Mass transport via grain boundary in Pr-based ZnO varistors and related electrical effects

Mass transport and the grain boundary formation behaviors in Pr, Co-doped ZnO ceramics are strongly affected by firing atmosphere and temperature. Drastic change of grain boundary morphology and mass transport behavior were observed in the specimens sintered between 1350 and 1500°C which is ascribed to the increase of the vaporization contents of zinc oxide and praseodymium oxide with the formation of liquid phase. The penetration of liquid (ZnO–PrO_x) into the grain boundaries of sintered, Co-doped ZnO pellets resulted in varistors with breakdown voltages per grain boundary in the 1–3 V range and nonlinearity coefficients of 15–58.