3.1. introduction synthesis techniquesshodhganga.inflibnet.ac.in/bitstream/10603/40351/8/08... ·...

Chapter III: Experimental Techniques

DEPARTMENT OF PHYSICS, SHIVAJI UNIVERSITY, KOLHAPUR Page 61

3.1. Introduction Synthesis Techniques

In the presented research work, the synthesis of NiO and GDC

nanopowders for anode material was carried out by using solution combustion

synthesis. A thorough introduction of combustion synthesis has been discussed

in Chapter I as the research work is fully dependent on this technique for the

synthesis of anode material for IT-SOFC. For synthesis of GDC10 thin film on

NiO-GDC substrates, Spray Pyrolysis Technique (SPT) was used. Also to form

the complete single cell assembly i.e. NiO-GDC/GDC/LSCF-GDC, screen

printing was used to form thick layer of LSCF-GDC cathode on NiO-

GDC/GDC structures. Hence both these techniques have been discussed in the

present chapter.

3.2. Solution Combustion Synthesis (SCS)

SCS of oxide materials was unexpectedly discovered during the reaction

between aluminum nitrate and urea. The schematic of the SCS process is

shown in Fig. 3.1. A mixture of Al (NO3)3.9H2O and urea solution, when

rapidly heated around 500 °C in a muffle furnace foamed and ignited to burn

with an incandescent flame yielding voluminous white product which was

identified as α-Al2O3 [1].

Figure 3.1 Schematic of the SCS process



To understand the highly exothermic nature of this reaction, concepts

used in propellant chemistry were employed [2]. A solid propellant contains an

oxidizer like ammonium perchlorate and a fuel like carboxy terminated

polybutadiene together with aluminum powder and some additives. The

specific impulse (Isp) of a propellant, which is a measure of energy released

during combustion is given by the ratio of thrust produced per pound of the

propellant.

It is expressed as

�� = �� (3.1)

The highest heat Tc is produced when the equivalence ratio (Ψ = oxidizer/fuel

ratio) is unity. The equivalence ratio of an oxidizer and fuel mixture is

expressed in terms of the elemental stoichiometric coefficient.

� = ∑�� !!"#" $%�!�&"'"("$) * + $%"$�� #"!"#!�,+-*./0�1.* $#2/�34/∑�� !!"#" $%�!, '-#"$) * + $%"$�� #"!"#!�,+-*./0�1.* $#2/

(3.2)

A mixture is said to be stoichiometric when Ψ = 1, fuel lean when Ψ >

1, and fuel rich when Ψ < 1. Stoichiometric mixtures produce maximum

energy. The oxidizer/fuel molar ratio (O/F) required for a stoichiometric

mixture (Ψ = 1) is determined by summing the total oxidizing and reducing

valences in the oxidizer compounds and dividing it by the sum of the total

oxidizing and reducing valences in the fuel compounds. In this type of

calculation oxygen is the only oxidizing element; carbon, hydrogen, and metal

cations are reducing elements and nitrogen is neutral. Oxidizing elements have

positive valances and reducing elements have negative valences [3].

In solution combustion calculations, the valences of the oxidizing

elements was modified and considered as negative, and the reducing elements



as positive, similar to the oxidation number concept familiar to chemists.

Accordingly, the elemental valency of C, Al, and H is +4, +3, and +1,

respectively, and oxidizing valency of oxygen is taken as −2. The valency of

nitrogen is considered to be zero [4].

3.2.1. Role of Fuels

Urea is familiar as an ideal fuel for the combustion synthesis of high

temperature oxides like alumina and alkaline earth aluminates. However, a

need to employ alternate fuels to prepare oxides which are unstable above 1000

°C such as transition metal aluminates was necessary. In this regard, hydrazine-

based fuels like carbohydrazide (CH), oxalyl dihydrazide (ODH) and malonic

dihydrazide (MDH) which have low ignition temperature and are combustible

due to the presence of N–N bond that decomposes exothermically to N2 (N≡N)

were found to be suitable.

These fuels serve the following purposes

1. They are the source of C and H, which on combustion form

simple gaseous molecules of CO2 and H2O and liberate heat.

2. They form complexes with the metal ions facilitating

homogenous mixing of cations in solution.

3. They break down into components from which they are formed.

These components in turn decompose to produce combustible

gases like HNCO, NH3 which ignite with NOx .

The simple compounds such as urea and glycine are recognized as

potential fuels. The compounds containing N–N bonds in their molecular

formulae are particularly found to assist the combustion better. Some of the

important criteria that qualify an ideal fuel should

� Be water soluble.

� Have low ignition temperature (< 500 °C).



� Be compatible with metal nitrates, i.e., the combustion

reaction should be controlled and smooth and not lead to

explosion.

� Evolve large amounts of gases that are of low molecular

weight and harmless during combustion.

� Yield no other residual mass except the oxide in question.

� Be readily available or easy to prepare.

A combustion synthesis reaction is influenced by the type of fuel and the

fuel-to-oxidizer ratio. The exothermic temperature of the redox reaction (Tad)

varies from 1000 °C to 1500 °C. Depending upon the fuel used and the type of

metal ion involved, the nature of combustion differs from flaming (gas phase)

to nonflaming (smoldering and heterogeneous) type. Flaming reactions could

be attributed to the generation of gaseous products like nitrogen oxides (NOx)

by metal nitrates and HNCO, NH3, CO, etc., generated by fuels like urea.

Interestingly, some of these fuels were found to be specific to a particular class

of oxides. Urea for example is specific for alumina and related oxides.

