Chapter 1INTRODUCTION

1.1 Ceramics - An Overview

Ceramics have been in use even before the beginning of written history. They

predate metals. The long process of moving ceramics from a tradition-based craft to a

science-based technology was underway in the 1800s and has continued to the present

day. Ceramics have been the focus of increased interest during the last century since

they exhibit better hardness, stiffness and chemical stability compared to many other

materials. The word ceramics originated from "Keramos" meaning burnt stuff.

Ceramics cover a vast area of inorganic, non-metallic materials including

whitewares, structural clay products, refractories, glass and glass-ceramics, cement,

concrete, lime, foundry sand, oxide ceramics, non-oxide ceramics such as boride,

carbide and nitride, and also cermets. Developments in the 20th century that stimulated

progress in ceramics include advances in science and technology in general, the rise of

new industries, advances in military technology and also the overwhelming concern for

health, safety and environment [1-12].

1.2 Classification of ceramics

According to the area of applications of advanced ceramics, they are classified

into traditional (conventional) ceramics and modem (new) ceramics. The former

constitute a range of products based on natural materials such as clays and silicates

while the latter embrace ceramics consisting of materials such as alumina, zirconia and

titania. The modem ceramics are further classified into structural (engineering),

electronic, environmental related ceramics and ceramic coatings. Emphasis is given in

modem ceramics to special properties or advanced features, which increase their

commercial value. Hence they are represented as advanced (high technology )

ceramics [3].

2

Advanced ceramics comprise a broad spectrum of recently developed inorganic

materials whose special properties such as ability to withstand high temperature, high

strength and hardness, wear and corrosion resistance, biocompatibility, special optical

and electronic properties etc. are revolutionizing industry and opening new vistas for

future technologies. Hence successful development and commercialization of high

performance ceramic materials have become the center of attraction at an international

level. Major applications of advanced ceramic materials are summarized in Table 1.1

and the world market for advanced ceramics in Fig 1.1 [13, 14].

Table 1.1 Advanced ceramic applications

Branch

Structural ceramics

Applications

Heat engine components..................................................................................................................................................................................

Cutting tool inserts..................................................................................................................................................................................Wear-resistant parts

..................................................................................................................................................................................Heat exchangers..................................................................................................................................................................................High temperature and energy related components

..................................................................................................................................................................................Aerospace and Defence

..................................................................................................................................................................................Bioceramics

Insulators............................................................................................................._ .Substrates and IC packages..................................................................................................................................................................................

Electronic ceramics ~.~~~~.~~?.~~ .Piezoelectric ceramics.................._ .Ferrite magnetic materials

..................................................................................................................................................................................High temperature superconductors.........................................................................................................................................................................................................Aircraft and aerospace engines..................................................................................................................................................................................Land-based turbine engines

..................................................................................................................................................................................

Ceramic coatings ~~~?~..~~.~~.:.~..~~.~ ..~~~.~~ ..~~~~.~~~ .Heat exchangers and high temperature wear parts

.................................J,mgjn.d.!J.~tr..!~.I..!!p..QH£!!ti.QD.~ ..Cutting tool inserts.........................................................................................................................................................................................................MembranesEnvironmental related ..Catalyst supportsceramICS .Filters, catalysts

