Shingo Tachibana

Pressure-composition-temperature relationships of the Sm2Fe17N system

The pressure-composition-temperature (P-C-T) relationships of the Sm 2 Fe 17 -N system were measured volumetrically at temperatures between 623 and 673 K. Clear pressure plateaux could be reproducibly measured between [N]/[Sm 2 Fe 17 ]=2.0 and 3.0, indicating the coexistence of Sm 2 Fe 17 N 2 and Sm 2 Fe 17 N 3 phases. As the N concentration [N]/[Sm 2 Fe 17 ] approaches from 0 to 2.0, the N chemical potentials and the lattice parameters increase sharply, indicating the induction and accumulation of internal stresses caused by the uptake of N. The inclinations of the measured plateaux suggest that the formation of Sm 2 Fe 17 N 3 from Sm 2 Fe 17 N 2 seems to proceed under highly constrained lattice conditions. The Lacher model was fitted to the measured P-C-T curves and a partial phase diagram of the Sm 2 Fe 17 -N system was constructed. The critical temperature below which Sm 2 Fe 17 N 3 exists was estimated as 793±30 K. The exposure of a stoichiometric Sm 2 Fe 17 sample to N 2 gas at 673 K over a period of 1 month exhibited no appreciable disproportionation. The partial molar enthalpy and entropy calculated for the formation of Sm 2 Fe 17 N 3 were -46.9±2.4 kJ (mol N 2 ) -1 and -25.4±1.3 J K -1 (mol N 2 ) -1 respectively.

A Samarium-Iron Nitride Magnet Fabricated by Shock-Compaction Technique

A samarium-iron nitride Sm2Fe17N3 powder was shock-consolidated by using a propellant gun with metal-plate impactor system. Disk-like magnets were consolidated under each experimental condition. At an optimum condition in this study, apparent density of shock-compacted Sm2Fe17Nx magnets was 97% of solid density, and X-ray diffraction patterns of these magnets showed no appearance of the decomposition of rhombohedral Sm2Fe17Nx structure. The highest value of the maximum energy product obtained so far was 10.5 MGOe. Microstructural analyses suggested the possibility of improving magnetic properties of the magnets.

Diffusion behavior of N atoms in Sm2Fe17

The rate of N diffusion in Sm 2 Fe 17 was measured using an Sm 2 Fe 17 N 2.4 sample with a particle size of 5 μm. At this N concentration, the two Sm 2 Fe 17 N 2 and Sm 2 Fe 17 N 3 phases were assumed to coexist. The measured diffusion coefficient yielded a value, D=2.7x10 -12 cm 2 s -1 at 623 K which is much higher than those measured for larger particle sizes of the sample. The marked change in the diffusivity can be attributed to the contribution of the pre-exponential factor D 0 to the diffusion coefficient. The sample size dependence of the N diffusivity suggests strong interactions of the diffusing N atoms with defects induced and accumulated in the nitrided sample. The rate of the N diffusion seems to change depending on the readiness of strain relief of the sample in the nitride formation, resulting in a different diffusion process depending on the N concentration and the particle size.