Effects of low-temperature annealing on the microstructure and electrical properties of doped-ZnO varistors

We report in the present paper that fully dense doped-ZnO varistors with submicronic grain size can be fabricated by a strategic two-stage pressureless thermal processing method, at temperatures as low as 825 degreesC in air. Densification in a two-stage sintering is believed to be based on a rapid rearrangement of the solid ZnO nanoparticles in the presence of a liquid phase at the first stage up to 900 degreesC. In the second stage, on cooling at 825 degreesC, the liquid phase recedes improving, thus, ZnO-ZnO direct contacts and suppressing the grain boundary migration while keeping grain boundary diffusion very active to densify. Full density by a close-packed arrangement of the grains was obtained at the second stage for 10h. The current-voltage characteristics of such fully dense varistors were as high as 270 for die non-linear coefficient alpha, and the breakdown field V-b (at 1 mA/cm(2)) of about 20 kV/cm.