Kutz M. Handbook of materials selection

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3 ADVANCED PROCESSING 1117

3 ADVANCED PROCESSING

When it comes to advanced ceramics, we can assume that there is value added

in the product that justiﬁes advanced processing. If there is no value added, then

we might as well follow the methods developed thousands of years ago for

pottery. It would be foolish to suggest that we are abandoning pottery and

whitewares or that we ever will. To the contrary, whitewares constitute a major

part of the ceramic industry, and there are continuing improvements in their

manufacture, with respect to increasing their environmental friendliness, recy-

cling, performance, and economics.

The need for improving conventional processing has been well served [see,

e.g., Lange (1989), Lewis (2000), Sigmund et al. (2000)]. When it comes to

conventional powder processing, the needs are water-based systems to replace

organic-solvent-based systems, denser suspensions, better powder stabilization

schemes, and more in situ diagnostics. While addressing these issues is critical

to the health of the ceramic industry, the long-term survival and growth of ce-

ramics require some radical new processing schemes. The new technologies are

the basis for the remainder of this chapter. The interested reader will ﬁnd that

conventional ceramics are treated thoroughly in many excellent textbooks [see,

e.g., Reed (1995)] and there is not enough room in this review for a full treat-

ment.

In looking for what is new in ceramic processing, three directions appear.

One can look for new energy sources for processing, such as microwave heating,

self-propagating reactions, mechanochemical processes, or reaction bonding.

One can look for new ways to control particle characteristics, such as size (e.g.,

nanoparticles), shape (e.g., equiaxed particles), metastable phases (e.g., rapid

solidiﬁcation), or templating (e.g., seeding). Alternatively, one can look for new

building blocks, such as polymer precursors or sol–gel processing.

3.1 New Energy Sources

Microwave Processing

What is microwave processing? From the initial attempts in around 1979 to sinter

ferrites with microwaves, it has been know that certain materials show heating

effects through dielectric loss when irradiated with microwaves at a frequency

of 2.45 GHz. Ordinarily, microwaves, which are located on the electromagnetic

spectrum between infrared and radio waves, correspond to energies characteristic

of molecular rotation. This explains the usefulness of microwaves in reheating

food, which contains water molecules. In addition, selected ceramic composi-

tions show rapid heating through the use of microwaves. While materials with

low loss tangents are transparent to microwaves and materials, such as metals,

with high loss tangents reﬂect microwaves, it is the materials with intermediate

loss tangents that can beneﬁt from microwave treatment. In the case of ceramics,

where an ion jump relaxation mechanism gives rise to polarization in an applied

electric ﬁeld, there is the possibility to use microwaves to assist sintering [see,

e.g., Katz (1992), Clark and Sutton (1996)]. The advantages of microwave heat-

ing are experienced through volumetric heating, and rapid heating, resulting in

increased kinetics. The danger of microwave heating in ceramics is nonuniform

susceptibility, leading to localized hotspots, nonuniform grain growth, or ag-

1118 ADVANCED CERAMICS PROCESSING

glomeration. Microwave sintering has been investigated in oxide ceramics pri-

marily, as well as nonoxides. The greatest use of microwaves in ceramics

processing is in drying, but the use of microwaves in sintering is promising in

cases where volumetric heating and small grain size are desirable.

Self-Propagating Synthesis

What is self-propagating high-temperature synthesis, often abbreviated SHS? It

is a method for preparing composites that involves a chemical reaction that is

exothermic. Once the reaction is initiated, the conversion of chemical energy to

thermal energy is used to consolidate a powder compact into a composite. The

product of SHS is rarely homogeneous or free of pores, but the kinetics of the

consolidation is very rapid.

