Guo-Zhong Zang

Novel TiO2·WO3 varistor system

Abstract The nonlinear electrical behavior and dielectric properties of TiO 2 -based ceramics with various WO 3 contents have been investigated. It was found that the 0.25% WO 3 +99.75% TiO 2 has an optimal nonlinear coefficient α of 9.6, a breakdown electrical field of 44.5 Vxa0mm −1 , and an ultrahigh relative dielectric constant of 7.41×10 4 (measured at 1 kHz). The theory of defects in the crystal lattice was introduced to explain the nonlinear electrical behavior of the TiO 2 ·WO 3 system. A grain boundary barrier model that is analogous to the grain boundary defect model for ZnO varistors are introduced in the article. From this point, the nonlinear electrical behavior of the TiO 2 system is explained.

Electrical nonlinearity of (Cu, Ni, Nb)-doped SnO2 varistors system

Abstract A best nonlinearity properties is obtained in the varistors of Sn–Ni–Nb, via the investigation of the varistors with compositions of (99.2− x )% SnO 2 +0.75% Ni 2 O 3 +0.05% Nb 2 O 5 + x % CuO. The sample doped with 1.0 mol% CuO exhibits the highest electrical nonlinear coefficient ( α =25) and the highest relative density (96.1%). SEM micrographs show that grain size increases with increasing CuO content. The phenomenon can be explained by interface defect model, the substitution of Cu 2+ for Sn 4+ facilitates the formation of the defect barriers and facilitates the grain growing. Breakdown voltage increases with increasing CuO when doping less than 1.0 mol% CuO, and reaches a maximum (946 Vxa0mm −1 ) at 1.0 mol% CuO dopant then decreases with increasing CuO. It can also be explained by the effect of Cu 2+ substituting for Sn 4+ .

Effects of Ta2O5 on the grain size and electrical properties of SnO2-based varistors

The effects of Ta2O5 on SnO2-based varistors were investigated. It was found that Ta2O5 significantly affects the grain size and the electrical properties. The average grain size decreases from 9.3 to 3.8 µm, the breakdown electrical field increases from 246 to 1412 V mm−1 and relative electrical permittivity decreases from 1.9 to 0.42 k with an increase in Ta2O5 concentration from 0.10 to 1.00 mol%. The sample with 1.00 mol% Ta2O5 has the best nonlinear electrical property and the highest nonlinear coefficient (α = 52.6) among all samples. The reason for grain size decrease with increasing Ta2O5 concentration is explained. To illustrate the grain–boundary barrier formation of (Co, Ta)-doped SnO2 varistors, a modified defect barrier model is introduced.

Nonlinear electrical properties of SnO2.Li2O.Ta2O5 varistors

Abstract The electrical properties of (Ta, Li)-doped SnO2 ceramics as a new varistor material were investigated. The sample 98.90% SnO2·1.0% Li2O·0.10% Ta2O5 (mol fraction) sintered at 1500xa0°C possesses the highest density (ρ=6.63 g/cm3) and nonlinear electrical coefficient (α=10.8). Effect of dopants and sintering temperature on the properties of the samples were investigated. The substitution of Sn4+ with Li+ and the variation of sintering temperature play very important effects on the densities, dielectric constant, nonlinear electrical properties and other characteristics of the samples. The samples sintered at 1500xa0°C exhibit better physical and electrical properties than the samples sintered at 1400xa0°C. The properties of the grain-boundary defect barriers and the microstructural characteristics were investigated to ensure the effect of the dopants and the sintering temperature. A grain-boundary defect barrier model was used to illustrate the grain boundary barrier formation in SnO2·Li2O·Ta2O5 varistors. PACS numbers: 74.40.Lq; 72.20.Ht

Effects of Cr2O3 on the properties of (Co, Nb)-doped SnO2 varistors

The effects of Cr2O3 on the properties of (Co, Nb)-doped SnO2 varistors were investigated. The samples with different Cr2O3 concentrations were sintered at 1350 °C for an hour. The properties of (Cr, Co, Nb)-doped SnO2 varistors were evaluated by determining their I–V and e–f relations, measuring their resistivities, scanning electron microscopy. It was found that the breakdown electrical field increases from 400 to 1000 V mm−1 and relative electrical permittivity decreases from 2022 to 147 with increasing Cr2O3 from 0.00 to 0.07 mol%. Nonlinear coefficient presents a peak of α=52 and grain boundary barrier becomes highest when 0.06 mol% Cr2O3 was added. Electrical permittivity and grain size decreases with increasing the content of Cr2O3. In order to illustrate the grain boundary barrier formation in this varistor system, an interface defect model was introduced.

Electrical nonlinearity of (Ni, Ta) doped SnO2 varistors

Abstract The electrical nonlinearity of (Ni, Ta) doped SnO 2 varistor system was investigated. The nonlinear coefficient α and the barrier height of this varistor system were calculated. It is found that the variations of electrical nonlinear coefficients with acceptor concentrations are in accordance with that of the barrier heights. 0.75 mol% Ni 2 O 3 doped sample sintered at 1370xa0°C exhibits a high nonlinear coefficient α of 21 and a high breakdown electric field (697 Vxa0mm −1 at 1 mAxa0cm −2 ), but presents a relatively low densification. Among the samples sintered at 1420xa0°C, the sample doped with 0.75 mol% Ni 2 O 3 and 0.05 mol% Ta 2 O 5 possesses the highest breakdown electric field (469 Vxa0mm −1 at 1 mAxa0cm −2 ) and the highest electrical nonlinear coefficient α of 16.3, which is consistent to its highest defect barrier φ B of 0.693 eV. An atom mode for the acceptors and donors to penetrate into SnO 2 lattice was put forward. To illustrate the grain-boundary barrier formation of (Ni, Ta) doped SnO 2 varistors, a modified defect barrier model was introduced, in which the negatively charged acceptors substituting for Sn ions should not be located at the grain interfaces instead at SnO 2 lattice sites of depletion layers.

Effects of In2O3 on the properties of (Co, Nb)-doped SnO2 varistors

The effects of In2O3 on the properties of (Co, Nb)-doped SnO2 varistors were investigated. It was found by characterizing the samples sintered at 1350°C that the nonlinear coefficient presents a peak of α = 20.4 for the concentration of 0.05 mol% In2O3, the average grain size decreases from 8.4 to 3.9 μm, the breakdown electrical field increases from 206 to 821 V mm−1 and relative electrical permittivity decreases from 2.3 to 0.16 k with increasing In2O3 from 0.00 to 0.10 mol%. The increase of the breakdown electrical field with increasing In2O3 concentration is mainly attributed to the decrease of the grain size. The reason why the permittivity decreases with increasing In2O3 concentration was originated from the ratio of the grain size to the barrier width. To illustrate the grain-boundary barrier formation of (In, Co, Nb) doped SnO2 varistors, a modified defect barrier model was introduced, in which the negatively charged acceptors substituting for Sn ions should not be located at the grain interfaces instead at SnO2 lattice sites of depletion layers.

Nonlinear electrical behaviour of the WO3-based system

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