3

Environmentalrelated ceramics

Ceramic coatings 18% Structural ceramics

~~~

Electronicceramics

68%

Fig.1.1 World Market for Advanced Ceramics

Although the structural ceramics form only six percent of the modem ceramics,

in the utility point of view they steal the first position in priority because of their

strength, stiffness and environmental stability. Fiber reinforced ceramics and cermets

fall under this category. Commonly available structural materials include oxides and

non-oxide ceramics. Ah03 is the most widely used ceramics and finds application in

electroceramics, wear resistant components and dental and prosthetic implants. Zr02

and MgO are used as refractories. Among non-oxide ceramics, the most widely studied

and sought after materials are silicon carbide, boron carbide, silicon nitride, boron

nitride and sialon. They are promising candidates for high temperature structural and

wear-resistant materials. Other practical applications are as anticorrosive materials,

bearing in automotives, microwave light polarizers and in nuclear industry. Medical

needs have led to the development of a variety of bioceramics, including bioactive

glasses and hydroxylapatite coatings which assist bone growth [4].

Electronic ceramics as shown in Fig.I.! form 68.1 % of the world market. They

find applications in microwave communication as ferrites in circulators, garnets in

phase shifters and dielectrics and insulators and also as elements in microwave

integrated circuits. Ferrites are widely used in high-frequency power supplies,

4

transformers and magnetic read/write heads for computer back-up tape drives.

Piezoelectric ceramics have manifold applications as diverse as submarine detection to

medical ultrasonic imaging. The discovery of superconductors has revolutionized the

industrial scenario smce it can conduct electricity with zero energy loss. The

development of low loss optical fiber has revolutionized high-capacity

communications. Sensors are another fascinating area of application of ceramics. The

zirconia oxygen sensor is an essential element in all modem automobile engines which

controls the air-to-fuel ratio. The major environmental uses of ceramics include

honeycomb structure as catalyst support for emission control devices in automobiles.

Ceramic filters for water purification appear to be finding a niche market. Ceramic

mesoporous membranes of 1-2 mm thickness are used for liquid filtration, air­

separation and gas-separation in petrochemical processes [3,4].

A great deal of research has gone into the production of structural ceramic

materials having superior mechanical properties. However, high cost of raw materials

and difficulty in manufacturing complex shaped components limit their use.

1..2.1 Processing of ceramics

Forming or fabrication of ceramIC components from the particulate raw

materials is of extreme significance. Powder processing and shaping techniques are

discussed below.

1.2.1.1 Powder Preparation Techniques

The methods developed for the synthesis of ceramic powders can be broadly

divided into three categories viz., solid, liquid and gas phase. In solid phase synthesis, a

mixture of hydroxides, oxalates, oxides or carbonates are blended and allowed to

undergo a solid-state reaction. Repeated grinding and firing may be required to

increase the homogeneity of the product. To obtain characteristic requirements for

advanced ceramics, a few processing techniques have been developed and

commercialized. These include solution technique, vapor phase technique, salt

decomposition (spray-pyrolysis) and reaction formed synthesis [15, 16].

G.326'10 5

Solution techniques include sol-gel, precipitation-co-precipitation technique and

hydrothermal synthesis. The main advantages of sol-gel process are: better

homogeneity in products, high purity, ease of preparing ultra fine powder, lower

reaction temperature, possibility of many new materials with new composition and

versatility to prepare bulk solid, fiber or film [16, 17].

Vapor phase synthesis can produce high purity, fine unaggregated powders with

excellent homogeneity. The vapor phase techniques that are in use are Chemical Vapor

Deposition (CVD) and Physical Vapor Deposition (PVD). CVD is the synthesis

through the chemical reaction of the vapor of a metallic compound. Therefore it is

widely used for the formation of SiC and ShN4• PVD is the method equivalent to the

atomized liquid synthesis method. Raw materials are vaporized at high temperatures

by an arc or pl~sma, and the pro~uct is. condensed to a fine powder by~~g.

Plasma synthesIs and laser synthesIs are m development stage [18-21]..?'_f: ',' ....i,;;.<,:~J,'. , "'.

, r%~'~~~>::'"VrJ:'/('·J f"\l\ (. '. /: .~'

......... ~ . '". . \' ~~"

1.2.1.2 Forming (shaping) techniques

Forming processes, which are carried out at ambient temperature, are known

as "Cold Forming Methods" and are predominant in ceramic industry. Die pressing,

slip casting and extrusion constitute a few of the examples of cold forming techniques.

However, the use of processes at which ceramic parts are formed at elevated

temperature is increasing. Such processes include hot pressing, hot isostatic pressing,

and injection moulding [22].

The forming method used in the production of ceramics may be divided into

two broad categories, viz., plastic deformation and casting. Die pressing and extrusion

fall under the former while slip casting, gel casting, tape casting and fusion casting

under the latter. Dry pressing is the most economic shaping process. Extrusion is

restricted to long rods and tubes with small diameters with various profiles. The

injection moulding technique is suitable for use in the fabrication of small parts with

complicated shapes [4,23]. Hot pressing involves simultaneous application of heat and

... ,"

6

