Nd-Fe-B계 및 Fe-Co계 영구자석용 재료의 보자력 향상 연구

Abstract

Permanent magnet is an essential material in the area of energy saving and clean environment technology. Especially, Nd-Fe-B-type magnet is used in the traction motor of HEV (Hybrid Electric Vehicles), EV (Electric Vehicles) and wind turbine generator. The Nd-Fe-B-type magnet for traction motor applications definitely needs to have high room temperature coercivity. Because of the problem of the rare-earth element price and supply, it is also important that finding new 3d-transition metal compound alloy with high MCA and Ms. In this study, feasibility of the electrophoresis deposition (EPD) technique for homogeneous and adhesive deposition of DyF3 particles on the Nd-Fe-B-type particles was studied, and coercivity enhancement in the diffusion-treated Nd-Fe-B-type particles deposited with DyF3 by EPD was investigated. EPD led to more homogeneous coating of DyF3 particles on the surface of Nd-Fe-B-type particle with respect to conventional dip-coating. Coercivity enhancement by diffusion of DyF3 coated by EPD was more profound with respect to dip-coating. Secondly, an attempt was made to enhance electrical resistivity and to improve performance of the Nd-Fe-B-type die-upset magnet by doping with eutectic DyF3-LiF salt mixture. The enhancement of electrical resistivity was achieved in the magnet doped with eutectic (DyF3-LiF) salt mixture with respect to the magnet doped with single DyF3 salt, and this enhancement was attributed to the homogeneous and continuous coverage of the interface between flakes by the low melting point dielectric salt mixture. Coercivity was enhanced by the doping of eutectic (DyF3-LiF) salt mixture or DyF3, and this was attributed largely to the enhancement of anisotropy field of (Nd,Dy)2Fe14B-type grains in the Dy-diffused shell on the flake surface. Finally, (1) bulk-type body-centered-tetragonal Fe-Co alloy was synthesised by utilising a conventional alloy preparation technologies, such as melting, solidification, and homogenising treatments, and its magnetic properties were investigated. In the (Fe100−Cox)1−Cy alloy, the composition range, from which single phase body-centered-tetragonal alloy (martensite phase) was obtained, was severely limited: Co content x = 2.5, and C content y = 0.062. Tetragonality (c/a) of the synthesised body-centered-tetragonal (Fe97.5Co2.5)0.938C0.062 alloy was 1.05. (2) Realization of the coercivity in the Re-substituted (FeCo)2B-type compound alloy prepared mainly by mechanical milling was investigated. Phase evolution in the (FeCo)2B-type alloy during mechanical milling and annealing was also investigated. Single phase Re-substituted (FeCo)2B-type material was prepared from stoichiometric (Fe0.675Co0.3Re0.025)2B by the combination of suction casting and mechanical milling. Heavily milled and fully annealed stoichiometric (Fe0.675Co0.3Re0.025)2B and boron-excess alloys had similar coercivity of around 1.0 kOe.