Microstructure-property relationships of rare-earth-zinc-oxide varistors

The microstructure and properties of ZnO varistors containing Ba, Co, and rare-earth-metal oxides, which give values of α [α=d(log I)/d(log V)] as high as 29, are examined. Mean ZnO grain size is 11 μm, and the grains are uniformly doped with Co. The barium and rare earth metals concentrate into 1.5-μm-wide particles embedded in a matrix of ZnO grains. Within the grains and at grain boundaries, the barium and rare-earth-metal concentration is below the detection limit of the energy-dispersive spectrometer technique (about 0.5%). No intergranular films, amorphous or crystalline, are detected, to within 10 Å resolution. These results are shown to be consistent with the grain boundary charge depletion model for the voltage barrier formation and breakdown.