Tungsten Oxide Ceramics Mass Transfer Process

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.

In vacuum sintering, the concentration of oxygen vacancies in the material is much greater than that in air and oxygen atmospheres. However, its grain size and relative density are relatively small. This may be related to the mass transfer path. Related studies have shown that when the initial sintering mechanism of tungsten oxide ceramics is evaporation condensation, the shrinkage of the material can be ignored. The mass transfer mechanism from the grain boundary to the neck through lattice diffusion will cause the shrinkage of the donor.

The shrinkage of the tungsten oxide ceramic sintered in vacuum is negative, and the shrinkage of the sintered tungsten oxide ceramic in the air is almost zero. The shrinkage rates of the tungsten oxide ceramics sintered and heat-treated in an oxygen atmosphere were 1.2% and 3.2%, respectively. Therefore, we believe that the tungsten oxide ceramic material's mass transfer path is dominated by evaporation condensation when sintering in vacuum and air.

The mass transfer path of the tungsten oxide ceramic material sintered and heat-treated in an oxygen atmosphere is mainly due to the mass transfer mechanism from the grain boundary through the lattice diffusion to the neck. During vacuum sintering, most of the W and O do not reach the neck, but because they are in a vacuum state, they escape from the solid material, which results in slow grain growth during sintering and a decrease in density. The samples sintered in air, and most of them reached the neck, so that their grain growth was significantly better than vacuum sintering.

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