Degassing rate constant for dilute solution of nitrogen in tantalum

Abstract

The degassing of nitrogen dissolved in tantalum has been studied over the pressure and temperature ranges 5 × 10^–5 to 10^–8 torr and 1600 to 2300°K respectively. A theory is developed based on the assumption that desorption from the surface in accordance with the second-order Arrhenius equation is the rate-limiting step at low concentrations. The theoretical predictions, (a) that the reciprocal of the square root of the pressure should be a linear function of the time, and (b) that the activation energy controlling this process is the heat of solution, are both confirmed by the data. With the aid of simple statistical models for the nitrogen atoms chemisorbed on the surface of the metal and for those in solution, the second-order rate constant is calculated to be 3.5 × 10^–3 cm² atom^–1 sec^–1. This value is of similar magnitude as for desorption of nitrogen from metals such as tungsten for which the solubility is negligible.