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The pressure dependence of the linear oxidation of niobium in the temperature range 450°–1050°C

Authors
Journal
Journal of the Less Common Metals
0022-5088
Publisher
Elsevier
Publication Date
Volume
5
Issue
3
Identifiers
DOI: 10.1016/0022-5088(63)90033-x

Abstract

Abstract The pressure dependence of the linear oxidation of niobium has been studied using both constant pressure and varying pressure techniques in the temperature range 450°–1050°C, for pressures in the range 10–760 mm Hg of oxygen. Below 600°C there is an initial slow linear rate during which a continuous oxide layer forms. After a time, a powdery oxide nucleates on the metal surface bursting through the continuous layer and the reaction accelerates to a faster linear rate. The breakaway time is very variable, but decreases as the temperature is raised, and above 560°C is too short to be detectable. The pre-breakaway rate is shown to involve an equilibrium adsorption step, with a low-pressure rate depending on the square root of the pressure. The post-breakaway rate depends on pressure to the power 0.7–0.8 and the activation energy of this step is about 17 kcal mole −1. In the range 600°–650°C the shape of the rate curves changes, the overall rate diminishing. This decrease is shown to be due to a third linear rate appearing at increasingly early times in this range, with a rate depending on the square root of the oxygen pressure and an associated activation energy of 28.0 kcal mole −1. Above 850°C a fourth linear rate appears, depending on pressure to the power 0.7–0.9. The pressure dependence increases linearly with temperature. The temperature dependence of the overall reaction rate is small, and is a function of pressure. The inapplicability of the Arrhenius equation to this process is demonstrated.

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