Similarly, CH is specific for zirconia and related oxides; ODH for Fe2O3 and

ferrites; TFTA for TiO2 and related oxides; glycine for chromium and related

oxides, etc. The fuel specificity appears to be dictated by the metal–ligand

complex formation, the thermodynamics of the reaction as well as thermal

stability of the desired oxide formed. Theoretically any redox mixture once

ignited undergoes combustion. All metal nitrates on pyrolysis yield

corresponding metal oxides. The decomposition temperature of the metal

nitrates is lowered by the addition of a fuel. So the choice of fuel is critical in

deciding the exothermicity of the redox reaction between the metal nitrate and

the fuel. Depending upon the exothermicity of the reaction, combustion is

smoldering, flaming, or explosive. For example, aluminum nitrate-urea reaction

is highly exothermic (Tad ∼ 1500 °C) but is not explosive, probably due to the

thermal insulating nature of the alumina formed. Whereas, the transition metal

nitrate-urea reaction is violent. By changing the fuel from urea to glycine, the



combustion is much more controlled due to the complex formation of the metal

ions with ligands like CH.

3.2.2. Purpose of Glycine

The glycine plays a dual role in the combustion process. Glycine has the

ability of forming complexes with the metal cations in the solution. The

complexation improves the solubility of the metal cations and prevents

selective precipitation during the removal of water by boiling. The metal ions

are kept in solution and randomly distributed until the point of ignition and

oxide formation. This gives an oxide with great compositional uniformity. In

addition to the complexation, glycine serves as a fuel for the combustion

reaction. The combustion is driven by the oxidation of glycine by the nitrate

ions. Hence, in this study we have used glycine as a fuel.

3.2.3. Advantages of the Solution Combustion Synthesis

SCS has emerged as a novel technique for the synthesis wide of

spectrum of the materials like advanced ceramics, phosphors, catalysts,

intermetallics, composites, and nanomaterials [5]. This technique has various

advantages as given below,

a. It is a simple, low cost and fast process of material synthesis and does

not need special instrument as in other SHS methods.

b. It offers control over structure, homogeneity, and stoichiometry of the

products.

c. It is suitable for the formation of high-purity products.

d. Desired size (micron to nano) and shape (spherical to hexagonal) of the

products can be achieved by this process.

e. For industrial application, it is economically attractive and easy to scale

up. It permits incorporation of desired quantity of dopants in the oxide


DEPARTMENT OF PHYSICS, SHIVAJI UNIVERSITY, KOLHAPUR 6

hosts to prepare industrially useful materials such as phosphors as well

as high Tc cuprates and SOFC (solid oxide fuel cell) materials.

The typical set up used for combustion synthesis in the present study is

as shown in Fig. 3.2. It consists of glass dome having 30 cm diameter and 50

cm height, fixed in the well designed chamber. It prevents the nano powder

from splitting openly into air. It also helps for collecting the powder after

combustion. In this set up, hot plate is fixed such that the distance between the

hot plate and dome can be varied accordingly with size of beaker used for

combustion.

Glass Dome

Hot Plate

Exhaust

Figure 3.2 Typical combustion set up used for synthesis of nanopowders



3.3. Spray Pyrolysis Technique

Thin film synthesis techniques can be broadly classified as physical and

chemical techniques. The physical methods include sputtering, physical vapor

deposition, laser ablation and molecular beam epitaxy. The chemical methods

involve gas-phase deposition method and solution techniques. Chemical vapor

deposition (CVD) and atomic layer epitaxy (ALE) are gas-phase deposition

methods. However, spray pyrolysis, sol-gel, spin and dip coating, selective ion

rinsing etc employ precursor solutions. In the present studies, we have used

spray pyrolysis technique for the deposition of the GDC10 electrolyte as

chemical methods have given freedom to tailor microstructure in turn tune the

property [6]. Spray pyrolysis is simple, versatile, cost effective technique used

for the deposition of oxide thin films. It offers capability to deposit continuous

and uniform films. Also as solution precursor is used in this process, one can

deposit thin films with desired stoichiometry.

3.3.1. Basic Principle

Spray pyrolysis is nothing but the pyrolytic decomposition of precursor

salts of desired compounds to be deposited on a preheated substrate.

In this technique, fine aerosols of a precursor solution are directed

towards preheated substrates. Depending on the method of the atomization,

spray pyrolysis technique is classified into electrostatic spray deposition, Flame

spray deposition; Pressurized spray deposition, Ultrasonic spray pyrolysis and

Mist spray pyrolysis. Out of all these pressurized spray pyrolysis is very simple

and uses pressurized air as carrier gas. Spray parameters can be easily

controlled. Hence in the present case this technique is employed to deposit

GDC10 electrolyte films.



3.3.2. Components of Spray Pyrolysis Unit

Fig. 3.3 shows the schematic diagram of the spray pyrolysis unit. It

consists of various parts like spray nozzle, rotor for nozzle motion, liquid level

monitor, gas regulator valve, hot plate, and air tight chamber. The details of the

above listed parts are given below.

a) Spray Nozzle

It is made up of glass and consists of the solution tube surrounded by the

specially designed glass bulb. It works on Bernoulli principle. With application

of the pressure to the carrier gas, pressure decreases at the tip of the spray

nozzle. Due to this pressure decrease, the solution is automatically sucked in

the solution tube. It facilitates the breaking of precursor solution into droplets.

