Study on Oxidation and Coercivity of Nd2Fe14B Compound Crystal

Oxidation of the Nd2Fe14B compound crystal and its effect on the coercivity of the fine Nd2Fe14B crystal particles were investigated. Oxidation kinetics of the Nd2Fe14B compound crystal was investigated using an excessively grown Nd2Fe14B grains in the Nd15Fe77B8 alloy ingot. Oxidation of the Nd2Fe14B compound crystal occurred by dissociation of the phase into multi-phase mixture of alpha-Fe, Fe3B, and Nd oxides. Oxidation rate of the Nd2Fe14B compound crystal showed no dependence on the crystallographic direction. The oxidation reaction was modeled according to simple linear relationship. Activation energy for the oxidation of Nd2Fe14B compound crystal was calculated to be approximately 26.8 kJ/mol. Fine Nd2Fe14B crystal particles in near single domain size was prepared by ball milling of the HDDR-treated Nd15Fe77B8 alloy, and these particles were used for investigating the effect of oxidation on the coercvity. The near single domain size Nd2Fe14B crystal particles (= 0.3 mu m) had high coercivity over 9 kOe. However, the coercivity was radically reduced as the temperature increased in air (< 2 kOe at 200 degrees C). This radical coercivity reduction was attributed to the soft magnetic phases, alpha-Fe and Fe3B, which were formed on the surface of the fine particles due to the oxidation.