Investigation of the possibility of Pr₂Fe₁₄B/α-Fe composite obtaining by Pr–Fe–B alloy oxidation in a fluidized bed jet mill

The paper presents the results of studies into the possibility of obtaining Pr₂Fe₁₄B/α-Fe composites by Pr–Fe–B alloy oxidation in a fluidized bed jet mill. It is shown that  Pr₂Fe₁₄B/α-Fe composites with high magnetic characteristics can  be obtained using the standard powder metallurgy technology supplemented by Pr–Fe–B alloy oxidation in a fluidized bed jet mill for rare earth hard magnetic materials. It is found that there is an increase in residual induction (B_r) with a slight drop in the coercive force ( jH_c) during the fine powder production according to the proposed technology in argon containing up to 0.2 vol.% of oxygen. This effect causes the  maximum  energy product (BH)max to  increase  by 5 %.  With  further increase  in  the oxygen concentration,  the Pr_xFe, high-praseodymium phase almost completely oxidizes. This leads to a sharp drop in the coercive force and, as a consequence, to a drop in (BH)_max. α-Fe particles resulting from magnetic material oxidation form at the boundaries between the Pr₂Fe₁₄B phase grains. Maximum magnetic characteristics are achieved when α-Fe particles are separated from the main magnetic phase grains by thin layers of non-magnetic phases. This allows maintaining a high  coercive force jH_c  for sintered hard magnetic material samples. The optimal thickness of α-Fe layers is 0.2–0.3 μm. α-Fe layers were significantly thicker (0.8 to 1.1 μm) for samples obtained at an oxygen content of 0.3 vol.%. As a result, the coercive force of samples reduced by almost 10 %, while other magnetic parameters (B_r , (BH)_max)  decreased by 3–7 %. Therefore, it is possible to change the thickness of the α-Fe phase layer formed in the Pr₂Fe₁₄B/α-Fe composite and  control its magnetic parameters by adjusting the oxygen content in the jet mill medium.

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