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

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immerse the discs and coat them with solder. The preliminary warming lessens

the thermal shock (see Section 5.3) of immersion in molten solder. The time of

immersion must be brief since silver dissolves rapidly in the molten metal. The

units are next coated with a polymer, usually an epoxy resin, baked to cure the

resin and tested for capacitance value and voltage breakdown.

Tubes are formed by extrusion. They have the advantage of being less fragile

than ﬂat pieces and are more suitable in some types of circuit assembly. After

sintering, the tubes are completely coated with silver. Automatic machines then

grind the silver from one end and remove a ring of silver from the outer surface

near the other end (Fig. 5.9). The leads are looped round each end and soldered

in place by immersion in solder.

As an alternative to solder, nickel can be deposited from a solution containing

nickel salts and strong reducing agents such as sodium hypophosphite. This

process is termed ‘electroless plating’. The surface of the ceramic must ﬁrst be

activated by successive immersion in dilute stannous chloride and palladium

chloride solutions. The adhesion of the nickel is inferior to that of ﬁred-on silver

but adequate to secure a wrapped-round wire on a tube. Nickel has the

advantage of insolubility in solder but acquires a tough layer of oxide if exposed

to air for a prolonged period and cannot then be wetted by solder.

Lead and electrode inductance can be somewhat less in discs than in tubes so

that discs have some advantage at higher frequencies. The two shapes are similar

in volumetric eﬃciency since their bulk largely consists of the encapsulating

resin. The tubular geometry is suited to the manufacture of ‘feed-through’

capacitors. A schematic diagram of the cross-section of such a capacitor is shown

in Fig. 5.10. These are used as bypass capacitors in television and FM tuners.

Disc and tubular shapes are used for all classes of dielectric since they are

lowest in cost. Using Class I dielectrics they cover the 0.1–1000 pF range, with

Class II/III they cover 1000–100 000 pF and with Class IV they cover 0.1–2 mF.

Except for Class IV, the safe working voltages are usually at least 100 V although

in electronic circuits they are not likely to encounter applied voltages of more

than 10 V. Dielectric thicknesses lie in the 50 mm to 2 mm range with the thicker

units able to withstand the normal mains supply. Discs range in diameter (or

side) from 2 to 30 mm whilst tubes may be 5–60 mm long61–10 mm in diameter;

these are bare dimensions without an encapsulant.

262 DIELECTRICS AND INSULATORS

Fig. 5.9 Longitudinal section showing silvering of a tubular capacitor.

TEAMFLY

Team-Fly

Multilayer capacitors A critical step in the manufacture of multilayer

capacitors is, of course, the barium titanate-based starting powders, and the

various routes for producing these are described in Section 3.4. The multilayer

capacitor structure (Fig. 5.11) enables the maximum capacitance available from

a thin dielectric to be packed into the minimum space in a mechanically robust

form.

Broadly speaking there are two signiﬁcant manufacturing processes, the ‘dry’

and the ‘wet’; there is a third, the ‘fugitive electrode’ process, developed to

replace the expensive palladium-containing electrodes with a cheap metal, but it

has ceased to be of any commercial signiﬁcance. The steps in the various

processes are summarized in Fig. 5.12.

CAPACITORS 263

Fig. 5.10 Cross-section of ‘feed-through’ tubular capacitor.

Fig. 5.11 Schematic diagram of a multilayer ceramic capacitor construction.

The thickness of the dielectric layers produced in the ‘dry’ process is typically

in the range 5–8 mm with 3 m m being achieved and  2 mm a target for some

manufacturers, the objective being to maximize volumetric eﬃciency. The

dielectric layers manufactured by the ‘wet’ process are thicker, typically  20 mm.

The overall capacitor dimensions range from approximately 1.060.560.5 mm

to 66662.25 mm, and the corresonding capacitance values from 1 pF to 300 mF.

Generally speaking the dielectrics employed are Class I or Class II/III.

In the ‘dry’ process the dielectric powder is formed into either an organic- or

water-based slip and tape-cast (see Section 3.6.6) to form a continuous strip. The

264 DIELECTRICS AND INSULATORS

Fig. 5.12 Outline of the fabrication process for multilayer ceramic capacitors.

cast tape is cut into sheets onto which the electrode pattern is printed as shown in

Fig. 5.13. The electroded sheets are stacked and consolidated under pressure at

about 70 8C. The consolidated stack is diced by cutting along lines such as AA’

and BB’, to expose electrodes of successive layers at opposite end-faces. The

polymer binder, which may consist of up to 35 vol.% of the green body, is next

carefully removed by heating in air. This is a critical step which must be

accomplished without any disruption of the structure. Following removal of the

organic material, the ‘chips’ are ﬁred to the ceramic sintering temperature during

which process the electrodes must remain solid and in place.

