Influence of Ca amount on the synthesis of Nd2Fe14B particles in reduction–diffusion process

• We employ spray drying for the preparation of precursor powders.
• Critical amount of Ca for the reduction of Nd-oxides is revealed quantitatively.
• The formation of Nd2Fe14BHx (x=1–5) occur in the Ca to powder ratio above 2.0.
• Final products after washing and drying show more than 10 MGOe of (BH)max.
• We propose the optimum amount of Ca in reduction–diffusion process.

Nd2Fe14B alloy particles with high coercivity of more than 10 kOe were successfully synthesized by adjusting the amount of Calcium (Ca) in reduction–diffusion (R–D) process. Calcium oxide (CaO) and unreacted Ca remained after R–D process in particles prepared by heat treatment in Hydrogen (H2) atmosphere at previous step. In the ratio of 0.4 of Ca to powders (Ca/powders, wt%), residual Ca was not detected from X-ray diffraction pattern. On the other hand, Ca appeared above the ratio of 1.0 and below the ratio of 0.2, amount of Ca was not enough to reduce Nd oxide. Moreover, excess Ca affected magnetic property of final products obtained after washing, because residual Ca gave rise to evolution of H2 gas during disintegration with water and it led to the formation of Nd2Fe14BHx (x=1–5). Finally, Nd2Fe14B magnetic particles were synthesized after washing in de-ionized water with a mean size of 2 μm and their maximum energy product showed 15.5 MGOe.