Analysis of ZnO varistors prepared from nanosize ZnO precursors

Varistors in the system ZnO–Bi2O3–Sb2O3–Co2O3–MnO2–Al2O3 were fabricated from doped ZnO nanopowders prepared by a previously developed sol-gel process. The nanopowders were compacted under two different conditions and then conventionally sintered at 950°C for 2 h. Varistor samples with densities > 0.98 theoretical and grain sizes < 2 μm were obtained by high-pressure cold-isostatic pressing followed by sintering. These varistors displayed superior electrical properties, i.e., higher value of the nonlinear coefficient (α > 45), lower leakage current (IL < 1 μA), and higher critical electric field value (V1mA/mm > 1000 V), compared to the samples pressed uniaxially under 50 MPa followed by sintering. The advantages of the samples pressed isostatically were due to their greater structural homogenity, higher density, and smaller grain size.