Chapter PREPARATIVE TECHINQUES 2 2.1 GENERAL INTRODUCTIONshodhganga.inflibnet.ac.in/bitstream/10603/79500/8/08_chapter-2.pdf · PREPARATIVE TECHINQUES 2 2.1 GENERAL INTRODUCTION Conventional

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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.