Moulson A.J., Herbert J.M. Electroceramics: Materials, Properties, Applications
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10. Assuming that a glaze will ‘craze’ if the tensile strain is greater than 0.1%, find the
critical relationship for the onset of crazing between the mean expansion coefficients
of a glaze and of a thick section ceramic body to which it is applied. The
transformation temperature of the glaze can be assumed to be 725 8C and room
temperature 25 8C [Answer: a
g
a
b
4 1:43 MK
1
].
11. (i) Calculate the ratio of the partial pressures of CO and CO
2
at 1400 8C that would
provide an oxygen partial pressure of 10
15
bar given that
2CO þ O
2
¼ 2CO
2
DG
1673
¼260 kJ
(ii) Estimate the oxygen partial pressure in wet hydrogen (dew point, 25 8C) at 1 atm
and 1700 8C given that
H
2
þ
1
2
O
2
¼ H
2
O
DG
1973
¼138 kJ
(vapour pressure over water at 25 8C ¼3.17 kPa). [Answers: 2:76 10
3
;4:9 10
11
bar]
12. Estimate the oxygen partial pressure below which alumina will be reduced to the
metal at 1700 8C, given that
2Al þ
3
2
O
2
¼ Al
2
O
3
DG
1973
¼1053 kJ
[Answer: 2:5 10
19
bar].
13. What precautions could be taken to prevent the take up of impurities when reducing
the particle size of zirconia from 10 mmto1mm? Describe the beneficial effects that
addition of a deflocculating agent might be expected to have on the ball-milling
process.
14. A product is to be made by cold pressing. Describe the attributes of a good binder
and the benefits expected from its use.
15. What shapes are suited to fabrication by extrusion, cold pressing or slip-casting?
What are the advantages and disadvantages of isostatic-pressing?
16. Discuss the control of grain-growth during sintering.
17. Because of the volatility of PbO at sintering temperatures it is not easy to control the
composition of ceramics containing it. What steps might be taken to alleviate the
problem?
18. A slip for band-casting contains, on a weight basis, 100 parts alumina powder, 9
parts non-volatile organics and 35 parts toluene. The thickness of the dried film
132 PROCESSING OF CERAMICS
TEAMFLY
Team-Fly
®
which adheres to the band on which it is cast is 0.6 of the cast thickness. What is the
porosity of the tape (i) after drying and (ii) after the binder has been removed? On
firing the shrinkage is a further 5% in thickness and the final total porosity is 5%.
Estimate the shrinkage in the plane of the film. [Answers: 17%; 38%; 17%]
19. A few grammes of 100 mm diameter polythene spheres are warm-pressed into a solid
block. Describe the expected electrical characteristics of the block. In a separate
experiment each sphere is first coated with a nanometre thick layer of aluminium
prior to compaction. Describe how the electrical properties of the block would be
modified and estimate the quantity of aluminium in the ‘composite’ on the bases of
volume and weight. Comment on the relevance of your finding to electroceramics
research and development 30 p.p.m.; 90 p.p.m.
[Answers: Insulator; conductor; about 30 p.p.m on volume basis.]
Bibliography
1. Reed, J.S. (1995) Principles of Ceramic Processing, 2nd edn, J. Wiley and Sons Inc.,
New York.
2. Rahaman, M.N. (1995) Ceramic Processing and Sintering, Marcel Dekker Inc., New
York.
3. Cahn, R.W. et al. (eds) (1996) Materials Science and Technology, Volumes 17A and
17B, R.J. Brook (ed.), ‘Processing of ceramics’, Parts 1 and 2, Verlagsgesellscaft
mbH, Weinheim.
4. Shaw, D.J. (1980) Introduction to Colloid and Surface Chemistry, 3rd edn,
Butterworths, London.
5. Segal, D. (1996) Chemical preparation of powders. In: Materials Science and
Technology, R.J. Brook (ed.), ‘Processing of ceramics’, Part 1, Verlagsgesellschaft
mbH, Weinheim. 17A, pp. 69–98.
