NONEQUILIBRIUM GAS-PHASE NITROGENATION

Non-equilibrium nitrogenation of transition-metal rich rare-earth intermetallics can be used to increase the nitrogen content beyond its equilibrium concentration. For example, nitrogenation of Sm2Fe17 using ammonia yields volume expansions up to 8.3 vol %, which correspond to a composition Sm2Fe17Nx with x almost-equal-to 4. This effect is related to catalytic action at the metal surface where ammonia is decomposed into nitrogen and hydrogen, with nitrogen absorption on a site where it is weakly bound, as an intermediate step. For overloaded Sm2Fe17Nx with x>3 the net reaction energy U(o) = 5 +/- 10 kJ/mole indicates a nearly temperature-independent occupation of the non-9e sites.