Figure 3.3 Schematic diagram of the spray pyrolysis unit

Thermo-

couple

Heating Plate

Compressed air

Mechanical assembly for

nozzle motion

Substrates

Heater

Temperature

Controller

Electrical Supply

Compressor

Nozzle

To Exhaust



b) Rotor for nozzle motion

The to and fro motion of the nozzle and its speed is controlled by using

stepper motor controlled with microprocessor based controller. With this

arrangement one can monitor the movement of the nozzle over the required

length of hot plate. During to and fro motion of the nozzle provides sufficient

time for the nucleation which helps to form adherent deposit.

c) Liquid level monitor

The arrangement for changing the height of the precursor solution forms

the liquid level monitor. Height of the solution is measured from the upper

level of the solution to the tip of the nozzle. At constant carrier gas pressure,

spray rate of the solution depends on the height of the solution. Proper

monitoring of the spray rate is very important in the process of the spray

pyrolysis

d) Gas regulator valve

Gas regulator valve is used to regulate and control the gas flowing

through the nozzle. It is attached to the gas flow meter designed from the glass

tube of length 25 cm and diameter 1.5 cm. Since pressure depends on the size

of the air flow meter, the air flow meter should be calibrated from nozzle to

nozzle.

e) Hot plate

Hot plate is made up of rectangular iron plate of size 5 cm x 18 cm x 0.7

cm. It is heated with 2000 W heater coil. The temperature of the hot plate is

controlled by using temperature controller. The chromel- alumel thermocouple

is attached at the front side of the iron plate.

f) Air tight chamber

In spray pyrolysis technique fine aerosols are formed from the precursor

solutions. Some time precursor solutions contains toxic elements or number of

toxic gases are evolved during the thermal decomposition process. So it is

necessary to fix the spray pyrolysis system inside air tight chamber. The outlet



of the chamber is provided with the exhaust fan to remove the gases evolved

during the thermal decomposition process.

3.3.3. Steps Involved in Thin Film Formation

Viguie and Spitz [7, 8] classified chemical spray deposition process

according to the type of reaction. In all, variation in parameters like, solution

concentration, solution flow rate, carrier gas flow rate , droplet radius , nozzle

to substrate distance, substrate temperature were considered. When substrate

temperature is more than the decomposition temperature of the precursors, thin

film formation takes place in the following steps.

A) In the first step, an aqueous precursor solution is converted into

aerosols (droplets) by spray nozzle and the solvent evaporation takes

place.

B) In this step, vaporization of the solvent leads to the formation of

precipitate as the droplets approaches the substrates.

C) Pyrolysis of the precipitate occurs in succession before the

precipitate reaches the substrates.

D) When the precipitate reaches the substrate, nucleation and the growth

of the metal oxide thin films on the substrate takes place.

E) Finally, the growth of the nuclei leads to the formation of the

continuous thin layer of metal oxide.

3.4. Characterization Techniques

3.4.1. Introduction

Characterization plays very important role in optimizing as well as

monitoring the desired properties of the materials. Day by day instrumentation

is improving and modern experimental techniques are helping scientist to

understand more about the samples under study. These techniques provide data



that can be analyzed for efficient use of the materials. Generally structural and

morphological characterizations are carried out at room temperatures.

However, for application of the materials, properties are tested as per the

requirement of the temperatures, air or gas atmospheres. Present chapter

describes the working principle, specifications of the setup, advantages, and

disadvantages of the instruments used for the characterization of NiO-GDC

samples and investigating their properties. The various instrumental techniques

used are listed below.

A. Thermogravimetric Analysis (TGA) and Differential Thermal

Analysis (DTA)

B. X-ray Diffraction (XRD)

C. Scanning Electron Microscopy (SEM) and Field Emission Scanning

Electron Microscopy (FE-SEM)

D. Transmission Electron Microscopy (TEM)

E. Fourier Transform Infra Red Spectroscopy ( FTIR)

F. Fourier Transform Raman Spectroscopy ( FT-RAMAN)

G. DC electrical Measurement- Two probe and Four probe

H. AC impedance spectroscopy

3.5. Thermogravimetric and Differential thermal analysis (TGA-DTA)

The classical meaning of the analysis is dissolution, separation; break up

into constituent elements. In more comprehensive sense, it is nothing but

quantitative and qualitative determination of a physical quantity with

presentation of the measuring method, processing and interpretation of the

measured values within the framework of the model concept. More

specifically, thermal analysis is nothing but a group of techniques in which

property of the sample is monitored against time or temperature in a specific

gaseous atmosphere [9]. In thermogravimetric analysis, change in the weight of

the sample is continuously measured as a function of time or temperature.



However, in differential thermal analysis (DTA), change of the difference in

temperature of the sample under study and reference sample is measured.

3.5.1. Thermogravimetric Analysis (TGA)

Principle of Operation

The schematic of the instrument is shown in Fig. 3.4. The system is

based on dual beam horizontal design that supports both TGA and DTA

measurements. A bifilar-wound furnace provides uniform controlled heating

upto 1500 °C. The sample contained in a pan and the reference (typically an

empty pan) sits on platinum sensor at the end of the each balance arm.

The TGA weight is measured by a tautband meter movement located at

the rear of each balance arm. An optically active servo arm loop maintains the

balance arm in the horizontal (null) position by regulating the amount of the

current flow through the transducer coil. An infrared LED light source and pair

of photo-sensitive diode detect movement of the arm. A flag at the end of the

balanced arm controls the amount of light reaching each photo sensor. As

Figure 3.4 Dual Beam electrobalance



weight is lost or gained, the beam becomes unbalanced, causing unequal light

to strike the photo-diodes. A restoring current is generated to eliminate this

imbalance and return to the null point. The amount of restoring current is a

direct measure of the weight change. The sample balance monitors actual

sample weight, while reference balance is used to correct the TGA

measurement for temperature effects like beam growth via a proprietary

algorithm.

