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

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exceed the breakdown strength of air ( 3MVm

) causing an arc to form

across the dry band which may grow until ‘ﬂash over’ occurs from line to earth.

This may cause an interruption of power supply to the consumer, or reduced

voltage, and can also result in serious damage to the conductors, towers and

insulators. Both RI and ‘ﬂash-over’ can be alleviated by using glazes having

surface resistivities in the range 1–100 MO/

(see Section 4.2).

Radio interference is reduced by coating the insulator surface in the line-

insulator contact area with a semiconducting glaze to smooth the voltage

gradient. Glazes for this purpose usually have a zinc ferrite (see Section 4.4.1)

conducting phase, although antimony-doped tin oxide (see Section 4.1.4) is

sometimes used. Close control of surface resistivity is not necessary.

The eﬀective use of semiconducting glazes to avoid pollution-induced ﬂash-

over is less simple. The glaze needs to have a closely controlled resistivity,

designed to pass a leakage current of approximately 1 mA under the service

voltage. This ensures that the voltage drop across the dry band does not exceed

the air breakdown voltage. The temperature coeﬃcient of resistance must be

small (semiconducting glazes usually have a large negative temperature

coeﬃcient of resisitivity) and the glaze must be very durable. Where insulators

are, broadly speaking, disc-shaped, the current density will vary with the radius

272 DIELECTRICS AND INSULATORS

Fig. 5.17 A 36 kV porcelain rod insulator of the variety used to position the overhead power

lines for railways. Overall length 500 mm. (Courtesy of New Zealand Insulators, Temuka,

NZ.)

Table 5.2 Physical properties of electrical porcelains

Property Siliceous Aluminous

Density/Mg m

2.4 2.6–2.8

Cross-breaking strength/MPa 100 150

Coeﬃcient of linear expansion/MK

6 5–6

Dielectric Strength/MV m

25 25

Volume resistivity/O mat208C  10

 10

Tan d/10

4

at 20 8C  150  150

TEAMFLY

Team-Fly

leading to a non-linear voltage gradient, and grading of the glaze thickness is

needed to compensate for this. Glazes with an antimony-doped tin oxide

semiconducting phase are almost invariably used. Although they impart to the

insulators better pollution performance than any other type, they are expensive

and produced by only a few manufacturers.

Recently insulators comprising a core of glass ﬁbre-reinforced polymer and a

sheath of a weather-resistant elastomer (‘composite insulators’) have become

available and now have a promising performance history. In particular, some

silicone elastomers have strongly hydrophobic surface characteristics which are

maintained for long periods (420 years) and these give good performance in

polluted conditions because the surface never gets wet. Such insulators are

replacing porcelain for very high voltage transmission, but at lower voltages

(566 kV) are uneconomic unless pollution conditions are severe. Insulators of

toughened glass are also popular, and where a standard design in very large

numbers is demanded have a signiﬁcant price advantage over porcelain.

Nevertheless, in excess of 500 000 tonnes of porcelain insulators are manufac-

tured world wide each year.

Varistors play an important role in protecting high voltage distribution

equipment (see Section 4.3) against voltage surges caused by lightning strikes,

switching operations and similar hazards. Superconducting fault current-limiters

(see Section 4.7.4) are also used to protect equipment from current surges.

Low-voltage applications of electrical porcelain exist where an insulating

material unaﬀected by high temperatures or ultraviolet radiation is required.

Holders for discharge lamps such as those used for street lighting, and housings

for fuses are examples. The porcelain types are typically of similar composition

to those discussed above, but the shapes are commonly formed by pressing damp

(15 wt.% moisture) body granules in a steel die.

Talc-based

Talc-based ceramics are important electrical porcelains that have major

crystalline components of the ﬁred ceramic lying in the ternary phase diagram

shown in Fig. 5.18. The principal raw material used for these ceramics is talc

(Mg

H

O), which is the softest of minerals (no. 1 on the Mohs scale). Talc

is also called steatite, and ceramics using it as a raw material are termed ‘steatite

porcelains’ although the talc contained in them has been changed in crystal

structure during sintering. Blocks of the mineral steatite, which is also known as

soapstone, can readily be machined to shape and, on ﬁring, undergo a change in

crystal structure that results in a very small overall expansion accompanied by a

large increase in hardness and strength.

