Degradation mechanism of non-Ohmic zinc oxide ceramics

Abstract

Microstructure and phase transformation in a highly non−Ohmic metal oxide varistor ceramicThe degradation phenomena caused by dc and ac biasing in non-Ohmic ZnO ceramics are studied from the viewpoints of voltage (V)-current (I) characteristics, dielectric properties, and thermally stimulated current (TSC). As a result, it is concluded that the degradation caused by dc biasing is attributed to the asymmetrical deformation of Schottky barriers, due to ion migrations in Bi z 0 3rich intergranular layers and in the depletion layers of the Schottky barriers; and that the degradation caused by ac biasing is attributed to the symmetrical deformation of the Schottky barriers, due to ion migration in the depletion layers of the Schottky barriers. Also, the relationship between the thermal runaway life of non-Ohmic ZnO ceramics and biasing conditions, such as biasing temperature and bias voltage, is obtained.

Electrical degradation of double-Schottky barrier in ZnO varistors

“…During the degradation process, the I-V characteristics of ZnO varistor may drift nonlinearly. Under the applied DC voltage, the I-V curve of ZnO varistor deforms asymmetrically, 5,16 as shown in FIG. 1 Conversely when AC voltage is applied for a period of time, the I-V characteristic curve of ZnO varistor deforms symmetrically.…”

“…1 Conversely when AC voltage is applied for a period of time, the I-V characteristic curve of ZnO varistor deforms symmetrically. 16 Moreover, no matter DC or AC voltage is applied, the I-V characteristics in pre-breakdown region under low electric field drift more severely than the case involving breakdown region under medium electric field.…”

“…16 The change in the value of dielectric constant directly leads to the change of capacitance of varistor 27,28 indicating the complexity of ageing in ZnO varistor under the effect of long-term operation voltage.…”

“…Summarized Degradation Mechanism Eda et al [16][17][18] ion migration process Takahashi et al 19 chemical desorption of oxygen ions at the grain boundaries Chiang et al 20 asymmetrical distribution of impurity ions in grain boundary region Sato et al 21,22 electrons captured by traps inside depletion layer and thus accumulated near grain boundary Gupta et al 15 migration process of zinc interstitial (the seminal grain-boundary defect model) Sonder et al 23 high mobility of oxygen ion at high temperature, and flaws like micro-cracks in varistor act as conduits for oxygen Bui et al 24 partial discharges causing the formation of HNO 3 that degrades the Schottky barrier Ramírez et al 25 the lost of oxygen species and β-Bi 2 O 3 phase during degradation process leading to abnormal degradation phenomena and determining its degradation rate, 26 whereas knowledge from atomic scale is quite limited on the role of element that is individually doped and, worse, on the co-doping effect.…”

Condition Monitoring of Zinc Oxide Surge Arresters

“…Figure 4 shows the V-I characteristics of ZnO element which are divided into three regions, being low current region (I), operating region (II) and high current region (III). (Eda, 1980) The current density-electric field J-E, relationship in region I is expressed from the physical point of view as follows:…”