Moulson A.J., Herbert J.M. Electroceramics: Materials, Properties, Applications

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body. It can therefore be considered to be part of the mixing process. The

calcination conditions are important factors controlling shrinkage during

sintering. The required ﬁnal phases may not be completely formed but the

remaining chemical gradients may assist sintering. The main requirement is that

calcination should yield a very consistent product.

Calcination can be carried out by placing the mixed powders in shallow

saggers in a batch or continuous kiln. The saggers may need to be closed if any of

the constituents are volatile, as is the case with lead oxide. The container surfaces

in immediate contact with the powder must not react with it both to avoid

contamination and to permit reuse of the sagger. The thermal conductivity of

powdered materials is always low, so that a suﬃciently uniform temperature can

only be obtained through a depth of a few centimetres when the period at

maximum temperature is, as is usual, only 1 or 2 h.

Calcination can also be carried out continuously in a rotary kiln of the type

shown in Fig. 3.1. This consists of a slowly rotating tube mounted with its axis at

an angle to the horizontal such that powder fed in at the top emerges at the

bottom in the desired state. Heat is usually supplied by the combustion of oil or

gas inside the barrel.

The calcined material has usually undergone a limited amount of sintering and

must be milled to give a powder or slip suitable for the shaping stage. The

machinery and problems are essentially the same as those discussed above in

relation to mixing. The calcine is usually coarser and more abrasive than the raw

materials so that precautions against contamination are of more importance. The

102 PROCESSING OF CERAMICS

Fig. 3.1 Industrial-scale rotary kiln. (Courtesy of Philips Technical Review.)

TEAMFLY

Team-Fly

initial size reduction to produce suitable particles for the ﬁner grinding processes

can be carried out using any one of the wide range of pieces of equipment, jaw-

crushers, roller-mills, hammer-mills, developed for ‘ore-dressing’.

3.6 Shaping

The treatment of the milled powders depends on the method of fabricating

shapes from it. The forms in which it is required for various shaping processes

are given in Table 3.1.

Unless the material concerned contains a substantial quantity of clay (usually

10% or more) it is necessary to incorporate an organic binder. The primary

function of the binder is to give the dry shape suﬃcient strength to survive the

handling necessary between shaping and sintering, but it may also be essential to

the method of shaping as, for example, in items (iii)–(ix) of Table 3.1.

One of the most important requirements for a binder is that it should be

possible to eliminate it from the compact without any disruptive eﬀect. When

particles are in high concentrations in a ﬂuid they tend to form a continuous

network with points of direct, or almost direct, contact between them. These

points of contact remain when the binder is volatilized or burned out and

provide, through van der Waals forces, suﬃcient strength to resist the

disintegrating eﬀect of small stresses. It can be seen that in the initial stage the

binder at the surface of a shape is removed and the fragile porous outer layer so

formed is held together by the bulk of the body which still contains binder. The

binder-free layer grows inwards allowing gases from the interior to escape

through its pores and, as its bulk increases, contributes to the restraining forces

holding the interior particles in place. The same tendency to aggregation that

SHAPING 103

Table 3.1 Feed materials for various shaping methods and the type of product

Shaping method Type of feed material Type of shape

(i) Dry-pressing Free-ﬂowing granules Small simple shapes

(ii) Isostatic pressing Fragile granules Larger more intricate shapes

(iii) Calendering; viscous

plastic processing

Plastic mass based on an

elastic polymer

Thin plates

Simple shapes

(iv) Extrusion Plastic mass using a viscous

polymer solution

Elongated shapes of constant

cross-section

(v) Jiggering Stiﬀ mud containing clay Large simple shapes

(vi) Injection moulding Organic binder giving ﬂuidity

when hot

Complex shapes

(vii) Slip-casting Free-ﬂowing cream Mainly hollow shapes

(viii) Band-casting Free-ﬂowing cream Thin plates and sheets

(ix) Screen-printing Printing ink consistency Thin layers on substrates

may make the dispersion of powders diﬃcult is essential to the mechanical

stability of a body from which the binder has been removed.

To achieve adequate binder ‘burn-out’ is a matter of experience, and size of

component, ‘green’ particle packing, manner of oven loading, temperature/time

schedule and atmosphere are all determining parameters. Removal of the ﬁnal

traces of binder residues may be essential especially in the case of some

electroceramics, for example superconductors (see Section 4.7.4) where residual

carbon could be detrimental to properties.