LOW-PERMITTIVITY CERAMIC DIELECTRICS AND INSULATORS 273

The various porcelain types fall into three regions of the phase diagram and

will be discussed in turn; typical properties are summarized in Table 5.3 and

examples of components are illustrated in Fig. 5.19.

Cordierite ceramics are best known for their low expansion coeﬃcient which

imparts excellent thermal shock resistance. The major phase developed during

ﬁring is cordierite Mg

(2MgO2Al

5SiO

). Clay and talc are the

principal ingredients; the talc content is in the range 20–40 wt% but there are

many modiﬁcations to adjust ﬁring temperature and range. Components are

shaped mainly by extrusion and dry pressing, and typical ﬁring temperatures lie

in the range 1150–1400 8C. Uses for the ceramic are many and varied, but it is

274 DIELECTRICS AND INSULATORS

Fig. 5.18 Phases present in the Al

–MgO–SiO

system (in wt%).

Table 5.3 Typical properties of common electrical ceramics

Material e

tan d/10

at 1 MHz

/MK

at 20–1000 8C

l/Wm

at 25 8C

‘Low-loss’ steatite 6.1 7 8.9 3

Cordierite 5.7 80 2.9 2

Forsterite 6.4 2 10.7 3

96Al

9.7 3 8.2 35

99.5BeO 6.8 2 8.8 250

AlN 8.8 5–10 4.5 100

Glass 4–15 2–22 0.8–9 0.5–2.0

particularly suited to applications where there is a need for good thermal shock

resistance combined with high electrical resistivity, for example for high-power

electrical fuse holders and supports for high-power wire-wound resistors and fan-

heater elements.

Steatite ceramics were introduced into electronic components during the 1920s

to meet the demands of the rapidly developing radio industry. Their principal

attribute is low dielectric losses which are necessary for higher frequencies. As for

the cordierite ceramics, the major constituents are clay and talc but with the

composition adjusted so that enstatite, a modiﬁcation of MgSiO

, crystallizes. A

typical composition is about 85 wt% talc, 15 wt% clay and 2 wt% calcium

carbonate (chalk). The calcium carbonate acts as a ﬂux and is used instead of

feldspar to avoid the introduction of alkali metals which would increase dielectric

losses. The shrinkage on sintering can be adjusted by calcining up to three-quarters

of the talc and varying the proportion of calcined material to compensate for

changes in the raw materials. The soft raw talc has a lubricating action that greatly

reduces tool wear during fabrication compared with other ceramic powders.

Most steatite ceramics are either dry mixed and pressed, or wet-mixed, spray-

dried and pressed, but they can also be wet-mixed and extruded. Typical ﬁring

temperatures lie close to 1300 8C. Many small parts are made for the electronics

components industry where low dielectric losses are required, for example for tie-

bars and other parts for ganged capacitors, small trimmer capacitors, high-power

capacitors (see Section 5.6.3), coil formers, lead-throughs and substrates for

LOW-PERMITTIVITY CERAMIC DIELECTRICS AND INSULATORS 275

Fig. 5.19 Selection of magnesium silicate parts (parts supplied by Morgan Advanced

Ceramics).

some types of resistor and circuit. The ceramic is also commonly supplied with

nickel metallizing for terminal connector blocks.

The sintered bodies consist of crystals of protoenstatite (or mesoenstatite),

which is a polymorph of MgSiO

, a small amount of cordierite and a continuous

glassy phase surrounding the crystalline phases. Protoenstatite is the thermo-

dynamically stable phase above 985 8C and is stable at room temperature

provided that it is in the form of small crystals (510 mm) covered with a layer of

glass. However, prolonged heating at 500 8C converts protoenstatite into

clinoenstatite, which is the thermodynamically stable form below 985 8C. The

dimensional changes in the crystals due to the transformation result in an overall

expansion and a marked decrease in strength. Badly formulated material, or

material that has been overﬁred so that excessive crystal growth has occurred

and the amount of glass has been diminished, may be unstable under humid

conditions at room temperature. In bad cases the surfaces of bodies become

white and blotchy, they expand and their strength is reduced. The stability of

steatite components can be tested by exposing them to steam at 100 8C for 24 h

and then checking that their cross-breaking strength is unaltered.

