5.8.2 Frequency response of ZnO material
Early measurements [39, 52, 114] on the frequency response of ZnO material ceramics were achieved using a General Radio capacitance bridge, and because of the very small size of the tested samples, the applied voltage was merely 1 V r.m.s. In the frequency range 10–105 Hz, the calculated relative permittivity from capacitance measurements is unexpectedly high (1000–1600) considering that the relative permittivity for ZnO is in the range εr = 8 to 10, and for the additives is in the range
εr = 16 to 25. This can be partially accounted for by the microstructure of ZnO material. As used in surge arresters, the material consists of ZnO grains (approximately25 μm) of high conductivity separated by a very thin layer (