EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Zinc oxide is a widely applied ceramic for electronics due to its semiconductor properties. An example is the ZnO varistor which possesses a non-ohmic conductivity over several orders of magnitude. ZnO varistors are usually made with Sb2O3 and Bi2O3 as sintering aids. In addition, oxides of Cr, Mn and Co, for example, are included as dopants, and a complex microstructure with several secondary phases is the result. This finally controls the properties, e. g. the non-ohmic behavior of the varistor. Although it is understood which dopants are necessary to achieve the desired conductivity characteristics, little is known about the mechanisms being involved in the doping process. This includes the (solid state) chemical behavior of the dopant species as a function of parameters like dopant concentration or sintering regime.
This contribution deals with principal investigations of doping effects in the pure secondary phases commonly found in ZnO ceramics made with Sb2O3 and Bi2O3 as sintering aids. Namely, these are a Zn7Sb2O12 spinel and a Zn2Bi3Sb3O14 pyrochlore phase. Doping of these compounds with trivalent Al, Sc, Cr, Mn, Fe and Ga ions is presented with a focus on the structural evolution and the thermodynamic stability of these phases, on the one hand, and the effect on the (electronic) properties as a function of the dopant concentration and the processing parameters, on the other. Finally, an outlook will be given about the effect of the M3+ doped spinel and pyrochlore secondary phases on the properties of the corresponding ZnO ceramics.