Electrical And Microstructural Properties Of Tungsten Trioxide Ceramic Doped With Rare Earth Oxides

Developmemts of industry and academic research of varistor materials, especially these varistor materials for low voltage applications have been introduced. The advantages and disadvanntages of Tungsten trioxide (WO3) ceramic as low voltage varistor materials have been concluded in the paper. The physical mechanism and sintering behavior, microstructural properties and electrical characteristics have been investigated based on the pure and Pr6O11, Bi2O3 doped WO3 ceramics.Varistor behavior based on undoped WO3 ceramic have been studied in detail and it was found that compared with the conventional ZnO, SnO2 based varistor ceramics, WO3 showed a very different varistor properties and this may be abscribed to different varistor behavior mechanisms, in addition, varistor behavior in WO3 ceramic was very sensitive to atmosphere treatments. AES results revealed that there was an enrichment of oxygen on the grain surface of WO3 ceramic, formed the surface states and then the Scottky barriers at the grain boundary which is the origin of varistor behavior of WO3 ceramic.Doping with large radios and low melting temperature transition metal oxides, Pr6O11 and Bi2O3, we fabricated high density WO3 ceramic for the first time. Based on the results of sintering behavior of pure WO3 ceramic, it was found that WO3 was very difficult to densitify, due to its low metlting temperature and high evaporation. By doping with Pr3O11 and Bi2O3, TMA (Thermal Mechanical Analysis) results showed that WO3 could be densitified because of the liquid phases and solid state reaction during the sintering process, and therefore, it is believed that liquid phase sintering is an effective way to fabricate high dense WO3 ceramic samples. Microstrural and electrical characteristics of dense WO3 ceramic has been studied for the first time. Experimental results revealed that Pr6O11 and Bi2O3 dopants can promote the sinterability forming high dense ceramic structure from prous ones and the grain size increased with dopants.Based on the results, it was demonstrated that Pr6O11 and Bi2O3 dopants can enhance the density of WO3 ceramic, however, there was no improvements on varistor properties of ceramic samples. It was noted that varistor behabior of cereamic samples doped with Pr3O11 degrated quickly until the low resisitivity ohmic behavior was observed. While doping with Bi2O3, varistor properties of WO3 ceramic have been enhanced, the nonlinear coefficients increased with Bi2O3 and breakdown voltage decreased even at only 3.0V/mm this demonstrated that Bi2O3 doping could promote the varistor properties of WO3 in low voltage applications. Based on the above results, it can be concluded that varistor properties of WO3 ceramic can not be improved by only one element doping and for practical application of this material more suitable doping elements are required which has been well supported by the results observed in conventional ZnO, SnO2 based varistor systems.