Diffusion of Nitrogen in Niobium, with Special Reference to Temperature Dependence of the Activation Energy

The Kinetics of nitrogen diffusion in niobium were studied in the temperature range 900∼1300°C by gas-metal absorption experiments. The hardness profile was correlated with the concentration of nitrogen in niobium. It was found that the diffusion kinetics in this temperature range could be represented by a single Arrhenius type rate equation. The activation energy and the pre-exponential factor were found to be 24.9 kcal/mol and 0.0023 cm2/sec, respectively.The analysis of available experimental data for diffusion of nitrogen in niobium shows that the apparent activation energy decreases with increasing temperature. An explanation can be formulated in terms of a temperature dependence of the motion enthalpy Hm of interstitial nitrogen atoms. Hm is found to decrease from 38.6 kcal/mol to 24.7 kcal/mol between 280 and 650°C. Above this temperature range the activation enthalpy stays constant up to 1300°C, then it increases suddenly to 50 kcal/mol.