In the ‘wet’ process a slip carrying the ceramic powder is laid down, by screen-

printing for example, onto a temporary carrier such as a glass tile. The process

can be repeated to build up the required thickness of the dielectric onto which the

electrodes are screen-printed. The next dielectric layer is then laid down and the

process repeated. The mutlilayer structure is diced as described above, and the

individual chips removed from the tile for the subsequent stages, as for the ‘dry’

process.

Extreme care has to be exercised to avoid contaminating the tape-cast sheets

with ‘dust’ which would seriously reduce yield and impair reliability, and a high

level of cleanliness, including ‘clean room’ standards, are required for processing

the thinner layers.

The inks for screen-printing the electrodes contain the sub-micron metal

powder, either a Ag–Pd alloy or a base metal, usually nickel (melting point,

1455 8C) but sometimes copper (melting point 1084 8C). Palladium (melting

point, 1554 8C) and silver (melting point, 961 8C) form solid solutions with

melting points approximately proportional to the content of the end members.

The sintered chips are next ‘terminated’ which involves coating the ends with a

paint, in the case of Ag–Pd electroding consisting of a mixture of powdered silver

and glass frit, and a suitable organic vehicle, which is ﬁred on at about 800 8C.

CAPACITORS 265

Fig. 5.13 Stacking arrangement and cutting of laminated printed sheets for forming

multilayer ceramic capacitor.

Thin (2 mm) layers of nickel and tin are electroplated over the silver

termination, the former to act as a barrier preventing dissolution of the silver

during the subsequent ‘wave-soldering’ operation when the chips are ‘surface-

mounted’ onto a substrate. The tin layer ensures good wetting of the termination

by the solder. In the case of the BME capacitor (see below) the termination is

copper ﬁred on under reducing conditions and covered with an electroplated

layer of nickel.

The terminations must make good electrical contact with the exposed alternate

electrodes, connecting up the stack of plate capacitors in parallel. After testing

the terminated chips can be put into bandoliers and sold ready for automatic

mounting onto printed circuits (Chapter 4, Fig. 4.5). Alternatively, leads can be

soldered to the terminations and the chip encapsulated in a suitable polymer and

then bandoliered.

The high cost of palladium has been a stimulus to reducing its content in the

Ag/Pd alloy or of eliminating it entirely. The former strategy necessitates

developing ceramic dielectrics which sinter at correspondingly lower tempera-

tures (see Section 5.7.2) or, in the case of base metal electroded (BME) MLCCs,

are stable against reduction ﬁring (see Section 5.7.3).

BME technology presently (2002) accounts for at least 50% of total MLCC

production and the trend seems set to continue for as long as the price of

palladium dictates. The thermodynamic and chemical factors involved in the

production of a reliable BME capacitor are discussed in Section 5.7.3.

As in the case of the BME capacitor, the ‘fugitive electrode’ process was

developed to avoid the high expense involved in using palladium. In this process

the ‘electrodes’ are printed using an ink consisting of an organic vehicle carrying

coarse ( 10 mm) uncalcined powders (BaCO

, TiO

, ZrO

). Following sintering,

porous ‘electrode’ cavities 2–5 mm thick are retained in the ceramic. After the

chip is terminated with porous silver it is evacuated and then, under pressure, the

electrode cavities are impregnated with a low melting metal or alloy, for example

lead. The technology is not now exploited to a signiﬁcant extent, if at all.

High-voltage and high-power capacitors From the point of view of

sustaining high voltages, the structures so far discussed have the disadvantage

that the electrodes remain parallel up to their edges. For example, in a parallel-

plate capacitor the field is normal to the electrodes in central regions remote

from their edges but extends into the dielectric beyond the electrode edges with

the result that there is an electric field concentration near the edges as indicated

in Fig. 5.14(a). As a result the field strength close to the electrode edge may be as

much as twice the average value. This is of little consequence when the average

field is a small fraction of the breakdown strength but becomes extremely

important in high-voltage units.

The ﬁeld at the edges can be reduced by shaping the dielectric into the form of

two cups as shown in Fig. 5.14(b). This shape ensures that the electrode edges are

266 DIELECTRICS AND INSULATORS

CAPACITORS 267

Fig. 5.14 (a) Converging lines of force at electrode edges; (b) dielectric shaping for high

voltage capacitors.

(a)

(b)

Fig. 5.15 Sections through high-voltage high-power capacitors illustrating electrode edge

conﬁgurations to avoid ﬁeld concentration; (a) pot, and (b) cylinder.

separated by several times the dielectric thickness in the centre of the unit. It also

increases the path length across the surface between the electrodes and so lessens

the possibility of surface breakdown. Surface breakdown occurs at lower ﬁelds

when the dielectric is exposed to the atmosphere. Therefore in high-voltage

applications it is essential to encapsulate materials in closely adherent glaze or

polymeric substances.

