Effect of Milling Atmosphere on the Preparation of Sm₂Fe₁₇N_x Powder by Mechanical Grinding

Abstract

The preparation of Sm₂Fe₁₇N_x permanent magnet powders has been tried by mechanical grinding (MG) for a short time without crystallization heat treatment which was usually needed to obtain a Th₂Zn₁₇ structure in the conventional method by MG or MA. MG was carried out in three kinds of atmospheres; i.e., Ar, N₂ and NH₃ gas. Non-treated and hydrogenated Sm_2.5Fe₁₇ alloy powders were used as the starting materials. The α-Fe precipitation was detected in the powder after MG for over 14.4 ks. The grain size of powder obtained by MG for 10.8 ks was around 10 nm which was much finer than the single magnetic domain size of about 300 nm. The existence of hydrogen promoted not only the nitriding of powders but the oxidation. The hydrogenated powder after MG in NH₃ and nitriding had a high oxygen content of about 13000 ppm. This oxidation prevented the absorption of nitrogen during the nitriding process. The non-treated powders after MG in NH₃, however, were nitrided efficiently up to 38000 ppm at a low temperature of 623 K, and the oxygen content was about 7000 ppm. The maximum coercivity of 0.93 MA·m⁻¹ (11.7 kOe) was obtained with the non-treated powder after MG in NH₃ and nitriding at 623 K for 10.8 ks in N₂.