Nitride layer growths on liquid thorium and on solid thorium carbon alloys

Nitride layers were formed on thorium‐carbon alloys by reaction with nitrogen (1–1600 Torr) at temperatures between 1400° and 2400°C. Values of scale thicknesses of face‐centered cubic (fcc) on liquid thorium and of on solid alloys with were found to be proportional to the square‐root of time of reaction indicating growth rates are controlled by diffusion. Calculated composition‐average values of concentration ‐gradient type interdiffusion coefficients in pure at 1900°, 2100°, and 2400°C can be represented by the equation . Diffusion coefficients in the Th(C,N) layers are approximated by the same method: cross effects due to presence of carbon are neglected. The results show that the nitrogen diffusion rate in increases with carbon content and with decreasing nitrogen pressure. Nitrogen‐rich layers formed on alloys with are preceded by a carbon‐rich front containing precipitate. This observation together with the assumption of negligible mobility of thorium atoms leads to the conclusion that carbon diffuses "uphill" against a concentration gradient. The phenomena also occurs in the analogous phase during reaction under similar conditions. With increasing carbon content, nitrogen diffusion is suggested to change from a vacancy mechanism in pure to a substitution mechanism in carbon‐rich .