The ceramic in a high-power capacitor is disc shaped, cylindrical or pot shaped

(see Figs 5.15 and 5.16) with thicknesses of up to 10 mm. It is essential that the

dielectric is free from cavities and has a uniform density in order to avoid plasma

discharges in pores and local high-ﬁeld regions. Pressing is therefore best

undertaken hydrostatically with the ﬁnal shaping of the edges carried out on the

unﬁred piece on a lathe.

Metallization usually comprises three stages: a layer of ﬁred-on silver with a

high frit content to secure adhesion, a second layer of ﬁred-on silver with a low

frit content to give maximum conductivity and an electroplated layer of copper

to prevent solder dissolving the silver. The leads on the larger units are thick

copper foils ﬁxed to the metallization with solder. The soldering is necessarily a

prolonged process to minimize the risk of failure by thermal shock. The edges of

the electrodes and the unelectroded portions of the ceramic surface are glazed,

and the glaze and the metallization act as an encapsulating layer. Anything

268 DIELECTRICS AND INSULATORS

Fig. 5.16 A range of high-power transmitter capacitors (components supplied by Morgan

Electro Ceramics).

thicker would diminish heat dissipation and limit the power which the unit could

pass.

Only Class I dielectrics are used in high-frequency (50 kHz–4100 MHz) power

capacitors. Typical ranges of parameter values are as follows: capacitance, 2 pF

to 12 nF; peak voltage, 3–25 kV; rated power, 0.25–500 kVA; tan d, 0.0005–0.001.

PART II PRINCIPAL CERAMIC TYPES AND

APPLICATIONS

5.5 Low-Permittivity Ceramic Dielectrics and

Insulators

Low-permittivity (e

515) dielectrics are widely used for straightforward insula-

tion. In this case their mechanical properties may be more important than their

dielectric properties, and, when required in tonnage quantities, they must also be

based on a low-cost material. When used as substrates for components their

dielectric properties become of greater importance. They ﬁnd some applications as

capacitor dielectrics where very small capacitances are required for use at higher

frequencies or, as can be the case with capacitors passing high current, where larger

size is an advantage because it leads to a higher rate of heat dissipation.

The siliceous ceramics are produced from naturally occurring minerals that

have been puriﬁed to only a limited extent, for instance by washing out soluble

impurities or removing iron-containing contaminants magnetically. The purer

oxides used in capacitors require more elaborate processing. The discussion

starts with insulating components based on natural minerals.

5.5.1 Electrical porcelains

Clay-based

Ceramic insulators used for the distribution of electrical power at high voltage

will be taken as an example of an insulating siliceous clay-based ceramic.

Electrical power is transmitted at high voltage because, for a given power

carried on a line, when the voltage is doubled the current is halved. As a result

the quantity of conductor material can be reduced to a quarter for the same

power loss (I

R) in the line (see Eq. (2.27)). Therefore, since the cost of conductor

materials is high compared with other costs, very signiﬁcant economies result

LOW-PERMITTIVITY CERAMIC DIELECTRICS AND INSULATORS 269

from an increase in voltage. Materials of high tensile strength such as steel-cored

aluminium can be used to replace copper, allowing relatively longer spans and

fewer towers. This cost-saving more than oﬀsets the cost of the extra conductor

volume required to compensate for the higher resistive materials. Many factors

inﬂuence the choice of operating voltage, but up to 1000 kV is used.

Transmission by d.c. is sometimes used, since with a.c. systems peak voltages

and currents have to be accommodated while only r.m.s. (see Eq. (2.94)) values

represent useful power transmission. However, the cost of rectiﬁcation and

conversion back to a.c. of the power at high voltage results in d.c. transmission

only being economic for very long (4500 km) lines.

As an example of relative costs, in a conventional 275 kV a.c. line, the towers

represent 20–40% of the cost, the conductors 15–35%, while insulators represent

only approximately 5–10%.

The electrical porcelain used for such insulators is chosen because of its

excellent durability in outdoor conditions, its low cost, its adequate mechanical

strength (insulators rated to safely withstand tensile loading up to 90 tonnes are

produced) and the ability to produce large monolithic units (the largest produced

to date is 12 m long and nearly 2 m in diameter). The dielectric strength (see

Section 5.2) is less critical, since the insulators are normally used in air, which

limits the electrical stress which can be applied.

