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Chapter PREPARATIVE TECHINQUES
2
2.1 GENERAL INTRODUCTION
Conventional solid state reaction described as ceramic
technique, do not provide a high level of homogeneity in the
product compounds of spinel ferrites. These materials are
generally produced in the form of powders and thin layers
with a requisite grain morphology and controlled particle
size and shape.
Preparation of ceramic oxides is a long time challenge for
materials scientists. Materials scientists have invested a
long period for the synthesis of magnetic oxides. There are
several methods available now a day for the synthesis of
magnetic oxides. The properties of ferrite are known to
depend upon preparation technique and preparation
parameters. It has been reported that the properties of
material prepared by two different techniques are different.
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By changing the preparative technique one can bring
changes in the properties of a material.
Synthesis of nano grain size particles proves to be one
of the most interesting and important technique in the field
of material science, as the small grain size particles have
some of the interesting properties when compared to bulk
particles in material processing and technological
applications. These particles have improved magnetic,
dielectric, catalytic properties, as they possess high
resistivity and negligible eddy current losses [1-5]. The
preparation technique plays important role in modifying the
properties of spinel ferrite. Curie temperature (Tc) can be
varied by substitution of non-magnetic cations. Magnetic
nano particles exhibit some interesting properties which are
useful in high frequency devices, magnetic fluids, high
density recording, colour imaging etc [6-8].
The various processing techniques, which are used for
the synthesis of spinel ferrite powders include, microwave
refluxing [9], sol-gel [10-13], hydrothermal [14,15],
co-precipitation [16], spray pyrolysis [17], In fact there are
numerous research papers on synthesis of nickel ferrite by
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various methods. In our present investigation we have
employed sol-gel auto-ignition method to synthesize
powders of nickel cadmium ferrite. The sol-gel auto-ignition
method is best to speed up the synthesis of complex
materials. It is a simple process, a significant saving in time
and energy consumption over the traditional methods, and
requires low sintering temperature. This method is employed
to obtain improved powder characteristics, low homogeneity
and have a narrow particle size, thereby influencing
structural, electrical, and magnetic properties of spinel
ferrites. Small crystalline size of the resultants may have an
important influence on the properties of the materials
prepared.
In the present chapter the various preparative
methods are briefly discussed.
2.2 DETAILS OF PREPARATIVE TECHNIQUES
1) Ceramic method
The most common method of preparing metal oxides
and other solid materials is by the ceramic method. In the
ceramic method [18, 19] very pure and fine grains
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constituents in oxide forms are taken. Then they are
thoroughly and uniformly mixed. This mixture is sintered
for prolonged time at specific temperature so as to facilitate
solid-state chemical reaction among the oxides and the
formation of chemical compound.
Pre-sintering of the samples can be done at about
9000C and final sintering of the ferrite sample can be done
at above 11000C depending on the constituents. The ceramic
method consists of the following steps.
a) Weighing and thorough mixing of constituents in
stoichiometric proportion.
b) Grinding of the mixed powder for three to four
hours.
c) Presintering at the temperature slightly lesser
than the solid state chemical reaction
temperature,
d) Powdering and pressing into desired shape using
hydraullic press
e) Final sintering at elevated temperature
(>10000C) and slow cooling.
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All these steps involved in the preparation of ferrite are
depicted in the flow sheet as shown in Fig 2.1. Several
mixed oxides, sulphides, phosphides etc. normally can be
prepared by this method. Knowledge of the phase diagram is
generally helpful in fixing the desired composition and
condition for synthesis. Caution is taken in deciding the
choice of container.
The ceramic method has some inherent drawbacks
such as,
i) Poor compositional control,
ii) Chemical inhomogeneity,
iii) Coarse particle size,
iv) Introduction of impurities during grinding,
v) Time consuming process.
vi) Works at high temperature (>10000C).
Various modifications in the ceramic technique have
been employed to overcome some of the limitations of this
method.
By using co-precipitation, sol-gel, freeze drying, spray
drying etc. wet chemical routes, the particle size can be
brought down at much lower temperature compared to
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ceramic technique and which helps to have intimate mixing
of reaction of the constituent reactants. These wet chemical
methods are reproducible, low cost and requires low
temperature.