Despite this potential instability, steatite ceramics have been widely used and

adequate precautions in manufacture have prevented all but a very few cases of

deterioration during component lifetimes of many years in a wide variety of

environments.

In forsterite ceramics the mineral forsterite (Mg

SiO

) crystallizes. They have

excellent low-dielectric-loss characteristics but a high thermal expansion coeﬃcient

which imparts poor thermal shock resistance. During the 1960s they were

manufactured for parts of rather specialized high-power devices constructed from

titanium and forsterite and for which the operating temperature precluded the use

of a glass–metal construction. The close match between the thermal expansion

coeﬃcients of titanium and forsterite made this possible. Today alumina–metal

constructions have completely replaced those based on titanium–forsterite and the

ceramic is now manufactured only to meet the occasional special request.

5.5.2 Alumina

Structure, fabrication and properties

Alumina is a widespread component of siliceous minerals. It occurs as single

crystals in the form of sapphire, and with chromium ‘impurity’ as ruby, and in

large deposits as the hydrated oxide bauxite (Al

2H

O). The dehydration of

this and other hydrated oxides at temperatures below 1000 8C leads to the

formation of g-Al

which is converted to a-Al

above 1000 8C. The

transformation is irreversible and the a-polymorph is stable from absolute zero

to its melting point at 2050 8C.

276 DIELECTRICS AND INSULATORS

a-alumina, in powder form, is a byproduct of aluminium production. Bauxite,

which is naturally contaminated with other oxides, principally SiO

and Fe

,is

puriﬁed by the Bayer process. This involves the dissolution of a low-silica-

content ore in caustic soda (NaOH) solution under pressure, ﬁltering oﬀ the

insoluble hydroxides (mostly of iron), and then precipitating Al(OH)

by diluting

the solution at atmospheric pressure and adding a small amount of Al(OH)

as a

nucleating agent. Most of the silica remains in the solution. The hydroxide is

washed and then calcined at temperatures in excess of 1000 8C to produce a-

. Because of the involvement of sodium in the extraction process the

calcined alumina contains 0.1–0.2% Na

O. This impurity has important

consequences as far as the manufacture of electrical ceramics is concerned and

care is exercised to keep it to a minimum. Practically all the powder for the

production of alumina ceramics is produced in this way, but a purer product, for

instance for growing artiﬁcial sapphire crystals, can be made by preparing

ammonium alum (NH

Al(SO

)

12H

O) by dissolving 99.999% aluminium metal

in sulphuric acid and neutralizing the excess acid with ammonia. The alum is

allowed to crystallize, which helps to purify it, and is then calcined below 1000 8C

to yield g-Al

, which is preferred for some methods of sapphire preparation, or

at higher temperatures to give the a form.

The crystallites in alumina ceramics are mainly a-Al

, mineralogically

known as corundum (a synonym for sapphire). Corundum is 9 on the Mohs scale

of hardness, i.e. it is next hardest to diamond, at 10. Examples of the

microstructures of a high-purity alumina and a debased alumina are shown in

Figs 5.20(a) and 5.20(b) respectively. The latter consists of a-Al

crystallites

embedded in a glass–crystalline matrix usually composed of calcium and

magnesium silicates.

The less pure aluminas are blended with silicates so that they can be sintered at

1350 8C or less. The highest-purity materials require a temperature of 1750 8Cat

atmospheric pressure or hot-pressing. As cost increases with sintering

temperature, the grade used in practice is usually the least pure that has

adequate properties.

Table 5.4 gives the properties of a range of aluminas and shows the

improvement in properties with purity. The most striking change is in thermal

LOW-PERMITTIVITY CERAMIC DIELECTRICS AND INSULATORS 277

Table 5.4 Typical values of the electrothermal properties of various grades of alumina

ceramic

Alumina content/% 85 90 96 99.5 99.9

at 1 MHz 8.2 8.8 9.0–9.3 9.7 9.8–10.1

tan d/10

at 1 MHz 9 4 1–3 3 0.4–2

Resistivity/O m at 300 8C 4.6 610

1.4610

3.1610

2.0610

1.0610

Thermal conductivity/

at 20 8C

14 16 24–35 35 40

278 DIELECTRICS AND INSULATORS

conductivity which indicates the superiority of the purer grades in applications

where the transfer of heat is of importance, e.g. in substrates, discussed below.