For example, a composite of Al

–ZrO

–Nb can be produced [see, e.g., Holt

and Dunmead (1991), Munir (1988), Yi and Moore (1990)]. Initially, reactant

powders are milled together and pressed into a simple shape. Beginning with a

compact of niobium oxide and an intermetallic such as Al

Zr, it is possible to

heat the compact to around 900

⬚C to ignite the reaction. An ignitable powder in

this case might be a mixture of aluminum, iron oxide, boron, and titanium on

the surface or in a layer in the compact. Reactions in the igniter layer at the

ignition temperature set off a self-propagating wave, which generates an adia-

batic temperature as high as 2300

⬚C. This temperature is sufﬁcient to melt a

eutectic of Al

and ZrO

. The wave propagation velocity depends on the

particle size and the presence or absence of liquid phase. The propagation is

rarely steady state, and while it is hard to control, it is reproducible. The resulting

composite contains Nb metal dispersed in an Al

–ZrO

matrix. Depending on

the size of the starting powders, the ignition can generate stable wave propa-

gation through the compact that inﬂuences the matrix composition. The SHS

process, as practiced in ceramic matrix composites, has been described as burn-

ing a cigar in a bell jar. The approach has been expanded to many technologi-

cally important systems, well beyond the original class of thermite reactions.

Other ceramic composites produced by SHS include TiC–NiAl and TiB

–NiAl,

as well as, compounds such as MgCr

and MgAl

The advantages of SHS are the rapid nature of the process and the reduced

energy consumption. These advantages are balanced by the difﬁculty experi-

enced in controlling the process and the restriction to systems where exothermic

reactions occur. Nevertheless, there are many solid–solid and solid–gas systems

that experience exothermic reactions once they are ignited, and further work is

being pursued.

Mechanochemical Synthesis

What is mechanochemical synthesis? It is a process that uses mechanical acti-

vation to bring about chemical reactions instead of using calcination at moderate

to high temperature to accomplish the same result [see, e.g., McCormick and

Froes (1998), Gilman and Benjamin (1983)]. For example, to prepare a multi-

component oxide such as lead zirconate titanate (PZT), it is possible to mix

powders of PbO, TiO

, and ZrO

, beginning with high-purity commercial pow-

ders, ﬁrst in a ball mill in ethanol for 48 h, followed by drying, and ﬁnally in

3 ADVANCED PROCESSING 1119

Fig. 4 Schematic ﬂowchart for mechanochemical processing of PZT.

a shaker mill at 900 rpm for up to 25 h (Lee et al., 1999). The high-energy

milling leads to mechanical activation. A schematic ﬂowchart of the process is

shown in Fig. 4. At the same time, the surface area is increased, and the so-

called mechanical alloying of the powders yields crystal sizes that give broad-

ened X-ray diffraction peaks. The powders that exist following the milling do

not give crystal patterns for the individual components. Rather, the appearance

of the X-ray diffraction pattern suggests an X-ray amorphous or metastable form

of the PZT phase. When carried out properly, the mechanically activated powder

converts to crystalline PZT upon heating without the usual intermediate phases

of PZ or PT. In addition, the mechanically activated powder sinters more readily

than conventional powders. While the effects of high-energy ball milling are

generally favorable, there are some drawbacks associated with handling ﬁne

particles, particle aggregation, and lead loss.

Originally, mechanochemical synthesis was designed for alloying metals and

intermetallics. Nowadays, many multicomponent ceramics, especially technolog-

ically important ferroelectric compositions, are prepared using mechanochemical

activation. The beneﬁts are seen as (a) a particle size reﬁnement, (b) partial

amorphization, and (c) and creation of distributed nuclei of the desired phase

during the mechanical treatment. The ‘‘chemical’’ part of mechanochemical

processing is said to be the chemical reactions that yield the nuclei of the equi-

librium phase, according to the phase diagram. The solid-state reactions occur

during the mechanical activation, rather than during thermal activation. This

claim is supported by the observation that the nuclei increase in number and

size as the time of the mechanical treatment increases, where thermally activated

growth would require much higher temperatures. Similarly, the exotherm ob-

served for the formation of perovskite in the thermal analysis of a conventional

powder is not observed for the mechanically activated powder because the per-

ovskite phase already exists. Consequently, the desired phase, typically perov-

skite, can be developed without signiﬁcant pyrochlore, where conventional

processing may require high pressure and high temperature to develop perov-

skite.

Many ternary oxide systems have been investigated and the mechanochemical

treatment has been found to be beneﬁcial. Where mechanochemical treatment is

not recommended is systems that are not reactive, those that show agglomeration

rather than size reduction, and those that require some additive to activate the

powder where the additive, in turn, becomes a contaminant. Alternatives to me-

1120 ADVANCED CERAMICS PROCESSING

chanochemical synthesis are, therefore, some of the other methods described,

such as sol–gel processing, hydrothermal synthesis, pulsed laser ablation, or

metal organic decomposition.

