Anisotropic atomic diffusion mechanism of N, C and H into Sm 2Fe 17

Abstract

The preparation of Sm 2Fe 17N 3 involves the slow diffusion of nitrogen atoms from the surface to the bulk of the material. The atomic diffusion mechanism operating in this case is voidal diffusion. The nitrogen atoms are located inside the 9(e) octahedra, which share Sm corners but no faces between them. Nitrogen atoms migrate by jumping from a 9(e) site into a thermodynamically unstable tetrahedral 18(g) site, and subsequently into a new 9(e) site. In such a migration path, nitrogen atoms have to contend with an enormous energy barrier, accounting for the energy needed to overcome the strong bonding from its nearest neighbors (Fe and, especially, the Sm atoms) and, more importantly, for the strain energy needed to break out through the octahedral face (Fe(f)Sm(c)Fe(h)) and in through the tetrahedral face (Fe(h)Sm(c)Fe(h)). Although the 18(g) sites cannot accommodate nitrogen atoms in an equilibrium fashion, their presence plays a key role for the diffusion of the nitrogen atoms. The diffusion of nitrogen atoms is anisotropic, as a result of the anisotropic crystal structure of Sm 2Fe 17. Bitter domain patterns have been used to map the nitrogen diffusion fields of nitrogenated particles, and clearly show the expected anisotropic behavior of the diffusivity. Grain boundaries with their open structure provide free paths, behaving essentially the same way as free surfaces exposed to nitrogen gas. The presence of hydrogen facilitates nitrogen diffusion but, more importantly, it fractures the particles such that nitrogen effectively penetrates towards the centre of the particles. The use of ammonia gas causes severe morphological changes to the grains, resulting in very distinct fine microstructures.