The wide range of thermal conductivities reported for 96% Al

requires

some explanation. Thermal conductivity depends on the transfer of lattice

vibrational energy in the form of phonons. Defects in the lattice inhibit this

process, and it is found that the incorporation of impurities much in excess of

0.1 mol.% in the lattice appreciably reduces the thermal conductivity. However,

if impurities are present as separate phases and have not entered the lattice of the

main phase their eﬀect will be in accordance with one of the mixture rules (cf.

Section 2.7.4). Which rule will depend on the distribution of the minor phases,

for example whether they form continuous layers round each grain of the main

phase or are present in separate discrete regions. High conductivity only occurs

in 96% alumina when the raw materials and processing conditions are such that

the additives to lower the sintering temperature do not enter the Al

lattice in

more than minor quantities.

There is considerable uncertainty about the room temperature values of the

electrical resistivity of good insulators; the best estimates are probably derived by

LOW-PERMITTIVITY CERAMIC DIELECTRICS AND INSULATORS 279

Fig. 5.20 Microstructures of (a) 99.9% Al

(Courtesy of E.W. Roberts), (b) 95% Al

(Courtesy of R. Morrell), (c) chemically etched electrical porcelain: note the partially dissolved

quartz grains and mullite precipitates (Courtesy of S.T. Lundin).

extrapolation of the linear log r–1/T plots of the type shown in Fig. 5.21. Also,

the reliable measurement of the resistance of very high resistance specimens is

made diﬃcult because of the relatively low resistance of the ceramic surface due

to adsorbed impurities, and even that of the surrounding air or other gas. This

can be circumvented by using a properly guarded measurement method.

The principle of a guarded measurement is illustrated in Fig. 5.22 which shows

a guard ring around one of the measuring electrodes. When the eﬀects of gas

conduction have to be guarded against the ring must be extended into a cylinder.

The guard ring and one of the centre electrodes are connected to the voltage

supply but only the current through the central electrode is measured.

Satisfactory measurements can be made in this way at 200 8C and above, but

with high resistivities and lower temperatures the polarization and reordering of

defects and impurities result in an initially high current that takes many hours to

fall to a steady state.

Very careful measurements have been made by F.G. Will et al. [4] on high

purity (535 p.p.m. impurities) sapphire over the temperature range 400–1300 8C

and at an oxygen partial pressure of 1 mPa (10

711

atm). They ﬁnd the

conduction mechanism to be electronic with negligible ionic contribution. The

280 DIELECTRICS AND INSULATORS

Fig. 5.21 Dependence of resistivity on temperature for various grades of Al

. *[4]

measured activation energy of 4.8 eV is consistent with the determined optical

band gap of 9.5 eV.

In the case of polycrystalline alumina the matter is complicated by charge

transport along grain boundaries. It is generally found that s increases with both

large and small p

values, with a minimum at about 10 mPa (10

4

atm) at about

1600 8C. This behaviour seems to be independent of the type of dopant (acceptor

or donor) or whether the material is single crystal or polycrystalline. Figure 5.23

illustrates the expected general behavioural pattern, but the positions of the

boundaries deﬁning the ‘ﬁelds’ for the various mechanisms is largely speculative

and will, of course, depend strongly on the natures and amounts of impurities

present.

The eﬀects of deliberately added donors, such as titanium, and acceptors, such

as iron and magnesium, on electrical conductivity have been studied. Doping

with aliovalent ions aﬀects the concentration of intrinsic defects and, in

consequence, the diﬀusivity of Al and O. In the case of variable-valency dopants,

changes in p

change the fraction of dopants in the aliovalent state and the

nature and concentration of the defects. For example, the dopant Ti substitutes

for Al and, in the fully oxidized state, produces the defect Ti

, compensated by

000

, so that

½Ti

¼½3V

000

ð5:16Þ

and ionic conductivity by transport of V

000

is encouraged. However, at low p

is in the isovalent state, Ti

, and electron conduction occurs according to

Ð Ti

þ e

ð5:17Þ

LOW-PERMITTIVITY CERAMIC DIELECTRICS AND INSULATORS 281

Fig. 5.22 Schematic diagram of a guard ring and circuit for measurements of high resistivity.