Fabrication of Nanocrystalline Soft-Magnetic Bulk Cores through

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We have investigated synthesize of soft-magnetic bulk cores with nanocrystalline bcc structure by powder consolidation for Fe-(Zr, Nb, Hf)-B nanocrystalline alloys with both high saturation magnetization and good soft-magnetic properties.(1) Amorphous powder was effectively produced by mechanically crushing at about 100 K the melt-spun amorphous ribbons which were embrittled by annealing. On the other hand, no amorphous powder was obtained by a high-pressure gas atomization where the cooling rate was insufficient for obtaining amorphous phase.(2) Nanocrystalline compacts with high densities above 99.5% were obtained by the direct consolidation of the amorphous powders through a uniaxial hot-pressing technique or warm extrusion. On the contrary, the consolidation of nanocrystalline powders which were prepared by annealing the amorphous powder was unsuitable for production of nanocrystalline compacts with high density, resulting in nanocrystaline compacts with a densities below 90 %.(3) The best soft-magnetic properties were obtained in a nanocrystalline Fe-Zr-B-Ni compact, where the deterioration of the soft-magnetic properties by residual strain was suppressed because of the its zero magnetostriction. The nanocrystalline Fe_<88>Zr_7B_3Ni_2 compact with a nearly full density of 99.9% exhibited the saturation magnetization B_s of 1.58T,coercive force H_c of 14A/m and effective permeability mu_e of 2000 at 1 kHz and 0.4 A/m.(4) The soft-magnetic properties of the nanocrystalline compacts were improved by coating the amorphous powder particles with SiO_2 thin films using sol-gel method. It is for this reason that eddy current loss is reduced by electrically insulating the powder particles.We may, therefore, reasonably conclude that the consolidation of amorphous powders will promote the further development of soft-magnetic bulk cores.