Reaction Bonding

What is reaction bonding? To answer this, a good example is an aluminum-

oxide-containing composite, referred to as reaction bonded aluminum oxide

(RBAO). This technique begins with a porous alumina preform that is inﬁltrated

with aluminum metal powders, or a compact of aluminum oxide and aluminum

metal powders [see, e.g., Wu et al. (1993)]. The aluminum metal is oxidized as

the compact is heated up to about 1000

⬚C and the compact densiﬁes on contin-

ued heating to about 1400

⬚C and higher. The process is ‘‘near-net shape’’ in that

the part changes by sometimes as little as 1% in dimension. The mechanism for

the growth of oxide is the diffusion of oxygen through the polycrystalline alu-

mina. The oxidation kinetics can be controlled by additions of other oxides, such

as zirconia.

A variation of reaction bonding is the ‘‘directed metal oxidation,’’ or DIMOX,

process [see, e.g., Nagelberg et al. (1992)]. In this case, there is a growth front

presented by the breakdown of the alumina ﬁlm on fresh aluminum melt. The

oxidation proceeds by diffusion of oxygen through the melt until the metal is

depleted. Similar to RBAO, the kinetics are controlled by alloy additions, in this

case, commonly MgO.

In all cases where reaction bonding is used, including earlier studies on re-

action-bonded silicon nitride (RBSN), there have to be favorable thermodynamic

conditions for the reactions to occur. The reactions bear some similarity to SHS.

The advantage of the process is the near-net shape capability for composites that

are difﬁcult to densify by conventional means. The difﬁculty, however, is that

the approach is limited to a few reactive systems. Nevertheless, there are critical

applications for these materials, such as lightweight armor, that cannot be sat-

isﬁed in any other way.

3.2 New Shapes

Nanotechnology

What aspects of nanotechnology are relevant to advanced ceramic processing?

For a given application, there are now several suitable methods for preparation

of nanostructured materials with controlled chemical and physical characteris-

tics. One should ask, however, what is the critical nanograin size that enhances

a material’s properties for a target application? These preliminary questions need

to be asked before entering the expanding area of nanopowders and nanomater-

ials.

The motivation for synthesizing nanostructured materials is the discovery of

physical and chemical behavior exhibited at this size dimension that is neither

described by single molecules nor by conventional materials [see, e.g., Siegel

(1991), Gleiter (2000)]. Nanopowders have been prepared using vapor- and so-

lution-based methods. The reason is that these methods can be adjusted to avoid

hard aggregation and morphology control problems.

3 ADVANCED PROCESSING 1121

One vapor phase technology is a combustion ﬂame system, based on a ﬂat-

ﬂame burner, which is capable of synthesizing nonagglomerated nanopowders.

The temperature distribution in the ﬂame precisely deﬁnes the zones of nucle-

ation, growth, and aging to enable the formation of monodisperse nanoparticles.

Equiaxed ceramic nanopowders with a narrow particle size distribution as small

as 10 nm have been prepared (Hahn, 1993, 1997).

One solution-phase technology is the hydrothermal method, where solutions

or suspensions are heated under pressure to precisely control the nucleation,

growth, and aging of the crystallites. In comparison to other solution-based tech-

nologies, hydrothermal methods offer the ability to prepare anhydrous crystalline

multicomponent oxides with controlled particle size. Similarly, hydrothermal

synthesis leads to control of nanopowder morphology.

Molecular-based technologies focus on the preparation of the nanostructured

mesoscale powders directly from the gas or solution phase. One molecular-based

vapor-phase technology is pulsed laser deposition in which ceramic material is

vaporized by a laser and vapor species are deposited on a substrate. This tech-

nology has been used primarily for thin-ﬁlm preparation. Another uses combus-

tion ﬂames or chemical vapors to deposit molecular species directly onto a

rotating or stationary substrate (Glumac et al., 1999). Sol–gel processing, an-

other molecular-based synthesis route, is treated separately.

It is also possible to hybridize various methods. For instance, a surface pre-

treated via pulsed laser deposition can be used to seed a nanostructure for a sol–

gel powder. Combinations of hydrothermal synthesis and microwave heating

have been tried with promising results.

