Various Factors Affect the Sintering Process 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.

Sintering the ceramic sintered in an oxygen-deficient atmosphere into air or an oxygen atmosphere for a second time can re-grow the crystal grains of the tungsten oxide ceramic and increase the density of the tungsten oxide ceramic. During this process, the neutral lattice oxygen adsorbed at the grain boundary combines with the electrons in the ceramic to form a negatively charged lattice oxygen ion. Due to the large amount of oxygen vacancies in the tungsten oxide material during the sintering process, the grain boundary can adsorb oxygen elements more than the stoichiometric ratio. Oxygen ion radius is large and mobility is poor. As a result, they accumulate in the grain boundary and form a negatively charged ridge in the grain boundary. Since the electrons in the surrounding lattice are combined with the neutral lattice oxygen, an electron depletion layer with a certain width is formed. Such a tungsten oxide ceramic grain boundary forms a Schottky barrier, resulting in pressure sensitive properties. The ceramic sample obtained by sintering the tungsten oxide in an oxygen-deficient atmosphere and then placed in air or oxygen atmosphere for a second time, the varistor is significantly increased, the leakage current is reduced, and the nonlinear coefficient is increased.

The leakage current of the tungsten oxide varistor ceramics decreases during the aging test, and the varistor voltage increases and the like is related to the defects in the barrier. Neutral lattice oxygen and electrons have strong binding ability, which makes tungsten oxide ceramics can absorb oxygen atoms in the air or oxygen-rich atmosphere, and form oxygen ion adsorption, making the barrier height increase, and thus the above phenomenon occurs. The phenomenon of poor repeated behavior of tungsten oxide electrical properties that appeared in previous experiments may be related to this. The varistor properties of tungsten oxide ceramics rapidly decrease with increasing temperature, which is related to the desorption of oxygen ions at grain boundaries.

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