3.5.2. Differential Thermal Analysis (DTA)

It is a technique in which the change of the difference in the temperature

between sample and a reference sample is monitored against time while they

are exposed to a temperature alteration. In this analysis, DTA curve is

obtained by plotting the temperature difference (∆T) on the ordinate with

endothermic reactions downwards (exothermic reactions upwords) and time t

or temperature T increasing from left to right. The shape and size of a typical

DTA curve is determined by the environment surrounding the sample and

reference materials , mechanism controlling the reaction and the material

characteristics. Practically one can carry out DTA of samples like minerals,

polymers, organic and inorganic materials. However, care must be taken that

reference material used in these experiments should be inert over the

temperature range studied, unreactive to the crucible or thermocouple and

preferably, similar in thermal properties to the sample. The reference

materials most frequently used in DTA of minerals and inorganics are α-

alumina, Al2O3 (which must be heated to around 1500 °C to remove

adsorbed water), titania, TiO2, or carborundum, SiC. These have thermal

conductivities in the range 3 to 40 W m-1 K-1 in the temperature range up

to 1000 °C. In order to make the sample and reference more similar, the

sample may be diluted with the reference, although this will decrease the

sample signal. Care must be taken that no reaction takes place between

sample and reference diluent. The use of inert gases such as nitrogen,



argon or helium, and reactive gases, particularly air and oxygen, is

possible with most of DTA instruments. However, their flow rates should be

adjusted properly as the thermal conductivity of the gases used will affect the

response of the sensors [10].

Specifications:

Model: Trans Analytical Instruments USA: SDT-2960

Temperature range: Ambient to 1500 °C

Sample capacity (max): 200 mg (350 mg including sample holder system)

Balance sensitivity: 0.1 µg

Heating rate: 0.1 to 100 ˚C/min 0.01 °C/min increments (ambient to 1000 °C)

0.1 to 25˚C/min 0.01 °C/min increments (ambient to 1500 °C)

3.6. X-Ray Diffraction (XRD)

X-ray diffraction is an indispensible tool for crystalline materials [11]. It

is found to be most powerful in determination of the crystal structure of the

materials. In general, characterizations of material can be classified into phase

(structural and compositional), physical and chemical properties. However

more emphasis should be paid on characterization of phase of the materials.

3.6.1. Principle of Diffraction

Basic principle of the X-ray diffraction can be found in text books by

various authors e.g. Buerger [11], Klug and Alexander [12], Cullity [13],

Tayler [14], Guinier [15], Barrett and Massalski [16].

The phenomenon of bending of radiation towards the geometrical

shadow of an aperture is called diffraction. Phenomenon of the diffraction can

be explained by the Huygen’s wave theory of radiation. According to this

theory, diffracted beams can interfere with each other giving rise to bright and



dark fringes in the shadow region depending upon their phase difference. The

diffraction grating shows such effect in visible radiations. In 1912, Max von

Laue demonstrated such effect with X-rays. Equivalent to the diffraction

grating, crystal lattice consist of the parallel lines of the atoms and inter planar

spacing between them could be successfully determined from the separation of

the fringes of the diffraction pattern. According to the Bragg’s diffraction law,

constructive interference of the diffracted beams takes place when it satisfies

Bragg’s condition,

2d sin θ = nλ (3.3)

where,

λ = wavelength of X-ray

θ = glancing angle (called Bragg angle)

d = inter planer spacing

n = order of diffraction

Further, successful efforts on the development of theory and

experimental procedure made the X-ray diffraction a unique tool for the study

of the solids.

3.6.2. Phenomenon of Diffraction

In 1895, German scientist Wilhelm Conrad Rontgen produced X- rays.

X-ray being electromagnetic wave, they interact with the electron cloud of the

atom. Thus, the fluctuating electric field of the X-rays makes the electrons of

the atom to vibrate with same frequency as that of X-rays. Hence, they emit

radiations of same frequency as that of incident X-rays which made them to

vibrate. Hence incident X-rays appears to be scattered by the atom. This X-ray

scattering takes place in all directions. Intensity of these scattered X-rays

depends upon the number of electrons in the atom. More number of electrons



more intense will be the diffraction. This scattering of X-rays by the electron is

given by the Thomson equation and called as the scattering power of an

electron �= /. Thus scattering power of the atom at rest is usually expressed as

scattering factor or form factor and is equals to the atomic number (Z) of the

atom. It is well known that at any temperature atoms in the unit cell vibrates at

their mean position. The scattering factor decreases with their amplitude of

vibration and also with the increase in the diffraction angle.

Thus scattering factor of an atom is given by the relation,

f = f?eABCDEFG

HF (3.4)

Where,

=? = Scattering factor of an atom when it is at rest and at 0°

λ = Wavelength of x-ray

θ = Angle of diffraction

B = A constant (called isotropic temperature factor). This factor is given by

B =8JKLK, where, LK= mean of square displacement of the atom from its mean

position. The exponential term in equation 3.2 is called as Debye- Waller

factor)

3.6.3. X-ray Diffraction Setup

X-ray diffraction experimental setup consists of X-ray source, sample

under investigation and the detector to pick up the diffracted X-rays. The

radiation source can be monochromatic or polychromatic and X-ray

diffractions can be carried out on single crystal or polycrystalline sample.

Single crystal samples are studied by Laue method, Weissenberg photograph

methods or most commonly with automated four circle diffractometer. The

powder samples are studied by Debye Scherrer photographic methods by the



diffractometer and radiation detection can be made with the help of

photographic film or radiation counter.

However for fast and most efficient detection, modern diffractometer use

image plates or CCD. Fig. 3.5(a) shows a typical block diagram of XRD unit.

