11 Chapter 2

CHAPTER-2

EXPERIMENTAL METHODS FOR SYNTHESIS

Experimental Methods for Synthesis

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CHAPTER-II

2.0 EXPERIMENTAL METHODS FOR SYNTHESIS

In this chapter an attempt is made to give a systematic survey of the possible

preparation methods for nano-sized Barium Titanate. Part of these preparation

methods have been used as starting points for the work, described elsewhere in

this thesis. The preparation of Barium Titanate is mainly performed in two ways:

(1) Mixed oxide (solid state) preparation. (2) Wet-chemical preparation method,

so called because of the use of a solvent.

The solid state synthesis is not in use as mentioned above due to several

limitations. Moreover, as mentioned above there are different wet chemical

methods designed and are in practice to obtain crystalline or amorphous ceramic

powders.

The general description of some processes used widely for the preparation of

nano Barium Titanate mentioned below.

2.1 Co-precipitation Method

The basic principle of this procedure is the simultaneous precipitation from

solution of different ions in the form of insoluble highly dispersed Hydroxides or

Carbonates which are converted to final crystalline particles by thermal

treatment. This method has been applied in the preparation of Barium Titanate.

Synthesis routes based on the coprecipitation of complex metal salts remain one

of the most widely used commercial routes for the synthesis of BaTiO3.


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The precipitation is performed mostly through the hydrolysis at low temperatures

(25-100 C) of metal-alkoxide solutions, such as Barium Isopropoxide and

Titanium amyloxide. The Clabaugh process is a wet chemical technique where

solutions of BaCl2, TiCl4 and oxalic acid are mixed and particles of barium titanyl

oxalate (BTO) precipitated [2.1]. After synthesis the particles are calcined to

drive off the oxalate and form BaTiO3. An advantage of BTO is that there is little

change in particle size during the conversion from BTO to BaTiO3 if the

calcination step is properly controlled. In addition, because the Clabaugh

process is primarily a solution-based synthesis route, good mixing and near

atomic scale homogeneity are possible. However, there are two critical issues

associated with the Clabaugh process: (1) Oswald ripening during synthesis and

(2) agglomeration and crystallite growth during calcination. To overcome the two

issues different researchers have used different approaches to correct the

problems during synthesis process.

A modified Clabaugh process has been studied by Kimel et al. [1.25] and

Szepesi [2.2]. In the modified process, a small-volume high-shear mixing

chamber is used to create turbulent fluid flow which permits particle nucleation

while limiting particle growth. After precipitation the particles are directly injected

into a quenching solution which coats the particle surface to inhibit Oswald

ripening. This method has produced BTO particles as small as 10 nm.


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Yamaura et al. [2.3] and Park et al. [2.4] used alcohol based oxalic acid solution

during synthesis of BTO. The BTO exhibits a lower solubility in alcoholic solution

compared to aqueous solution and therefore growth by Oswald ripening is

limited. No notable growth was observed for BTO particles prepared in alcoholic

environments. However, since the solubility of Ba and Ti in alcohol solution is

incongruent and it was difficult to precipitate homogenous BTO powders.

To understand particle evolution during calcination it is important to understand

the decomposition reactions of BTO. Under isothermal conditions during

calcination BTO is believed to decompose by the following reactions [2.5-2.6]

BaTiO ( C2O4 )24 2 Ba2Ti2O2 ( C2O4 ) CO33 2 CO( g )+

2 Ba2Ti2O2 ( C2O4 ) CO33 BaCO3 TiO2 BaTiO3 CO2 CO( g )+++ 33 +

..(2.1)

On contrary the chemical reaction mentioned below can also take place

simultaneously during synthesis.

BaTiO ( C2O4 )2 BaCO3 TiO2 CO( g )+ 2+ 1/2 O2 ( g ) ++ CO2( g )

..(2.2)

Independent of the decomposition reaction, BaCO3 and TiO2 must react to form

BaTiO3

.

+ CO2( g )BaCO3 TiO2+ BaTiO3 .. (2.3)


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The reaction can also lead to agglomeration and crystallite growth during

calcination. Wada et al. [1.47, 2.7] developed a two step calcination of BTO.

