The physics of zinc oxide varistors

Abstract

This paper presents a physical description of the action of ZnO varistors, which are complex ceramic bodies of ZnO grains sintered in an oxide flux; their conductivity is very low at low voltage, but becomes high after a certain breakdown voltage is reached. It is found that depletion layers in the ZnO adjacent to the intergranular layers of oxide flux are the principal barrier to conduction at low voltage. These depletion layers are formed because electron traps in the intergranular layer absorb electrons from the ZnO, and the oxide flux itself is found to be more conductive than the neighboring barriers; conduction within the flux is observed at low temperatures. Electrical breakdown is associated with the completion of trap filling in thin regions of the intergranular layer. This model is supported by a wide range of physical phenomena.

Electrical switching of some zinc oxide based ceramics

“…Contrary to the observations in this study and for the varistor materials just cited, it has also been observed by other investigators that the resistivity of single crystal ZnO decreases upon being doped with Co [42]; however, the experimental procedure and the Co doping concentration were not specified.…”

Dielectric properties of microwave flash combustion derived and spark plasma sintered CaCu3Ti4O12 ceramic: role of reduction in grain boundary activation energy

“…The decrease in the activation energy with an increase in dc-biasing is associated with the reduction in the potential energy height of Schottky barrier. The reduction in potential energy of Schottky barriers leads to the increase in width of depletion zone, which results in the decrease in the capacitance of grain boundary [28,[30][31][32][33][34][35][36][37]. This effect is seen in Fig.…”

Electronic Structure of Grain Boundaries and Interfaces in Polycrystalline Zinc Oxide

“…This elec-tron transfer, followed by considerable orbital rehybridization within the valence band at the O,:ZnO interface, results in an empty conduction band and partially occupied 0 r*-like states coinciding with the top of the valence band. 4 The latter states are represented by the 9e ,orbital of the model o ZnO -cluster in Fig. l(b) having the orbital topology shown in Fig.…”

“…Recent high resolution transmission electron microscopy [2] shows, however, this not to be the case. Although a considerable number of alternative models have been proposed in recent years [3,4] they all propose in common the existence of a charged grain boundary interface bounded by space charge layers penetrating into the adjoining grains. All models either implicitly or explicitly rely on the existence of low-lying interface states or traps at the intergranular layers to proyide for the formation of the potential barrier at the grain/grain-boundary interface.…”