Study On The Electrical Properties And Related Problems Of Tungsten Trioxide-based Functional Ceramics Doped With Rare Earth Elements

Tungsten trioxide has received much attention because of its potential for manytechnology applications, such as electrochromic device, gas sensors and photocatalyticdegradation. In recent years, it has been found that doped WO3 ceramics show goodnonlinear electrical property characterized by low breakdown voltage and low operatingcurrent. Moreover, the dielectric constant of WO3 is high. So WO3 is a new kind ofcapacitor-varistor material, which is possible for potential application in micro-electronicarea. Because of its structural complexity, however, it is unclear for the physical mechanismabout the electrical property in WO3 ceramics up to now. In addition, its instable electricalproperty must be solved before using WO3 as a kind of varistor in practice. Previous studiesindicated that the electrical behavior of sintered WO3 ceramics was unstable and had serioushysteresis effect, i.e. the current under constant voltage decayed with time andcurrent-voltage characteristics depended on the electrical history of the sample. Thisproblem is an obvious blockage for practical application of WO3 as a varistor material. Wewant to improve the electrical properties of WO3 ceramics by doping rare earth elementsand understant the related physical mechanism. More importantly, though China has therichest resourse of tungsten and rare earth in the world, the research about tungsten and rareearth is lagged behind other countries such as the Occident and Japan. So it has scientificand practical significant to investigate the detailed properties of tungsten and rare earthelement. In this thesis, starting from fabricating WO3 ceramics doped with rare earthelement oxides, the nonlinear electrical properties are mainly investigated to understand thephysical mechanism of WO3 functional materials. Firstly, the microstructure, phase structure, nonlinear electrical properties and dielectricproperties of WO3 ceramics doped with rare earth elements were investigated and the resultsinclude: (1) Doping rare earth elements in WO3 ceramics can influence the growth of the grains.Small dopant of Gd and Ce can inhibit the grain growth and large dopant can promote thegrain growth. Both Dy and La can promote the grain growth. Only Yb inhibits the graingrowth. The grain size is in the range of 10～20μm for most of the samples. As dopants,rare earth elements mainly segregated at the grain boundaries. (2) There is only monoclinic phase of WO3 existing in rare earth elements-doped WO3ceramics. The electrical properties under high electrical field become stable, which can be III华 中 科 技 大 学 硕 士 学 位 论 文astribed to its mono-phase structure characteristic. And the electrical properties under lowelectrical field are also stable indicting that ion migration in the depletion layer can beignorable. (3) WO3 ceramics doped with rare earth elements have low breakdown voltage andbarrier votage indicating that WO3 is more suitable for low voltage varistor. (4) Doping of rare earth elements can not increase nonlinear coefficient of WO3. Thenonlinear coefficient is mainly in the range of 2~5. (5) Doping of rare earth elements can obviously increase the dielectric constant of WO3ceramics, about one order of magnitude on the whole. High dielectric constant makes itmore suitable as a capacitor-varistor material. (6) There are some porous slice-shaped materials in Dy-doped samples andclub-shaped materials in La-doped samples. It is believed that slice-shaped materials(Dy-rich phase) are amorphism with weak conductivity to form the Schottky barrier.However, club-shaped materials (La-rich phase) are crystalline state with good conductivityso that the barrier is broken down, leading to the nearly linear current-voltage characteristics.The GBBLC model cannot account for the high dielectric constant of La-doped samplesbecause of the good conductivity of grain boundary materials. (7) A Schottky barr...