2.3 DETAILS OF WET CHEMICAL TECHNIQUES
The various wet chemical methods are briefly
discussed as follows,
2.3.1 Precursor Method
Synthesis of complex oxides is done by the
decomposition of precursor compounds [20]. For example
thermal decomposition of precursors LaCo(CN)6⋅5H2O and
LaFe (CN)6⋅6H2O in air readily yield La COO3 and LaFeO3
respectively LiCrO2 can be prepared from the hydrate of Li
[Cr (C2O4)2]. In general alkoxides and carboxylates are the
precursors employed in the synthesis of metal oxides.
Hydrazinate precursors have been employed to
prepare a variety of oxides metal. Ceramic composites have
been prepared by the thermal decomposition of complex
ammonium oxalate precursors. Carbonate solid solutions are
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ideal precursors for the synthesis of monoxide solid solutions
of rock salt structure. Organo-metallic precursors have been
used widely for the synthesis of semi conducting compounds
such as GaAs and InP.
2.3.2 Combustion Synthesis
Combustion Synthesis or the self propagating high
temperature synthesis is a versatile method for the
synthesis of a variety of solids [21]. The method makes use
of a highly exothermic reaction between the reactants to
produce a flame due to spontaneous combustion which then
yields the desired product.
Borates, carbides, oxides and other metal derivatives
have been prepared by this method. For combustion to occur
it is ensured that the initial mixture of reactant is highly
dispersed and contains high chemical energy. A fuel and an
oxidizer can be added by the combustion method, to yield
the product.
For combustion synthesis, the powder mixture of
reactants (0.1-100m particle size) is generally placed. Then
appropriate gas medium that favour an exothermic reaction
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ignition. The combustion temperature is between 1500K and
3000K depending on the reaction. Reaction times are very
short, since the desired products are obtained soon after
combustion. A gas medium is not necessary for the synthesis
of borides, silicates and carbides. A large number of oxides
have been prepared by using nitrate mixture with a fuel
such as glycine, urea and hydrazine. Superconducting
cuprates, ferrites and various oxides can be prepared by this
method.
2.3.3 Wet chemical co-precipitation method
Materials of the same composition but with very
different properties can be prepared at low temperature
(550C) as wet ferrites by chemical co-precipitation method
from aqueous solutions of the corresponding hydroxide.
The preparation of ferrite powders by the oxidation
method consists of oxidation by bubbling air through an
aqueous solution containing ferrous ion and other divalent
ions after an alkaline solution has been added.
Fe2++M2++ROH+O2→M1-xFe2+xO4
where, ROH is NaOH, KOH, NH4OH etc.
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Thus, ferrite powders with high homogeneity and
purity is obtained. Wet chemically prepared ferrites have
been extensively studied by many workers. [22-24].
Ferrites are prepared by air oxidation of an aqueous
suspension containing constituent’s cations in stoichiometric
proportions. The starting solutions are prepared by mixing
50ml of aqueous solution by respective sulphate in
stoichiometric proportions. A two molar (2M) solution of
NaOH is prepared and used as a precipitant. In order to
achieve simultaneous precipitation of all the hydroxide, the
starting solution (pH ≈3) was added to the solution of NaOH
and a suspension (pH = 11) containing dark green
precipitate and kept at low temperature (600C), while
oxygen gas is bubbled uniformly into the suspension to stir
it and to promote the oxidation reaction. The stirring is
continued till all the intermediate precipitates turn in to
dark brownish precipitates of the oxides of soft ferrites. The
samples are filtered, were washed and then dried. Fig 2.2
gives the flow chart of wet chemical method.
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2.3.4 Sol gel Method
Sol gel method is an important mean of preparing
inorganic oxides [25]. It is a wet chemical method and a
multistep process involving both chemical and physical
processes. A sudden increase in viscosity is the common
feature in sol-gel processing, indicating the onset of gel
formation.
The important features of the sol-gel method are.
a) better homogeneity
b) high purity
c) lower processing temperature
d) better size and morphological control
d) reproducible
e) easy and low cost
The six important steps in sol gel synthesis are as follows,
i) Hydrolysis
The process of hydrolysis may start with a mixture of
metal alkoxide and water in a solvent usually alcohol at the
ambient or slightly elevated temperature.
ii) Polymerization
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This step involves condensation of adjacent molecules
where in H20 and alcohols are eliminated and metal oxide
linkages are formed. Polymeric networks grow to colloidal
dimensions in the liquid (sol) state.
iii) Gelation
In this step, the polymetric networks link up to form a
three-dimensional network throughout the liquid. The
system becomes some what rigid, on removing the solvent
from the sol. Solvent as well as water and alcohol molecules
remain inside the pores of the gel.
iv) Drying
Water and alcohol are removed at moderate
temperatures leaving a hydroxylated metal oxide with
residual organic content.