pressure during sintering. In isostatic pressing, a mould is filled with powder and then

subjected to high pressure transmitted through liquid in a pressure vessel.

Injection moulding is a plastic forming technique in which ceramic powder is

added as filler to an organic polymer, usually a thermoplastic, to form a plastically

deformable mixture that is injected, by using a combination of heat and pressure, by a

plunger into the mold. Injection molding is used extensively in the plastics industry

and has the potential for production of components with complex shapes such as

turbine blades [23].

1.3 Natural raw materials: a necessity

Quality products require advanced technology and high-grade raw materials. A

great deal of work has been carried out in this area. It has been understood that clay

mineral is an excellent raw material for various high temperature ceramic requirements.

The ceramic properties of clays are largely governed by the crystal structure and the

crystal composition of their essential constituents and the nature and amount of

accessory minerals present. Since silicates and aluminosilicates are easily available,

they are also inexpensive and thus provide the backbone of high tonnage products in

ceramic industry [24, 25]. India ranks fifth among the major clay producing countries

[26].

The principal clay mineral groups are kaolinite, smectite and palygorskite. Clay

minerals can be divided into chain and layer structures. The layer structures are

branched into 1:1 and 2: 1 (dimorphic and trimorphic). Classification of clay minerals is

indicated below [27].

7

Two layer type( 1:1 )

Three layer type( 2:1)

Regular mixed­layer type

Chain structuretype

(Homblende like chains)e.g. Attapulgite, sepiolite

palygorskite

Elongate(Montmorillonite group)

(ordered stacking of altematelayers of different types)e.g. Chlorite group

Equi dimensional(Montmorillonite group)

(sheet structures composed of twosilica layers and one alumina layer)

(sheet structures composed of onesilica layer and one alumina layer)

e.g. Kaolinite, DickiteNacrite etc..·

e.g. Halloysite e.g. MontmorilloniteVermiculite, Sauconite

e.g. Montmorillonite, NontroniteSaponite, Hectorite

1.4 Kaolin

The tenn kaolin is derived from Kau-ling, a locality in Jiangxi Province, China,

where the Chinese used white clay to make porcelain. Mineralogically, it is a group of

hydrated alumininosilicate minerals, which includes kaolinite, dickite, halloysite and

metahalloysite [28].

The major mineral in kaolin, Kllolinite is chosen as the basic precursor

materials for the preparation of advanced ceramic materials dealt in the present work.

Hence details regarding its natural fonnation, availability, properties, structure and

industrial applications are briefly discussed.

1.4.1 formation of Kaolin

Clays have been fonned in nature principally by mechanical and chemical

weathering of igneous or metamorphic rocks. The basic rocks from which clays are

fonned are complex aluminosilicates. During weathering these become hydrolysed, the

8

alkali and alkaline earth ions form soluble salts and is leached out, the remainder

consists of hydrated aluminosilicates and free silica. This remainder is therefore more

refractory than the original igneous rock. Weathering process does not alter quartz;

therefore it is a common constituent of clays. The weathering process can be

I'epresented by chemical equations, which is a convenient way of obtaining a picture of

the process [6].

K20 Ah03 6Si02+ 2H20 ~ Ah03 6Si02H20 + 2KOH(Feldspar)

Ah03 2Si02H20 + H20 ~ Ah03 2Si022H20(kaolinite)

1.4.2 Availablllty of Kaolin

(hydrolysis)

(desilication)

(hydration)

World resources of kaolin are estimated at close to 20,000 million tons.

Important kaolin deposits are in Asia, Africa, Australia, Brazil, China, Czech Republic,

England, Germany, Indonesia, Japan, Mexico, New Zealand, Spain, Ukraine and

United States [29]. In India, kaolin deposits are estimated to be around 9860 lakh tons

as per 1990 survey of Indian Bureau of Mines. Rajastan is the leading producer of

china clay (29%) followed by West Bengal (17%), Kerala (12%), Delhi (10%),

Haryana and Orissa (3%) each [26]. The major china clay deposits of Kerala are

located in the districts of Thiruvananthupuram, Kollam, Kannur and Kasargod [30].

1.4.3 Properties of Kaolin

Kaolin develops plasticity when mixed with water. They vary over a wide limit

m chemical, mineralogical and physical characteristics. The most important

characteristic property is their crystalline layer structure, consisting essentially of

electrically neutral aluminosilicate layers, which leads to a fine particle size and plate

like morphology and allows the particles to move readily over one another, giving rise

to an easy cleavage. Clays have two important functions in ceramic bodies: (1) their

characteristic plasticity is basic to many of the forming processes and to maintain their

9

shape and strength during firing is unique, (2) they fuse over a temperature range,

depending on the composition, in such a way to become dense and strong without

losing their shape at temperatures which can be economically obtained.

The most common clay mineral of primary interest to ceramists (since it is the

major component of high-grade clays) is kaolinite, Ah(ShOs)(OHk To understand the

properties of clays, it is necessary to consider both the crystal structure and surface

chemistry [1, 31, 32].

1.4.4 Structure of kaolin

The structure of kaolinite is based on the combination or condensation of two

layer structures - a silica layer and gibbsite layer. A schematic representation of

the layer structure is shown in (Fig. 1.2).

Fig.1.2 Schematic representation

of 1:1 layer structure of kaolinite

Kaolin

Silica layer is composed of silicon and oxygen atoms, the oxygen being at the

comers of the regular tetrahedron. The silicon carries four +ve charges, and each

