Effects of additives on hydrogenation, disproportionation, desorption and recombination phenomena in Nd₂Fe₁₄B compounds

Hydrogenation, desorption, disproportionation and recombination (HDDR) phenomena are very attractive for the production of anisotropic NdFeB powders with high coercivity. HDDR phenomena are closely related to the characteristics of hydrogen absorption and desorption which correspond to the disproportionation and recombination reactions respectively. The effects of additional elements on these characteristics have been studied. According to the results for Nd_12.6Fe_81.4B_6.0 alloys, Nd₂Fe₁₄B compounds disproportionate into a mixture of NdH₂, α-Fe and Fe₂B phases at around 650 °C, which recombine to give the original compound at around 1050 °C under atmospheric pressure of hydrogen. The Nd₂Fe₁₄B compound exhibits two hydrogen desorption peaks corresponding to the recombination reaction. Judging from this result, it is considered that the recombination reaction occurs by a two-stage process. The temperature of the hydrogen absorption peak corresponding to the disproportionation reaction shifts to a higher value on the addition of Zr. Alloys with Co or Ga added show lower optimum temperatures of the recombination reaction than those of the alloy without additional elements. It can be said that Zr addition makes the disproportionation reaction sluggish, and Co or Ga addition enhances the recombination reaction in HDDR phenomena.