3.6.1 Dry-pressing

Dry-pressing is carried out in a die with movable top and bottom punches

(Fig. 3.2). A cavity is formed with the bottom punch in a low position and this is

ﬁlled with free-ﬂowing granulated powder (Fig. 3.2(a)) which is then struck oﬀ

level with the top of the die. The top punch then descends and compresses the

powder to a predetermined volume (Fig. 3.2(b)) or, in more elaborate presses, to

a set pressure (75–300 MPa or 750–300 atm). Both punches then move upwards

until the bottom punch is level with the top of the die and the top punch is clear

of the powder-feeding mechanism (Fig. 3.2(c)). The compact is then removed, the

bottom punch is lowered and the cycle is repeated (Fig. 3.2(d)).

The feedstock is usually contained in a hopper attached to the press, and if the

granules have a range of sizes they may segregate as the die ﬁlls and cause density

variations. Also, during pressing the granules must ﬂow between the closing

punches so that, ﬁnally, the space between them is uniformly ﬁlled.

Spherical granules of uniform size can be produced by spray-drying. A spray-

drier (Fig. 3.3) consists of a large tank with a cylindrical upper part and a conical

lower section. The slip is sprayed in at the top and meets hot air injected into the

centre of the tank. The droplets are dried and fall into the conical section from

which they can be removed. Any very ﬁne particles are carried out by the hot air

and recovered. In order to obtain granules of a suitable size, usually 0.1–1 mm,

tanks with diameters of 2–4 m are necessary. Smaller tanks do not allow

suﬃcient residence time for the removal of moisture from the larger descending

droplets. The method is therefore best suited to large-scale operations involving

100–1000 kg batches.

Alternatively, a stiﬀ paste of ceramic and binder solution can be forced

through a wire mesh and granules of the required size extracted from the dried

product by sieving. The ﬂow of the granules can be enhanced and die-wall

friction reduced by ‘tumbling’ with a small quantity of a powdered lubricant such

as calcium stearate.

Once the granules are properly distributed in the die they must crush readily

into small fragments so that their structure is not apparent in the ﬁnal pressing,

since this would result in the formation of large pores. Compaction must not be

104 PROCESSING OF CERAMICS

so rapid that the air entrapped within the powder fails to escape; if this should

happen there is likely to be a crack across the compact perpendicular to the

pressing direction. Friction between the compact and the die walls must be

minimal, since it results in a reduction in the pressure, and therefore in the

density, at points remote from the moving punch (or punches if both move

within the die). The green density is not usually greatly increased by applying

SHAPING 105

(b)

Fig. 3.2 The stages in dry pressing.

Fig. 3.3 Schematic diagram of a spray-drier.

pressures exceeding 74–150 MPa but in practice pressures as high as 300 MPa

may be used to minimize density gradients due to die-wall friction and so

minimize distortion during sintering. High pressures also ensure the destruction

of granule structures. Highly polished die and punch surfaces help to reduce wall

friction, and the tools are made of hardened steels to minimize wear and

maintain surface ﬁnish.

Shapes with a uniform section in the pressing direction are the easiest to

produce by dry pressing. Pieces that vary in section require very careful powder

preparation and may need special press facilities such as ﬂoating dies, i.e. dies

free to move relative to the punches, or dies that split open to allow easy

extraction of the compact. The time taken by a pressing on an automatic

machine varies from 0.2 s for pieces of diameter around 1 mm to 5 s for large

complex shapes.

3.6.2 Isostatic-pressing

Many of the diﬃculties encountered in dry-pressing can be avoided by some

form of isostatic-pressing. Ideally, this simply involves the application of

hydrostatic pressure to powder in a ﬂexible container. Powder movement is

minimal and side-walls are absent. In practice shapes are often produced by the

use of rigid mandrels as illustrated in Fig. 3.4. Powder is weighed into a rubber

bag with a rigid mouth and a mandrel is then inserted and makes a seal with the

mouth. Pressures of 20–280 MPa (200–2800 atm) can be applied through either

liquid or gas media. The pressure must be released slowly since the air originally

106 PROCESSING OF CERAMICS

Fig. 3.4 Dry-bag isostatic pressing.

within the powder is compressed within it. Pressure can usefully be applied a

second time when there is far less air within the compact.

This method gives highly uniform densities and freedom from laminations.

The powder does not have to be in the form of the free-ﬂowing granules required

for automated punch and die pressing so that the risks of pores associated with

granules are avoided. This method is used in the mass production of spark-plug

insulators and ceramics for high-voltage devices. The green compacts can be

machined when more complex shapes are required.

3.6.3 ‘Jolleying’

‘Jolleying’ is used for the manufacture of domestic crockery, typically cups and

other symmetrical hollow shapes. It is also exploited in the manufacture of clay-

based porcelain insulators. The plastic body is introduced into a rotating, porous

mould, commonly plaster. The mould is shaped to form one surface of the

article, for example the outer surface of a cup or insulator. A stationary proﬁling

steel tool is brought close to the inner surface of the rotating mould, spreading

the body to the required thickness. The surface in contact with the mould loses

some moisture to the plaster and is readily detached. The process is well adapted

to producing shapes having simple symmetry but is little used for shaping

electroceramics since a high clay content seems essential for the necessary

rheological properties.