oxygen carries two negative charges, so that the tetrahedron as a whole carries a net

charge of minus 4 ie. [Si04t. This structure can be extended indefinitely in the a and

b directions (Fig 1.3) [1,24].

10

(a)

(b)

Qand-,

I I

'-"Oxygens • and 0 Silicons

Fig.1.3 Structure of silica tetrahedral sheet in kaolinite [27,33]

(a) single silica tetrahedron (b) asheet of silica tetrahedra arranged in a hexagonal network

O and.-' Hydroxyls.. " • Aluminium, Magnesium etc

Fig.1.4 Structure of alumina octahedral sheet

(a) asingle octahedron and (b) asheet of octahedra arranged in ahexagonal network [27.33]

The basic building block of the gibbsite layer is an aluminium atom surrounded by six

hydroxyl groups, the hydroxyls being at the corners of a regular octagon. Hence

gibbsite layer is referred to as the octahedral layer (Fig 1.4). The two types of forces

11

that hold the sheets together are (a) weak Van der Waal's attractive forces, which exist

between all particles of matter, and (b) weak hydrogen bonds between the hydrogens of

the hydroxyl groups in the gibbsite layer. The hydrogen acts as a bridge as shown in

(Fig 1.5). Although the forces between kaolin layers are weak, they are strong enough

to hold the layers in fixed positions and give a constant basal spacing of7.2A 0.

OOxygens

@HYdrOXYls

eA'uminums

eOSilicons

Fig.1.5 Structure of Kaolin [27,33]

Since the bonds between the kaolin layers are weak, the numbers of layers that

can be stacked together are limited and the clay crystals tend to be in the c-direction.

The bonds between the octahedral and the tetrahedral layers and in the a and b

directions are strong. Thus the crystals of the clay minerals are long and wide but thin.

The hexagonal structures in both the tetrahedral and octahedral layers result in flat,

hexagonal crystals [1]. The card house structure of hexagonal kaolin plates are as

shown in Fig.1.6 [34].

Fig.1.6 Card house

Structure in kaolinite [34] *

12

In chemical composition kaolin approaches the formula Al203 2Si02 2H20.

The particles are extremely small, most particles lying in the range 1-10 Ilm. The small

size, plate like nature of crystals and high specific surface area are responsible for many

of the important properties of kaolinitic clay minerals. Kaolin is highly refractory clay

having a melting point about 1973 K [1].

1.4.5 Industrial Applications of Kaolin

The most important use of kaolin is in paper making and is also extensively

used in ceramics, rubber, paint, ink, plastics, adhesives, pesticides, pharmaceuticals,

cosmetics, fertilizers, animal feed, polishes and as a sources of aluminium and

aluminium compounds. The area of application of kaolin is presented in Fig 1.7 [35,

36].

o Paper filling+coalingIIRefractorieso CeramicsI!J Fiber glasso CementI1lI Rubber+plasticsoPaint• CatalysismOthers

Fig.1.7 Kaolin use by end·market

1.4.6 Effect of heat on Kaolin

The transformation of kaolinite to mullite is the most important reaction in the

entire field of ceramic technology. Brindley and Nakahira have extensively studied the

above reaction series [37]. When kaolin is heated, an endothermic reaction takes place

at temperature 773-873 K resulting in the formation of an amorphous phase called

metakaolin. The reaction is represented by the following equation [26].

Ah03 2Si02 2H20 ----+ Ah03 2Si02+ 2H20

13

During further heating, an exothermic reaction occurs in the temperature range

1223 K-1273 K, by the formation of a spinel type of phase concurrent with the discard

of amorphous silica as per the following equation.

2 [Ah03 2Si02] --+ 2Ah03 3Si02+ Si02Spinel

The second exothermic reaction at 1323-1373 K completely transfers the spinel

structure to mullite with further discard of silica. Any further reaction at higher

temperatures result in the continued development of mullite to the composition 3Ah03

2Si02and cristobalite as represented by the following equation [1, 6].

3 (2Ah03 3Si02) --+ 2 (3Ah03 2Si02) + 5Si02Mullite cristobalite

Kaolinite-mullite reaction has great industrial importance since mullite is an

essential component (phase) in alumino silicate based ceramic products. Formation of

other phases along with mullite is observed depending on the ingredients present in the

starting composition. Making use of this highly advantageous property, attempts were

made to fabricate advanced ceramic materials from kaolinite (aluminosilicate).

1.5 Aluminosilicate ceramics

A major segment of the silicate ceramic industry includes cement, whitewares

and porcelain. Another distinct group is the structural clay products, which consist

mainly of bricks and tiles. Natural fireclays were the first raw material for refractories.

The principal impurities in refractory fireclays are silica, in the form of quartz and iron

oxide. The aluminosilicate range of compositions of natural fireclay by the addition of

non-clay high-alumina raw materials such as sillimanite, kyanite and andalusite (all are

different forms of Ah03Si02) and corundum, Ah03 etc. The main advantages of the

Ah03-Si02 binary systems are many. The first is that both silica and alumina are

highly refractory, having melting points of 1983 K and 2073 K respectively, but they

form an eutectic mixture, which melts at 1818 K. Secondly, the composition of the

eutectic lies very near to the silica end. Thirdly a refractory compound, i.e. mullite,

3Ah0 32Si02 having melting point at 2083 K occurs in the system itself [6, 38].

14

Thomas e/ al. have studied the refractoriness of clays available in the southern part of

Kerala and found that they are. suitable for blast furnace linings [39]. Production of

fireclay bricks from kaolinitic clays has also been reported [40].

Apart from conventional ceramICS, aluminosilicates can be promlSlng

candidates for the preparation of advanced ceramic materials. In the present study,

