Grain growth of zinc oxide in the ZnO-Bi2O3-Sb2O3 ceramic system

Abstract

ZnO-based varistors are known for their non-ohmic properties characterized by an electrical resistance which decreases as the applied field increases, and are widely used as a protective device against power and voltage surges. These varistors protect by providing a path across the power supply that takes only a small current under normal conditions but takes large currents if the voltage rises abnormally. ZnO varistors are manufactured by sintering ZnO microcrystals with several kinds of insulating oxides. Matsuoka reported a varistor composition of 97 mol% ZnO, 1 mol% Sb₂O₃, and 0.5 mol% each of Bi₂O₃, CoO, MnO and Cr₂O₃. The microstructure of non-ohmic ZnO ceramic consists of ZnO phase dissolving Co and Mn, spinel phase of Zn₇Sb₂O₁₂, dissolving Co, Mn and Cr, and mixed phases of Bi and d-Bi₂O₃ For a ZnO-Bi₂O₃-Sb₂O₃ system, formation of pyrochlore phase occurs below 700°C. The pyrochlore phase moderates grain growth and keeps it uniform. At about 1000°C the pyrochlore phase reacts with the ZnO matrix forming the spinel phase, Bi₂O₃ being libcrated as a liquid phase at this temperature. Upon cooling the reactions are reversed depending upon the cooling rate.
It has been established that the voltage breakdown per unit thickness in a varistor is directly related to the ZnO grain size. For high voltage applications grain size is kept small, which is achieved by Sb₂O₃ additions, whereas for low voltage applications, coarse-grained microstructure is preferred, with small additions of TiO₂. Therefore, kinetic studies of the grain growth of various ZnO systems are of great importance. This paper aims to determine the kinetic parameters such as the grain growth exponent n, the activation energy Q, and the constant of equality K₀, in the grain growth equation: Gⁿ − G₀ⁿ = K₀ t exp(−Q/RT), where G is the average grain size at time t, G₀ is the initial grain size, R is the universal gas constant and T is the absolute temperature.