0
x = 0.2
x = 0.4
x = 0.6
x = 0.8
x = 1.0
112
In Figure 6.19, variation in ac conductivity at different frequencies for Co1-xCdxFe2O4 is
given. AC conductivity is higher for higher frequencies of applied electric field.
0.0 0.2 0.4 0.6 0.8 1.0
-0.002
0.000
0.002
0.004
0.006
0.008
0.010
0.012
ac (
-cm
)
Concentration x
1 kHz
10 kHz
100 kHz
1 MHz
Figure 6.19 Effects of Cd concentration (x) at different frequencies of the applied
electric field on ac conductivity (ac) for Co1-xCdxFe2O4
The graphs for change in ac conductivity with respect to the Jonscher’s power law
[113] are shown in Figure 6.20. The particular values of ‘n’ between ‘0’ and ‘1’show
the hopping mechanism of ac conduction.
113
0.0 0.5 1.0
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
Po
we
r n
Concentration x
Figure 6.20 Variation in ac conductivity with respect to power law with variation in
concentration of composition Co1-xCdxFe2O4 (x=0.0, 0.2, 0.4, 0.6 0.8, 1.0)
For all the compositions Nyquist plots are shown in Figure 6.21. The Nyquist plots
provide very useful information about the reactive and real components of the
material. To differentiate the contribution of grains and grain boundaries the complex
plane plots has been drawn in the frequency range of 100Hz to 3MHz at room
temperature. Nyquist plots were attained by plotting the () against (). Two
semicircles trend, which are attributed to the resistance of the grains and grain
boundaries can be modeled approximately with an ideal equivalent circuit consisting
on two parallel R–C elements in series. The plots shown in Figure 6.21 indicate that
the resistance of the grain boundaries is higher than that of entire grains.
114
Figure 6.21 Nyquist plots for Co1-xCdxFe2O4
0 2000 4000 6000 8000 10000
0
2000
4000
6000
8000
10000
12000
Die
lectr
ic L
oss F
acto
r (
'')
Dielectric Constant (')
x= 0.8
0 5000 10000 15000 20000 25000
-5000
0
5000
10000
15000
20000
25000
30000
35000
40000
Die
lectr
ic L
oss F
acto
r (
'')
Dielectric Constant (')
x = 0.6
0 50000 100000 150000 200000
-50000
0
50000
100000
150000
200000
250000
300000
350000
400000
Die
lectr
ic L
oss F
acto
r (
'')
Dielectric Constant (')
x = 0.4
-500 0 500 1000 1500 2000 2500 3000 3500
0
1000
2000
3000
4000
5000
Die
lectr
ic L
oss F
acto
r (
'')
Dielectric Constant (')
x = 1.0
0 50 100 150 200 250 300 350 400
0
100
200
300
400
500
600
Die
lectr
ic L
oss F
acto
r (
'')
Dielectric Constant (')
x = 0.0
0 20000 40000 60000 80000 100000 120000
-20000
0
20000
40000
60000
80000
100000
120000
140000
160000
180000
200000
Die
lectr
ic L
oss F
acto
r (
'')
Dielectric Constant (')
x = 0.2
115
Table 6.1 Crystallite size d(311), lattice constant(a), Cell volume (V), hopping lengths
LA(Å) and LB(Å) X-ray density (ρx), measured density (ρm), porosity (P) for different
values of ‘x’ in Co1-xCdxFe2O4
Parameter x = 0.0 x = 0.2 x = 0.4 x = 0.6 x = 0.8 x = 1.0
d(3 11)(nm) 35(4) 18(2) 16(2) 18(2) 18(2) 19(3)
a(Å) 8.362(1) 8.401(1) 8.443(3) 8.501(2) 8.553(1) 8.602(2)
V(Å3) 584 593 601 614 625 636
LA(Å) 3.619(1) 3.638(1) 3.654(3) 3.680(2) 3.702(1) 3.723(2)
LB(Å) 2.955(1) 2.970(1) 2.982(3) 3.003(2) 3.021(1) 3.041(2)
ρx(g/cm3) 5.336 5.365 5.376 5.367 5.263 5.405
ρm(g/cm3) 3.570 2.517 2.537 2.989 2.340 2,819
P 0.331 0.473 0.463 0.467 0.439 0.427
116
Summary
Co1-xCdxFe2O4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) nanocrystalline particles were
synthesized by a wet-chemical co-precipitation technique. Specific thermal transition
temperatures were analyzed by differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA) to estimate the sintering temperature. After
analysis of DSC and TGA plots the 600°C temperature was chosen for sintering. We
obtained pure phase spinel crystal structure. FCC spinel crystal structure was
confirmed by X-ray diffraction analysis. The crystallite sizes of the samples were in
the range from 162 nm to 354 nm. Lattice constant calculations show that lattice
constant increases with increase in Cd concentration gradually. Cation distribution
was found by XAFS analysis which is a powerful technique for determination of local
