Nonlinear electrical behaviour of the WO3-based system

Abstract

Varistors can limit high transient voltage surges and can repeatedly endure such surges without being destroyed, therefore they are usually used to protect electronic circuits from voltage pulse shocks. The most important property of a varistor is its nonlinear voltage–current characteristic. This can be expressed by the equation I = K V α , where α is the coefficient of non-linearity, the essential parameter to scale the nonlinearity. Commercial varistors used in protection systems are based on SiC or ZnO. Varistors based on SiC have low coefficients (of non-linearity) [1]. ZnO varistors exhibit high coefficients (of nonlinearity), but the degradation problem of ZnO varistors has not been resolved [2, 3]. While efforts to improve the temperature stability of ZnO varistors are being made, the search for new varistor materials is ongoing. In 1995, Pianaro found a new varistor material, (Co, Nb)-doped SnO2, which is single phase with the rutile structure [1]. In 1999, Kim et al. found a one-step-air-fired SrTiO3-based ceramic which had varistor characteristics [4]. In 2000, Wang found that only one oxide (Sb2O3)-doped TiO2 ceramics shows varistor behavior [5]. Following Wang, in 2002, Su found another TiO2 varistor, doped with only one oxide (WO3) [6]. In this letter, the processing of a new WO3-based varistor system, WO3·Na2CO3·CuO/CdO/Bi2O3/ Sb2O3, as well as its nonlinear properties (electrical field as a function of current density) and dielectric properties are described. The raw chemicals used in this study were analytical grades of WO3 (99%), Na2CO3 (99.8%), CuO (98%), CdO (99%), Bi2O3 (99%), Sb2O3 (99.9%) and obtained from Shanghai Chemical Company. The composites investigated in the present work contain a molar ratio of WO3:Na2CO3:X = 96.5:0.5:3, where X = CuO, CdO and WO3:Na2CO3:Y = 98:0.5:1.5, where Y = Bi2O3, Sb2O3. Varistors were obtained by conventional ceramic processing. The mixed raw chemicals were milled in nylon pot for 15 h with ZrO2 balls and some distilled water, dried, mixed with 0.5 wt% PVA binder and pressed into disks 15 mm in diameter and 1.5 mm in thickness at 180 MPa. After burning out the PVA binder at 650 ◦C, the disks were sintered in air at 1000 ◦C for 60 min and then slowly cooled to room temperature. During sintering, compacts were surrounded with powders of matching compositions and covered with crucibles to reduce evaporation. The frequency dependence of the relative dielectric constant and complex impedance spectra were obtained using an impedance analyzer (Agilent 4294A). For electrical characterization of current density as a function of applied electrical field, a semiconductor I–V grapher (QT2) was used. The coefficient of non-linearity α was obtained from