The picture shows SEM images of sintered tungsten oxide ceramic samples with different concentrations of hafnium tetraoxide. The doped hafnium tetraoxide can significantly increase the crystal grain size of the ceramic, and the gap between the crystal grains is significantly reduced. It indicates that the doped hafnium tetraoxide contributes to the sintering of tungsten oxide ceramics.

Sintering in an oxygen-deficient atmosphere such as vacuum and nitrogen atmosphere produces a large amount of oxygen vacancies in the ceramic. However, due to the absence of oxygen atoms, the interplanar spacing increases, and the XRD diffraction peak of tungsten oxide shifts to a small angle corresponding to the angle. This may be the reason why the tungsten oxide ceramics coexist at a room temperature or coexist in two phases. In addition, the missing oxygen atoms are not replenished, so that grain growth and densification are suppressed. The tungsten oxide sintered in an oxygen-deficient atmosphere has a high electrical conductivity, and the electrical conductivity conforms to Ohm's law.

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.

Rare-earth doping methods can affect the microstructure, phase structure, nonlinear electrical properties, and dielectric properties of tungsten oxide. According to related studies, the main conclusions of the influence of rare earth doping on tungsten oxide are:

Tungsten oxide ceramic sintered under nitrogen gas has the least resistance and the best conductivity. This shows that sintering in a nitrogen atmosphere can promote the formation of oxygen vacancies in tungsten oxide ceramics, like the effect of vacuum sintering with tungsten oxide. Samples 2 and 3 had higher resistance than that of sample 1. The volt-ampere characteristic curve of Sample 4 coincides with that of Sample 1, and the resistance of the sample is the largest. It has non-linear electrical properties.

In the figure, SEM images of tungsten oxide ceramics prepared by five different sintering processes are shown. As can be seen from the figure, nitrogen has a great influence on the topography of tungsten oxide ceramics.

Each of the mass transfer phenomena and reaction processes occurring in the tungsten oxide ceramic material are related to various defects in the tungsten oxide crystal lattice. And the mass transfer coefficient or reaction rate constant is directly related to the type and concentration of atomic defects. The more atomic defects in the solid phase, the greater the corresponding mass transfer capacity.

The aging process of tungsten oxide ceramics can reflect its anti-aging ability, and its anti-aging ability is mainly determined by the leakage current value of the tungsten oxide ceramics.

High oxygen partial pressure can inhibit the formation of oxygen vacancies in tungsten oxide ceramics, thereby affecting the mass transfer rate. However, a tungsten oxide ceramic sintered in an oxygen atmosphere has a relatively high density. The radius of oxygen ions is 0.14 nm, which is much larger than that of hexavalent tungsten ions.

For stable and efficient detection of nitrogen dioxide gas at room temperature. The use of organic substances to activate tungsten oxide is a better choice, for example, polypyrrole and tungsten oxide are combined. Polypyrrole-tungsten oxide composites, depending on the amount of polypyrrole incorporated, can exhibit characteristics from semiconductors to metallic conductors.