crystal structure with elemental specification. XANES spectra shows that an
increasing fraction of Fe goes to tetrahedral sites as a function of Co and Co occupies
the octahedral B sites. Pure Cd ferrites have normal spinel structure. Crystal structure
transforms gradually from normal spinel to inverse spinel with increasing contents of
Co. The dc electrical resistivity for all compositions was measured by two probe
method from 313 K to 700 K temperature range. It was observed that dc electrical
resistivity increases significantly with increase in Cd concentration. Activation
energies calculated by linear fitting of resistivity measurements for all the samples
confirms that the conduction in Cd doped Co nanoferrites is due to polaron hopping.
The drift mobility of charge carriers increased with increase in temperature but
decreases with increase in Cd concentration. The dielectric dispersion and loss tangent
exhibits normal behavior in the frequency range 100Hz-3MHz at room temperature. It
was found that the replacement of Co with Cd enhances the value of , ε'', tan δ and
ac up to x=0.4 and then decreases gradually with further increase in Cd contents. The
ac electrical conductivity ( σac ) increases with increase in applied frequency of the
electric field. AC conductivity is inversely proportional to hopping length, but is
directly proportional to the hopping rate of electrons. With increase in ferrous ions at
octahedral sites, the conductivity increases but as the jumping length increases, it
leads to the decrease in conduction mechanism. The Nyquist plots indicate that the
resistive and capacitive properties are mainly endorsed as a result of the processes
associated to the grains and grain boundaries.
117
Chapter 7
7 Comparison of doping effects
118
We have observed the change in structural and electrical properties of the CoFe2O4
nano crystalline ferrite particles when the Co element was replaced gradually by Zn,
Mn and Cd. A comparison of these changes is presented here.
7.1 Thermal Studies
Thermal studies for particular transition temperatures were done by differential
scanning calorimetry (DSC) and Thermogravimetric Analysis (TGA) for M Fe2O4
(M= Co, Zn, Mn, Cd). The analysis was done for as synthesized powder before any
further heat treatment for all compositions in temperature range from room
temperature to 1000° and graphs are shown in Figure 7.1 and 7.2.
Figure 7.1 The differential scanning calorimetric (DSC) thermogram of A Fe2O4
(A= Co, Zn, Mn, Cd)
0 200 400 600 800 1000
-0.025
-0.020
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015
0.020
0.025
Exo Up Temprature (°C)
MnFe2O4
ZnFe2O4CoFe2O
4
CdFe2O4
Heat
Flo
w (
W/g
)
119
Figure 7.2 The thermogravimetric analysis (TGA) thermogram of AFe2O4 (A = Co,
Zn, Mn, Cd)
For all compositions in the DSC graphs a wide exothermic peak without any sharp
peak for any kind of transition indicates the temperature range for crystal recovery
temperature. TGA graphs for all synthesized compositions shows that almost all
major weight losing phenomena have been completed before 600°C whereas in case
of MnFe2O4 the weight loss was completed at about 800°C. At initial stage from room
temperature to about 100°C the weight loss can be associated to dehydration of the
material then up to about 300°C weight loss is attributed to the decomposition of
nitrates and at this stage the crystallization of sample materials started. In the next
stage the weight loss is due to the removal of structural water. Above 600°C the major
weight loss processes were completed for CoFe2O4, ZnFe2O4 and CdFe2O4 but the
weight loss for MnFe2O4 was completed up to about 950°C. Also from the combined
analyses of DSC and TGA the estimated temperature of 600°C as sintering
0 200 400 600 800 1000
60
65
70
75
80
85
90
95
100
Temprature (°C)
ZnFe2O
4
CdFe2O
4
CoFe2O
4
MnFe2O
4
We
igh
t (
)
120
temperature was selected to have maximum phase with same sintering conditions so
that their properties could be compared. Sintering at 600°C for 3 hours gives cubic
spinel phase for all compositions.