3.6.4 Extrusion

Components with a uniform section and large length-to-diameter ratios can be

produced by extrusion (Fig. 3.5). In the case of bodies containing 10% or more

clay, a suitable paste for extrusion can generally be obtained by mixing with

water and passing through a de-airing pug mill, which is a screw-fed extruder

with a means for extracting air. Clay-free starting materials need the addition of

a viscous liquid such as water containing a few per cent of starch, polyvinyl

alcohol, methylcellulose etc. An entirely organic soap is also added as a lubricant

and wetting agent. Very thorough mixing is essential. The paste is ﬁrst forced

through a die with a large number of small oriﬁces and the resulting ‘spaghetti’ is

loosely packed into the barrel of an extrusion press which is evacuated before

pressure is applied from the ram. The binder content has to be adjusted so that

the extruded body is suﬃciently strong to be broken oﬀ and moved to a drying

rack without signiﬁcant distortion. It requires experience to achieve this but,

once the correct conditions are established, large quantities of material can be

extruded very rapidly (about 10 cm s

1

). Rods and tubes are readily produced.

Dielectric sheets down to a fraction of a millimetre thick and up to a metre wide

SHAPING 107

have been produced by extrusion. There is an appreciable shrinkage on drying

and generally a greater shrinkage on sintering than for dry or isostatically

pressed pieces, so that very precise dimensions are diﬃcult to maintain without

machining after ﬁring.

3.6.5 Colloidal processing: slip-casting

Slip-casting has been a major part of traditional ceramics’ processing technology

for at least 250 years. It was introduced for the casting of alumina ware in the

early part of the last century and since then has been extensively exploited for

forming advanced ceramics.

In relatively recent years the term ‘colloidal processing’ has been coined to

describe processing technologies in which the colloidal properties of ceramic

powders are exploited. J.A. Lewis [6] comprehensively reviews colloidal

processing and its potential.

In the slip-casting process the ceramic powder is suspended in a ﬂuid vehicle,

usually water. The suspension, or ‘slip’, has a high solids content, typically

50 vol.%, and the individual particles are ﬁne, usually less than 10 mm.

Deﬂocculants, which modify the electrical environment of each particle so that

they repel one another, are added to the slip. The ﬁneness of the powder and

consequent high surface area ensure that electrostatic forces dominate gravity

forces so that settling does not occur. When exceptionally heavy powder particles

108 PROCESSING OF CERAMICS

Fig. 3.5 Extrusion of (a) a rod and (b) a tube.

are involved the viscosity of the suspending medium can be increased to hinder

settling. A plaster of Paris mould is made by casting round a model of the

required shape, suitably enlarged to allow for the shrinkage on drying and ﬁring.

The inner surface of the plaster mould must have a very smooth ﬁnish free from

holes originating from air bubbles in the plaster so that the cast article can be

removed without damage. The mould is dried and the slip is poured into it.

Water passes into the porous plaster leaving a layer of the solid on the wall of the

mould. When a suﬃcient thickness is cast, the surplus slip is poured out and the

mould and cast are allowed to dry. Slips containing a high percentage of clay give

casts that shrink away from the mould and are easily extracted from it. Most

other materials give only a small shrinkage and therefore greater care is needed in

mould design and preparation.

The casts are usually suﬃciently dense to yield low-porosity (5% or less)

bodies on sintering. The relatively slow dewatering process evidently results in

close-packed particles and, compared to dry powder compaction routes, with less

risk of introducing defects.

This process oﬀers a route for the manufacture of complex shapes and, in the

traditional pottery industry, is the accepted method for the production of

teapots, milk jugs, ﬁgurines and large articles such as wash-hand basins. It may

be necessary for the mould to be made up of a number of pieces so that the cast

article can be removed.

Slip-casting of technical ceramics has been steadily introduced over the past 60

years or so, and now it is standard practice to cast alumina crucibles and large

tubes. The process has been successfully extended to include silica, beryllia,

magnesia, zirconia, silicon (to make the preforms for reaction-bonded silicon

nitride articles) and mixtures of silicon carbide and carbon (to make the preforms

for a variety of self-bonded silicon carbide articles). Many metallics and

intermetallics, including tungsten, molybdenum, chromium, WC, ZrC and

MoSi

, have also been successfully slip-cast.

