Microstructure and magnetic properties of Nd-Fe-B sintered magnets with P addition

In this paper, the effects of P doping on magnetic properties and microstructure were studied in Nd-Fe-B sintered magnets. With P doping, the grain size gets refined and the distribution of the main phase is optimized due to the reduction of the liquidus temperature. The liquidus temperature for the 0.05wt% P-doped magnets is 1022K, while that for the P-free magnets is 1038K. As P content increases, the liquidus temperature significantly decreases. Clear and continuous grain boundary phases are formed in the P-containing magnets with smaller grain size. The optimized microstructure with average grain size of 8.43m is obtained in the 0.05wt% P-doped magnets, which is approximately 0.69m smaller than that of P-free sintered magnets (9.12m). Though P is usually thought to be an impurity element, it might be beneficial in Nd-Fe-B sintered magnets with proper addition. The coercivity of the 0.05wt% P-doped magnets could be increased to 1283kAm(-1), with slight changes of the remanence and the maximum magnetic energy product. NdPO4 phases in the grain boundary are of hexagonal structure, while those at the triple junctions have monoclinic structure. Activated sintering is achieved by doping proper P element in the Nd-Fe-B sintered magnets.