Optical method for determining the grain resistivity in ZnO‐based ceramic varistors

An optical technique for evaluating the grain resistivity in ZnO ceramic varistors is described. It is based on the existence of a rather sharp minimum in the infrared reflectance whose wavelength position depends on the free‐electron density and hence on the ZnO grain resistivity. Values of resistivity obtained from optical measurements are in good agreement with those obtained using alternative evaluation techniques.