6. Lewis, J.A. (2000) Colloidal processing of ceramics. J. Am. Ceram. Soc. 83, (10),
2341–59.
7. Hellebrand, H. (1996) Tape casting. In: Materials Science and Technology, R.J.
Brook (ed.), ‘Processing of ceramics’, Part 1, Verlagsgesellschaft mbH, Weinheim.
17A, pp. 189–265.
8. Su, B., Pearce, D.H. and Button, T.W. (2001) Routes to net shape electroceramic
devices and thick films. J. Eur. Ceram. Soc. 21, 2005–9.
9. Evans, J.R.G. (1996) Injection moulding. In: Materials Science and Technology, R.J.
Brook (ed.), ‘Processing of ceramics’, Part 1, Verlagsgesellschaft mbH, Weinheim.
17A, pp. 268–311.
10. Bowen, L.J., et al. (1993) Injection moulded fine-scale piezoelectric composite
transducer. 1993 IEEE Ultrasonics Symposium, pp. 499–503.
11. Will, J. et al. (2000) Fabrication of thin electrolytes for second-generation solid oxide
fuel cells. Solid State Ionics 131, 79–96.
12. McMillan, P.W. (1979) Glass-ceramics, 2nd edn, Academic Press, New York.
13. Lewis, M.H. (ed.) (1989) Glasses and Glass-ceramics, Chapman & Hall, London.
BIBLIOGRAPHY 133
14. Gentile, A.L. and Ainger, F.W. (1996) Single crystals. In: Materials Science and
Technology, R.J. Brook (ed.), ‘Processing of ceramics’, Part 1, Verlagsgesellschaft
mbH, Weinheim. 17A, pp. 313–42.
15. Remeika, J.P. (1954) A method for growing barium titanate single crystals. J. Am.
Chem. Soc. 76, 940.
134 PROCESSING OF CERAMICS
4
CERAMIC CONDUCTORS
Chapter 2 provides the elementary background physics essential to an under-
standing of the following discussion of conductive ceramics. These include
conductors capable of sustaining their mechanical integrity at high (>1500 8C)
temperatures, ohmic resistors with properties little affected by temperature and
voltage, resistors with voltage-dependent resistivities, resistors with large
negative temperature coefficients, resistors with large positive temperature
coefficients, fast-ion conductors, sensors and superconductors.
The discussion may be supplemented by reference to the text edited by R.C.
Buchanan [1].
4.1 High-temperature Heating Elements and
Electrodes
The principal application for high-temperature heating elements and electrodes is
as furnace elements for temperatures at which most materials either oxidize or
melt. Strictly speaking, for this application the term ‘resistor’ is more apt than
‘conductor’ since resistance is necessary for Joule heating. It is advantageous for
such components to be ohmic in their behaviour, but it is not essential as suitably
controlled power supplies can be devised to compensate for both non-linear
voltage–current characteristics and high temperature coefficients of resistance.
Resistivities of 0.01–1 O m are convenient as this allows the impedance of rods up
to 1 m long and 0.5–2 cm in diameter to match into readily available power
supplies; it is possible but impracticable to form ceramics into replicas of the wire
windings that utilize metals with their much lower resistivities ( 10
7
O m). The
nearest to this is to form a ceramic tube and then to cut a spiral slot through the
wall with a pitch of about 1 cm. Suitable transformers for feeding very low
resistances are available if necessary. Highly resistive elements necessitate high
Electroceramics: Materials, Properties, Applications. 2nd Edition. Edited by A. J. Moulson and J. M. Herbert.
& 2003 John Wiley & Sons, Ltd: ISBN 0 471 49747 9 (hardback) 0 471 49748 7 (paperback)
voltage power supplies which lead to difficulties in furnace design since
refractories all become conductive when hot so that it is difficult to avoid
current leakage and often an accompanying risk of thermal breakdown (see
Section 5.2.2).
