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Introduction of Varistor Ceramics

Zinc oxide () varistor, which is a kind of polycrystalline semiconductor ceramic composed of multiple metal oxides and sintered using conventional ceramic technology, is a voltage-dependent switching device, which exhibits highly nonohmic current-voltage characteristics above the breakdown voltage. Basic information on ZnO varistors, including the fabrication, microstructure, and typical electrical parameters, is introduced. The history and applications of ZnO varistors are also presented. The panorama of alternative varistor ceramics for Bi2O3-based ZnO varistors is mapped out. Especially, the ceramic-polymer composite varistors with lower breakdown voltage, incorporating varistor particles such as semiconducting particles, a combination of metal and semiconducting particles, and ZnO microvaristors, in a polymeric matrix are reported. Now, varistors are available that can protect circuits over a very wide range of voltages, from a few volts for low voltage varistors in semiconductor circuits to 1000?kV AC and ±1100?kV DC in electrical power transmission and distribution networks. Correspondingly, they also can handle an enormous range of energies from a few joules to many megajoules.

1.1 ZnO Varistors

A varistor is an electronic component with a "diode-like" nonlinear current-voltage characteristic, which is a portmanteau of variable resistor [1]. Functionally, varistors are equivalent to a back-to-back Zener diode and are typically used in parallel with circuits to protect them against excessive transient voltages in such a way that, when triggered, they will shunt the current created by the high voltage away from sensitive components.

The most common type of varistor is the metal oxide varistor (), which contains a ceramic mass of ZnO grains, in a matrix of other metal oxides, such as small amounts of bismuth, cobalt, and manganese, sandwiched between two metal electrodes. The boundary between each grain and its neighbor controls the current according to the applied voltage, and allows current to flow in two directions. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. A varistor's function is to conduct significantly increased current when voltage is excessive. Only nonohmic variable resistors are usually called varistors [1].

In normal use, a varistor is subject to a voltage below its characteristic breakdown voltage and passes only a tiny leakage current. When the voltage exceeds the breakdown voltage, the varistor becomes highly conducting and draws a large current through it, usually to ground. When the voltage returns to normal, the varistor returns to its high-resistance state [2]. The result of this behavior is a highly nonlinear current-voltage characteristic [3-5], in which the MOV has a high resistance at low voltages and a low resistance at high voltages; usually, the resistivity of a ZnO varistor is more than 1010 O cm below the breakdown voltage, whereas it is less than several ohm-centimeters above the breakdown voltage [6]. The switch is reversible with little or no hysteresis although it can degrade under electrical loading [2]. A varistor remains nonconductive as a shunt-mode device during normal operation when the voltage across it remains well below its "clamping voltage"; thus varistors are typically used for suppressing line voltage surges. However, a varistor may not be able to successfully limit a very large surge from an event such as a lightning strike where the energy involved is many orders of magnitude greater than it can handle. Follow-through current resulting from a strike may generate excessive current that completely destroys the varistor [1].

ZnO varistors are voltage-dependent switching devices, which exhibit highly nonohmic current-voltage (I-V) characteristics above the breakdown voltage. The nonohmic I-V characteristics are usually expressed logarithmically, as shown in Figure 1.1 [6]. The degree of nonohmic property is usually expressed by a nonlinear coefficient a defined by the following equation:

Figure 1.1 I-V characteristics of a typical ZnO varistor.

Source: Adapted from Eda [6].

Empirically, the following simple equation is used:

where C is a constant. Typical a values of ZnO varistors are from 30 to 100; therefore, the current varies by orders of magnitude with only small changes in voltage. A more accurate measure of the nonlinearity is the dynamic conductance, the differential of the characteristic, at each voltage [2]. On the contrary, a values of conventional varistors such as SiC varistors do not exceed 10.

The I-V characteristics of ZnO varistors are classified into three regions, as shown in Figure 1.2 [6]. In region I (low electrical field region, or pre-breakdown region), below the breakdown voltage (typically a voltage at 1 µA?cm-2), the nonohmic property is not so prominent and can be regarded as ohmic, and the leakage current is highly temperature dependent. In region II (medium electrical field region, nonlinear region, or breakdown region), between the breakdown voltage and a voltage at a current of about 100?A?cm-2, the nonohmic property is very prominent and almost independent of temperature. In region III (high electrical field region or upturn region), above 100?A?cm-2, the nonohmic property gradually decays, and the varistor is again ohmic. These three regions in engineering applications are also called as the low current region, medium current region, and high current region, respectively.

Figure 1.2 The wurtzite structure of ZnO.

Source: Adapted from Addison [7].

ZnO varistors are characterized by the magnitude of the a values and the width of the range where highly nonohmic property is exhibited. In contrast to the pre-breakdown region, the nonlinear region and upturn region are little affected by temperature. The I-V characteristics below 100?mA?cm-2 are usually measured using a DC electric source, whereas those above 1?A?cm-2 are measured by an impulse current source to avoid heat generation and thermal breakdown. The waveform of the impulse current is 8/20 µs with an 8 µs rise time and 20 µs decay time up to one-half the peak value [6], which is used as a standard impulse current to test ZnO varistors. The I-V characteristics measured by the impulse currents show voltages higher than those measured using DC. The discrepancy is usually 10-20%, as shown by arrows in Figure 1.1 [6]. This discrepancy is caused by the delay in electrical response in the ZnO varistors. The response delay is speculated to be caused by electron trapping and hole creation at the grain boundaries.

1.2 Fabrication of ZnO Varistors

ZnO has a wurtzite structure in which the oxygen atoms are arranged in a hexagonal close-packed type of lattice with zinc atoms occupying half the tetrahedral sites, as shown in Figure 1.2; Zn and O atoms are tetrahedrally coordinated to each other and are equivalent in position. The ZnO structure is thus relatively open with all the octahedral and half the tetrahedral sites empty. It is, therefore, relatively easy to incorporate external dopants into the ZnO lattice. The open structure also has a bearing on the nature of defects and the mechanism of diffusion, and the most common defect in ZnO is the metal in the open interstitial sites [8]. Pure ZnO without any impurities or dopants is a nonstoichiometric n-type semiconductor with linear or ohmic behavior, and with a wide band gap (3.437?eV at 2?K) [8,9].

ZnO varistors are semiconducting ceramics fabricated by sintering of ZnO powders with small amounts of various metal oxide additives such as Bi2O3, CoO, MnO, and Sb2O3. The nonohmic property comes from grain boundaries between semiconducting ZnO grains. These oxides are added to control one or more of the properties such as the electrical characteristics (breakdown voltage, non-linearity, and leakage current), grain growth, or the sintering process [10-18]. Bi2O3 is the most essential component for forming the nonohmic behavior, and addition of CoO and MnO enhances the nonlinear properties [11,1219-22]. A very low concentration of Al2O3 increases the ZnO grain conductivity while Sb2O3 controls the ZnO grain growth [23-32]. Combination of additives such as Cr2O3, MnO, Bi2O3, and CoO produces greater nonlinearity than a single dopant [10]. During high temperature sintering different phases are formed and the microstructure of a ZnO varistor comprises conductive ZnO grains surrounded by electrically insulating grain boundary regions.

Dopants play at least three major roles in forming varistors by affecting grain growth during sintering, the dewetting characteristics of the liquid phase during cooling, and the electronic defect states that control the overall varistor...