The X-ray tubes use different targets like Cu, Co, Fe, Cr, Mo, Ag and

W. However, the selection of the X-ray tube depends upon the requirements,

sample character (absorbance and unit cell parameters), resolution required and

angle of coverage. Higher wavelength will give more resolved data but

sometimes it is difficult to collect full data. The wavelength of X-rays from

copper target is 0.1542 nm and is generally used for standard applications. This

target produces X-rays with two wavelengths Kα and Kβ. These β radiations

can be filtered with the help of the filter made up of element having atomic

number Z-1 of the target. i.e. in case of Cu target Ni is used as filter.

The sample in the form of powder either in form of smear or compact

flat pack or a capillary is exposed to monochromatic beam of X-ray and the

diffracted beam intensity is collected in the range of two theta angles with

Figure 3.5 (a) A typical block diagram of a powder XRD unit

Voltage Stabilizer

V High

voltage X-Ray

Tube

I

D

Monochromato

Sample

Collimato

Slits

Current Stabilizer

Amplifier Computer Printer



respect to the incident beam. The intensity corresponding to a constructive

interference of the diffracted beam from a crystallographic plane is observed as

a peak. A typical diffraction pattern is shown in Fig. 3.5 (b)

Each peak in the pattern can be assigned miller indices (h k l) which

equate the inter-planer spacing with the unit cell parameters. This typical

assignment of the (h k l) values to diffraction peaks is called as indexing. One

can determine crystallite size using the broadening of the diffraction peaks.

This broadening of the peaks takes place due to incomplete destructive

interference. Thus according to Debye Scherer formula,

D = ?.OPQ �R (3.5)

where, D = crystallite size, λ = wavelength, β = FWHM, θ = Bragg’s angle

Figure 3.5 (b) A typical XRD pattern



3.6.4. Applications of XRD

There are various applications of XRD data. Such as study of the nature

of the sample (crystalline or amorphous), phase identification, phase transition,

monitoring the solid state reactions, unit cell determination, crystallite size

determination, structure and strain determination, study of defects. Since the

XRD pattern is the finger print of materials, it can be used to identify the

materials.

The specifications of XRD unit used for the characterization of NiO-

GDC samples are given below.

Specifications:

Model: PW1710/PW3710 PHILIPHS Holland

Angle: 2θ = 10° to 100°

Target: Cu

3.7. Scanning Electron Microscopy (SEM)

Interaction of the electrons with elements is well understood and has

been extensively used for characterizing the materials. Interesting fact is that

electrons can be focused to micro to submicron size, hence are well suited for

analyzing submicron sized areas or features. When electron strikes the atom,

variety of interactions takes place. Scattering of the incident electrons from the

electron of the atom gives rise to the backscattered electrons and secondary

electrons. Some electrons may get transmitted if the sample is thin. However

primary electrons with sufficient energy may knock out the electron from the

inner shells of atom and excited atom may relax with the liberation of Auger

electron or X-ray photons. All these interactions of an electron carry

information about the sample. Scanning electron microscopy uses secondary

electrons for studying the topography of the sample.



3.7.1. Principle of Operation

SEM is an instrument, which is used to observe the morphology of the

sample at higher magnification, higher resolution and depth of the focus as

compared to the optical microscope. Herein, an accelerated mono-energetic

beam of electrons is incident on the surface of the sample and scanned over

small area of the sample. Several signals are generated and are collected as per

the mode of operation. Detected signals are amplified and made to form the

synchronous image on the cathode ray tube (CRT). The camera is used to

photograph the images or it may be digitized and processed on a computer.

Magnetic

Lens

Scanningc

oil (X,Y)

Magnetic

Lens

Sample Current

Sample

Detector BE

Scanning

Generator

Image C.R.T

Magnification Generator

Video

Amplifier SE

Cathode

Whenlt

Anode

Beam

6V-100V 5-50KV

Figure 3.6 A typical line diagram of scanning electron microscope.



The general outline of the SEM is shown in Fig. 3.6. The essential

features are a) electron gun b) electron optics c) electron signal detectors d)

specimen stage and e) vacuum system [17]

a) Electron gun

The electron gun used to get electrons is a triode gun as shown in

Fig.3.7. Tungsten filament works as a cathode. In order to emit the electrons

from the tip it is bent in ‘V’ shape. The diameter of the filament wire is of the

order of 0.1 mm. The filament is heated up by passing current through it. Thus

electrons are emitted by the thermionic emission.

The emission current is controlled by the wehnelt cylinder around the

filament and kept at negative voltage V. The electron beam is accelerated due

to anode having potential V. However for the safety of the operator it is

grounded. Like tungsten, lanthanum hexaboride (LaB6) is used as a cathode for

the emission of the electrons due to its lower work function (2.7 eV) as

compared to the tungsten. As emission of electrons from the LaB6 filament

takes place a lower temperature than the tungsten filament, its life span in

longer.

R

_V

Anode

Filament

Wehnelt

Figure 3.7 Ray diagram of electron gun



b) Electron optics

The electron beam is produced from the filament is focused using

electromagnetic lenses.

c) Electromagnetic lenses

The electron lenses help to focus the electron beam on the sample. The

size of the probe on the sample depends on the reduction factor of the lenses

used. The initial size of the beam at the aperture of the anode is 50µm is

reduced to 5 nm to 1 µm on the sample by using 2 or 3 lenses.

d) Electron Detectors

Backscattered electrons and the secondary electrons are detected using

electron detectors. Generally, Everhart-Thornly detectors are used for the

detection of the back scattered electrons. The grid near the sample is kept at -50

V to avoid secondary electrons from being collected. Back scattering

coefficient increases with atomic number Z. Two types of contrasts are

obtained from the backscattered electrons namely i) chemical contrast and ii)

topological contrast. Secondary electrons are detected with the scintillators

coupled with photomultiplier. These detectors are kept near the sample. The

topographical contrast due to secondary electrons results from the collected

intensity, increase with the angle of inclination of the sample. So the samples in

SEM are inclined.