materials viz., lithium aluminiosilicate (spodumene) and magnesium aluminosilicate

(cordierite); fiber reinforced ceramic composites, cermets and silicon carbide-mullite

composites have been prepared from the hydrous aluminosilicate (kaolinite). The

bibliographic survey on the above topics has been presented below.

1.6 Mullite ceramics

Good thermal shock resistance, hot-Ioad-bearing characteristics, chemical

inertness and high refractoriness make mullite a favourable material to ceramic, glass,

steel, aluminium and petrochemical industries. Production of mullite includes

conventional methods, hydrothermal processing, sol-gel processing and spray

pyrolysis. Synthesis of mullite from kaolinite by conventional sintering is well known.

Reports regarding kaolinite-mullite reaction series show that the presence of

mineralizers can considerably reduce the temperature of mullite formation [41-52].

Several authors claim to have succeeded in increasing the mullite content of ceramic

wares by the judicious use of certain mineralizers [53]. A comparative study of mullite

formation at high temperatures from kaolinite and equivalent alumina-silica mixture in

presence of impurities such as Ti02, Fe203, CaO, Na20 and K20 have shown that

kaolinite based mixture always produced acicular mullite while Ab03 - Si02 mixture

gave mullite crystals of rectangular shape. Also, the presence of CaO and larger

amounts of Fe203 added to both natural and synthetic aluminosilicate mixtures caused

an increase in crystal size [54]. According to Bulens e/ al. crystallinity of the stacking

kaolinite is an important parameter that influences the reaction sequence of mullite

[55]. Studies regarding the relation between mode of introduction of mineralizers and

their influence on the behavior of kaolin bodies have shown that the state of dispersion

of a given mineralizer in kaolin strongly alters its effect on sinterability and mechanical

strength as well as the recrystallisation phenomena [56]. The influence of mineralizers

15

on shrinkage of kaolin bodies is explained in terms of hyperbolic shrinkage law by

Lamatire e/a![57].

Effect of incorporation of mineralizing agents viz, ZnO, Ti02 and Fe203 in

triaxial porcelains constituting china clay, quartz and feldspar on the development of

crystalline and glassy phase was studied by Chaudhuri e/ a! [58-59]. According to

Albert, the transition temperature of cristobalite formation will generate stresses, which

persist at higher temperatures and cause cracking during temperature cycling [60]. The

factors that influence the strength of whiteware bodies include mullite formation, firing

process, particle size distribution and flux chemistry. Studies regarding the influence of

mineralogical composition on the thermal properties of hard porcelains have shown that

thermal expansion of these samples is a function of asymmetry of size distribution of

mullite crystal [61]. Hermansson e/ a! have reported that the crystallization of glassy

phase can be achieved only in presence of nucleating agents and the increase in

addition of K20 content enhances the glassy phase formation [62]. The basis of

understanding the reaction steps that occur in natural kaolinite during high temperature

reaction have been critically analyzed by Pask e/ a! [63]. The influence of impurity

viz., CaO and K20 in kaolinite during sintering have shown that these minieralizers

cause shift in the exothermic reaction temperature and accelerate the cristobalite

formation [64]. Reports regarding the microstructrual evolution of mullite crystals

from kaolin powder compacts during sintering have shown that these crystals show

preferred orientation which is in correlation with the orientation of kaolin powder [65].

Yamuna et a! have reported that phase pure mullite can be prepared from kaolinite by

the incorporation of K2C03 as a mineralizer [66].

1.7 Lithium aluminosilicate (LAS) (P-spodumene)

Kaolin, which is 1:1 aluminosilicate, is used as an effective precursor for

making refractory materials like SiC-mullite, cordierite-mullite and spodumene-mullite

composites [67-70]. Induced mullitization of kaolinite has been widely studied using

fluxes such as oxides, carbonates, oxalates, fluorides, phosphates, tungstates and

sulphates to impart improved physical properties to the sintered material [45]. Lithium

aluminosilicates are used as raw materials in the production of thermal shock resistant

16

whitewares because of their strong fluxing effect [71]. Spodumene finds excellent

application in glass and ceramic industry as a lithia bearing flux and low expansion

filler material [72, 73]. It also makes ceramics harder, smoother and more resistant to

chemical attack and thermal shock [74]. Synthesis of lithium aluminosilicates using

various molar ratios of oxides of Li, Al and Si has been performed by many

investigators with a view to determine their refractoriness and thermal expansion [75].

The Li20-Ah03-Si02 system is known for its low or even negative thermal expansion

coefficients [76-78]. The thermal expansion properties of whiteware bodies of

spodumene-kaolin and petalite-kaolin systems at higher temperatures have been

reported by Fishwick etof [79].

The phase equilibrium studies of LhO-Ah03-Si02 were carried out by many

authors in order to verify the relationships in the ternary system [80-84]. The ternary

compounds p-eucryptite, P-spodumene and solid solutions of p-eucryptite and p­

spodumene can be prepared by reaction of LhC03, Ah03 and Si02 at 1573 K [85]. The

optimization of ceramic parameters as well as structural characterization of bodies of p­

eucryptite, p-spodumene and solid-solution porcelain prepared by conventional ceramic

techniques with different lithia: alumina: silica ratios have been presented by Abdel et

of [86]. Densification studies of mullite in presence of P-spodumene have been

