저융점 절연성 산화물 첨가를 이용한 Nd-Fe-B계 영구자석의 전기비저항 향상

Abstract

Ceramics-bonded Nd-Fe-B-type magnet with high electrical resistivity was fabricated with the intention of suppressing induction of eddy current thus lowering operating temperature of the magnet used as rotor magnet in high speed motor. The ceramics-bonded Nd-Fe-B-type magnets were fabricated by consolidating mixture of melt-spun flake and HDDR-treated powder and oxide ceramics with low melting point. The ceramics-bonded Nd-Fe-B-type magnets had remarkably enhanced electrical resistivity with respect to the magnet without ceramics binder. Coercivity of the isotropic ceramics-bonded magnet decreased with increasing the addition of ceramics binder, and this was attributed to the increased demagnetizing factor. Thin oxidized layer on the flake surface formed by reaction between the flake and oxide binder also contributed to reducing coercivity in the ceramics-bonded magnet. Highly resistive ceramics-bonded magnet containing 30 vol% ceramics binder still had good magnetic performance and high mechanical strength at 175 ℃ : iHc = 5 kOe, Mr = 4.8 kG, (BH)max = 4.3 MGOe, and over 900 MPa. In the anisotropic ceramics-bonded magnet, thanks to low temperature consolidation of Nd-Fe-B-type particles using oxide ceramic binder with low melting point the detrimental reaction between magnetic particle surface and oxide was profoundly suppressed, thus retaining the high coercivity of initial magnetic particles even in the ceramics-bonded magnets. Ceramics-bonded (15 vol%) magnet, which had good room temperature magnetic performance (iHc = 12.7 kOe, Mr = 9.4 kG, and (BH)max = 17.0 MGOe), still had reasonably good performance(iHc = 5.0 kOe, Mr = 8.7 kG, and (BH)max = 10.2 MGOe) at 150 ℃.