Nanopowders derived from vapor- and solution-phase sources can be proces-

sed in both dry and wet environments. Dry nanopowder assembly processing

encompasses spray deposition, ﬂuidized bed, and fused deposition of ceramics.

Spray deposition methods include thermal spraying and cold gas dynamic spray-

ing. Thermal spray methods are suitable for powders that can endure the high

temperatures in the ﬂame or plasma. Cold gas dynamic spraying projects par-

ticles at supersonic velocities and at low temperatures, so that particles form

bonds with a surface upon impact. This is effective for nanopowder agglomerates

with ductility. Another low-temperature dry nanopowder assembly process is

magnetically assisted ﬂuidized-bed deposition. In this process, magnetic forces

ﬂuidize magnetic particles, which in turn ﬂuidize the desired nanopowders. This

process is very effective in coating large numbers of objects in a very uniform

fashion.

Wet nanopowder assembly processing affords the capability to disperse nan-

oparticles as singlets, something that cannot be achieved with dry nanopowder

assembly processing. There are three requirements for effective nanopowder as-

sembly processing: deagglomeration, dispersion, and particle assembly forma-

tion. For deagglomeration, processes such as eccentric rotating cylinder shearing,

impinging jets, and ultrasonic mixing are important technologies (Riman, et al.,

1998; Gelabert, et al., 2000). Dispersions based on a variety of polymer chem-

istries provide stability for nanopowder suspensions. To control the positions of

nanopowders in suspension, another approach is polymer self-assembly [see,

e.g., Brinker et al. (1999), Agarwal et al. (1997)].

1122 ADVANCED CERAMICS PROCESSING

Densiﬁcation efforts focus on developing methods for controlling crack for-

mation and porosity while maintaining a nanoscale grain size. Conventional

solid-state and liquid-phase sintering lead to densiﬁcation of nanostructured un-

ﬁred mesoscale coatings at low ﬁring temperatures. In one case, a metastable

ceramic nanopowder has been densiﬁed, by exploiting a pressure-induced phase

transformation, to produce a fully dense nanocrystalline material, where the ﬁnal

grain size of the stable phase is comparable to or less than the particle size of

the starting powder. This was demonstrated for alumina and titania using low

temperatures (

⬃0.4 T

) and high consolidation pressures (up to 8 GPa).

One ﬁnal method, relevant also to layered manufacturing, applies advanced

laser rastering methods to densify nanopowders in two dimensions. Unlike pre-

vious work with laser sintering, the laser beam is designed to incorporate a

temperature distribution that induces, according to the patterned scanning of the

laser, densiﬁcation normal to the surface in a continuous, defect-free fashion.

This leads to rapid densiﬁcation times, localized heating, and the capability of

changing temperature quickly allows densiﬁcation of complex, ﬁne-scale pat-

terned structures. Thus, assemblies of nanopowders can be densiﬁed and the

remaining unﬁred materials can be removed by various means such as ultrasonic

cleaning.

Layered Manufacturing

What is layered manufacturing or solid freeform fabrication (SFF)? In recent

years, the cost of making machine tools or dies to make a prototype has become

exorbitant, and an alternative has been found that allows the preparation of the

part without dies, thus the name ‘‘freeform.’’ Using a computer-aided design

program, a solid object is cut into layers and a computer ﬁle is created that can

direct the building-up of the object layer by layer [see, e.g., Bandyopadhyay et

al. (1997), Song et al. (1999)]. One approach to solid freeform fabrication is

ink-jet printing. Borrowing the concept that is popular for creating written text,

an ink-jet printer can print the object layer by layer, to create three-dimensional

solids. In the form that this printing technology relates to ceramics, the required

steps are (a) the ink preparation, (b) the ink delivery through a nozzle, (c) the

break up of the ink stream into droplets, (d) the rastering of the nozzle over the

substrate, (e) the registry of successive layers, and (f) the consolidation of the

layers and sintering of the object to a fully dense ceramic.

Typical dimensions of each layer are

⬃0.5

␮

m. The ﬁnest nozzle is about 75

␮

m, although micropen nozzles are being developed. A typical ink formulation

contains dispersant, binder, and plasticizer with 30 vol% ceramic powder. The

ink is forced through the nozzle and broken into droplets usually with electrodes.

The proper placement of the drops is critical to producing the desired dimensions

and aspect ratio in the object. Typically, the objects call for hundreds of layers.

