Origin of radical coercivity reduction in fine Nd-Fe-B-type hydrogenation, disproportionation, desorption, recombination particles and its recovery

Abstract

Coercivity variation of fine Nd-Fe-B-type particles prepared by ball milling of HDDR (hydrogenation, disproportionation, desorption, recombination)-treated Nd-Fe-B-type material was investigated using a HDDR-treated Nd12.5Fe80.6B6.4Ga0.3Nb0.2 material, and it was compared with that of fine particles prepared by mechanical milling of sintered Nd10Pr2.5Fe80.4B6Ga1Cu0.1 magnet. Coercivity of the fine particles prepared from the HDDR-treated Nd-Fe-B-type material was radically reduced as particle size decreased. In order to determine the major cause for the radical coercivity reduction, surface oxidation, structural damage (micro-strain), and crystallographic change (lattice shrinkage due to residual hydrogen desorption) in the milled fine particles were examined. The major contributory factor for the observed radical coercivity reduction was surface oxidation. Chemical etching proved an effective means for the recovery of the reduced coercivity in the milled fine particles prepared from the HDDR-treated Nd-Fe-B-type material. In contrast to fine particles prepared from sintered Nd-Fe-B-type magnet, the fine particles prepared from the HDDR-treated material could have reasonably high coercivity even as a very fine powder.