Chapter - 2
Crystal structure
and
preparation techniques for Hexaferrite
This chapter describes crystal structure, molecular
arrangement and magnetic properties of different types of
ferrite like spinel, garnet, ortho and hexaferrite. Particularly
crystal structure of different compositions of barium
hexaferrite; BaFe12O
19, Ba
2Me
2Fe
12O
22, BaMe
2Fe
16O
27, Ba
3Me
2Fe
24O
41,
Ba2Me
2Fe
28O
46, Ba
4Me
2Fe
36O
60 are explained with diagrams.
Magnetic moment at different lattice sits and magnetic
anisotropy of hexaferrite are briefly explained. General
experimental methods used to prepare ferrites and their
composites are also included in this chapter.
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 26 - Ph.D. Dissertation
2.1 Crystal structure of ferrite
The crystal structure of a ferrite can be regarded as an interlocking network of
positively-charged metal ions (Fe3+
, Me2+
) and negatively charged divalent oxygen
ions (O2-
). The arrangement of the ions or the crystal structure of the ferrite plays a
most important role in determining the magnetic interactions and therefore, the
magnetic properties. According to their different crystal types, ferrites can be
classified into four groups, namely, spinel, garnet, magnetoplumbite or hexaferrite
and orthoferrite [1-7].
2.1.1 Spinel ferrite
Fig. 2.1. Diagrammatic sketch of spinel crystal structure [6]
There is a kind of crystal structure with the same structure as the mineral
spinel (MgAl2O4), which is called spinel structure. Analogous to the mineral spinel,
the spinel has the general formula MeFe2O4 where Me is the divalent metal ion like
Mn2+
, Ni2+
, Cu2+
, Co2+
, Fe2+
or more often, combinations of these. The spinel lattice
is composed of a close-packed oxygen arrangement in which 32 oxygen ions form a
unit cell that is the smallest repeating unit in the crystal network. Between the layers
of oxygen ions, there are interstices that may accommodate the metal ions. Now, the
interstices are not all the same; some which we will call A (tetrahedral) sites are
surrounded by or coordinated with 4 nearest neighboring oxygen ions whose lines
connecting their centers form a tetrahedron.
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 27 - Ph.D. Dissertation
B (octahedral) sites are coordinated by 6 nearest neighbor oxygen ions whose
center connecting lines describe an octahedron. The unit cell contains eight formula
units MeFe2O4. In a full unit cell there are 64 - A sites, of which 8 are occupied by
divalent metal ions, and 32 - B sites, of which 16 are occupied by trivalent metal ions.
If in the process of preparation, the ratio of metal ions to oxygen ions is too small,
some of the B sites will be unoccupied. These sites are then referred to as vacancies
[2,4,6].
Spinel ferrites can be divided into two types; normal ferrites and inverse
ferrites, depending on the distribution of the divalent ions on A and B sites. In some
spinel ferrites, the divalent ions have a strong preference for the A sites, leaving all
the trivalent Fe3+
ions on the B sites. These ferrites are called normal spinel ferrites.
Other ferrites belong to the class of inverse spinel ferrites, in which the divalent ions
are on B sites, and the trivalent ions are equally divided between A and B sites.
Completely normal or completely inverse spinel ferrites represent extreme cases. In
general, the structure is a mixed spinel with an intermediate structure. The distribution
of cations over A- and B-sites is determined by their ionic radius, electronic
configurations and electrostatic energy in the spinel lattice [2-9].
2.1.2 Garnet ferrite
The crystal structure is that of the garnet mineral, Mn3Al2Si3O12. The magnetic
garnets include Fe3+
instead of Al and Si, and a rare earth cation (R) substitutes Mn, to
give the general formula R3Fe5O12. The crystal structure has cubic symmetry and is
relatively complex. In contrast with spinels, the oxygen sublattice is not a closepacked
arrangement, but it is better described as a polyhedral combination. Three kinds of
cation sites exist in this structure: dodecahedral (eight fold), octahedral (six fold), and
tetrahedral (four fold) sites. Rare earth cations, R, occupy the largest, dodecahedral
sites, while Fe3+
cations distribute among the tetra and octahedral places. There are 16
octahedral, 24 tetrahedral and 16 dodecahedral sites in a unit cell containing 8 formula
units. One formula unit, 3M2O35Fe2O3 is distributed as follows: 3M2O3-dodecahedral,
3Fe2O3-tetrahedral, 2Fe2O3-octahedral [2-5,7,8,10,11].