Following the buildup of the object, the problems with drying and ﬁring are no

different from those using more conventional processing and binder burnout.

Fused deposition of ceramics (FDC) is another useful technology for making

nanocomposites. This process uses a ﬁlament of inorganic particles dispersed in

a polymer as a means to deliver a molten stream of the composite onto a sub-

strate. The droplets can be as small as 125

␮

m and can be deposited as multi-

layers for thicker coatings with a spatial positioning precision of

Ⳳ2.5

␮

3 ADVANCED PROCESSING 1123

Current instrumentation can process as many as four different ﬁlaments to allow

for the deposition of several materials on the same substrate.

To make nanostructured mesoscale coating depositions, we need to expand

the applications of ink-jet printing and FDC. These technologies should be useful

not only for deposition on a ﬂat surface but also for administering multiple

components. This enables the assembly of neighboring components as well as

layered components of different nanostructures. Like fused deposition process-

ing, computers equipped with fast processors can be used to assemble complex

patterned structures for multifunctional systems.

Thus, one can envision multifunctional systems consisting of nanostructured

capacitors, resistors, conductors, and even patterned power sources. Because the

structures are computer generated, a large number of unique structures are pos-

sible.

Overall, the beneﬁts of layered manufacturing include the ability to fabricate

complex shapes, fabrication without molds, automated fabrication, and near-net

shape processing. The potential drawbacks are the cost of the binder needed in

the initial layering, the dimensional accuracy, the binder burnout, and the trans-

ferability of a binder delivery system to a variety of ceramic materials. When

performance is more important than cost, then layered manufacturing used for

a low-volume product or small number of units may be competitive with con-

ventional processes such as forging, investment casting, or machining.

3.3 New Precursors

Polymer Precursors

What are the advantages of using preceramic polymers or precursors that py-

rolyze to ceramic? The advantage is the ability to use polymer forming methods

before carrying out the pyrolysis. The polymers are available as liquids, resins,

and thermosets. A typical preceramic polymer would be polycarbosilane, which

decomposes to SiC plus excess carbon and gaseous by-products such as hydro-

gen and hydrocarbons [see, e.g., Wynne and Rice (1984), Blum et al. (1989),

Wu and Interrante (1992), Greil (1995)]. Other preceramic polymers contain Si–

Si (silane), Si–N (silazane), or Si–O (siloxane) bonds. There are suitable pre-

ceramic polymers for aluminum, zirconium, and titanium carbides, nitrides,

borides, and phosphides as well. The pyrolysis atmosphere may be nitrogen,

ammonia, argon, or air. Typically, the ceramic product is porous. Nevertheless,

suitable precursor chemistry and heat treatment can lead to dense materials.

Another beneﬁt of the chemical approach is the high purity achievable and the

control over stoichiometry. In some cases, there are fewer steps involved with a

preceramic polymer than with powder processing. For example, the pyrolysis

can lead to a porous perform that requires inﬁltration by reaction bonding (as

in RBSN, reaction bonded silicon nitride), chemical vapor deposition (CVD), or

chemical vapor inﬁltration (CVI).

Sol–Gel Processing

Of all of the advanced ceramic processing methods, why has sol–gel processing

become so prevalent? When sol–gel processing emerged about 20 years ago,

there were many that thought it was a laboratory curiosity without much future.

1124 ADVANCED CERAMICS PROCESSING

The cost of raw materials was frequently raised as being prohibitive. Some 20

years later, the interest in sol–gel processing has not diminished. Perhaps, the

reason is that there are many applications for sol–gel processing, where con-

ventional processing does not work as well, if at all. Certainly, in optical ma-

terials, ceramic membranes, and coatings, there are many outstanding examples

[see, e.g., Klein (1988, 1994)].

The sol–gel process refers broadly to room temperature solution routes for

preparing oxide materials [see, e.g., Brinker and Scherer (1990), Pierre (1998)].

In most cases, the process involves the hydrolysis and polymerization of metal

alkoxide precursors of silica, alumina, titania, zirconia, as well as other oxides.

The solutions of precursors are reacted to form irreversible gels that dry and

shrink to rigid oxide forms.

