In order to reveal the conduction mechanism of ZnO varistor, the pulsed electroacoustic (PEA) measurement is employed to investigate the space charge distribution characteristics of ZnO varistors under different current densities in this paper. Experimental results show that the relationship between the space charge and the current density is consistent with the voltage-current characteristics, and can reflect the nonlinear characteristics of ZnO varistor. The space charge increases linearly with the increase of the electric field strength and the current density in the small current region, this reflects the decrease of the surface states of the grain boundary and changes of the Schottky barrier. There exists a transition region between the small current and middle current regions, where the current is composed of the current by the emission of thermally activated electrons and the tunneling current. In the middle current region, the space charge will decrease as the current increases, and will disappear with the depletion layer decreasing to zero.
The AC aging characteristics and mechanism of ZnO varistors have been widely concerned for a long time.In this paper,accelerated aging test of ZnO varistors was carried out based on the "ten degrees and a half" rule,power loss and thermally stimulated current(TSC) of samples were tested during each aging stage.The test results show that with the increasing of aging time,the power loss increases,the trapped charges increase and the trap level becomes deeper.In addition,the increase of power loss and the increase of trapped charge with the aging time have the same trend.A preliminary analysis shows that the migration of interstitial zinc ions will change the space charge distribution and decrease the Schottky barrier height to cause the change of the trap level.