investigated by Low and co-workers [87]. The presence of spodumene provided an

effective sintering flux to enhance the degree of mullitization and hence imparted better

physical and mechanical properties to mullite. The sintering studies of aluminium

titanate ceramics in presence of zero to 30 weight percent spodumene have shown that

spodumene is an excellent additive for liquid phase sintering of aluminium-titanate

ceramics [88]. Processing of dense ceramic structures using alumina and p-spodumene

has also been reported recently by Bayuseno et of [89]. Subramanian and co-workers

have reported the preparation of anorthite, cordierite and P-spodumene- based ceramic

substrates from ion-exchanged zeolites [90].

1.8 Magnesium aluminosilicate (MAS) (cordierite)

Cordierite (Mg2AI4SisOls) is mainly used in glass-ceramic compositions utilized

in multilayer electronic circuit substrates and boards, sound insulating boards, filters for

17

separating solids from fluids, kiln furniture, thermal insulation material applications

requiring controlled porosity, for the removal of nitrogen oxides from the exhaust gas

filters etc [91].

Cordierite powders can be prepared by the solid-state reaction of stoichiometric

amount of oxides of magnesium, aluminium, silicon or by glass recrystallization [92].

Method of preparation of cordierite with stoichiometric composition of clay, talc and

chlorite has been studied by Grosjean [93]. Extensive work has been carried out by

many authors in the area of natural mineral resource as the raw material for the

preparation of cordierite [94-96]. Okazaki et a!. have developed a cordierite body

comprising halloysite and plate shaped talc particles, which showed high crystallinity

and low coefficient of thermal expansion value suitable for the fabrication of catalyst

honeycomb support [97]. Kumar et a!. undoubtedly proved that fly ash could be used

as a substitute for clay in the synthesis of cordierite for refractory applications [98].

Without sintering aids cordierite is difficult to sinter because sintering is accomplished

by the liquid-phase process. Moreover, the optimum sintering temperature range

approaches the melting point of cordierite [99]. Sano has reported that addition of

zircon in the MAS system is an efficient way to widen the range of sintering

temperature [100]. A cordierite composition in which a portion of the SiOz is replaced

by germanium oxide is found to have low CTE, excellent formability and better thermal

shock resistance which make the composition especially suitable for mirror substrates

[101]. The use of cordierite as high power superconducting devices and effect of

surface porosity on dielectric properties have been studied in details [102-104]. Das

Gupta and co-workers extended their studies on improving the mechanical properties of

cordierite ceramics by the addition of ZrOz [105]. An investigation on the sintering

behavior of a mixture of kaolinite and basic magnesium carbonate to form dense

a-cordierite ceramics has been carried out by Sumi eta!. [106].

Bernier et a!. have studied the preparation of cordierite from inorganic and

organic precursors by the sol-gel route and found that this method is able to give

cordierite having 95% theoretical density at a lower temperature [107]. Synthesis of

nanocrystalline cordierite by sol-gel method has been reported by many authors [108­

115]. Oh et a!. have studied the effects of Al / Si ratio, the crystallization behavior and

sintering of cordierite ceramics prepared by sol-gel technique. They came to the

18

conclusion that a decrease in Al / Si ratio will decrease the sintering temperature of

cordierite formation and vice versa [116]. Vue et aL have reported that doping of

magnesium aluminosilicate gel derived glass with B20 3 and P20 S can reduce the

sintering temperature of cordierite to 1173 K [117]. The mechanical property

improvement of cordierite ceramics [118] and dilatometric and dielectric properties of

alkali doped cordierite [119] have also been studied. Synthesis of chemically

homogeneous cordierite powder by spray-drying technique has been studied by Douy

[120]. Grinding effect on the synthesis and sintering of cordierite have been reported

by Awano et aL. According to them, grinding of calcined powder enhanced the

homogeneity of the resulting powder and caused the accumulation of internal energy as

crystal strain and its subsequent relaxation also improve sinterability [121]. Cordierite

and cordierite-mullite composite powders by the combustion route have been described

by Gopichandran et aL [122,123]. Synthesis of amorphous cordierite powder at 773 K,

within a short span of time has also been reported [124].

1.8.1 Use of cordlerite as substrate material for catalytic converters

Tremendous effort to improve the quality of the air we breathe by reducing

automobile emission is being continued in research laboratories and testing centers.

Most outdoors air pollution in urban areas comes from combustion of fossil fuels in

industrial processes, for heating and generation of electricity and by motor vehicles.

The major pollutants from petrol driven vehicles are carbon monoxide (CO),

hydrocarbons (HC) and oxides of nitrogen (NOx). Unbumt HC and CO are present in

the exhaust. High combustion temperature promotes reactions leading to NOx

formation. The harmful effects of these pollutants on human health are many.

Control on automobile emission is a very complex task. But, emissions are not

constant and depend on the driving mode. Acceleration will cause an increase in CO2,

NOx and HC. NOx will be high during cruising and HC will increase during

decelaration. Various factors such as air-fuel mix determine the quantity and

composition of emissions. Mixtures richer than the chemically correct one give rise to

high carbon monoxide, high hydrocarbon and low NOx. Toxic substances in the