7.2 Structural Changes
For all compositions Co1-xMxFe2O4 (M= Zn, Mn, Cd & x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)
the structural analyses was done by XRD and XAFS techniques. The variation in all
structural parameters are given as under.
7.2.1 Crystal Structure
The XRD patterns confirmed FCC spinel structure with space group fd3m for all
synthesized nanocrystalline particles with compositions Co1-x Mx Fe2O4 (M= Zn, Mn,
Cd & x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0). Crystallite sizes of all sample materials were in
range the range from 10nm to 36nm.
7.2.2 Lattice Constant
For all compositions lattice constant was found to increase with increasing contents of
the dopant elements which were Zn, Mn and Cd at the expense of Co concentration
This behavior was expected because of the replacement of Co with atoms ( Zn, Mn
and Cd ) having larger atomic radii as compared to that of Co. Atoms with larger
atomic radii created expansion in the lattice structure. Hence lattice constants were
increased. This increase in lattice constant also influenced the hopping lengths and
electrical properties of the prepared materials. This is due to the fact that the ionic
radii of the Co2+, Zn2+, Mn2+ and Cd2+ are in the order Co2+ < Zn2+ < Mn2+ < Cd2+.
Therefore when we replace Co2+ by an element of larger ionic radius gradually the
lattice increases. Here the expansions occur linearly with every dopant element
without upsetting the crystal structure in accordance with Vegard’s law.
The lattice constant values for all compositions are given here in the table 7.1.
121
Table 7.1 Lattice constants “a” in Angstroms (Å) for all compositions Co 1-xMxFe2O4
(M= Zn, Mn, Cd & x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)
Composition x = 0.0 x = 0.2 x = 0.4 x = 0.6 x = 0.8 x =1.0
Co1-x Znx Fe2O4 8.362(1) 8.368 (5) 8.393(1) 8.411(3) 8.425(1) 8.443(1)
Co1-x MnxFe2O4 8.362(1) 8.3641) 8.373(2) 8.384(8) 8.465(8) 8.408(1)
Co1-x Cdx Fe2O4 8.362(1) 8.401(1) 8.443(3) 8.501(2) 8.553(1) 8.602(2)
7.2.3 Hopping Lengths
The distances between cations at tetrahedral A sites (LA) and between octahedral B-
sites (LB) were calculated for all the sample compositions and are given here in the
table 7.2. Hopping lengths play an important role in the conduction mechanism
because the charge carriers have to hop between ions of the same element existing in
different valence states at equivalent lattice sites for conduction in ferrites. The
change in the hopping lengths LA and LB are shown in the table 7.2. As explained in
the above section this is due to the fact that the ionic radii of the Co2+, Zn2+, Mn2+
and Cd2+ are in the order Co2+ < Zn2+ < Mn2+ < Cd2+ . Hence it was expected that
with replacement of Co2+ by elements of larger ionic radii the increase in lattice
constant will increase the hopping lengths. Hopping lengths increases gradually with
increase in dopant concentrations as the increase in dopant fractions causes more
lattice expansion.