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 28 - Ph.D. Dissertation
Fig. 2.2. Crystal structure of the garnet [12]
2.1.3 Hexagonal ferrite
The group of ferrites possessing hexagonal crystal structure is referred to
hexagonal ferrites. Six types of hexagonal ferrites are distinguished and indicated as
M, W, Y, X, Z and U as shown in the composition diagram in Fig. 2.3. They
correspond to (MO + MeO)/Fe2O3 ratios of 1:6, 3:8, 4:6, 4:14, 5:12 and 6:18
respectively. Where M can be the ions Ba, Sr, Pb, Ca, La, etc. whilst Me is a transition
cation (Zn, Mg, Mn, Co, etc.) or a combination of cations as it would occur in spinels.
Moreover the substitution of Fe3+
ions with other trivalent cations such as Al3+
, Ga3+
,
Sc3+
, In3+
etc. is also possible. One can obtain an extremely large number of
compounds with considerably different magnetic properties. This fact makes the
hexagonal ferrites attractive for different technical applications and interesting for
basic studies on the magnetic interactions in insulators [13].
The crystalline structure of the hexagonal ferrites is the result of a close
packing of oxygen ion layers. The divalent and trivalent metallic cations are located in
interstitial sites of the structure, while the heavy Ba or Sr ions enter substitutionaly the
oxygen layers. All the known hexagonal ferrites have a crystalline structure which can
be described as a superposition of three fundamental structural blocks namely S, R
and T. The S*, R
* and T
* are the rotational symmetry of S, R, and T at 180
o around the
hexagonal c-axis. The repeating unit ‘S’ has composition of either [Me22+
Fe43+
O8]0
(S0) or [Fe6
3+ O8]
2+ (S
2+) with neutral or uncompensated charge of +2 per subunit,
respectively. The ‘R’ sub-unit has the composition [Me2+
Fe63+
O11]2−
whereas the ‘T’
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 29 - Ph.D. Dissertation
unit is [Ba22+
Fe83+
O14]0. The sub-unit ‘R’ combines with ‘S
2+’ to give the neutral
block (RS), with the total composition MeFe12O19 (M-phase). Similarly, ‘T’ sub-unit
combines with the S0 to give the neutral block (TS), with the total composition
Ba2Me2Fe12O22 (Y-phase). Other stacking sequences of cubic and hexagonal basic
units leading to different compositions such as W, X, Y, Z and U-types hexaferrites
are also known as shown in table 2.1 [3-5,7,13-15].
Fig. 2.3. Ternary diagram showing the composition of the main barium hexagonal ferrites [13]
Table 2.1 Types of hexaferrites [9]
Symbol Composition Crystallographic
build up
No. of
molecules
/unit cell
c-axis
(Å)
M BaFe12O19 RSR*S* (MM*) 2M 23.2
Y Ba2Me2Fe12O22 3TS 3MeY 43.5
W BaMe2Fe16O27 MSM*S* 2MeW 32.8
Z Ba3Me2Fe24O41 MYMY 2MeZ 52.3
X Ba2Me2Fe28O46 MM*S 3MeX 84.0
U Ba4Me2Fe36O60 MM*Y* MeU 38.1
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 30 - Ph.D. Dissertation
Fig. 2.4. The three fundamental structural blocks S, R and T of the hexagonal ferrites [13]
(i) M-type hexaferrite
M-type hexaferrite is a solid solution written in molecular form MeO·6Fe2O3
or MeFe12O19 and possesses the same structure as the natural mineral
magnetoplumbite [2, 3]. Where, Me can be the divalent ions Ba2+
, Sr2+
or Pb2+
. The
magnetoplumbite structure can be built up from spinel blocks of two oxygen layers
being blocks S and S* which are connected by a block R containing barium or
strontium ion. The layer containing the barium is hexagonally packed with respect to
two oxygen layers at each side. The four oxygen layers between those containing the
barium ion are cubically packed. The basal plane containing the barium ion is a mirror
plane of R block and consequently the block preceding and succeeding the R block
must be rotated over 180o with respect to each other. Five oxygen layers make one
molecule and two molecules make one unit cell. The crystallographic structure can be
described as RSR*S
* and the space group is denoted as P63/mmc (D
46h) [16].
