High-field ZnO-based varistors

The role of principal additives is investigated in an attempt to obtain high-threshold-field ZnO-based varistors in the traditional way. In the order of effect level, these are Sb 2 O 3 , Co 3 O 4 , Cr 2 O 3 and MnO 2 . In the range of 0.5-5 mol%, the increase of additive content decreases the average ZnO grain size. As a result, the number of grains per unit thickness increases, which gives rise to the threshold field. The value of barrier voltage v b calculated from experimental data is about 1.4-2 V. The potential barrier height determined from the temperature dependence of the current-voltage characteristics is 0.3-0.5 eV. The abnormally large v b of a sample doped with 8 mol% Sb 2 O 3 is quantitatively interpreted by the influence of the spinel phase through the energy-band-bending effect caused by the Bi 2 O 3 -rich phase and the hypothesis of a 'preferential path' of the current.