122
Table 7.2 Hopping lengths between cations at tetrahedral A sites (LA) in Angstroms
(Å) and between octahedral B sites (LB) in Angstroms (Å) for all sample compositions
Co1-xAx Fe2O4 (A= Zn, Mn, Cd & x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)
Composition
Parameter x = 0.0 x = 0.2 x = 0.4 x = 0.6 x = 0.8 x = 1.0
Co 1-x Znx Fe2O4
LA (Å) 3.619(1) 3.624(5) 3.632(1) 3.641(1) 3.645(1) 3.654(1)
Co 1-x Znx Fe2O4
LB(Å) 2.955(1) 2.959(5) 2.966(1) 2.973(1) 2.976(1) 2.983(1)
Co1-xMnxFe2O4
LA(Å) 3.619(1) 3.619(1) 3.624(1) 3.629(8) 3.663(8) 3.637(1)
Co1-xMnxFe2O4
LB(Å) 2.955(1) 2.955(1) 2.959(1) 2.963(8) 2.991(8) 2.970(1)
Co1-xCdxFe2O4
LA(Å) 3.619(1) 3.638(1) 3.654(3) 3.680(1) 3.702(1) 3.723(1)
Co 1-xCdxFe2O4
LB(Å) 2.955(1) 2.970(1) 2.982(3) 3.003(1) 3.021(1) 3.041(1)
7.3 Occupancy of interstitial sites
As we have discussed that spinel ferrites are classified as normal, inverse and
intermediate spinel ferrites depending upon the allocation of divalent and trivalent
metal cations at Tetrahedral (A) and octahedral (B) interstitial sites. In the
composition Co Fe2O4 when the Co is replaced gradually by Zn the crystal structure
transforms itself from inverse spinel to normal spinel gradually depending upon the
amount of Zn contents at the expense of Co. Similarly when Co is replaced by Mn
and Cd, the crystal structure transforms gradually. It is a very important result
revealed from our study that the cation distribution is compositional dependent even
when the crystallite sizes are ranging from 10nm to 35 nm in all compositions. These
results evidently suggest that we can change the degree of inversion sequentially by
changing the stoichiometry. Hence, we can control the material properties as per need
by choosing the appropriate composition.
123
7.4 DC electrical properties
7.4.1 DC electrical resistivity
The dc electrical resistivity for all the sample compositions was measured by the two-
probe method in the temperature range of 313 K to 700 K. It was observed that the dc
electrical resistivity increased significantly with increase in hoping lengths for charge
carriers. The variation in the electrical resistivity depends upon the location of the
cations in the spinel structure and also by the microstructures of the material. When
Co was replaced by Zn, Mn and Cd the dc electrical resistivity increased significantly
with increase in dopant contents gradually. The composition with Cd as a dopant
element showed higher electrical resistivity as compared to the other two
compositions with Zn and Mn as a dopant element. Cd has higher ionic radius as
compared to Zn and Mn, hence it increases hopping lengths for charge carriers by
creating larger expansion in lattice. Hence another important result is revealed here
that considering the variation generated in hopping lengths and site occupancy we can
modify the electrical conduction as per required for a particular application. Variation
in dc resistivity with composition at 423K, 473K, 573K and 673K as a function of
concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd) are given in Figures 7.3 to 7.6
respectively.
124
0.0 0.2 0.4 0.6 0.8 1.0
0
1x105
2x105
3x105
4x105
5x105
6x105
7x105
-cm
)
-cm
)
Concentration x
Zn Doped
Mn Doped
423K
-2.0x109
0.0
2.0x109
4.0x109
6.0x109
8.0x109
1.0x1010
1.2x1010
1.4x1010
1.6x1010
Cd Doped
Figure 7.3 Variation in dc resistivity with composition at 423K as a function of
concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).
0.0 0.2 0.4 0.6 0.8 1.0
0.0
2.0x104
4.0x104
6.0x104
8.0x104
1.0x105
1.2x105
1.4x105
1.6x105
1.8x105
-cm
)
-cm
)
Concentration x
Zn Doped
Mn Doped
473K
0.0
5.0x108
1.0x109
1.5x109
2.0x109
Cd Doped
Figure 7.4 Variation in dc resistivity with composition at 473K as a function of
concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).
125
0.0 0.2 0.4 0.6 0.8 1.0
2.0x103
4.0x103
6.0x103
8.0x103
1.0x104
1.2x104
1.4x104
-cm
)
-cm
)
Concentration x
Zn Doped
Mn Doped
0.0
5.0x106
1.0x107
1.5x107
2.0x107
2.5x107
Cd Doped
573K
Figure 7.5 Variation in dc resistivity with composition at 573K as a function of
concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).
0.0 0.2 0.4 0.6 0.8 1.0
0.0
5.0x102
1.0x103
1.5x103
2.0x103
2.5x103
-cm
)
-cm
)
Concentration x
Zn Doped
Mn Doped
-2.0x105
0.0
2.0x105
4.0x105
6.0x105
8.0x105
1.0x106
Cd Doped
673K
Figure 7.6 Variation in dc resistivity with composition at 673K as a function of
concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).
