Synthesis of Hard Magnetic Nd-Fe-B Powders by a Thermochemical Process

Abstract

Fabrication of Nd-Fe-B magnetic powders by R-D process using spray-dried precursors was tried. Due to stable formation of a-Fe as a primary phase as same as that in ingots, the hard magnetic Nd₂Fe₁₄B phase was not formed in the alloy powder with the composition of stoichiometric Nd₂Fe₁₄B unless excessive amount of Nd precursor or Ca was added in the process. However, this excessive addition, especially for Ca, led to abnormal grain growth of Nd₂Fe₁₄B. On the contrary, Nd₂Fe₁₄B was successfully obtained as a major phase from the alloy powder with the composition of Nd₁₅Fe₇₇B₈ without such excessive addition. The size of spherical shape Nd₂Fe₁₄B particles mostly consisted of one or more subgrains was less than 1, um. The coercivity measured from this powder was relatively low (-3 kOe) mostly due to residual CaO and soft magnetic phases.