| Tungsten trioxide (WO3), as one kind of functional materials, has been investigated systematically because of its potential for technological applications, such as electrochromic devices, gas detection and chemical catalyst. In recent years, it has been found that doped WO3 ceramics show good nonlinear electrical properties characterized by low breakdown voltage and low operating current, which is possible for potential applications in micro-electronic area. However, the structure complexity of WO3 makes it difficult to clear the physical mechanism about electrical transport in WO3 ceramics. As for varistor material, WO3 exhibits instable electrical property characterized by poor recurrence of I-V curve and serious hysteresis effect. It has been pointed out that the instability mentioned above is concerned with polarization resulted from different phase coexistence of WO3. In this thesis, starting from fabricating WO3 ceramics doped with alkali-earth and transition element oxides, the nonlinear electrical properties are mainly investigated to understand the physical mechanism of WO3 functional materials. The principal research contents and results include:(1) The influence of doped with alkali-earth element oxides on WO3 ceramics is studied. It has been found that the addition of Ca, Sr or Ba into WO3 can all promote the grain growth. Moreover, the grain shape for Sr and Ba doped samples varied from globe- to plate-like morphology. Monoclinic phase and triclinic phase coexist in alkali-earth element oxides doped WO3 ceramics, which results in instable electrical property. When sintered in air, samples doped with Ca or Ba reveal poor electrical nonlinearity. But the Sr-doped samples exhibit good nonlinearity with maximum nonlinear coefficient about 8.7. On the other hand, improved electrical nonlinearity is observed in Ca-doped WO3 ceramics, which are sintered in oxidizing (O2) atmospheres with nonlinear coefficient about 13.(2) The effect of MnO2 and Cr2O3 on WO3 ceramics is investigated. MnO2 doping can not only promote grain growth of WO3, but also improve electrical nonlinearity of WO3 ceramics with maximum nonlinear coefficient about 9.5. The introduction of Cr2O3 into MnO2·WO3 system greatly inhibits grain growth of WO3 and leads to a lower nonlinear coefficient about 5 as well as a stable electrical property.(3) Based on the fact of the mentioned experiment, a simple double Schottky barrier model without other electrical background can not give a proper explanation for electrical transportation in tungsten oxide based ceramics, and some important factors such as ferroelectric polarization and phase coexistence should also be taken into account. |
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