3.6.6 Tape-casting

Tape-casting is also called doctor-blading or band-casting. A slip is spread on a

moving band (Fig. 3.6), dried, peeled from the band and reeled up prior to

further processing. The slip diﬀers from that used for slip-casting because it has

to act as a far stronger binder for the ceramic particles when the liquid phase is

removed. A water-based slip may contain polyvinyl alcohol as a binder, glycerine

as a plasticizer and ammonium polyacrylate as a deﬂocculant. It is more usually

based on a mixture of organic solvents containing, for instance, polyvinyl

alcohol, dibutyl phthalate and fatty acid deﬂocculants. Air bubbles must be

removed from the slip by the application of a vacuum and steps taken to exclude

foreign particles (e.g. from clothes or skin) that would leave pores on sintering.

SHAPING 109

The band may consist of highly polished stainless steel or may bear a carrier

layer of a suitable polymer. It moves under a hopper containing the slip with one

edge raised suﬃciently for the slip to be carried out. The evaporation of the

solvent may be assisted by heating and by a draught of ﬁltered air. It is diﬃcult

to dry the thicker (40.5 mm) ﬁlms successfully because the surface ﬁlm that

forms ﬁrst shrinks laterally and cracks as it is restrained from motion at its edges,

which dry more rapidly than the rest. With thinner layers and suitable solvent

compositions the ﬁlm dries out uniformly. Adhesion to the band prevents

sideways contraction so that, at this stage, shrinkage occurs entirely in thickness.

Hence the lateral shrinkage during sintering is about four times greater than the

shrinkage in thickness (e.g. 20 and 5%).

There are variations on the above approach. For instance, the hopper may

move over a stationary carrier ﬁlm. It is also possible to draw a polymer ribbon

through a slip so as to pick up thin layers on both surfaces.

The cast ﬁlm is porous because of the evaporation of solvent and contains up

to 30 vol. % of organic solids as binder. It is suﬃciently strong for handling and

the screen-printing of electrode patterns (see Section 5.4.3). Despite a relatively

high porosity after burning out the organic material the cast ﬁlm can be sintered

to a high density. The review by H. Hellebrand [7] is comprehensive.

3.6.7 Calendering and vi scous polyme r processing

Thin tape can also be prepared by calendering (see Fig. 9.45). In this process a

mixture of powder and an elastomeric polymer (e.g. rubber) is fed to a pair of

rollers which are rotating at diﬀerent speeds. As a consequence of this speed

diﬀerential the mixture is subjected to a strong shearing action as it passes

through the gap (pinch) between the rollers. The shearing action disintegrates the

agglomerates and forces the elastomer into close contact with the surfaces of the

grains. Dispersion is enhanced by reducing the pinch. Considerable energy is

dissipated in the process and the rollers are water cooled. The mixture is

110 PROCESSING OF CERAMICS

Fig. 3.6 Schematic diagram of the doctor-blade tape-casting process.

eventually reduced to the required thickness by running it through a set of rollers

rotating at equal speeds.

The process has the advantage that the green tape is free from porosity since

there is no solvent loss and the ceramic powder is very well dispersed with the

elastomer uniformly distributed. The tape can be further processed for substrates

or multilayer capacitors in the same way as band-cast material.

Over the past decade there has been impressive progress in ‘viscous polymer

processing’ (VPP), a technology related to calendering but applied to the

fabrication of complex shapes. The technology involves generating very high

shear rates in a highly viscous, plastic/ceramic powder mix to form a ‘dough’

from which the required net shape is formed. B. Su et al. [8] describe its

application to the fabrication of a variety of electroceramic devices, some having

features of size 5150 mm.

3.6.8 Injection-moulding

Injection-moulding, widely used in the plastics industry, is ideally suited to long

production runs of complex shapes. The process has been adapted for forming

ceramics and the last 20 years or so have seen it ﬁrmly established as an attractive

route for forming structural ceramics.

The ceramic powder is blended with a thermoplastic binder which, after

forming the shape, has to be removed by careful burning out; a combination of

solvent extraction and burning out may also be employed. The volume fraction

of ceramic powder (V

) lies in the range 0.55V

50.7, the optimal loading having

to be determined empirically. The high sacriﬁcial polymer loadings lead to high

‘green’ porosities and in consequence high green-to-sintered shrinkages. Despite

linear shrinkages as high as 18% good control over ﬁnal dimensions can be

achieved. For example the dimensions of silicon carbide turbine blades (thickness

10 mm) can be maintained to within 50 mm. The technology is reviewed by

J.R.G. Evans [9].

Recently injection-moulding has been exploited by L.J. Bowen [10] to fabricate

a range of piezoceramic–polymer composites (see Section 6.4.6) for the active

elements of sonar devices.

3.6.9 Films and layers

The strong research and development activity in fuel cell, sensor and ferroelectric

memory technologies (see Sections 4.5, 4.6 and 5.7.5) has stimulated eﬀorts to

fabricate ﬁlms and supported and self-supporting layers of electroceramics

spanning the sub-micron to 200 mm thickness range. The essentials of the various

technologies are summarized below. They can be classiﬁed according to whether

SHAPING 111