Degradation of oxide varistor ceramics in air atmosphere containing NO2 at elevated temperatures

The heat treatment of commercial zinc oxide based varistor ceramics at temperatures of 200-300 °C in air with 1000 ppm of NO2 causes strong shift of voltage-current characteristic (VCC) to low voltages. Subsequent exposure to pure air leads to partial restoration of VCC. Scanning electron microscopy analysis showed microstructural changes of samples after heat treatment in air with 1000 ppm of NO2. Chemical reactions between NO2 molecules and oxygen chemisorbed or Bi-rich oxide phases at grain boundaries followed by the lowering of grain boundary potential barriers can be the reason of observed degradation.