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Sintering and grain growth of nonstoichiometric rutile. Thiriar, Jacques Pierre Jean

Abstract

Rutile powders in flaked form were pressed and heated at different temperatures (1000°C to 1300°C) under reducing (H₂/H₂O) atmospheres to study the rate of weight loss, the grain growth and the densification. The weight loss measurements for reduction of rutile to two non-stoichiometric compositions of TiO₁.₉₂ and TiO₁.₉₈ yielded an activation energy for weight loss of 82 ± 2 kcal/mole. No attempt was made to identify the rate-determining step. Previous weight loss measurements carried out in equilibrium conditions produced an enthalpy of 83 ± 10 kcal/mole for the formation of an oxygen ion vacancy. This could suggest that the rate-determining step might be the formation of an oxygen ion vacancy. The grain growth study revealed that the non-stoichiometric composition of TiO₁.₉₂ did not obey the theoretical relation of Burke. The results can be expressed by the following D² - D₀² = Kt°·⁶exp (- 78,000/RT) This activation energy for grain growth is equal to the activation energy for oxygen ion diffusion in TiO₂. This suggests that the oxygen ion diffusion may be the rate-controlling step for grain growth. The densification on sintering was evaluated from linear shrinkage measurements of the compacts during reduction to TiO₁.₉₂. A few models were tried, to find the best fit for the present data. While the photomicrographs suggest the Coble model for bulk diffusion, and the values for the diffusion coefficients are of the right order of magnitude, the activation energy for the rate determining step is about 118 kcal/mole, which is not in agreement with the previous sintering study on rutile. From grain growth data for those compacts reduced to TiO₁.₉₈ at 1200°C and those sintered in open air, it was seen that the diffusion coefficient was not significantly affected by variation of the oxygen partial pressure. This discrepancy in the activation energy value may be explained by a possible error in measurement and other unknown variables which may control the densification process.

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