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 31 - Ph.D. Dissertation
Fig. 2.5(a). The crystal structure of hexagonal M-type Strontium hexaferrite [17]
Fig. 2. 5(b). Five Fe sites with their surroundings [17]
The M-type hexaferrite crystallizes in a hexagonal structure with 64 ions per
unit cell on 11 different symmetry sites. The unit cell contains 38 oxygen ions, 24
ferric ions and 2 Me ions (Me = Ba2+
, Sr2+
, Pb2+
and La3+
). The 24 ferric ions are
distributed over five distinct sites i.e. 2a, 2b, 4f1, 4f2 and 12k. Out of these five, 2a, 4f2
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 32 - Ph.D. Dissertation
and 12k are octahedral, 4f1 is tetrahedral and the last in which the ferric ion is
surrounded by five oxygen atoms forming a trigonal bipyramid (2b) site. The oxygen
ions occupy 4e, 4f, 6h, and 12k sites form a closed pack lattice. The Me ions
(Me = Ba, Sr, Pb and La) occupy 2d sites. The 12 Fe3+
are arranged as: 6 Fe3+
are in
12k site having the spin up, 2 ions in 4f2 and 4f1 having spin down and 1 ion in 2a and
2b site having spin up. So the 8 Fe3+
are in the upward direction and 4 in the
downward direction. So 4 upward and downward cancel each other and the net
moment is obtained of 4 Fe3+
per formula units. According to the configuration of
Fe3+
, there are 5 unpaired electrons in the 3d orbital, each Fe3+
ion has the magnetic
moment of 5 μB and the total moment is 20 μB per formula unit [13,18,19].
Table 2.2 Number of Fe3+
ions in sublattices of hexaferrites and their spin [18]
Site Geometry No. of Fe3+
ions Spin
12K Octahedral 6 Up
2a Octahedral 1 Up
4f1 Tetrahedral 2 Down
4f2 Octahedral 2 Down
2b Trigonal
bipyramidal
1 Up
(ii) W-type hexaferrite
Fig. 2.6. Crystal structure of W-type hexferrite (BaMe2Fe16O27) [7]
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 33 - Ph.D. Dissertation
The molecular unit of W ferrite is composed of two S blocks and one R block,
so it is similar to the M structure but not identical. There are now two S blocks above
and below the R block, but again there is a mirror plane in the R block and the unit
cell consists of two molecular W units to give SSRS*S
*R
*. The cell length of Fe2W is
32.84 Å, it is a member of the space group P63/mmc, and the structure is shown in
Fig. 2.6 [7,13,20,21].
(iii) Y-type hexaferrite
The molecular unit of Y-type hexaferrite is one S and one T unit, with a total
of six layers, the unit cell consists of three of these units, with the length of the c-axis
being 43.56 Å, and is a member of the space group R3m [20]. The T block does not
have a mirror plane, and therefore a series of three T blocks is required to
accommodate the overlap of hexagonal and cubic close packed layers, with the
relative positions of the barium atoms repeating every three T blocks. This gives the
unit cell formula as simply 3(ST), and the structure is shown in Fig. 2.7 [13, 21-24].
Fig. 2.7. Cross section view of Y-type hexaferrite (Ba2Me2Fe12O22) structure [7]
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 34 - Ph.D. Dissertation
(iv) X-type hexaferrite
Fig. 2.8. Cross section view of the X-type hexaferrite (Ba2Me2Fe28O46) structure [7]
The crystal structure of X-type hexaferrite is similar to that of W-type
hexaferrite, being composed of one M and one W molecular units (Fig. 2.8), to give
the structure SRS*S
*R
*, with the blocks of the W section rotated through 180
o relative
to the M section. The unit cell is constructed from three identical units to give the
crystal structure 3(SRS*S
*R
*), c = 84.11 Å, and it is a member of the R3m space group
[13,20,21].
