| As a low-voltage varistors, the applications of WO3ceramics in the protections of microelectronic circuits are becoming important, so the researches on the practical utilization of them are essential. The attention should be focused on the ageing process of the ceramics, ascribed to the alteration of the grain boundary potential barrier, when subjecting the WO3ceramics to pratice use. However, the formation mechanism of the WO3grain boundary potential barrier is still unclear. Based on the theory of the chemical defects and the quantities of the experimental data of our group, the manuscript proposed a grian boundary potential barrier model of WO3based varistor ceramics, which was proved to be reasonable through a series of relavent experiments. The ageing phenonmenons of the varistor property of WO3ceramics were clarified according to the proposed grian boundary potential barrier model, and the degradation resistance of WO3varistors was improved through doping rare-earth oxides. The main conclusions are as follows:(1) The sintering atmosphere has a significant effect on the microstructure and the electrical property of the WO3ceramics. The oxygen deficient atmosphere sintering, such as in vavuum and in nitrogen, made more creation of oxygen vacancies in WO3ceramics, causing the increase in the distance of the crystal face and leading to the shift of the diffraction peaks of WO3to the smaller angle. As had been proved, the oxygen vacancies in WO3could result in changes in W-O splitting which were the main differences among the phase structures of WO3. This might be responsible for the coexistence of two phase structures, even mutiphase structures in the WO3ceramics at room temperature. In the oxygen deficient atmosphere sintering, the lack of the oxygen in the ceramics structure could not be occupied, depressing the grain growth and the densification of WO3ceramics. However, the creation of oxygen vacanxies in WO3ceramics improved the electrical conductivity, the behavior of which followed the ohmic law. The grain growth and the densification of WO3ceramics sintered in the oxygen deficient atmosphere could be enhanced through the resintering in the air or in the oxygen atmosphere. In the resintering process, the neutral oxygen adsorpted in the grain boundaries combined with electrons to form the Schottky potential barrier at the grain boundaries of WO3ceramics. The samples sintered in the oxygen deficient atmosphere could obtain an obviously enhanced breakdown voltage, a lower leakage current and a larger nonlinear coefficient through resintered in the air or the oxygen rich atmosphere.(2) Ambient atmosphere has an obvious effect on the ageing process of the WO3ceramics, and doping WO3with rare earth element can improve the ageing resistance. During an ageing stress, unlike the traditional varistors such as ZnO and SnO2, the leakage current of WO3ceramics decreased and the breakdown voltage of them increased, which was ascribed to the defects in the grain boundaries potential barrier. In the air or in the oxygen rich atmosphere, WO3ceramics could adsorpt the oxygen atom to form the oxygen ions, enhancing the height of the potential barrier, which caused the above phenonmenon. The oxygen ions desorption at the grain boundaries was responsible for the poor duplication of the electrical properties and the deteriorated varistor properties. At high temperatures, the metal oxides with multivalance could release or absorb oxygen to affect the creation of the oxygen vacancies in WO3ceramics, which had notable influences on their microstructure and the electrical properties. Doping WO3with other oxides made their grain boundary potential barrier no longer depnd only on the oxygen adsorption, reducing the effects of atmosphere on the potential barrier, improving the stability of the electrical properties of WO3ceramics. In addition, the response of WO3ceramics under high voltage need more time than ZnO, which was the limit of their practice use and woth researching in the future study. |
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