New ZnSnO3-based varistor system
Semiconducting ceramics are widely used in electrical industries involving mobile communications, computers, signal processing, power transport, and control systems, because of their unique and useful electrical characteristics [1]. Varistors which are generally used to protect electronic circuits from voltage shock can sense and limit high transient voltage surges and can repeatedly endure such surges without being destroyed. The most important property of a varistor is its nonlinear voltage–current characteristic. It can be expressed by the equation I = K V α . The α coefficient gives the degree of nonlinearity and the constant K depends on the microstructure and is related to the electrical resistivity of the material. Commercial varistors used in protection systems are based on SiC or on ZnO. Varistors based on SiC have low nonlinearity coefficients [2–4] and ZnO varistors exhibit high nonlinear coefficients, but the degradation problem of ZnO varistors has not been resolved [5–7]. Therefore, the efforts to find new varistor materials have been ongoing. In 1983, N. Yumaka, M. Masuyama found that SrTiO3-based ceramics made by a two-step process had varistor characteristics [8]. In 1995, S. A. Pianaro found a new varistor material [4], (Co,Nb)-doped SnO2, which has only a single phase, rutile structure. In 2000, J. F. Wang found one oxide (Sb2O3)-doped TiO2 ceramic to have varistor behavior [9]. Following Wang, W. B. Su found, in 2002, another TiO2 varistor doped with only one oxide (WO3) [10]. In this work, we found a new varistor material, (Nb,Si)-doped ZnSnO3, and investigated the effects of SiO2 on the properties of the (Nb,Si)-doped ZnSnO3 varistor. The materials used were analytical grades of SnO2 (99.5%), Nb2O5 (99.5%), ZnO (99.5%), and SiO2 (90.0%). The compositions were SnO2 + ZnO + 0.2%Nb2O5 + x%SiO2 in molar terms, where x = 0, 0.25, 0.4, 0.5, 1.0. The varistors were prepared by conventional ceramic processing. The mixed raw chemicals were milled in a nylon kettle with ZrO2 balls and some distilled water, dried, mixed with 0.6% weight of PVA binder and pressed into disks 15 mm in diameter and 1.5 mm in thickness at 160 MPa. The disks were sintered at 1427 ◦C for an hour and cooled to room temperature after burning out the PVA binder at 650 ◦C. To measure the electrical properties, silver electrodes were made on both surfaces of the sintered disks. For microstructure characterization, the surfaces of the samples were observed by scanning electron microscopy (SEM) and the phases were analyzed by X-ray diffraction (XRD). For electrical characterization of current density versus applied electrical field, an I–V grapher (QT2) was used. The complex impedance dependent on frequency is measured using an Agilent 4294A impedance analyzer. According to the XRD analysis (Fig. 1), no apparent second phase was observed and SnO2 and ZnO should be synthesized according to the following reaction [11]: