Using percolation to design ZnO composites with hBN modified grain boundaries to obtain varistor-like behavior

Conventional varistors rely on the formation of a Double Schottky Barrier within the intergranular region of ZnO via acceptor doping and a Bi₂O₃ phase. This work has been able to yield varistor-like behavior via cold sintered ZnO composites by placing 2D hBN flakes on the grain boundaries within the ZnO matrix. Above the percolation threshold, a network of resistive hBN barriers is formed which prevents current from flowing through the more conductive ZnO. However, at a given voltage, electrons can tunnel through the hBN if the layers are kept thin enough. Within this narrow band of hBN content, samples have been fabricated with α values as high as 9.5. The composite system demonstrated Schottky conduction at low fields before switching to Fowler-Nordheim tunneling at high fields. This microstructural design was able to show greater nonlinearity compared to previous attempts at creating varistor materials through the unique cold sintering process (CSP).