Tungsten Oxide Ceramic Application Problems

Tungsten oxide ceramics is an extremely important functional material. However, our understanding of the physical properties of tungsten oxide ceramics is not very deep. And lack of coherence. As a varistor material, to make the tungsten oxide ceramics practically used, there is another problem that must be solved, namely the electrical stability of tungsten oxide.

Compared with other varistor materials, the repeatability of the volt-ampere characteristic curve of the tungsten oxide ceramic is not good, and the subsequent measurement cannot repeat the previous measurement result. There is also a serious electrical relaxation phenomenon, the current decays with time at a constant voltage. For the stability problem of tungsten oxide, the electrical instability of tungsten oxide results from the coexistence of two phases of its phase structure. At a temperature range of -40°C to 740°C, tungsten trioxide has at least 5 structural transformations. With the increase of temperature, the corresponding crystal structures are low temperature monoclinic phase and triclinic phase. Monoclinic phase, orthogonal phase and square phase at room temperature.

However, the phase transition temperature is related to the thermal history of the material and there is a significant thermal hysteresis effect. Therefore, many studies have shown that triclinic and monoclinic phases coexist in tungsten oxide sintered ceramics. Polarization caused by coexistence destroys the stability of the electrical properties. Therefore, how to make the phase structure of tungsten oxide single and thus stabilize the electrical performance becomes a major problem to be solved at present. For electrical stability problems, it has been found that in the tungsten oxide-manganese dioxide-sodium carbonate system, the incorporation of alumina can significantly improve its electrical stability, but at the same time it also loses its nonlinearity. When rare-earth oxides are doped with tungsten oxide, not only the phase of the tungsten oxide can be simulated, but also its electrical properties become stable, without losing nonlinearity.

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