Study On The Electrical Properties Of Gd,Ce-WO₃ Functional Ceramics And WO₃ Fabricated By Using Namopowders

Tungsten trioxide has received much attention because of its potential for many technology applications, such as electrochromic device, gas sensors and photocatalytic degradation. In recent years, it has been found that doped WO3 ceramics show good nonlinear electrical property characterized by low breakdown voltage and low operating current. Moreover, the dielectric constant of WO3 is high. So WO3 is a new kind of capacitor-varistor material, which is possible for potential application in micro-electronic area. But its instable electrical property must be solved before using WO3 as a kind of varistor in practice. Previous studies indicated that the electrical behavior of sintered WO3 ceramics was unstable and had serious hysteresis effect, i.e. the current under constant voltage decayed with time and current-voltage characteristics depended on the electrical history of the sample. This problem is an obvious blockage for practical application of WO3 as a varistor material. We want to improve the electrical properties of WO3 ceramics by doping rare earth elements and understand the related physical mechanism. So it has scientific and practical significant to investigate the detailed properties of tungsten and rare earth element. The principal research contents and results include: (1)Gd2O3-WO3 ceramics are fabricated by the conventional solid-state reaction process. With increasing temperature were measured the electrical characterized and dielectric constant of the samples, the microstructure, phase structure were investigated. Based on the phase transition of tungsten trioxide, the electrical properties of ceramics can be explained. (2)anometer tungsten trioxide powders were prepared by a wet chemical method. The sizes of WO3 particles characterized by the TEM are below 30nm. (3)The effect of sintering temperature on electrical properties of WO3 ceramic prepared by nanopowders was studied in detail. The most obvious characteristic of I-V curve of the polycrystalline WO3 ceramic is that negative-resistance behaviors exist. And the negative-resistance behaviors are not stable. With the sintering temperature increasing, the electrical property of sample displays stable nonlinear current-voltage characteristics, but this nonlinear electrical property is weekly; (4)Using nm-level powders fabricated by a wet chemical method as precursor, the CeO2-doped WO3 ceramics were prepared by the conventional solid state reaction at sintering temperatures from 600 to 1100?C. There is no negative-resistance behaviors exist. With sintering temperature increasing, the electrical property of samples displays an interesting transformation from linear to nonlinear behavior. When the sintering temperature is low, we can see that unstable I-V curves under repeated measurements is unstable ,and this implies that there are obvious grain boundary regions with various defects and space charges. In contrast, the samples sintered beyond 1000?C exhibit single semicircle in impedance spectrum and a good stability, which indicate that the grain boundaries in these samples are stable and developed suitable,and the electrical properties of samples. The measurements of SEM, complex impedance and electrical stability indicate that a lot of grain boundary region in the samples sintered at low temperatures influence strongly the electrical transportation. the electrical nonlinearity observed in Ce-doped WO3 prepared by nm-level powder as precursor may be explained according to the back-to-back Schottky barrier model; (5)Based on the analysis of the mentioned experimental data, it is found that the conventional theory of nonlinear electrical transport based on double Schottky barriers at grain boundary in semiconductive functional ceramic is too simple to be used to explain exactly the nonlinear characteristic of tungsten oxide polycrystalline ceramic, we must think over the phase transition and the coexistence of a lot of phase of WO3.