Powders with particle size ranging from 17 to 100 nm were reported.

Another group of researchers found that precise control of the heating rate is

necessary to control the final particle size of the BaTiO3 [2.4-2.8]. Using an

intermediate heating rate it yields the proper control of nucleation rate with limited

growth. At low heating rates the nucleation rate is low and the duration of the

reaction is long enough for substantial growth to occur whereas, high

temperature promotes growth of particles [2.9]. Under optimum conditions

BaTiO3 powder with a particle size ranging from 20-40 nm can be synthesized

Thermal Decomposition of Double Metal Salts

Other synthesis methods based on the thermal decomposition of double metal

salts have been presented, but the most common is the Pechini method, or the

citrate method [2.10-2.11]. This method is similar to the Clabaugh process

except that citric acid is used instead of oxalic acid to form a complex double

metal salt. The decomposition reactions involved in the Pechini method are

more complex than that of the Clabaugh process. Since both the Clabaugh

process and Pechini method are based on carboxylic acids, the formation of

BaCO3

during thermal decomposition is unavoidable [1.35].


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2.2 SOL-GEL Process

Sol-gel is similar to that of co-precipitation. The main difference, however, is the

formation of a polymeric precipitate instead of the fine-grained powders obtained

by coprecipitation. The precursors used in sol-gel processing are metal-organic

compounds, mostly metal alkoxides. Several research groups have used a sol-

gel method for the preparation of nanoscale BaTiO3 powders [1.23-1.25, 1.27,

1.29 and 2.12-2.13]. Most sol-gel routes begin with the formation of non-aqueous

sols using high purity Ba and Ti reagents, commonly organo-metallics.

Hydrolysis of metal alkoxides at high pH causes the nucleation of Hydroxide or

oxide powders directly from solution, whereas hydrolysis at low pH produces a

gel. The nonaqueous solution in which the alkoxides are dissolved undergoes

gelation by the addition of an excess of water at temperature below 100 C, and

the resulting polymeric gel has to be converted to the final powder by annealing.

The properties of the resulting gel depend on many factors, including water

content, pH, and temperature. After gelation, the gels are dried and calcined at

high temperatures to remove the chemically bound water and crystallize the

amorphous gel. The calcination temperature is lower than that of solid-state

routes, and therefore agglomerates formed are weaker and easier to reduce

during milling. The main disadvantage of sol-gel routes is that the processes are

costly with low yields. To further reduce the particle size and tailor the particle

size distribution, Hempelmann and co-workers [2.14-2.15] performed sol-gel

synthesis in a microemulsion system. By using such a system the nucleation and

growth of the particles was confined to the aqueous phase of a water-in-oil


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microemulsion and controls the particle size. Using different surfactant systems

and varying the experimental conditions, narrow particle size distributions with

mean sizes ranging from 3 to 16 nm were synthesized. The general reaction map

of sol-gel process is shown in Figure 2.1.

Figure 2.1 General reaction scheme of sol-gel process

Vapor phase synthesis

During exhaustive literature survey it is also found that efforts have been focused

on vapor phase synthesis routes for nanocrystalline BaTiO3 [2.16-2.18]. The

synthesis uses vapor phase Ba and Ti sources such as liquid precursors that are


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either boiled or have inert gas bubble through them, the vapors are then mixed at

elevated temperatures and quenched. Because of the high quenching rates

growth of the particle after nucleation is severely limited. It is also possible to

use electron beam evaporation or sputtering of solid precursors to generate the

vapor [2.16]. Particle sizes less than 20 nm have been reported. One of the

major issues during synthesis is controlling the mixing of the vapors and the

chemical stoichiometry (i.e. Ba: Ti ratio) of the particles. The formation of BaCO3

is also a problem if the atmosphere is not properly controlled [2.1].

During the literature search the direct wet-chemical synthesis routes based on

precipitation have been presented. The techniques are named as low

temperature aqueous synthesis (LTAS), low temperature direct synthesis

(LTDS), solvent refluxing, or hydrothermal synthesis, all these syntheses consist

of basic synthesis steps. Aqueous solutions of Ba and Ti sources are injected

into a high pH solution, and then aged as needed. The Ba and Ti sources, pH

solution, and temperature range in different techniques leading to powders with a

variety of physical properties. Work has been carried out to precipitate BaTiO3

The high pH solution during synthesis also leads to the incorporation of large

amounts of hydroxide defects into the lattice, and since the reaction is open to

directly in an aqueous environment at or near room temperature under ambient

pressure [1.27, 1.30 and 2.19-2.22]. The resultant solutions contain large

amounts of Na and Cl it is necessary to thoroughly wash the particles after

synthesis.


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the ambient atmosphere the presence of BaCO3 is difficult to eliminate. By

adjusting the synthesis variables (i.e. solution concentration, temperature, etc.)

particle size can be varied from 20-900 nm.

2.3 Hydrothermal Synthesis

Hydrothermal techniques are not new and were largely applied in the last century

for the synthesis of minerals of geological interest. In recent years, these

methods have attracted increasing attention for synthesis of dielectric and

piezoelectric ceramic powders [1.28, 1.31-1.32, and 2.23-2.29]. Under the

optimized synthesis conditions, powders with low defect concentrations and

controlled stoichiometry that require no further processing can be synthesized,

making hydrothermal synthesis an excellent choice for the commercial synthesis

of BaTiO3 [1.34]. In hydrothermal synthesis an aqueous solutions of barium and

titanium are mixed and sealed in a high temperature-pressure reaction vessel

and heated. Osseo-Asare et al. [2.30] and Lencka and Riman [2.31] studied the

thermodynamics of the hydrothermal formation of BaTiO3 and found that a basic

environment is necessary for precipitation of BaTiO3

The hydrothermal synthesis of BaTiO

and pH was dependent on

the Ba concentration in the starting solution.

3 is extensively commercialized and

protected by a variety of patents [2.32-2.36]. The methods invented by Abe et

al.[2.32] and Menashi et al.[2.35] are two of the primary methods used for the

commercial synthesis of hydrothermal BaTiO3. The synthesis steps in each

method are similar with differences arising in the post-synthesis treatments.


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They used hydroxides of both Ba and Ti as the source material, which are mixed

in an aqueous solution and heated. After synthesis the powder is washed with

an acetic acid solution to remove BaCO3. However, the acid wash leads to Ba

dissolution from the particle and a Ba deficient surface. Stoichiometry is

controlled by a post-washing treatment with an insoluble Ba metal salt to adjust

to the desired Ba: Ti stoichiometry.

Menashi et al. [2.35] used an amorphous hydrous Ti-gel, Tiy(OH)x, as the Ti

precursor with Ba(OH)2 as the Ba precursor. After synthesis, the particles were

washed with a 0.005 to 0.02 M Ba(OH)2 solutions. The use of a Ba-rich wash

solution limits Ba dissolution and eliminates the need to adjust the stoichiometry

with a second treatment. Regardless of the method used to the synthesis, the

general reaction for the formation of BaTiO3

+ ++ BaTiO3Ba 2+

( g )TiO2 OH-2 H2O

during hydrothermal treatment is

shown in (equation 2.4).

.. (2.4)

Two rate-limiting mechanisms have been observed for the hydrothermal

synthesis of BaTiO3

The difference in formation mechanism is generally dependent on the phase of

the TiO

:

(1) Phase boundary and diffusion limited [2.26-2.28, 2.37]

(2) Nucleation and growth [1.28, 1.32, 2.24-2.25 and 2.29-2.38].

2 source. If TiO2 is crystalline or of large size, then the TiO2 particles


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have a low solubility and growth occurs by the reaction of Ba2+ at the surface of

the TiO2 followed by diffusion of Ba2+ into the lattice, eventually leading to the

conversion of the TiO2 to BaTiO3. A secondary effect of this growth mechanism

is that size and morphology are limited by the size and morphology of the starting

TiO2 particles [2.39]. Hertl [2.28] studied the kinetics of hydrothermal synthesis

using a crystalline TiO2 source. At low Ba concentrations diffusion of Ba into the

lattice of the TiO2 is the rate limiting step. In contrast, at higher Ba

concentrations, the reaction of the Ba with the surface of TiO2 particles is the rate

limiting step in BaTiO3 growth.

When a highly soluble TiO2 source is used, for example, a Ti-organometallic or

sol-gel derived Ti-hydrous-oxide gel, both the Ba and Ti exhibit high solubility at

elevated temperatures and synthesis proceeds by nucleation and growth. To

fully investigate hydrothermal growth under such conditions Kershner et al. [2.29]

used TEM to image particles synthesized using a TiCl4-based gel as the TiO2

source. At all stages of growth homogenous single crystal BaTiO3 particles were

observed. If a surface reaction/diffusion mechanism was responsible for growth,

then at the early stages of growth, inhomogeneous particles with a TiO2 core and

a shell of BaTiO3 are expected; however, this was not observed. This lack of

evidence for a surface reaction/diffusion mechanism was later confirmed with

kinetic studies from Moon et al. [1.28], which led to the conclusion that a

nucleation and growth mechanism controls the growth of hydrothermal BaTiO3

when a high solubility TiO2 source is used.


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The low temperature hydrothermal synthesis of BaTiO3 is of interest because of

the savings of time and energy. At low temperature the interface-diffusion growth

mechanism is kinetically limited. However, the mixing of the Ti and Ba is a

problem when using a Ti-gel precursor. When titanium isopropoxide is mixed

with water at high pH, a TiOy(OH)x gel readily forms. The local structure of the

gel is comprised of Ti-O-Ti bonds. It is necessary to break the Ti-O bonds for

complete mixing of the Ti and Ba [2.39]. Moon et al. [1.28] modified titanium

isopropoxide with acetylacetone which inhibits the hydrolysis of Ti and the

formation of TiOy(OH)x network [2.40-2.42]. This results in the Ti precursor

having greater water solubility and permits better mixing of the Ti and Ba. Using

a modified Ti precursor the BaTiO3 was synthesized at temperatures at 50 C

with particle sizes ranging from 50 to 350 nm.

Although high pH is necessary for synthesis it also leads to the greatest issue

with hydrothermal powders: hydroxide defects. During synthesis hydroxyl groups

are incorporated into the lattice of the particles [2.43]. After synthesis, heat

treatment of the powders is needed to remove the hydroxyl groups from the

lattice. The hydroxyl groups are compensated by the generation of oxygen

vacancies in the lattice [2.43]. If a large concentration of hydroxide defects is

present, during heat treatment the oxygen vacancies coalesce to form large

pores, which degrade electrical permittivity and physical properties, crystallinity

and density, of the bulk materials.


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In the synthesis of BaTiO3 the quality and physical properties of the powder must

meet high standards. Defects, contamination, and incorrect stoichiometry are all

problems which will affect the densification and sintering of bulk materials. Large

intragranular pores, exaggerated grain growth and secondary phase are all

possible if the physical properties of the powder are not well-controlled [2.44].

An advantage of hydrothermal synthesis is the ability to control particle

morphology. A variety of shapes have been reported, including tubes [2.45],

hexapods [1.33], and platelets [2.46] all in the nanoscale by hydrothermal

synthesis. By limiting growth in a specific direction an anisotropic morphology is

achieved. Crystal chemistry and the presence of specific adsorbates affect the

crystal growth. Bagwell [2.47] found the stable crystal habit in hydrothermally-

derived BaTiO3 changed from the {111} plane to the {100}, {110}, and {211}

planes with the addition of polymeric additives. Since the ferroelectric properties

of BaTiO3

Barium Titanate hydrothermal syntheses are based upon the reaction between

TiO

are strongly dependent on the crystallographic orientation of the

materials, these developments in morphology control could possibly lead to an

enhancement in the electrical properties of bulk samples prepared from these

powders.

