ZnO Varistors

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...

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 VDR behaviour in ZnO varistors is governed by electron states that are formed on the surfaces of crystals as a consequence of the discontinuity. These surface states act as acceptors for electrons from the n-type semiconductor. Electrons will be withdrawn from the region near the surface and replaced by a positive space charge. In this way oppositely oriented Schottky barriers will be created at the surfaces of neighbouring crystals so that a high resistance will be offered to electron flow in either direction (Fig. 4.10(a)). The situation with an applied field is shown in Fig. 4.10(b). With low applied fields small thermally... 

The a value for a ZnO varistor is 15 and ki = 9.3 x 10 31i, the potential dilference and current being measured in volts and amps respectively. Calculate the p.d. across the varistor for currents of 0.1, 1, 10, 100 and 1000 A. If a device needs protection against transient voltages in excess of 300 V, show how a varistor having the characteristic defined above can be connected to afford protection. [Answers 252, 294, 343, 400, 466 V]... 

The microstructure of a ZnO varistor is the key to its operation. Grains of about 15-20 pm in diameter are separated by a Bi-rich intergranular film (IGF) that varies in thickness from 1 nm to 1 pm, as illustrated in Figure 14.38. Varistor action is a result of a depletion region formed on either side of the IGF. To explain varistor behavior we use an approach very similar to that used to describe Schottky barriers in metal-semiconductor junctions. 

ZnO varistors are used for both low- and high-voltage applications. 

What do the stabilization of turquoise, the treatment of emerald, and ZnO varistors have in common ... 

Figure 14.2(a) Data from Chiang, Y-M., Kingery, W.D., and Levinson, L.M. (1982) Compositional changes adjacent to grain boundaries during electrical degradation of a ZnO varistor, J. Appl. Phys. 53, 1765. 

Both, spinel and pyrochlore accommodate excess dopants, the concentrations of which exceed their solubility limits in ZnO, and therefore they concentrate at grain boundaries [113]. The microstructure of a ZnO varistor will then comprise ZnO grains, a bismuth-rich phase, and spinel grains, which can be located either inter- or intragranularly (Figure 1.5). 

Figure 1.5 A backscattered-electron microscopy image of a polished ZnO varistor ceramics sintered at 1170 °C. The dark gray phase is ZnO, the light-gray is spinel, and the white is the bismuth-rich phase [131].

Levinson, L.W. (2004) ZnO varistor technology. Ceramic Materialsfor Electronics, 3rd edn (eds R.C. Buchanan), Marcel Dekker, New York. pp. 431-464. 

The essential principle of an active matrix display is that each pixel has associated with it a semiconductor device that is used to control the operation of that pixel. It is this rectangular array of semiconductor devices (the active matrix) that is addressed by the drive circuitry. The devices, which are fabricated by thin-film techniques on the inner surface of a substrate (usually glass) forming one wall of the LCD cell, may be either two-terminal devices (Fig. 6) or three terminal devices (Fig. 7). Various two-terminal devices have been proposed ZnO varistors, MIM devices, and several structures involving one or more a-Si diodes. Much of the research effort, however, has concentrated on the three-terminal devices, namely thin-film, insulated-gate, field-effect transistors. The subject of thin-film transistors (TFTs) is considered elsewhere in this volume suffice it to say that of the various materials that have been suggested for the semiconductor, only a-Si and poly-Si appear to have serious prospects of commercial exploitation. 

Two types of ceramic varistors are manufactured. Zinc oxide based ceramic varistors were developed in 1970. They exhibit a high non-linearity on voltage-current characteristics. Their a value is in the range of 40-50, and the adjustable to values in the range from 50 to 250 V/mm. Strontium titanate based varistors were developed in 1980. The feature of these varistors is their larger electrostatic capacitance compared with ZnO varistors. The SrTi03 ceramics are... 

FIGURE 2.1.11 Typical application of ZnO varistor as a transient protective device. 

Figure 92. ZnO varistor ceramic. Etched and unetched specimens.

Figure 10.20 Block model of a ZnO varistor having grain size d ( 20 jm) and Intergranular depletion barrier thickness t (-1000 A). D is the electrode separation. ...

L. M. Levinson and H. R. Philipp. Application and Characterization of ZnO Varistors. In Ceramic Materials for Electronics—Processing, Properties, and Applications. (R. C. Buchanan, Ed.) Marcel Dekker, New York, 1986, pp.375-402. 

Alamdari, H., Boily, S., Blouin, M., Van Neste, A., and Schulz, R. (2000) High energy ball milled nanocrystalline ZnO varistors. Mater. Sci. Forum, 343,... 

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