126
7.3.2 Drift mobility
Another important parameter is drift mobility which depends upon activation
energies. Drift mobility increases with temperature in all sample compositions.
Therefore conduction increases with rise in temperature as the conduction
measurements were done as a function of temperature. The Cd doped Co ferrites
nanoparticles shows lowest values of drift mobility as compared to that of the Co
ferrites with Zn and Mn as a dopant element.
7.4.2 Activation energies
The values of activation energies (E) were evaluated from the slopes of the linear
plots of dc electrical resistivity. In comparison the activation energies was higher
when Co was replaced by Cd in cobalt ferrites. The hopping depends upon the
activation energy, which is associated with the electrical energy barrier experienced
by the electrons during hopping. Variation in activation energy with composition as a
function of concentration x in Co1-xAxFe2O4 (A= Zn, Mn, Cd) is shown in Figure 7.7.
0.0 0.2 0.4 0.6 0.8 1.0
0.5
0.6
0.7
0.8
0.9
1.0
E
(e
V)
Concentration x
Zn Doped
Mn Doped
Cd Doped
Figure 7.7 Variation in activation energy with composition as a function of
concentration x in Co1-xAxFe2O4 (A= Zn, Mn, Cd).
127
7.5 AC electrical properties
7.5.1 Dielectric constant
The dielectric constant for Zn, Mn and Co substituted nanocrystalline cobalt ferrites
was measured at room temperature in the frequency range from 100 Hz to 3 MHz.
The characteristic features of the dielectric properties are known to arise from the
distribution and type of cations at the tetrahedral A sites and octahedral B sites of the
spinel structure. Dielectric properties are associated with charge polarization. The
dielectric constant increases gradually by increasing contents of Zn and Mn , but in
case of Cd doping it increases up to x = 0.4 and then decreases for further increase in
Cd contents in Co1-xMx Fe2O4 (M= Zn, Mn, Cd & x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0).
Here too the hopping length is doing effect. For increased hopping lengths due to Cd
doping the polarization process becomes slow which dimness the dielectric constant.
Dielectric constant is higher for Mn Fe2O4. For all composition the dielectric constant
decreases with increase in frequency. Variation in dielectric constant with
composition at 100 kHz, 1.2 MHz and 3 MHz frequency as a function of
concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd) is shown in Figures 7.8 to 7.10
respectively.
0.0 0.2 0.4 0.6 0.8 1.0
-300
0
300
600
900
1200
1500
1800
Die
lectr
ic C
on
sta
nt
(')
Concentration x
Zn Doped
Mn Doped
Cd Doped
100kHz
Figure 7.8 Variation in dielectric constant with composition at 100 kHz frequency as a
function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).
128
0.0 0.2 0.4 0.6 0.8 1.0
0
100
200
300
400
500
Die
lectr
ic C
on
sta
nt
(')
Concentration x
Zn Doped
Mn Doped
Cd Doped
1.2MHz
Figure 7.9 Variation in dielectric constant with composition at 1.2MHz frequency as
a function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).
0.0 0.2 0.4 0.6 0.8 1.0
-100
-50
0
50
100
150
200
250
300
Die
lectr
ic C
on
sta
nt
(')
Concentration x
Zn Doped
Mn Doped
Cd Doped
3MHz
Figure 7.10 Variation in dielectric constant with composition at 3MHz frequency as a
function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).
129
7.5.2 Dielectric loss (tan δ)
The change in dielectric loss (tan δ) as a function of frequency for all compositions at
room temperature was measured. In graphs for tan δ the peaks rises when hopping
frequency of localized electron matches with the externally applied alternating electric
field. Apparently the non-monotonic behavior is due to the dependence of dielectric
losses on particular resonance frequency. The peaks arise at different frequencies
depending upon the polarization mechanism. The peak’s positions changes with
frequency and composition because for each composition the resonance frequencies
will be different. The shifting of these peaks towards higher frequency region is
attributed to the increase of the hopping rate of the charge carriers as discussed in the
previous chapters. Dielectric losses also depend upon the entire microstructures and
density of the material. This type of behavior has already been reported for a family of
ferrites [124]. It was observed that in our prepared composition Co0.6Cd0.4Fe2O4 has
lowest dielectric losses values also less than the reported values for the nearest similar
composition [117]. Variation in dielectric loss with composition at 100 kHz, 1.2 MHz
and 3 MHz frequency as a function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn,
Cd) is shown in Figures 7.11 to 7.13 respectively.
