Dependence of the nonlinear behaviour stability of a ZnO nanoparticle-Bi O -Mn O -based varistor system on cooling rates during ceramic processing.

A conventional ceramic processing method was applied to manufacture high-density 20-nm ZnO-Bi O -Mn O varistor ceramics. Different cooling rates in the range of 135-540°C/h led to a relatively slight influence on the microstructure, varistor voltage, and leakage current. In contrast, these rates strongly affected the nonlinear exponent. The specific surface area of the 20-nm ZnO nanoparticle may have led to an intense solid-state reaction even at a low cooling rate. Superior nonlinearity, with 59.7 μA nonlinear current leakage and 273.5 μA leakage current, was achieved at the 135°C/h cooling rate. The differences in the cooling rates led to a remarkable change in the material's stability under direct current (DC)-accelerated aging stress in the following order: 135°C/hr ˃ 270°C/hr˃405°C/hr ˃ 540°C/hr.