19

exhaust gases can be rendered harmless by catalytic treatment. It is proven that

catalytic converter is the only techno economically feasible device to effectively

control the exhaust emission from automobiles. The catalytic converter is a device

installed between the exhaust manifold and muffler in an exhaust system that converts

pollutants to harmless by-products through a catalytic chemical reaction.

Ceramic substrates have been employed as automobile catalyst supports to

facilitate the conversion to less harmful gases of hydrocarbon compounds, carbon

monoxide and oxides of nitrogen. Cordierite ceramic honeycombs are currently

considered as leading candidates for trapping and oxidizing the carbonaceous

particulate emission from automobiles [125]. Many authors have studied the

fabrication of cordierite bodies having low coefficient of thermal expansion (CTE)

value and methods for producing honeycomb structure [126-131]. The catalyst selected

for purifying exhaust gas should have denitrification property [132-135]. Platinum and

palladium as catalysts are found to have a noticeable efficiency in automotive emission

control systems.

1.9 Fiber reinforced Ceramic matrix composites:

Extensive studies have been carried out in the area of ceramic matrix

composites [136-139]. As per the studies conducted so far it is understood that the

matrix chemistry and processing techniques play a critical role in determining the

composite behavior with a given reinforcement. Important attributes for matrix

selection are refractoriness, compatibility with fibers and composite fabricability.

Potential matrices include glass and glass-ceramics, crystalline oxides, carbides,

borides and nitrides. The availability of fibers that can be chosen for reinforcement are

carbon, boron, silicon carbide (nicalon), sapphire etc. Of these, sapphire is the least

studied because of its cost, whereas carbon and nicalon have been widely studied

because of their high modulus, strength and availability [140-143]. James et 01 have

studied the essential requirements for the processing of a ceramic matrix composite.

The fabrication and characterization of short alumina fiber reinforced mullite ceramic

matrix composites by conventional sintering at 1773 K showed that high loading of

20

alumina in the mullite matrix results in a dramatic reduction in the compacted and

sintered density [144].

All the successful techniques for the manufacture of ceramic matrix composites

(CMC) currently require processing at high temperatures. The problems of high

temperature reaction between fibers and matrix must be alleviated by interfacial

barriers but thermal expansion mismatch presents a more severe problem [145]. The

earliest and the most versatile method for controlling the chemistry of the interface

appear to be via the application of coatings to the fiber, prior to fabrication of the

composite. The most commonly used techniques for coating fibers are chemical vapor

deposition (CVD), organometallic precursor deposition and polymer precursor

deposition [146-149].

Tu and co-workers have put forward a new concept which permits the design of

damage tolerant CMCs without the requirement of a matrix-fiber interface [150].

Composite systems are inherently difficult to densify without applied pressure because

of two distinct physical effects viz., reduced sintering stress and densification limits.

Many authors have reported that, a broad and detailed understanding of the mechanical

properties of the composite materials is essential to enable prediction of performance

under service conditions [151-153]. A relation between the thermomechanical

functionality of fibrous ceramic composites and their microstructure has been

established. A great deal of research has been carried out in the area of CMCs with

discontinuous elongated reinforcements [154-157].

The commercial availability of continuous fine-grained polycrystalline SiC

fibers has facilitated the development of high strength and high fracture toughness

ceramic composites [158-160]. Many high temperature structural applications of

ceramic matrix composites require exposure to an oxidizing environment. Mah et al.

have been studied the degradation behavior of nicalon fibers above 1473 K in various

environments (vaccum, air, argon) [161]. The effect of thermochemical treatments on

the strength and microstructure of SiC have also been reported [162,163]. Non-oxide

fiber/non-oxide matrix composites such as SiC/SiC [164] and SiC-whisker/ShN4 [165],

or non-oxide fiber/oxide matrix composites such as SiC-whisker/Alz0 3 [166] have

21

received great attention. These composites show good low temperature strength and

toughness, but their poor oxidation resistance is a major limitation [167,168].

An investigation of the failure mechanisms in a unidirectional SiC fiber/glass

ceramic composite under tensile loading show that failure in tension occurs in different

stages viz, multiple matrix cracking, followed by fiber fracture and pull out [169]. A

detailed study has been undertaken by Prewo in SiC fiber reinforced lithium

aluminosilicate system [170]. Chemical and structural evaluation of SiC whisker

reinforced glass ceramic matrix composites show that nature of the interface has an

important influence on the thermal, environmental and mechanical properties of the

composite [171-173]. Fiber reinforced SiC composites with improved flexural strength

and toughness can be produced by chemical vapor deposition, reaction bonding, and

organosilicon impregnation with subsequent thermal decomposition [174,175]. A

phenomenological model which describes the kinetics of carbon interface formation at

SiC-glass interface show that composite processing influence the carbon layer thickness

and stability [176-179].

1.10 Ceramic - metal composites (cermets)