3.7.2. Sample Preparation

Charging of the sample surface affects the imaging process. So it can be

avoided by coating platinum over the surface of the sample. Carbon or copper

tapes are used to ground the sample surface. Biological sample are specially

treated to immobilize them. Proper vacuum and the low acceleration voltages

are used to avoid damages to the sample.



3.7.3. Applications

SEM is well suited for the morphological characterization of the

samples in varies fields like material science, biology, agriculture etc.

Specifications:

The instrument is of JEOL, Japan make JSM-6360. It uses an

acceleration voltage of 0.5 KV to 30 KV, a maximum magnification upto X

1,00,000 is possible. It offers a resolution of 3 nm. The instrument accepts a

specimen upto 150 mm in diameter.

3.7.4. Field Emission Scanning Electron Microscope (FE-SEM)

(a) Principle

Principle of the FE-SEM is same as common SEM. Only difference is

that it uses electrons librated by the field emission source. The object is

scanned by the electrons according to the zig-zag pattern.

(b) Source of electron in FE-SEM

In FE-SEM no heating but so called “cold” source is employed. In FE-

SEM, very thin and sharp tungsten (tip diameter 10-7 to 10-8 m) is used as a

filament. It works as a cathode in front of primary and secondary anodes. The

voltage of the order of 0.5 to 30 KV is applied across the cathode and anode.

This filament produces electron beam 1000 times smaller than the conventional

device. Hence resolution is better. One more advantage is that current flash

regularly decontaminates the electron source and life time of the filament is

more than conventional filaments.



(c) Image formation

Image formation process is same as in a conventional SEM. However

difference is that it detects secondary electrons from the surface of the object

under investigation and uses those for the image formation.

(d) Applications

Researchers in material science, biology and chemistry are using FE-

SEM for observing the features of small size of the order of 1 nm.

3.8. Transmission Electron Microscopy (TEM)

TEM is a tool which can provide information about the microstructural

characterization broadly means ascertaining the morphology of phases, number

of phases, structure of phases, identification of the crystallographic defects and

composition of the phases.

3.8.1. Principle

In TEM, specimen under investigation is illuminated with the electrons

emitted from the electron guns. These electrons are passed through the sample

and transmitted electrons from the sample are recorded.

3.8.2. Working of the TEM

Basically TEM consists of electron gun, electromagnetic lenses, vacuum

system, high voltage generator, recording devices and the associated

electronics.

Fig. 3.8 shows the ray diagram of the TEM. TEM operates on principles

similar to that of the optical microscope [18, 19].

The accelerated ray of electrons passes a drill-hole at the bottom of the

anode. The lens-systems consist of electronic coils generating an



electromagnetic field. The ray is first focused by a condenser. It then passes

through the object, where it is partially deflected. The degree of deflection

depends on the electron density of the object. The greater the mass of the

atoms, the greater is the degree of deflection. After passing the object the

scattered electrons are collected by an objective lense. Thereby an image is

formed, that is subsequently enlarged by an additional lens-system (called

projective with electron microscopes). Thus the formed image is made visible

on a fluorescent screen or it is documented on photographic material. Photos

taken with electron microscopes are always black and white.

Figure 3.8 Ray diagram of Transmission Electron Microscope



The lanthanum hexaboride (LaB6) and tungsten filaments are found to

be the best source of electrons. The electrons produced by the gun are

accelerated by the chosen voltages. This acceleration voltage determines the

wavelength of the electron beam produced.

The wavelength and acceleration voltage is related by an equation, λ=

(1.5)1/2

/ (V)1/2

where, λ is the wavelength in nm and V is the acceleration

voltage in volts. For higher accelerating voltages relativistic correction

becomes very important and thus should be taken into account.

The resolution of the electron microscope depends not only on the

wavelength of the electron beam but also governed by the aberration of the

image forming lenses. The spherical aberration coefficient of objective lens is

very important and it is related to the wavelength of the electron beam and the

resolution (ropt ) of the TEM by the equation, ropt= 1.21 λ3/4 (Cs)1/4.

In modern TEM, developments are going on to reduce the spherical

aberration to increase the resolution. Further improvement in the resolution is

done by adding field emission gun (FEG). This FEG reduces the instrumental

contribution to the chromic aberration. Hence information from the planes of

lower interplanar spacing is also transferred to the image.

Since the limit of resolution is in the order of a few angstroms, it is a

very useful and powerful tool for nanoparticles characterization. Low

resolution TEM can generally provide information regarding the size and

overall shape of the sample and is routinely used to elucidate such information.

3.8.3. Applications of TEM

TEM is useful for determining size, shape and arrangement of the

particles which make up the specimen. TEM is typically used for high

resolution imaging of thin films of a solid sample for nanostructural and

compositional analysis. The topographic information obtained by TEM in the

vicinity of atomic resolution can be utilized for structural characterization and

identification of various phases of mesoporous materials, viz., hexagonal, cubic



or lamellar [20]. TEM also provides real space image on the atomic distribution

in the bulk and surface of a nanocrystal [21].

3.9. Fourier Transform Infrared (FTIR) Spectroscopy

A variety of IR techniques have been used in order to get information on

the surface chemistry of different solids. With respect to the characterization of

metal oxides two techniques largely used, namely the transmission/absorption

and the diffuse reflection techniques. The infrared spectroscopy is the method

of qualitative analysis of organic material and it has broad application in

inorganic substances as well. The molecular vibrations in the sample can be

detected by the vibration spectroscopy. The roots of the vibrational

spectrometry are strengthened by the discovery of inelastic scattering of the

photons also known as “Raman effect” in 1928 by Raman and Krishnan [22].

