Study on Oxidation and Coercivity of Nd₂Fe₁₄B Compound Crystal

다국어 초록 (Multilingual Abstract)

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 α-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/㏖. Fine Nd₂Fe₁₄B crystal particles in near single domain size was prepared by ball milling of the HDDR-treated Nd₁?Fe??B? 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 ㎛) had high coercivity over 9 kOe. However, the coercivity was radically reduced as the temperature increased in air (＜2 kOe at 200 ℃). This radical coercivity reduction was attributed to the soft magnetic phases, α-Fe and Fe₃B, which were formed on the surface of the fine particles due to the oxidation.