Formation of varistor characteristics by the grain-boundary penetration of ZnO-PrOx liquid into ZnO ceramics

Penetration of a liquid (ZnO-PrOx) into the grain boundaries of sintered, cobalt-doped ZnO pellets resulted in varistors with breakdown voltages per grain boundary in the 1-2 V range and nonlinearity coefficients of 22-37. The varistors were fabricated by spreading a thin layer of Pr6O11 powder paste on the surface of ZnO pellets and heating to various temperatures (1200 degrees-1400 degrees C) and times (0-60 min). Comparing the varistor properties per grain boundary (e.g., threshold voltage, donor concentration, and barrier height) of liquid penetration to those of conventional method indicated the individual grain boundaries were electrically activated when the samples were heat-treated above liquid-phase formation temperature.