Twenty years ago, when the sol–gel process enjoyed resurgence in interest,

the emphasis was on the duplication of conventionally prepared ceramics and

glasses. Trying the sol–gel process was motivated by claims of the purity of the

starting materials and the generally lower temperatures for processing. Clearly,

the advantages of sol–gel processing outweigh the additional cost in enough

cases that the skeptics are satisﬁed for the moment.

In today’s terminology, sol–gel processing is a form of nanostructure proc-

essing. Not only does the sol–gel process begin with a nanosized unit, a mole-

cule, it undergoes reactions on the nanometer scale resulting in a material with

nanometer features. The concept behind the sol–gel process is that a combination

of chemical reactions turns a homogeneous solution of reactants into an inﬁnite

molecular weight oxide polymer. This polymer is a three-dimensional skeleton

surrounding interconnected pores. Ideally, the polymer is isotropic, homogene-

ous, uniform in nanostructure, and it replicates its mold exactly and miniaturizes

all features without distortion. The nanostructure and the nanophase porosity in

gels are, in fact, the features of interest, both scientiﬁcally and technologically.

The sol–gel process involves initially a homogeneous solution of one or more

selected alkoxides. Alkoxides are the organometallic precursors for silica, alu-

mina, titania, zirconia, among others. By far the most common system is one

composed of tetraethyl orthosilicate [TEOS–Si(OC

)

]–water–alcohol. Even-

tually, the solution reacts to a point where the molecular structure is no longer

reversible. This point is known as the sol–gel transition. The gel is an elastic

solid ﬁlling the same volume as the solution. Alkoxides react at different rates

according to the electronegativity of the cation.

Most studies of the sol–gel process deal with a single alkoxide. Each pre-

cursor has its own reaction rate and complicated interdependences of pH, con-

centration, and solvent. Even in the relatively straightforward case of alumina,

using aluminum-sec-butoxide (ASB), the expected reactions (1–4) are:

Al(OC H )

⫹ HO⫽ Al(OC H ) (OH) ⫹ CHOH (1)

493 2 492 49

2(Al(OC H ) (OH) ⫽ 2(AlO(OH)) ⫹ yCHOH (2)

492 49

2(Al(OC H ) (OH)) ⫹ 2H O ⫽ 2 Al(OH) ⫹ 2C H OH (3)

492 2 3 49

AlOOH or Al(OH) ⫽ Al O ⫹ zH O (4)

323 2

A catalyst is used to start reactions and control pH. The reactions are ﬁrst hy-

3 ADVANCED PROCESSING 1125

Table 2 Shapes Achieved by Sol–Gel Process

Shape Composition Typical Application

Thin ﬁlm Titania –silica Interference Filter

Membrane Alumina Ultraﬁlter

Fiber Alumina–zirconia–silica Reinforcement

Bulk

Xerogel Silica Lens

Aerogel Silica Thermal insulation

drolysis to make the solution active (reaction 1), followed by condensation

polymerization (reaction 2) along with further hydrolysis. These reactions in-

crease the molecular weight of the oxide polymer (reactions 3 and 4) resulting

in either the monohydroxide AlOOH (boehmite) or the trihydroxide Al(OH)

(bayerite). Nonaqueous sol–gel processes have been used to prepare most tran-

sition-metal oxides. In systems with multiple valence, the intermediate species

are oligomers that exist on the nanometer scale.

Aqueous colloidal sols also are used for sol–gel processing, recognizing that

the mechanism for accomplishing the sol–gel transition is quite different. In sols

such as Ludox, changing the pH or changing the concentration causes the ag-

gregation of sol particles. The sols can be gelled in such a way that the oxide

skeleton is a continuous linking of sol particles. These are discrete features that

make up the skeleton corresponding to the dimensions of the sol. The other

features are pores within secondary particles and pores between secondary par-

ticles. The chemical and structural differences between nonaqueous alkoxide

precursors and aqueous sol precursors become blurred at later stages of the sol–

gel process.

Mixing as a ﬁrst step applies to the single alkoxide, multiple alkoxide, and

colloidal sol processes. Absence of light scattering is a good indication of uni-

form mixing. Since the building blocks are nanometer in size, and smaller than

the wavelength of visible light, it is easy to follow, in a qualitative way, the

progress of the linking of building blocks. Gelling is often determined empiri-

cally as the time when the solution shows no ﬂow. This is referred to as the

time to gel. For this step, viscosity is a good diagnostic for the transition from

a viscous liquid to a rigid structure.

