The effects of precipitant on the defect structures and properties of ZnO varistor ceramics

The effects of different precipitants (NH₄HCO₃ and NaOH) on the performances of ZnO varistor ceramics which were prepared by coprecipitation method were analyzed. There are noticeable influences for different precipitants to the microstructure and electrical performances of ZnO varistor ceramics. The change of microstructure is caused by the properties of precipitants themselves while the change of electrical performance is caused by the influence of precipitants to the grain boundary potential barrier properties. Besides, the donor impurity ion Na⁺ introduced by NaOH in the ZnO varistor ceramic will increase the free electron concentration in the grain. So the densities of intrinsic defects (zinc interstitial and oxygen vacancy) are restrained. The density of zinc interstitial is more sensitive to donor impurity ions compared with oxygen vacancy. Therefore the choice of precipitant is important when using coprecipitation method to prepare the powder of ZnO varistor ceramics. Even little impurity ions will result in the obvious change of the performance of varistor ceramics. The impurity ions should be avoided in the powder of ZnO varistor ceramics.