| Tungsten trioxide (WO3) has received more attention because of its potential for many technology applications, such as varistors,electrochromic devices,gas sensors and photocatalytic degradation. The molecular formula of WO3is quite simple, but its electrical properties is complex, because that it undergoes several well-characterized phase transitions at different temperature and many intermediary suboxide. More importantly, the deep research about tungsten is lagged behind other countries such as the Occident and Japan. So it has scientific and practical significant to investigate the physical properties of tungsten oxide. Many investigations discovered that the electrical properties of sintered WO3ceramics are instable which lead to the coexistent of phase at room temperature. We found that the WO3ceramics prepared by nano-powders have only one phase. The electrical properties of WO3ceramics which prepared by nano-powders were studied in the room-to-high temperature range, combining with the National Natural Science Foundation of China. The research will enrich us to understand the electrical transport process.In this article, WO3namopowders have been prepared by precipitation method and sol-gel method, and prepared WO3ceramics by using nanopowders. We found the resistivity is1.05×104Ω·cm and51Ω·cm respectively at room temperature, and the analysis of the XRD indicated that the phase is different. In our present work, the article has two parts for two namopowders.In Chapter3, WO3nanopowdcrs were prepared by precipitation method. Effect of sintering temperature,sintering time and Ce-doped on electrical properties of WO3ceramics prepared by nanopowders was studied in detail. The XRD indicated the WO3ceramics which sintered at1000℃for1hour are monoclinic phase; and this is an important temperature section from700℃to900℃, the grain of samples rapidly grow up. A simple kinetics model of grain growing is established and the grain growth activation energy is calculated. The grain growth activation energy of pure WO3ceramics is43.94KJ/mol,and that doped by Ce is103.3KJ/mol, so the doping has hindered the migration of the grain boundaries; and we found that sintering time has little effect on the shape and size of the grains, the sintering time has made some contribution to the grain boundary at1000℃, the sintering time has little effect on electrical behavior of the samples sintered at600℃; the voltage-current characteristic of the samples sintered at600℃is the linearity at room temperature, and which is nonlinear at100℃, in this paper, we explain the change of the voltage-current characteristic of the samples sintered at600℃at different temperature with the oxygen adsorption and the oxygen vacancy.In Chapter4, WO3nanopowders were prepared by sol-gel method, the pure WO3ceramics and Dy2O3,Nd2O,3-doped WO3ceramics were prepared by the conventional solid state reaction at1000℃using nm-level powders, and the electrical properties of the samples were studied. The main phase of undoped WO3ceramics sintered at1000℃is triclinic,and the conductivity is higher, the measured conductivity is close to the specific conductivity of WO3crystallites. The electrical property of the samples doped by Dy2O3and Nd2O3is similar, when the amount of Dy2O3and Nd2O3is small, the voltage-current characteristic is the linearity,which is accord with the pure WO3ceramics, the new crystal boundary will appear with increasing Dy2O3and Nd2O3, and the voltage-current characteristic is a nonlinearity, which can be expained by Schottky barriers at grain boundary.In Chapter5, the analysis on the electrical behavior of WO3ceramics was explained simply. |
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