Microstructure Engineering to Drastically Reduce the Leakage Currents of High Voltage ZnO Varistor Ceramics

The energetic losses associated with the leakage currents can represent a serious problem for the efficient performance of ZnO-based varistor ceramics. However, by properly tailoring the microstructure of these ceramics, the leakage currents can be drastically reduced. Pursuing this goal, two intended doping strategies have been attempted over a standard high-voltage varistor formulation based on the ZnOBi2O3Sb2O3 ternary system. The first strategy focuses on the stabilization of the secondary Zn2.33Sb0.67O4 spinel particles; on cooling the ceramic, this spinel phase reversely transforms into a pyrochlore conductive structure whose presence favors the current flow in the varistor prebreakdown region (i.e., high leakage current values). A certain amount of TiO2 incorporated into the spinel lattice impedes its reverse transformation to pyrochlore, thus reducing the varistor leakage current. The second strategy leans on the insertion of an insulating phase in between the skeleton of Bismuth-rich phases. This skeleton provides a continuous network through the whole varistor microstructure for the current to flow, so the inclusion of an insulating phase may serve as a physical blockade to break off that connectivity. Such a phase is yielded by properly doping with SiO2. Eventually, the combination of both strategies leads to a decrease of two orders of magnitude in the varistor leakage current.