It is well known that, IR encompasses a spectral region from red end of

visible spectrum (12,500cm-1

, 0.8 mm) to the microwave (10 cm-1

, 1000 mm)

in the electromagnetic spectrum.

Figure 3.9 Block diagram of the FTIR



However, based upon applications and instrumentation involved, it is

divided into near (12,500 to 4000 cm-1

), mid-IR (4000 to 400 cm-1

) and far-IR

(400 to 10 cm-1). It is found that most of the fundamental molecular vibrations

occur in mid-IR region.

Modern Fourier transform IR spectrometers are superior to that of

dispersive IR spectrometers on several counts. Due to these advantages the

measurements of transmission, reflection or even emission spectra has become

significantly faster and even with higher sensitivity than ever before. FTIR

spectrometers are based upon Michelson interferometer.

Fig. 3.9 shows schematic of the spectrometer. A typical spectrometer

mainly consists of components like (a) radiation source, which is always Nernst

filament (ZrO2 + Y2O3) or Globar (SiC). (b) optical path and monochromator,

in which the beam is guided and focused by the mirrors aluminized or

silverized on their surfaces. (c) detectors, which detects the heat radiations .

To obtain an IR absorption spectrum, one mirror of the interferometer

moves to generate interference in the radiations reaching the detector. However

as all wavelengths are passing through the interferometer, interferogram is a

complex pattern. In these experiments the absorption/transmission spectrum as

function of wavenumber (cm-1

) is obtained from the Fourier transform of the

interferogram, which is the function of the mirror movement. Interestingly, this

design doesn’t have the reference cell which is generally used in the dispersive

IR instrument, so a reference spectrum is recorded and stored in the memory to

subtract from the sample spectrum.

3.9.1. Sample Preparation

Solid samples under test are mixed with KBr, palletized and used for

measurement. Liquid samples were taken in KBr/CaF2 window cells and gases

in gas cells.



3.9.2. Applications

It is used for the determination of functional group attached to atom or

molecule and compositional characterization of materials both organic and

inorganic and it is widely used in areas like material science, catalysis,

medicine, biochemistry, forensic science etc.

Instrument Specifications:

Model: PERKIN ELMER SPECTRUM 1

Range: 400 –4000 cm-1

3.10. Raman Spectroscopy

In 1928, Prof. C.V.Raman discovered that when a sample is irradiated

with an intense monochromatic light source ( Laser), most of the radiation is

scattered by the sample at the same wavelength as that of the incoming laser

radiation in a process known as Rayleigh scattering. However, a small

proportion of the incoming light – approximately one photon out of million – is

scattered at the wavelength that is shifted from the original laser wavelength. Is

known as the Raman effect.

Vibrational spectroscopy deals with the study of the vibrational energy

states in materials. These states involve vibration of atoms around their

equilibrium position. Vibrational spectroscopy can be classified as Raman and

infra red spectroscopy and used for characterization of samples in solid, liquid

ands gaseous form. In solids, characteristic vibrations come from the

individual atoms as well as vibration of any rigid or semi rigid molecular unit

present in the unit cell. However in gases or liquids, characteristic vibrations

belong to the atomic vibration in individual molecules. Total number of

independent vibrations depends on the number of atoms and given by 3N-5 and

3N-6 for linear and non-linear molecules respectively. In this case number 5 or

6 represents the transitional and rotational motion of the molecules. On the



other hand corresponding numbers for the solid are 3N-3, where 3 correspond

to the three acoustic modes. The vibrational states are the characteristic features

of these materials and hence can be used to identify the materials.

In solids, Raman scattering is different than liquids and gases because in

solids atoms are bound and hence they lose transitional and rotational degrees

of freedom. The atoms and the molecules in the unit cell are close enough to

interact with each other. This gives rise to the changing of the number of

Raman scattered lines and their intensity distribution. In solids, vibrational

states are governed by the phonon dispersion relation which represent the

phonon frequency or energy with the phonon wave vector [23].

3.10.1. Principle of Operation

Prof. C.V. Raman received Nobel Prize for the discovery of Raman

effect. In Raman spectroscopy, a monochromatic incident radiation is scattered

inelastically from the different vibrational states of the sample and spectrum of

the scattered radiation contains the information of the vibrational energy levels

in terms of energy differences between the incident and scattered radiation.

Raman spectroscopy is two photon second order process and now a days

Raman spectrometers can record almost all kinds of as prepared samples.

3.10.2. Experimental Setup of Fourier Transform Raman Spectrometer

In place of visible excitation lasers, an FT-Raman spectrometer uses a

laser in the near infrared – usually at 1064 nm. At this wavelength fluorescence

is almost completely absent, however because of the 1/λ4 relationship between

Raman scattering intensity and wavelength, the Raman signal is weak. In

addition, silicon CCD detectors cannot be used in this region of the spectrum.

FT-Raman uses sensitive, single-element, near-infrared detectors such as

indium gallium arsenide (InGaAs) or liquid nitrogen-cooled germanium (Ge)

detectors. An interferometer converts the Raman signal into an interferogram,



permitting the detector to collect the entire Raman spectrum simultaneously.

Application of the Fourier transform algorithm to the interferogram converts

the results into a conventional Raman spectrum.

FT-Raman spectroscopy is particularly well-suited for bulk sample

analysis and can be configured to accept samples in most common formats,

including vials, cuvettes, tubes, plastic bags, bottles, powders, films and solids.

