Grain growth analysis of ZnO in ZnO-Bi2O3-TiO2 ceramic

The objective of this study is to clarify the grain growth behaviors of zinc-oxide varistor ceramics with Bi2O3 and TiO2 additions. Zinc-oxide based ceramics have been widely used as transient surge suppressors for protecting electric equipments, because of their highly nonlinear voltage (V)-current (I) characteristics and excellent surge absorption capability. The nonohmic properties of these ceramics are highly affected by characteristics of the microstructures, such as ZnO grain size. A ZnO-6 mass% Bi2O3-0.55 mass% TiO2 ceramic has been selected for investigating the ZnO grain growth behavior. The experimental results show that the samples consisted of equiaxed ZnO grains and a second phases with the Bi2O3 and spinel crystal structure. It also indicated that the grain growth behavior could be observed to follow a time law for grain size omega of the form omega =k . t(n), where t is time and n is a kinetic exponent. The kinetic exponent for ZnO grain growth has been determined to be 1/2 for temperatures below 1303 K, but 1/5 above 1379 K. A convenient computer-aided system was developed for analyzing the grain growth behavior of sintered materials. It simulated the effects of heating rate and cooling rate on the ZnO grain growth in the ZnO-6 mass% Bi2O3-0.55 mass% TiO2 ceramic during the sintering process. As a result, it was clarified that the effect of heating rate on the ZnO grain growth was not neglected in comparison with the cooling rate in the range of practical use.