2 or Titanium Hydroxide and Ba(OH)2 at a high pH value in an aqueous

solution of NaOH. The reaction is carried out in a closed vessel at T >100C, so

that its pressure corresponds to the equilibrium vapor partial pressure at that

temperature. The temperature and concentration of reactants determine the time


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needed for the reaction. The final product is a very fine BaTiO3 powder (100

nm) with a cubic or pseudocubic structure at room temperature, always

presenting a certain amount of OH-

The solvothermal method provides a means of using solvents as temperatures

well above their boiling points, by carrying out the reaction in a sealed vessel.

The pressure generated in the vessel due to the solvent vapor elevates the

boiling point of the solvent. Typically solvothermal methods make use of solvents

such as ethanol, toluene and water, and are widely used to synthesize zeolites,

inorganic open frame structures and other solid materials. In the past few years,

solvothermal synthesis has emerged to become the chosen method to

synthesize nanocrystals of inorganic materials. Hydrothermal processes involve

using water at elevated temperatures and pressures in a closed system, often in

the vicinity of its critical point. A more general term, solvothermal, refers to a

similar reaction in which a non aqueous solvent (organic or inorganic) is used.

Under solvo (hydro) thermal conditions, certain properties of the solvent, such as

groups trapped in the crystal structure, which

are released after thermal treatment at moderate temperature (~500 C)

necessary to obtain the thermodynamically stable tetragonal structure. A

beneficial effect of hydrothermal synthesis is the improved chemical purity of the

final product because of the dissolution-recrystallization process occurring during

hydrothermal aging. In particular, the amount of iron in the final product is

strongly reduced. This procedure nowadays has important industrial applications

2.4 Solvothermal Method


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density, viscosity and diffusion coefficient, change dramatically and the solvent

behaves much differently from what is expected under ambient conditions. A

novel solvothermal route has been developed to synthesize highly dispersed

nanocrystalline Barium Titanate (BaTiO3), using a mixture of ethylenediamine

and ethanolamine as a solvent. The BaTiO3 nanoparticles obtained were highly

dispersed and crystalline with a cubic perovskite structure. The particle size

derived from the TEM ranged from 5 to 20 nm [2.48].

Table 2.1 represents a list of the most common synthesis routes used to produce

nanoscale BaTiO3 powders and their characteristics.


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Table 2.1 List of common techniques and their characteristics used in synthesis of nanoscale Barium Titanate [1.34]

Method Particle Size Impurities Advantages Disadvantages Mixed Oxide 400 nm to

100s m Large quantities of Impurities due to starting materials and milling method

Easy process to perform on large scale Relatively cheap starting materials

High Impurities level High Calcination temperatures Large amount of aggregation leading to large particle sizes Milling usually required Poor stoichiometric control from particle to particles

Coprecipitation 10nm 10s m

Chloride and other impurities present from starting materials. Contamination of milling is required

Low Impurity levels. Low reaction temperatures. Stoichiometric mixing approaches atomic level.

Usually requires a milling treatment to obtain desired particle size. More time consuming than mixed oxide method. Tedious washing required to remove chloride ions.

Sol-Gel 5- 100 nm Minimal contaminants from organic precursors. Small amounts of Si contamination from glass wares.

Very low impurity levels. Stoichiometric on atomic level. Low processing temperature 20-650 C

Relatively expensive starting materials. Low temperature methods are generally time consuming with low product yields.

Vapor phase 20 nm- micron level

Small levels of contamination from starting materials

Low processing temperature from 100- (-800) C. Easy to produce nanosized particles

Some precursor materials are costly. Collection without aggregation is difficult. Stoichiometric control can be difficult

Hydrothermal 3nm micron level

Small levels of contamination from starting materials and reaction vessel. Hydrothermal (OH) defects due to aqueous synthesis.

Low processing temperatures 60-500 C. Particles are formed in solution giving potential control over agglomeration. High purity and atomic scale stoichiometry Particle morphology easily controlled.

Some precursor materials are costly. Recovery from suspension without agglomeration. Re-dispersion of agglomerates.


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On going through the literature search one of the main disadvantages of several

routes is high-temperature calcination step which leads to the formation of hard

agglomerates.


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