The microstructure and conduction mechanism of the nonlinear ZnO varistor with Al2O3 additions

The microstructure and electrical properties of nonlinear ZnO varistors with Al₂O₃ additions are investigated. The variation of nonlinear behavior with Al₂O₃ additions is indicated from J-E and C-V measurement to be a result of the change of the interface defects density N_t at the grain boundaries and the donor concentration N_d in the ZnO grains. The optimum composition which has a nonlinear coefficient α of ~57 was observed in the sample with 0.005 wt.% Al₂O₃ additions. The conduction mechanism in the pre-breakdown region is consistent with a Schottky thermal emission process obeying a relation given by J∝exp[-(φ-βE^1/2)/kT] and the conduction process at the breakdown region follows Fowler-Nordheim tunneling