In cases where reducing atmospheres can be tolerated, graphite or refractory
metals such as molybdenum and tungsten can be used, while platinum and its
alloys can be used safely in air up to 1500 8C. Ceramics allow the use of air
atmospheres at relatively low cost.
4.1.1 Silicon carbide
Silicon carbide ceramic is noted for its hardness and for its high strength
maintained to high temperatures. A passivating silica layer protects the material
from corrosion in oxidizing atmospheres, the rate of reaction between SiO
2
and
SiC limiting the practical operating temperature in air to about 1650 8C.
It is widely used as an abrasive, as a refractory, for high temperature furnace
elements and for voltage-sensitive resistors (‘varistors’) (see Section 4.3). For
most purposes it is made by the method devised by Acheson in 1891. He
accidentally formed SiC, initially taking it to be a mixture of carbon and
corundum (Al
2
O
3
), or so it is believed, which explains the trade name
‘Carborundum’. The Carborundum Company was formed in 1891 to produce
SiC for grinding wheels. The Acheson process for producing SiC has not
changed significantly since 1891 and in its essentials involves passing a large
electric current through a graphite resistor packed around with a mixture of sand
and coke. Temperatures of approximately 2500 8C are reached in the reaction
zone producing SiC and CO.
Silicon carbide is covalently bonded with a structure similar to that of
diamond. There are two basic structures. One is a cubic form, b-SiC which
transforms irreversibly at about 2000 8C to one of a large number of hexagonal
polytypes, and the other is a rhombohedral form also with many polytypes. Both
the hexagonal and rhombohedral forms are commonly referred to as a-SiC.
The properties that have led to the established commercial importance of SiC
as a ceramic include its hardness (approximately 9 on the Mohs scale), its hot
strength and its electrical conductivity. The combination of high strength,
particularly at high temperatures, high thermal conductivity and low thermal
expansivity which combine to impart good thermal shock resistance, have placed
the ceramic in the fore as a candidate for advanced high temperature engineering
applications. In the present discussion its electrical properties are the prime
consideration.
Pure cubic SiC is a semiconductor with a band gap of approximately 2.3 eV
and, as expected, is transparent with a pale yellow appearance in transmitted
light. The hexagonal form has a band gap of 3 eV. The wide band gap of SiC
136 CERAMIC CONDUCTORS
HIGH-TEMPERATURE HEATING ELEMENTS AND ELECTRODES 137
Fig. 4.1 Heating elements
(i) SiC lump as produced by the Acheson process (approx. 150 mm across).
(ii) Sintered (recrystallized) element manufactured in 3 sections (700 mm long).
(iii) Reaction-bonded, spiralled element (150 mm long).
(iv) Illustrating-range of manufactured SiC elements.
(v) Molybdenum disilicide (MoSi
2
) elements.
(Courtesy of Kanthal AB Hallstahammer, Sweden.)
offered the realization of blue light emitting diodes (LEDs) and there was volume
production of the devices over the period 1990–5. However they have been
totally replaced by the far more efficient GaN variety.
There has been impressive progress in the technology of growing single crystal
SiC for semiconductor devices. Silicon and gallium arsenide melt and therefore
the Czochralski melt-growth (see Section 3.11) approach to growing single
crystals can be exploited. Because silicon carbide does not melt the same
approach cannot be exploited, but good quality wafers can be produced via
vapour phase epitaxy. Polycrystalline SiC is heated to about 2400 8C when it
sublimes, the vapour condensing on to a cooler SiC ‘seed’ crystal. At present SiC
single crystal wafers of diameter larger than 5 cm are commercially available.
This breakthrough, coupled with the wide band gap and excellent thermo-
mechanical properties of silicon carbide, has made available material having
potential for various devices. These include high-power transistors for switched
mode power supplies, improved microwave devices for radar and communica-
tions, radiation-hardened devices, UV sensors, and high temperature sensors for
management of automotive and aircraft engines.
As produced commercially as a mass of small crystals (see Fig 4.1) the colour
of SiC ranges from black, through shades of grey, to blue, green and pale yellow.