Table 2 summarizes the shapes available by the sol–gel process. All of the

options are more or less porous materials. In all cases, it is important that the

porosity remain interconnected, whether the form of the material is essentially

one-dimensional such as a ﬁber, two-dimensional such as a ﬁlm, or three-

dimensional such as bulk monoliths. These shapes can be divided into those that

show isotropic shrinkage from the preform to the ﬁnal form and those that show

anisotropic shrinkage. Bulk materials fall into the isotropic category. Thin ﬁlms

on substrates fall into the anisotropic category. High aspect ratio ﬁbers also show

anisotropic shrinkage. Sol–gel thin ﬁlms are exceedingly simple to prepare. A

solution containing the desired oxide precursors is applied to a substrate by

spinning, dipping, or draining. The process is able to apply a coating to inside

and outside of complex shapes simultaneously. The time to gel is short, indi-

cating ﬁlm formation, drying and creation of pores must be rapid. When it comes

to dip coatings, 50 to 500 nm coatings are easy to make but thicker coatings

1126 ADVANCED CERAMICS PROCESSING

are more difﬁcult. Similarly, ﬁbers can be drawn out of low-water-content so-

lutions. The sol–gel process allows one to bait and draw a string of gel about

the same diameter as the desired ﬁber directly from the solution.

The sol–gel process based on casting and molding can make bulk objects.

The process can be used to make a microporous preform that is near-net shape.

This preform is called a monolith to refer to its continuity. Monolithic gels can

be formed from a colloidal sol or from an alkoxide solution. The main difference

between colloidal gels and alkoxide gels is their pore structures. Alkoxide gels

have small pores (

⬍10 nm), while colloidal gels have bigger pores or voids

between particles. Monolith fabrication is arguably the most challenging dem-

onstration of the sol–gel process.

Having selected geometry and designed the formulations accordingly, there

are several further steps common to monoliths, ﬁlms, and ﬁbers. First of all, the

gels must be dried. For monoliths, drying is more difﬁcult because of the thicker

dimensions. Keeping in mind the nanostructured character of the material, sev-

eral drying treatments have been developed. One possibility is aerogels that are

dried in an autoclave by hypercritical techniques. That is, the solvent is removed

above its critical point. The resulting gel is about 10% dense and shows no

shrinkage. The more common case is xerogels that are dried by natural evapo-

ration. Xerogels are 60% dense and have reductions 40–70% in volume.

Following reacting, gelling, and drying, gel materials have many of the char-

acteristics of the corresponding ceramic oxide, but they are more or less porous.

Interconnected pores, which remain open at the surface until the gels are ﬁred

to temperatures well above 600

⬚C, allow the water and solvent to escape.

When the goal of the sol–gel process is a pore-free dense oxide, the ﬁnal

stage of processing is sintering. The high surface area of gels is looked at as a

high driving force for sintering, so sintering is likely to occur at lower temper-

atures than in conventional powder compacts. This drive to remove porosity in

the end produces a material similar to conventionally processed materials.

Remember that high purity and uniform nanostructure are trademarks of the

sol–gel process. The challenge of the sol–gel process is to exploit the nano-

structure aspects of the process to derive real beneﬁts. In single-component,

single-phase materials, by far the most important nanophase property is the po-

rosity. Porosity also means that there is surface area. Sol–gel processing has a

special contribution to multiphase material design and fabrication, in that the

porosity provides access to the nanoscale for processes such as liquid or vapor

inﬁltrations, physical or chemical depositions, and chemical reactions such as

pyrolysis or oxidation/reduction.

Because the sol–gel process is a low-temperature process, it is possible to

incorporate organic material. This is accomplished by inﬁltrating a previously

formed oxide gel with monomer, creating an organic–inorganic copolymer with

a metal alkoxide, or simultaneously polymerizing monomer and metal alkoxide.

Because of the scale of mixing of the organic and inorganic phases, these ma-

terials are nanocomposites. The products of some of these syntheses can be

classiﬁed as sequential interpenetrating networks or simultaneous interpenetrat-

ing networks.

Among the features of sol–gel processing, the one that should stand out at

this point is its nanometer scale. In the long run, the advantages for the sol–gel