Cermets are special ceramic materials, being a heterogeneous mixture of

ceramic and metal phases or particles finely intermixed with each other. Generally,

cermets are formed by physically combining separately pre-existing ceramic and metal

materials [180]. There are reports regarding ill situ formation of a metal phase in a solid

mass of ceramic material when contacted with the molten metal at 973-1173 K. These

types of cermets are called 'reaction cermets' which find application in electrical

conductors, refractory articles and abrasives [181-183]. Another important area of

application of cermets is as ceramic cutting tool. Most of the commercially available

ceramic cutting tools are based on aluminium oxide, and this usually contains MgO or

other metal oxide as sintering aid. But, their acceptance is limited due to the low

mechanical and thermal properties compared with carbide tools for conventional

cutting. Alloying additions of metals and carbides can considerably increase their

fracture toughness property [184]. Rudy has introduced aluminium oxide based cermets

from aluminium oxide and group IV refractory transition metal diborides [185].

22

Cennet compositions having high temperature, oxidation and abrasion

resistance, which are particularly designed to use as, seals for thennionic converters

and diodes have been reported by Fletcher et a! [186]. Ceramic sponge with a three­

dimensional cellular structure, obtained by using a high temperature coating technique

consisting essentially of molten-spray deposition by a plasma spray, was disclosed by

Perugini et a! [187]. An alumina metal cennet which contains 0.5 to 1.5 volume %

finely dispersed platinum or chromium by hot pressing technique, which finds use in

high temperature, thennal shock resistant electrical insulators, has been developed by

Morgan et a! [188]. The fabrication and evaluation of SiC composites containing

different volume fractions of TiC have also been reported [189,190]. Cennets procured

from powder mixtures of aluminium and titanium by sintering process has shown that

the interdiffusion between the elements of two materials is a significant factor [191].

The metal-oxidation process (of Lanxide Corporation) for composite preparation, their

key properties and applications have been reported by Urquehart [192]. Ab03/Al

cennet produced by the squeeze cast method shows significantly improved strength and

toughness values compared with monolithic ceramic composite [193]. High

perfonnance composites based on metal, ceramic and intennetallic matrices having

strength, ductility, toughness, fatigue resistance and oxidation resistance have been

reported by Evans [194]. Many authors [195-198] have extensively studied interfacial

properties of CMCs. Krell and Blank have prepared high-carbide Ab03/TiC

composites by liquid phase sintering process, using titanium dihydride as sintering aid

[199,200].

A self-supporting ceramic composite compnsmg a ceramiC matrix

obtained by the oxidation of an aluminium-zinc alloy has been disclosed by Ritland et

al. [201]. Embedding Ti, Nb and Ni particles in a partially stabilized zirconia!

cristobalite matrix has produced cennets with electrical conductivity [202]. According

to Martinelli and Sene these cennets can be used as crucibles in induction furnaces

[203].

1.11 Mullite- Silicon Carbide Composites

Silicon carbide is well known as a substance having numerous polytypes [204].

~-SiC can be prepared at 1523 K from simple mixtures of silicon and carbon or by

23

hydrogen reduction of organosilanes at temperature below 2073 K. In addition to the

reactants, sawdust is sometimes added to increase the porosity of the mix and to

facilitate the venting of the carbon monoxide formed [205]. Prochazka synthesized

submicron ~-SiC powder using direct reaction of high-purity silicon powder with

carbon black at temperatures between 1773 and 1923 K. He found that the material got

densified in presence of boron and carbon additions [206]. Production of ~-SiC by

decomposing gaseous or volatile compounds of silicon and carbon in reducing

atmospheres has also been reported. A great variety of reactants and several methods

of heating, such as d.c arc jet plasma, high frequency plasma, laser and thermal

radiation are possible. However, large-scale commercialization of these processes has

been limited by the very high co~t of production [207].

Carbothermal reactions, which were developed for the benification of oxidic

ores, have been recently extended for benification of silicate minerals like clay. Cutler

et.al separated the alumina from the silica part in clay, the silica being reduced to ~­

SiC. The reactions were catalysed using ferric, cobaltic and nickel salts as catalyst in

small amounts. By carrying out these reactions under nitrogen atmosphere kaolinite

was converted to p-sialon [208-210]. The carbothermal reduction of kaolinite In

nitrogen atmosphere has also been reported by Panda et.al [211,212].

1.12 Scope of present work

Kaolinite, an inexpensive natural raw material, due to its features, viz., fine

particle size, triclinic structure, good processability and high refractoriness can be a

promising candidate for the preparation of ceramic materials for special applications.

The present work explores the possibility of converting the indigenously available

natural raw material kaolin to advanced ceramic materials. The focus of the work is on

the preparation, characterization and applications of ceramic materials viz., spodumene,

cordierite, fiber reinforced ceramic composites, cermets and silicon carbide-mullite

composites by adopting appropriate fabrication methodologies and controlled high

temperature sintering techniques. The experimental details and results obtained are

described in detail in the following chapters.

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