The colours arise from a variety of impurities including boron, aluminium,
nitrogen and phosphorus, and are useful for selecting grades for particular
applications. The purer grades contain at least 99.5 wt% SiC, but with sufficient
impurities to render them conductive at room temperature.
There are three principal methods of manufacturing the types of SiC heating
element shown in Fig. 4.1:
1. pressureless sintering (also described as ‘recrystallization’),
2. reaction-bonding; and
3. in situ formation of SiC from carbon and SiO.
Method 1, which accounts for approximately 95% of element manufacture, is the
straightforward sintering (‘recrystallization’) of a-SiC grit which is formed into a
rod or tube and sintered in a carbon furnace at approximately 2500 8C. To give
the component the necessary low resistance cold-end terminations the pore
volume of a predetermined length of the end sections is infiltrated with silicon or
a silicon alloy.
The above outlines the manufacturing route for one-piece elements. In fact
most rod elements are of three-piece construction in which the low-resistance
cold end sections (silicon or silicon alloy infiltrated) are manufactured separately
from the high-resistance hot centre section. The three sections are then joined by a
reaction-bonding process. This is the most economic approach to manufacturing
138 CERAMIC CONDUCTORS
elements and, importantly, offers flexibility in quickly responding to customer
demand.
Method 2 produces what is usually referred to as self-bonded SiC by a process
essentially the same as that developed for nuclear applications [2] and known as
‘Refel*’ silicon carbide. Refel silicon carbide contains a finely dispersed residual
silicon phase accounting for between 5 and 10% of the total volume, the precise
amount depending upon the porosity of the original preform. In the case of self-
bonded silicon carbide elements the residual silicon is volatilized away by a final
heat-treatment at over 2500 8C when recrystallization occurs resulting in grain
growth and further bonding between primary and secondary carbide phases.
The element fabrication route starts with a mix of a-SiC grains, carbon powder
and plasticizers being formed into the required shape by standard ceramic
processing, usually extrusion or injection-moulding. The ‘green’ form is then
brought into contact with molten silicon which, by capillarity, permeates the
pore space, siliciding the carbon and forming a secondary phase of SiC, which
bonds to the primary SiC phase.
Although the melting point of silicon is about 1410 8C, because of the need to
remove the ‘native’ oxide layer coating the SiC so that the silicon wets the grains,
the infiltration is usually carried out at temperatures 42000 8C when a-SiC
forms. The removal of the native silica layer depends upon the ambient and in
neutral atmospheres reaction-bonding can be effected at lower temperatures
forming b-SiC.
The material is fired again in a recrystallization process which involves heat-
treating the rods at approximately 2500 8C in an inert atmosphere. This process
transforms any b-SiC to the a-modification and partially removes residual
silicon, permitting control over electrical resistivity. The resulting ceramic has a
low porosity and consequently a long service life.
The resistance of the hot section of a tubular element may be adjusted to the
required value by spiralling, which is readily accomplished in the ‘green’ state,
the resistance of the heating section being varied by adjusting the pitch of the cut.
Alternatively, for rods, low resistance ‘cold ends’ may be joined to high
resistance hot-zone lengths by reaction-bonding.
In method 3, which currently is followed only to an insignificant extent, a
carbon tube is heated at a temperature of about 1900 8C in a bed of sand (SiO
2
)
and coke (carbon). The tube may be directly resistance-heated or heated
indirectly by a sacrificial carbon tube of smaller diameter. Silicon monoxide
(SiO) from the SiO
2
-C reaction infiltrates the carbon tube transforming it to b-
SiC. After the siliciding stage the SiC tube is readily removed from the bed and
any residual carbon burnt out. In this form the tubes have a uniform electrical
resistance along their length and the necessary higher resistance heating section is
made by diamond-sawing a spiral through the tube wall. The tube has a porosity
HIGH-TEMPERATURE HEATING ELEMENTS AND ELECTRODES 139
* ‘Reactor Fuel Element Laboratories’.
of about 30 vol.% and a large internal surface area. To reduce the rate of
oxidation during use the tubes usually are protected by a silicate glaze fired to
about 1450 8C.