The raw materials used in the manufacture of electrical porcelains are clays,

ﬂuxes and ﬁllers. Clays are aluminosilicates in the form of small (about 1 mm)

platey particles. Kaolinite, with the composition Al

(OH)

{

, is the usual clay

mineral. Fluxes are commonly feldspars, a group of aluminosilicate minerals

containing potassium, sodium and calcium. The potassium form, orthoclase

(KAl Si

) is often preferred because of the better electrical and mechanical

properties it confers on the porcelain. Fluxes melt at relatively low temperatures

to form a glass, which bonds the structure of the ﬁred material, and on cooling

crystallizes to give a web of ﬁbrous mullite crystals. Common ﬁllers are ﬁnely

milled silica (SiO

) as quartz or cristobalite, or alumina (either as calcined

bauxite or reﬁned alumina). Where higher mechanical strength is required silica

is replaced by alumina thus limiting the strength-reducing internal micro-

cracking resulting from the silica crystal inversions which occur on cooling (Fig.

5.20(c)). The penalties attached to ‘aluminous porcelains’ are increased cost and

relatively poor manufacturing characteristics. Typical compositions lie in the

ranges: clays 40–60 wt%: ﬂuxes 15–25 wt% and ﬁllers 25–40 wt%. Control of the

ﬁneness of the ﬂuxes and ﬁllers is critical.

The raw materials are blended with water to form a slip from which most of

the water is removed by ﬁlter-pressing. The ‘body’ is then homogenized and de-

aired in a vacuum extrusion ‘pug-mill’. The extruded blanks may be further

270 DIELECTRICS AND INSULATORS

{

It is common practice to write the chemical formula of kaolinite as Al

2SiO

O and important to

understand that this is a convention (applied to many minerals of interest to the ceramist) and does not

mean that kaolinite is a mixture of alumina, silca and water!

formed by pressing or by ‘jolleying’, a process similar to shaping on a potter’s

wheel (see Section 3.6.3). More usually the blanks are dried to a rigid, but still

wet state (16 wt% moisture) or to a nearly dry (8 wt% moisture) state, and

then machined to shape by turning, milling, etc. Some manufacturers spray-dry

the slip and isostatically press blanks from the powder prior to ﬁnal machining.

After the forming stage the insulator is carefully dried and then coated with a

glaze consisting of a water slurry of milled quartz, feldspar and calcium

carbonate, plus zircon as an opaciﬁer and, if required, an oxide-based pigment

(commonly Mn, Ni, Cr, etc.) to give the desired colour. The glaze formulation is

chosen to give a lower thermal expansion than the body to ensure that the

surface is in compression after ﬁring. At this stage a grit of similar composition

to the body but of lower thermal expansion may be applied to areas where metal

parts are to be cemented. This is to eﬀect good adhesion of the cement in the ﬁnal

assembly stage. Following this the insulator is ﬁred to a temperature between

1180 and 1350 8C on a schedule which typically extends from 1 day to 2 weeks,

depending on the size of ware and type of body. Firing in oxygen-depleted

atmospheres (reduction ﬁring) is occasionally used to achieve vitriﬁcation at

lower temperatures, but this necessitates raw materials having a low iron content.

In general terms the microstructure (Fig. 5.20(c)) consists of the ‘ﬁller’

particles embedded in a glass and crystalline matrix (mullite: Al

) derived

from the clay and ﬂux. There must be no open porosity and total porosity must

be kept to a minimum, typically 4 vol.%, to avoid adverse eﬀects due to

ionization of the gas in the pores (see Section 5.2.2).

Most complete insulators comprise metal components cemented to the

porcelain insulator to join it to the line structure, or to build up a string of

insulators. Various cements are used depending on application, although

conventional Portland cement is most common. Galvanized malleable cast

iron is preferred for the metal since its thermal expansion is closer to that of

porcelain than most metals and it is comparatively inexpensive. None the less,

the cost of the metal components when used is often 40–80% of the cost of the

insulator. The form of the widely used rod-insulator is shown in Fig. 5.17.

Typical physical properties of the two main types of high voltage electrical

porcelain are listed in Table 5.2.

Electrical discharges may occur due to the very high electric ﬁelds which exist

in the vicinity of a high voltage insulator. For example corona discharge may

occur around the insulator–conductor interface, especially in the case of

insulators which carry the conductor directly, without intervening metal ﬁttings.

This creates interference (RI) to radio and television reception, which is

unacceptable in populous areas. Another form of discharge may occur when the

insulator surface is wetted under conditions causing the wet layer to become

electrically conductive such as when contaminated by sea-spray or polluted

atmospheres. The wet layer dries unevenly, forming dry bands of relatively high

resistance. Under some conditions the voltage appearing across a band can

LOW-PERMITTIVITY CERAMIC DIELECTRICS AND INSULATORS 271