Thermodynamics of Oxygen in the Fe-Nd-O System for Production of Dy Free Nd-Fe-B Magnet

Abstract

To improve the coercivity of Dy free Nd₂Fe₁₄B magnet, it is necessary to prevent oxidization of Nd-rich phase formed in grain boundaries on the basis of the information of oxygen behavior in this material which has been not well revealed so far. The present study aims to clarify the thermodynamic property of oxygen in the Fe-Nd-O system. The solubility of oxygen in molten Fe-Nd alloy coexisting with solid Nd₂O₃ has been measured at 1473 and 1673 K. On the basis of this results, the standard Gibbs energies for the dissolution of oxygen into molten Fe-Nd alloy at 1473 and 1673 K have been determined to be −397.3～−411.6 and −314.6～−396.8 kJ/mol, respectively. The dissolution behavior of oxygen into molten Fe-Nd alloy is thermodynamically discussed and found to be dominated by chemical property of neodymium. Addition of calcium or neodymium fluoride is found promising for effective deoxidation of the present alloy.