The reaction of tantalum with sulphur vapour and selenium vapour

Abstract

The kinetics of reaction of coupons of annealed tantalum with sulphur vapour and selenium vapour were studied by a thermogravimetric method over the temperature range 485–550 °C. Total sulphur pressures ranged from 0.5 to 40 kPa and total selenium pressures varied from 0.01 to 4 kPa. For the reaction with sulphur vapour, slow linear kinetics were observed under all conditions that yielded a TaS 2-type compound. The apparent activation energy calculated from the linear rate constants was 202 kJ mol −1. The rate increased as P S 2 0.25 where P S 2 is the partial pressure of S 2. It is postulated that the sulphurization process is chemically controlled at the surface of the TaS 2 scale. For the reaction with selenium vapour, parabolic kinetics were observed under all conditions that yielded a TaSe 2-type scale. The apparent activation energy calculated from the parabolic rate constants was 137 kJ mol −1. The parabolic rate constant increased as P Se 2 0.51 where P Se 2 is the partial pressure of Se 2, and this leads to the conclusion that the rate was controlled by the diffusion of tantalum vacancies through the constantly thickening scale.