Electric force microscopy investigations of barrier formations in ZnO-based varistors

The well known metal-oxide varistors (MOVs) are polycrystalline electronic ceramic materials whose electrical behavior is dominated by their grain boundaries. ZnO-based varistors are MOVs whose nonlinear properties are characterized by an electrical resistance that decreases as the applied voltage field increases. The objective of this work was to image the Schottky barriers in ZnO doped with 0.5 mol.% Cu and x wt.% G (G is a frit and x = 0, 1 and 5%). The frit is used to form a glassy insulating layer around grain boundaries. Samples were sintered at 850 °C and the microstructures were analyzed by atomic force microscopy (Nanoscope IIIa, VEECO Instruments). Electric force microscopy (EFM) experiments were conducted to map the electric field distribution on the surface of CuO–ZnO-based varistors. The formation of Schottky barriers was observed and their width measured.