(v) Z-type hexaferrite
The Z - unit is composed of Y + M, and therefore consists of ST + SR, with a
mirror plane in the R block and a repeat distance of 11 oxygen layers. Therefore, two
molecular units are required to form a single unit cell of Z - ferrite, one rotated 180o
around the c-axis relative to the other, to give STSRS*T
*S
*R
*, with a c axis length of
52.30 Å, and it is a member of the space group P63/mmc. Perspective views of the
unit cell are also shown in Fig. 2.9 [13,21,25,26].
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 35 - Ph.D. Dissertation
Fig. 2.9. Crystal structure of Z-type Barium hexaferrite [27]
(vi) U-type hexaferrite
Fig. 2.10. Cross section view of U-type hexaferrite (Ba4Me2Fe36O60) [7]
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 36 - Ph.D. Dissertation
The unit cell of the U-type compound formed by three molecules possesses the
rhombohedral structure belonging to space group R3m. The structure is built up by the
superposition of two M-blocks and one Y-block along the c-axis, which gives the
block structure as SRS*R
*S
*T. Cross section view of U-type ferrite is shown in Fig.
2.10 [28-30].
2.1.4 Orthoferrite
Orthoferrites are the component with formula RFeO3, where R represents rare
earth or transition metal. They have perovskite structure with a general form ABO,
where A is an twelve-coordinated oxygen site occupied commonly by magnetic ions
of the lanthanide (rare-earth) series, and B houses a transition metal ion, usually Fe3+
.
The ideal perovskite structure is simple cubic. The mineral perovskite is CaTiO3 and
is actually orthorhombic at room temperature, becoming cubic only at temperatures
above 900°C. Other ceramics with the perovskite structure include BaTiO3, SrTiO3,
and KNbO3. The unique magneto-optical properties of orthoferrites are a direct
consequence of their structure. The speciality of orthoferrites is that their unit cell is
slightly orthorhombic instead of cubic like BaTiO3. Indeed, the orthoferrite unit cell is
very nearly cubic, but instead the angle between the c-axis and the a-b plane is
slightly greater than 90o. This orthorhombic cell manifests itself as a slight canting in
the unit cell of the orthoferrite usually in the order of 0.6o [31].
Fig. 2.11. Representation of YFeO3 unit cell [32]
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 37 - Ph.D. Dissertation
An example of a common orthoferrite is YFeO3. It crystallizes in a distorted
perovskite structure with an orthorhombic unit cell. The distortion from the ideal
perovskite is mainly in the position of the yttrium ions, whereas the Fe3+
ions are
present in an essentially octahedral environment. The structure can be visualized as a
three-dimensional network of strings of FeO6 octahedra. One of the anions (O2-
)
forms the common apex of the two adjacent octahedra and provides the super-
exchange bond (Fe-O-Fe) between two iron ions. Thus each iron ion is coupled by
super-exchange to six nearest (iron) neighbours. Because the spin alignment of Fe is
not strictly antiparallel but slightly canted, a small net magnetization results, giving
rise to a weak ferromagnetic behavior [2, 3, 32].
2.2 Magnetism in hexaferrite
2.2.1 Magnetic moment
Magnetic moment depends on the electronic configuration and the distribution
of the substituted ions at different sites in the crystal structure [33]. In hexaferrite,
each S block consists of two layers of four oxygen atoms with three cations between
each layer, in octahedral and tetrahedral sites having opposing magnetic spins. There
are four octahedral magnetic moments and two opposing tetrahedral moments, giving
a net total of two moments. The R block has five octahedral moments, but due to the
effects of the large barium atom two of them are really distorted tetrahedral sites and
so they oppose the other three octahedral sites. The moment of the five-coordinate
trigonal bipyramidal site is aligned with three of the octahedral moments as it is a
distorted octahedral site, and so the total also results in a net of two moments. The T
section has six octahedral and two tetrahedral moments, but again two of the
octahedral moments are aligned with the tetrahedral, giving a net of zero magnetic
moments. The tetrahedral sites are formed by the two barium atoms distorting two
trigonal bipyramidal sites [7,9].