0.0 0.2 0.4 0.6 0.8 1.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Die
lectr
ic L
oss (
tan)
Concentration x
Zn Doped
Mn Doped
Cd Doped
100kHz
Figure 7.11 Variation in dielectric loss with composition at 100kHz frequency as a
function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).
130
0.0 0.2 0.4 0.6 0.8 1.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
Die
lectr
ic L
oss (
tan)
Concentration x
Zn Doped
Mn Doped
Cd Doped
1.2MHz
Figure 7.12 Variation in dielectric loss with composition at 1.2MHz frequency as a
function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).
0.0 0.2 0.4 0.6 0.8 1.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Die
lectr
ic L
oss (
tan)
Concentration x
Zn Doped
Mn Doped
Cd Doped
3MHz
Figure 7.13 Variation in dielectric loss with composition at 3MHz frequency as a
function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).
131
7.5.3 AC conductivity (ac)
AC conductivity studies for all synthesized compositions show that ac approximately
remains constant at lower frequencies and then starts increasing for higher
frequencies. In case of Cd doped cobalt ferrites the alternating current conductivity
(ac ) increase with increase in Cd contents upto x = 0.4 and then decrease for further
addition of Cd concentration. Where as in case of Zn doped cobalt ferrites ac
conductivity decreases with increase in Zn concentration. When Mn was used to
replace Co from Cobalt ferrite again ac decreases with increase in Mn concentration.
Variation in ac conductivity with composition at 100 kHz, 1.2 MHz and 3 MHz
frequency as a function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd) is
shown in Figures 7.14 to 7.16 respectively.
0.0 0.2 0.4 0.6 0.8 1.0
0.000
0.001
0.002
0.003
0.004
0.005
0.006
a
c (
S-c
m-1)
Concentration x
Zn Doped
Mn Doped
Cd Doped
100kHz
Figure 7.14 Variation in ac conductivity with composition at 100 kHz frequency as a
function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).
132
0.0 0.2 0.4 0.6 0.8 1.0
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
a
c (
S-c
m-1)
Concentration x
Zn Doped
Mn Doped
Cd Doped
1.2MHz
Figure 7.15 Variation in ac conductivity with composition at 1.2 MHz frequency as a
function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).
0.0 0.2 0.4 0.6 0.8 1.0
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
0.050
a
c (
S-c
m-1)
Concentration x
Zn Doped
Mn Doped
Cd Doped
3MHz
Figure 7.16 Variation in ac conductivity with composition at 3 MHz frequency as a
function of concentration x for Co1-xAxFe2O4 (A= Zn, Mn, Cd).
133
The graphs for change in ac conductivity with respect to the Jonscher’s power law are
shown in Figure7.17. The particular values of ‘n’ between ‘0’ and ‘1’show the
hopping mechanism of ac conduction.
0.0 0.2 0.4 0.6 0.8 1.0
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
Po
we
r n
Concentration x
Zn doped
Mn doped
Cd doped
Figure 7.17 Variation in ac conductivity with respect to power law with variation in
concentration of composition Co1-xAxFe2O4 (A=Zn, Mn, Cd) (x=0.0, 0.2, 0.4, 0.6 0.8,
1.0)
In present work, the dielectric constant appeared to decrease with increase in
frequency. Maxwell Wagner model is applicable in this case since the interfacial
polarization was observed and hence the relaxation peaks can also be seen in the
figures. It was also observed that by varying the doping content x, the magnitude of
the dielectric constant varies. This refers to the fact that by increasing the dopant x the
energy stored by the dielectric material was increased. The polarization is also vitally
affected by the number of Fe ions on octahederal sites since they play significant role
in electron exchange interactions Fe2+ Fe3+. The ac conductivity remains
constant at initial frequency but at higher frequency the ac conductivity increased due
to increased rate of electron hopping mechanism. The constancy of the ac
conductivity at lower frequency was due to low rate of Fe ions hopping. AC
conductivity is directly dependent on the dielectric constant, dielectric loss and
frequency and in the figures above this phenomenon is showing its clear dependence
on composition as well.