A typical Raman scattering setup consists of (a) an excitation source, (b)

a spectrometer/spectrograph, (c) a detector and (d) detection electronics with

output device as shown in Fig. 3.10. The excitation sources are usually in the

visible region and principle requirement is they should be highly

monochromatic (width < 0.1 cm-1

), power density a few KW/cm2 and

directional (beam divergence < 1 m rad). However, in compact and portable

Raman spectrometers, semiconductor and diode pumped solid state lasers are

used. The most common sources are 514.5, 532.0, 488.0 and 789.0 mm laser

lines in the visible region. The Laser light is incident on the sample and

Figure 3.10 Schematic diagram of the FT-Raman



scattered radiation is collected over a large solid angle by a collector lens and is

focused on the entrance of the spectrometer. This is collimated by a concave

mirror to fall on the grating which spatially separates the Raman lines. These

are then focused on another concave mirror on the exit slit and different Raman

lines are recorded by rotating the grating so that they are transmitted by the exit

slit in sequential manner. In a spectrograph, the exit slit is replaced by a broad

window with a multichannel detector, which can record full spectrum over a

finite range. Principal requirement of the Raman spectrometer is its stray light

rejection capacity. If more spectrometers are used in series one can reduce the

stray light but due to more monochromators in series will result in weak signal

due to finite reflectivity of mirrors and gratings. So for the practical purpose

compromise is done by using two monochromators.

The resolving power of the spectrometer is very important and is usually

described by linear dispersion at the exit slit and this depends on the number of

lines on the grating, focal length of the monochromator, number of stages of

the monochromator, and the wavelength.

3.10.3. Detector Assembly

Raman signals are weak hence very sensitive and low noise detectors are

needed. While choosing the detectors, one should consider (a) quantum

efficiency i.e. probability that incident photon on the detector generates the

measurable signal. (b) dark signal i.e the signal generated by the detector in

dark and (c) the response curve representing the quantum efficiency variation

with respect to wavelength.

Most commonly photomultiplier tubes (PMT) are used as a detector.

Raman spectrum is represented by plot of the counts/sec as a function of

Raman shift (cm-1

). In modern multi channel Raman spectrometers Charge

Coupled Device detectors are used. A CCD is a two dimensional array of

potential wells which stores the generated photo electrons and read out in a



very complicated fashion. Each well is known as a “pixel” and distance

between two consecutive pixels is ~ 25 mm.

3.10.4. Applications

Along with the identification of the materials, this technique is found to

be very much useful in studying the optical properties of the thin films,

superconductors, nano-crystals etc. Also it is widely used to study the phase

transition in materials under different external conditions like temperature,

pressure, electric and magnetic fields.

Instrument Specifications:

Model: MultiRam spectrophotometer

Source: Nd:YAG laser.

Resolution: 0.1cm−1.

3.11. DC Conductivity Measurement

There are various methods of measurement of the electrical conductivity

of the materials. It can be measured in terms of a dc or ac electrical conduction

in materials. The sample is biased by the dc power supply and variation in

current passing through the sample is measured. The resistivity of the sample is

given by the simple relation,

ρ = ST� (3.6)

where, U= resistance of the sample

V= area of cross section through which current is passing and

W= length of conductor.

One can easily determine the conductivity, as the reciprocal of the resistivity,



σ = 4ρ

(3.7)

The dc conductivity can also be represented as Arrhenius relation,

σ = σ?exp�− [\]�/ (3.8)

where, ^.is the activation energy for conduction

_ is the absolute temperature

K is the Boltzmann constant

σ? is the pre-exponential factor.

However, for the oxides, ionic conduction relation (3.8) can be

modified as,

σ = `σa� b exp�−[\]�/ (3.9)

In this case, conductivity is observed as a function of temperature. Depending

upon the resistance of the sample, dc conductivity can be measured by using

two probe and four probe technique.

3.11.1. Two Probe Technique

Two probe technique is simple to use but ohmic contacts are needed if

one wants to measure bulk resistance of the sample. Generally, this technique is

employed for the characterization of the samples with very high electrical

resistance. Figure 3.11 shows the schematic of the two probe technique.

The conductivity of the material is obtained by measuring the

conductance and the physical dimensions of the material. The material is cut in

to the shape known shape like rectangle having length�W/, width �c/ and

height�ℎ/.



Since two copper wires are used as two contacts to the sample, it is known as

the two probe technique.

3.11.2 Four Probe Technique

This technique uses four contacts to the sample and hence termed as

four probe technique of conductivity. This technique is often used for the

measurement of the conductivity of the samples with low electrical resistance

[24]. The schematic of the four probe technique is shown in figure 3.12

Ammeter

I

Current source

Figure 3.12 Schematic of four probe technique for measurement of conductivity

e

Voltmeter

f

V

g

V I

Voltage

source Ammeter

W c

ℎ

Figure 3.11 Schematic of two probe technique for measurement of conductivity



In this technique, known current I is passed through the outer probes and

voltage V developed across the middle probes is measured. In this case

separation between the middle probes is considered as a length W and used to

measure the area of the sample.

3.12. AC Impedance Spectroscopy

Theoretical background of this technique has been discussed in Chapter

II. For the present study the impedance spectroscopy was carried out using a

Solartron 1260 FRA (frequency response analyser) at frequencies between 5Hz

to 10 MHz. The current amplitude was chosen in such a way that the voltage

response never exceeded a value of 12 mV rms.

These measurements were performed at an interval of 25 °C, each

temperature being kept constant with an accuracy of ± 5 °C. Sufficient

stabilization time was ensured at each particular temperature. The data analysis

was done using Z-view frequency response analyzer.



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