V) Dehydration
This step is carried out between 670 K and 1070K to
take off the organic residues and chemically bound water,
yielding a glass metal oxide.
VI) Densification
For densification temperature in the range of 1200 to
1400 K are used to form the dense oxide product.
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The sol-gel technique has been used to prepare sub
micrometer metal oxide powders with a narrow particle size
distribution and unique particle shapes. Metal -ceramic
composites as well as organic- inorganic composites have
been prepared by the sol gel route.
Sol-gel synthesis includes three different techniques
namely, (a) auto-ignition, (b) auto-combustion, (c) Pechini
synthesis. The detailed description of the synthesis process
of the three methods is described as follows;
2.3.5 Sol-gel auto-ignition Method
In this method the initial compounds were taken in
the form of nitrates, as they dissolve easily in water, and if
the initial solution mixture is in liquid form one can get
very homogeneous powders. The flowchart is given in figure
2.3. The figure shows the detailed process in obtaining the
required ferrite powders by sol-gel auto-ignition method [26].
The nitrates were used as starting materials and citric
acid as chelating material. The molar ratio of metal nitrates
to citric acid has been taken in molar ratios. The metal
nitrates were dissolved together in a minimum amount of
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de-ionized water to get a clear solution. An aqueous solution
of citric acid was mixed with metal nitrates solution, then
ammonia solution was slowly added to adjust the pH at 7.
After thorough mixing of the chemical solution one has to
ensure that the solution is free from any unwanted
impurity. The mixed solution was moved on to a hot plate
with continuous stirring at 90°C. During evaporation, the
solution became viscous and finally formed a very viscous
brown gel. When finally all remaining water was released
from the mixture, the sticky mass began to bubble. After
several minutes the gel automatically ignited and burnt
with glowing flints. The decomposition reaction would not
stop before the whole citrate complex was consumed. The
auto ignition was completed within a minute, yielding the
brown colored.
2.3.6 Sol-gel auto-combustion Method
This method is similar to the method as described above
till the gel formation. Once the gel is formed the beaker with
gel is moved on to the mantle and the temperature is increased
to 300°C (Figure 2.4). As the temperature of the beaker
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reaches high the entire gel is transformed into glowing flints
and the entire process would not stop till the citric acid is not
consumed. The obtained precursor powders will also show
some interesting properties, but the structural changes,
which are taking place at low temperature i.e. the initial
phase of the compound formation, cannot be investigated.
This is because the obtained powders by this method are pre-
sintered at 300°C.
2.3.8 Pechini Method
This is also one of the sol-gel techniques employed
by pechini (27-30). The metal nitrate mixture was heated
to 900C, at which point ethylene glycol was added at a
mass ratio of 4060 with respect to citric acid (Figure 2.5).
The temperature was maintained constant up to gel
formation, which polymerized at 3000C.
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Weighing of consituent oxides
Through Mixing and grinding
Pelletization
Presintering at 900 C for 12 hours0
Regrinding
Pelletization
Final Sintering at 1100 c for 24 hours0
Slow cooling 2 c /min0
Final Ferrite product
Fig . 2.1 Flow chart of ceramic method.
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Preparation of molar constituent solution of sulphates, chlorides
Mixing of sulphate, chlorides solution
Initial pH measurement
Heating and stirring at 600 C
Simultaneous addition of NaOH and H2O2 to get pH >9
Filtering and washing
Heating at 1500 to remove water molecules
Final ferrite product
Fig. 2.2 Flow chart of wet chemical co-precipitation method.
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Figure 2.3 Flowchart for the preparation of ferrite
powders by Sol-Gel auto-Ignition
Ferrite Powder
Calcinations
Precursor
Auto- Ignition At 900C
Gel formation
Stirring and Evaporation at
900C
Nitrates Citric Acid
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Figure 2.4 Flowchart for the preparation of ferrite
powders by Sol-Gel auto-combustion
Ferrite Powder
Calcinations
Precursor
Auto- Combustion At 3000C
Gel formation
Stirring and Evaporation at
900C
Nitrates Citric Acid
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Figure 2.5 Flowchart for the ferrite powders by Pechini
method.
Ferrite Powder
Calcinations
Precursor
Polymerization At 3000C
Gel formation
Stirring and Evaporation at
900C
Nitrates Citric Acid
Adding Ethylene Glycol After Stabilizing the Solution Temp.