In most cases the ends of the elements are flame-sprayed with aluminium to
make good electrical contacts with the power leads.
SiC heating elements, depending upon the grade, are routinely used at
temperatures up to about 1500 8C in air, and even up to about 1650 8C for short
periods. The unglazed variety rely on a thin native passivating silica film for their
protection against oxidation. Service life therefore depends strongly on the
atmosphere in which they operate, which affects the stability of the film, and on
temperature which, of course, affects reaction rates.
The general form of the resistance–temperature characteristics for SiC elements
is shown in Fig. 4.2. Below approximately 800 8C the resistance varies from batch
to batch; above this temperature the variability is, practically speaking, absent.
Although the variety of SiC elements available precludes the possibility of
quoting a single electrical resistance characteristic it is useful to assume a
resistivity value at 1000 8C of about 10
3
O m.
The variability in the resistance–temperature characteristics at temperatures
below about 800 8C can be attributed to impurities, the materials behaving as
very complex, doped semiconductors (see Section 2.6.2 and Problem 2.10). The
positive temperature coefficient of resistance observed at the higher temperatures
suggests the effect of decreasing charge-carrier mobility with increasing
temperature (see Section 2.6.2), but because of the complex nature of the
materials both from the chemical and microstructural standpoints, this has to be
regarded as speculation.
140 CERAMIC CONDUCTORS
Fig. 4.2 Resistance–temperature characteristics of SiC elements prepared by various
methods.
As far as applications are concerned SiC heating elements are very widely used
and likely to be found wherever high-temperature technology is involved – in the
laboratory and particularly in the ceramics, glass and metallurgical industries.
The intrinsic hot strength of silicon carbide is an important factor in the use of
heating elements permitting horizontal installation without supports.
4.1.2 Molybdenum disilicide
Many metals form conductive silicides, which, like SiC, are resistant to oxidation
through the formation of stable ‘passivating’ layers of silicates or silica on their
surfaces at high temperatures. Molybdenum disilicide (MoSi
2
) has been
developed as a heating element for use in air at temperatures above 1500 8C.
Its resistivity behaves as is expected for a metal, increasing from about
2.5610
7
O m at room temperature to about 4610
6
O m at 1800 8C.
A commercial MoSi
2
heating element known as ‘Kanthal Super’ is a cermet
consisting of a mixture of particles bonded together with an aluminosilicate glass
phase which forms about 20% of the total volume. Common forms of element
are shown in Fig. 4.1. To fabricate the elements a mixture of fine MoSi
2
powder
with a carefully chosen clay is extruded into rods of suitable diameters for the
terminal sections and heating zones. The rods are dried, sintered and cut to
various lengths. The heating zones are bent to the required shapes at high
temperature and then welded to the larger-diameter terminal sections. The best
grade of MoSi
2
element is capable of operating up to 1800 8C.
Because of the nature of the phase composition a MoSi
2
heating element will
soften at high temperatures and therefore is usually mounted vertically; if the
furnace design calls for horizontal mounting then some refractory support has to
be supplied, as illustrated in the figure. This contrasts with silicon carbide
elements which maintain their rigidity at normal maximum operating
temperatures.
4.1.3 Lantha num ch romite
Lanthanum chromite (LaCrO
3
) was developed during the 1960s for electrodes in
magnetohydrodynamic (MHD) generators where hot electrically conducting gas
is passed through a duct across which there is a strong magnetic field. The e.m.f.
induced at right angles to both the gas flow and the magnetic field develops a
voltage difference between electrodes located on opposite sides of the duct. The
gas temperature must be close to 2000 8C and the gas seeded with potassium to
render it conducting. The electrode material is required to be electronically
conducting and resistant to corrosion by potassium and, allowing for some
cooling, it must be able to withstand a temperature of 1500 8C for at least
HIGH-TEMPERATURE HEATING ELEMENTS AND ELECTRODES 141