S block = 2↓ tetrahedral and 4↑ octahedral = 2↑
R block = 1↑ trigonal bipyramidal and 3↑ 2↓ octahedral = 2↑
T block = 2↓ tetrahedral and 4↑ 2↓ octahedral = 0
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 38 - Ph.D. Dissertation
In case of M-type barium hexaferrite, The magnetic moments of the Fe3+
ions
are arranged parallel to the hexagonal c-axis, but with opposite spin directions of the
sublattices. The iron ions in the 12k, 2a and 2b sites have their spins aligned parallel
to each other and the crystallographic c-axis, whereas those of 4f2 and 4f1 point in the
opposite direction. The resulting magnetization (M) at a temperature (T) of BaFe12O19
per formula unit can be approximated by simple summation according to the formula
(2.1)
Where, σi stands for the magnetic moment of the i-Fe3+
ion. Assuming a magnetic
moment of 5 μB per Fe3+
ion at 0K (μB is the Bohr magneton) the net magnetization
is of 20 μB per formula unit of barium hexaferrite [3,7,34,35].
In ferrites other than M type hexaferrite, and in doped M type hexaferrites,
some of the cations are other metals with different magnetic moments, which may
occupy different sites depending upon composition and temperature, and may occupy
on a fraction of the total number of a certain site. The opposing spins of the T block,
which is antiferromagnetic if all the ions are identical, lead to the lower magnetic
saturation values for the Y-ferrites compared to the other hexagonal ferrites. This
explains the many variations seen in the magnetic properties of the hexagonal ferrites
with temperature and composition, but it also means that to calculate the magnetic
moment of a compound the exact positions of all the cations must first be known.
However, the contribution towards the moment of each site in a compound can still be
summed up as [7,9]:
M = 1↑ trigonal bipyramidal + 7↑ 2↓ octahedral + 2↓ tetragonal = 4↑
W=1↑ trigonal bipyramidal + 11↑ 2↓ octahedral + 4↓ tetragonal = 6↑
X = 2↑ trigonal bipyramidal + 10↑ 4↓ octahedral + 2↓ tetragonal = 6↑
Y = 8↑ 2↓ octahedral + 4↓ tetrahedral = 2↑
Z = 1↑ trigonal bipyramidal + 15↑ 4↓ octahedral + 6↓ tetragonal = 6↑
U = 2↑ trigonal bipyramidal + 22↑ 6↓ octahedral + 8↓ tetragonal = 10↑
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 39 - Ph.D. Dissertation
2.2.2 Magnetic anisotropy and coercivity
The energy of a magnetic material depends on the orientation of the
magnetization with respect to the crystal axes, which is known as magnetic
anisotropy. The magnetic anisotropy affects strongly the hysteresis loop shape and the
values of the coercivity and the remanence. The magneto-crystalline anisotropy is an
intrinsic property of the ferrimagnetic materials which does not depend on the
particles’ shape and size. For a single crystal, it is the energy necessary to re-orient
the magnetic moment of the crystal from the easy magnetization axis of to the hard
magnetization axis. The existence of these two axes of magnetization arises from the
interaction between the spin magnetic moment and the crystal lattice (spin–orbital
coupling).
Generally, ferrites with hexagonal structure have two types of anisotropy,
namely c-axis anisotropy and c-plane anisotropy, which are associated with the easy
magnetization along the c-axis and in the c- plane, respectively. In the barium ferrite
family, only the Y-type barium ferrite has c-plane anisotropy, while the others have
c-axis anisotropy. The BaFe12O19 exhibits one of the highest values of the magneto-
crystalline anisotropy constant - K1 = 3.3 ×105 Jm−3 [33]. The energy Ek per unit
volume of the magneto-crystalline anisotropy for uniaxial anisotropy can be written as
follows:
(2.2)
Where, θ is the angle between the magnetization and the c-axis. K1 and K2 are the first
and the second anisotropy constant. The direction along which Ek has an absolute
minimum is called the easy magnetization axis [36-38].
