Varistors

Grain boundary defects are primarily responsible for the operation of zinc oxide (ZnO) varistors, a shortened form of variable resistor. The varistor behaves like an insulator or poor semiconductor at lower electrical field strengths, but at a critical breakdown voltage the resistance decreases enormously and the material behaves like an electrical conductor (Fig. 3.36). When a varistor is connected in parallel with electrical equipment, negligible power flows through it under normal low 

The microstructure of commercial varistors is extremely complex, and commercial preparations also contain other dopants, mainly oxides of cobalt, manganese, chromium, and antimony, that are used to fine tune the varistor characteristics. The transition-metal dopants are chemically similar to Zn2+ and mainly form substitutional defects within the ZnO grains, such as CoZn, that modify the n-type behavior of the grain interior. (See also Chapter 8 for further discussion of the electronic 

A varistor is used in an electric circuit as protection against large voltage pulses, i.e., a surge protector. Such devices are particularly important in protecting microelectronic devices. ZnO is the most widely used and important material for varistors. 

Under normal operating conditions a small current will flow through the varistor. If the voltage rises above some threshold value (for example, the result of a voltage 

FIGURE 30.15 Schematic representation of energy levels in a deficit semiconductor such as Cu2-xO. 

The exponent a is used as a figure of merit for the varistor and can be as high as 70 for ZnO, although values in the range 25-45 are more typical. For SiC a - 5. 

FIGURE 30.18 Proposed band diagram for two semiconducting ZnO grains separated by an IGF (a) showing the location of acceptor sites in the IGF (b) at equilibrium. Application of a potential decreases the barrier height. 

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Fig. 3. An overview of atomistic mechanisms involved in electroceramic components and the corresponding uses (a) ferroelectric domains capacitors and piezoelectrics, PTC thermistors (b) electronic conduction NTC thermistor (c) insulators and substrates (d) surface conduction humidity sensors (e) ferrimagnetic domains ferrite hard and soft magnets, magnetic tape (f) metal—semiconductor transition critical temperature NTC thermistor (g) ionic conduction gas sensors and batteries and (h) grain boundary phenomena varistors, boundary layer capacitors, PTC thermistors.

Several kinds of conduction mechanisms are operative in ceramic thermistors, resistors, varistors, and chemical sensors. Negative temperature coefficient (NTC) thermistors make use of the semiconducting properties of heavily doped transition metal oxides such as n-ty e Ti O andp-ty e... 

Nickel fluoride is used in marking ink compositions (see Inks), for fluorescent lamps (4) as a catalyst in transhalogenation of fluoroolefins (5), in the manufacture of varistors (6), as a catalyst for hydrofluorination (7), in the synthesis of XeF (8), and in the preparation of high purity elemental fluorine for research (9) and for chemical lasers (qv) (10). 

Varistors. Varistors are devices that exhibit nonlinear current—voltage behavior. At low voltages, current flow is minimal and the device behaves as an ohmic insulator. As the voltage approaches a critical value, the breakdown field (Fgj ), current flow increases and the device becomes highly... 

For a large number of applications involving ceramic materials, electrical conduction behavior is dorninant. In certain oxides, borides (see Boron compounds), nitrides (qv), and carbides (qv), metallic or fast ionic conduction may occur, making these materials useful in thick-film pastes, in fuel cell apphcations (see Fuel cells), or as electrodes for use over a wide temperature range. Superconductivity is also found in special ceramic oxides, and these materials are undergoing intensive research. Other classes of ceramic materials may behave as semiconductors (qv). These materials are used in many specialized apphcations including resistance heating elements and in devices such as rectifiers, photocells, varistors, and thermistors. 

Fig. 12. Log—log plot of current density,/, versus appHed electric field, E, for a ZnO varistor at room temperature, ia which the breakdown field. Eg, is iadicated. The exponent d equals the iaverse slope of the curve, log Ej J) = 1/a, and is a measure of device nonlinearity. Units of current density and the...

MiscelEneous. Small quantities of cobalt compounds are used in the production of electronic devices such as thermistors, varistors, piezoelectrics (qv), and solar collectors. Cobalt salts are useful indicators for humidity. The blue anhydrous form becomes pink (hydrated) on exposure to high humidity. Cobalt pyridine thiocyanate is a useful temperature indicating salt. A conductive paste for painting on ceramics and glass is composed of cobalt oxide (62). 

A book edited by Levinson (1981) treated grain-boundary phenomena in electroceramics in depth, including the band theory required to explain the effects. It includes a splendid overview of such phenomena in general by W.D. Kingery, whom we have already met in Chapter I, as well as an overview of varistor developments by the originator, Matsuoka. The book marks a major shift in concern by the community of ceramic researchers, away from topics like porcelain (which is discussed in Chapter 9) Kingery played a major role in bringing this about. 

The deposition temperature range is 300-500°C, the partial pressure of the alkyl is 0.5-2.5 Torr, and that of THF is 20-80 Torr. ZnO has found applications in piezoelectric devices, transducers, coatings for photoconductive devices, and non-linear resistors (varistors), and overvoltage protectors. 

Figure 3.36 Variation of current versus electrical field for a typical zinc oxide varistor.

Figure 3.37 Microstructure of a zinc oxide varistor (schematic).

As in the case of varistors, the effect is due to the defect structure of the solid, especially the presence of grain boundaries. There are a considerable number of variables that must be controlled to make a suitable PCT thermistor. 

The highest purity material is calcined with additives such as Bi203 and used in the manufacture of varistors [2.80]. The photoconducting properties of ZnO are used in photoreproduction processes. Doping with alumina causes a reduction in electrical resistance hence, it can be used in the coatings on the master papers for offset reproduction [2.81]. 

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