134
Chapter 8
Conclusions
135
8. Summary/Conclusions
In the present study nanocrystalline Cobalt ferrites, Cobalt-Zinc ferrites, Cobalt-
Manganese ferrites and Cobalt-Cadmium ferrites particles were synthesized and
investigated very precisely for site occupancy and cation distribution. The nominal
compositions were Co1-xAxFe2O4 (A=Zn, Mn, Cd and x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0).
Initially nanocrystalline Co Fe2O4 particles were prepared and then Co was replaced
gradually by Zn, Mn and Cd. Co-precipitation method was successfully used as a
synthesis technique for all the sample materials so that by equal synthesis conditions
their final properties could be compared. Characterization techniques like X-ray
diffraction, differential scanning calorimetry, thermogravimetric analysis, X-ray
absorption fine structure spectroscopy (XAFS), ac electrical properties (dielectric
constant, dielectric loss tangent tan δ, ac conductivity) as a function of frequency and
dc electrical properties as a function of temperature were used. Thermal analysis
provides useful information about specific critical temperatures. Thermal analyses
showed that after completion of almost all weight losing processes at temperature
about 600°C the synthesized material (CoFe2O4, ZnFe2O4 and CdFe2O4) get stability.
While in case of MnFe2O4 weight losing processes continue till 950°C. FCC spinel
crystal structure was achieved by sintering at 600°C of the samples prepared by Co-
precipitation method at reaction temperature of 70°C. FCC spinel crystal structure
with the space group Fd3m was confirmed by XRD. Crystallite sizes obtained though
Scherrer formula were in the nano regime. A clear increase in lattice constants was
observed with the gradual replacement of Co ions with the substituted ions of Zn, Mn
and Cd as Co, Zn, Mn and Cd has ionic radius in an increasing order respectively.
Cation distribution was found by XAFS technique for determination of local crystal
structure with elemental specification. It was found that although crystallite sizes were
in nano regime the shift from inverse spinel (CoFe2O4) to normal spinel structure
(ZnFe2O4) strongly obeys a regular shift pattern depending upon the concentration of
dopant element Zn in cobalt ferrite crystal system. XANES studies showed that
ZnFe2O4 has a normal spinel structure while CoFe2O4 has an inverse spinel structure.
The shift from inverse to normal spinel structure depends upon the site occupancy in
tetrahedral and octahedral sites. Similar pattern was observed in case of replacement
of Co with Mn and Cd. Hopping lengths for charge carriers increased gradually with
136
increase in lattice constants depending upon the dopant type and dopant
concentration. Increase in electrical resistivity was observed with increase of hopping
lengths. DC electrical resistivity decreased with increase in temperature. Activation
energy calculations showed that the conduction mechanism was polaran hopping. The
dielectric constant decreased with increase of frequency and also it was observed
composition dependent. AC conductivity was frequency dependent and increased with
increase in applied frequency. Microstructures and material density also do effect in
electrical properties The Nyquist plots indicated that the resistive and capacitive
properties are mainly endorsed as a result of the processes associated to the grain and
grain boundaries. Grains and grain boundaries have different resistance levels. It
revealed that we can control the crystal structural properties and conduction
mechanism by selecting the proper chemical composition in Co based nanocrystalline
ferrite particles. The lattice parameters in the crystal structure. These studies
addressed effectively the basic question that how we can change the ultimate material
properties by choosing a suitable composition in Co based nanoferrites because spinel
crystal structure has ability to receive a range of dopant elements.
8.2 Future work and recommendations
The applications of spinel ferrites prepared at nano length scale encompass an
inspiring range. These applications of ferrites are originated from the fundamental
characteristics of ferrites such as considerable saturation magnetization, a high value
of electrical resistivity, low down electrical losses, and inherited fine level of
chemical stability. Based on the present studies, we propose to prepare the gas sensors
by using the ferrite compositions like substituted cobalt–zinc spinel ferrite and doped
cobalt ferrites for humidity sensors and for sensing the most polluting gases such as
CO2, O3, NO2, SO2, and CO. Study of magnetic properties of the prepared samples
and their correlation to the structural and electrical properties will be an interesting
study.
137
Chapter 9
References
138
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