Degradation Analysis of the SnO₂ and ZnO‐Based Varistors Using Electrostatic Force Microscopy

Abstract

The degradation phenomena of ZnO and SnO 2 -based varistors were investigated for two different degradation methods: DC voltage at increased temperature and degradation with 8/20 ls pulsed currents (lightning type). Electrostatic force microscopy (EFM) was used to analyze the surface charge accumulated at grain-boundary regions before and after degradation. Before the degradation process, 85% of the barriers are active in the SnO 2 system, while the ZnO system presents only 30% effective barriers. Both systems showed changes in the electrical behavior when degraded with pulses. In the case of the ZnO system, the behavior after pulse degradation was essentially ohmic due to the destruction of barriers (about 99% of the interfaces are conductive). After the degradation with 8/20 ls pulsed currents, the SnO 2 system still presents nonohmic behavior with a significant decrease in the quantity of effective barriers (from 85% to 5%). However, when the degradation is accomplished with continuous current, the SnO 2 system exhibits minimum variation, while the ZnO system degrades from 30% to 5%. This result indicates the existence of metastable defects of low concentration and/or low diffusion in the SnO 2 system. High energy is necessary to degrade the barriers due to defect annihilation in the SnO 2 system.

Electrical degradation of double-Schottky barrier in ZnO varistors

“…Recently, Ramírez et al 12 use the electrostatic force microscopy (EFM) to analyze the surface charge accumulated at grain boundary regions before and after degradation. The topographic information was stored with a selectable height offset in EFM mode, during which the electric field data were collected simultaneously.…”

“…4 shows the EFM images with voltage steps from 0 and 2 V applied to the tip. 12 The bright grain regions are more resistive compared with dark grain and the electrical response of some grain boundaries are predominantly insulating. From FIG.…”

“…After electrical degradation with DC voltage at 110 • C (thermal runaway temperature), a significant decrease in the number of effective barriers to 5% is observed, there is also a decrease in the barrier height. In addition to influencing the grain boundaries by eliminating most of the effective barriers and reducing the barrier efficiency, the degradation process also affects the electrical response of the grains, 12 i.e. after degradation grains are dark colored indicating that they are qualitatively more conductive when compared with the original sample.…”

Recycling zinc oxide varistor blocks for electro‐active silicone composites

“…[1][2][3][4][5][6][7] Aging or catastrophic failure of equipment usually come from degradation of electrical insulation, especially in the vicinity of air-conductor-insulator interfaces where the equipotential lines concentrate. (Only the promising SnO 2 varistor system 14,15 exhibits comparable values.) [8][9][10][11][12][13] Several methods are used to overcome these problems.…”

Low-temperature sintering and electrical properties of Co-doped ZnO varistors

“…In this model, the potential barrier depends on the nature and concentration of the intrinsic and extrinsic defects localized at grain boundary [8]. On the other hand, a great deal of research concerning basically conduction and degradation phenomena in ZnObased varistors suggested that, the degradation process usually leads to the reduction in the potential barrier height and causes change in defect concentration near grain boundaries, which is based on the ion migration mechanism [9,10]. Moreover, they proposed that, the electrical properties of degraded ZnO varistors could be recovered after thermal treatment with oxygen flows, supporting the model of Schottky barrier formation by adsorption of oxygen species at grain boundary [11].…”