Coercivity is one of the most important characteristics of the hexaferrites in
what concerns their potential applications. It describes the stability of the remanent
state and gives rise to the classification of magnets into hard magnetic materials. The
fundamental characteristic of the coercivity is its dependence on the particles’ size,
which explains the unceasing development of techniques for preparation of
hexaferrite powders with high homogeneity and ever smaller particles’ size. Below a
certain critical size (Dcritical) the particle become monodomain; due to the
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 40 - Ph.D. Dissertation
hexaferrites’ magneto-crystalline anisotropy, this size is significantly higher than
that of ferrites with a spinel structures.
2.3 Preparation techniques of ferrites
It is known that the electrical, optical and magnetic properties of ferrites are
very sensitive to the particle sizes, shape and degree of crystallinity. At present,
tremendous efforts have been made in improving their magnetic capabilities by using
different synthesis methods [34]. At the same time, the research on their structural and
physical properties has continued [39, 40]. Recent studies have shown that physical
properties of nanoparticles are influenced significantly by the processing techniques
[41]. Since crystallite size, particle size distribution and inter particle spacing have the
greatest impact on magnetic properties, the ideal synthesis technique must provide
superior control over these parameters.
Table 2.3 Comparison between the wet methods and the conventional ceramic method [42]
No. Wet chemical methods Conventional ceramic method
1 Chemical of mixing of the raw
materials results in a homogeneous
mixture.
Mechanical mixing of raw materials;
difficult to get complete homogeneity.
2 Single phase ferrite formation and
small grain size can be easily
obtained.
Possibility of some phase segregation
cannot be ruled out
3 No impurity pick-up or material loss
during processing.
There is a possibility of impurity pick-
ups and loss of material during the
grinding process.
4 Lower temperature processing,
shorter sintering duration required
and no specific heating and or
cooling rate is required.
Processing requires a higher
temperature and longer durations due to
lower reactivity of the starting oxides,
the heating and/or cooling rates control
the particle size.
5 The raw materials are in the form
nitrate, citrate, acetates and
chlorides.
The raw materials are always in the
form of oxides or carbonates
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 41 - Ph.D. Dissertation
A typical method of obtaining ferrimagnetic hexagonal oxide particles in
general is the solid-state reaction, which involve physical mixing of hydroxide,
oxide, carbonate, or sulphate raw materials followed by high temperature treatment,
approximately 1100°C, for a lengthy period to enable the formation of the target
compound. This method results in powders with coarse-grained, agglomerated
structures with very low surface area. High temperatures needed for solid state
compound formation, give rise to poor sintering behavior, inhomogeneous
microstructures, possibly abnormal grain growth and lack of control of cation
stoichiometry. [7,43]
The conventional solid-state method for preparing BaFe12O19 is to fire an
appropriate mixture of α-Fe2O3 and BaCO3 at very high temperatures (1150 – 1250
oC). The resulting powder is then ground to reduce the particles’ size. Although
high-temperature firing assures the formation of the required ferrite phase, larger
particles (>1 μm) are often obtained in this firing process. It has been shown that the
theoretical intrinsic coercivities of ferrites can be approached only when the particle
sizes are below 1 μm [43]. On the other hand, grinding may introduce impurities into
the powder and cause strains in the crystal lattices, which has unfavourable effect on
the magnetic properties [44, 45]. To overcome these problems, various soft chemical
methods have been developed in order to reduce the particle size and obtain highly
homogeneous ultra fine single-domain particles of barium hexaferrite. Most common
chemical methods: hydrothermal processes [46], chemical co-precipitation [47,48,49],
micro emulsion [50], sol-gel [51,52,53], pyrolisis of aerosol [54], mechanochemical
method [55], sol-gel auto combustion [56,57,58], microwave combustion method
[59], ionic coordination reaction technique [60], solvothermal method [61],
sonochemical method [62], glass-ceramic processing[63], spray pyrolysis [64], pulsed
laser ablation [65], cryochemical method [66], colloidal synthesis [67], reverse
micelle technique [68], sparkplasma sintering [69], novel chitosan method [70]. In all
these processes, precursors are used that have ultra-fine size and high surface area;
thus conventional restrictions of phase equilibrium and kinetics can be easily
overcome, which leads to lowering of sintering and solid-state reaction temperatures
and increased sintering rate.
Crystal structure and preparation techniques for hexaferrite Chapter - 2
- 42 - Ph.D. Dissertation
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