Preparation And Properties Of Nd-Fe-B Magnets By Spark Plasma Sintering And Hot Deformation

Nd-Fe-B magnets have been widely used in various fields such as electromechanical equipments,information and medical devices,due to its excellent magnetic properties.For hot deformed Nd-Fe-B magnets,the fine grain and strong intergranular exchange coupling are beneficial in improving the temperature stability.In this dissertation,isotropic nanocrystalline Nd-Fe-B magnets were synthesized by spark plasma sintering?SPS?technique.The influences of addition of low melting point PrCu alloy on the microstructure and magnetic properties for SPSed Nd-Fe-B magnets were investigated in details.Based on the SPS precursors,anisotropic nanocrystalline magnets were prepared by SPS followed by hot deformation.The grain boundary diffusion process?GBDP?is employed to improve magnetic properties of hot deformed magnets.Eventually,hot deformed nanocrystalline Nd-Fe-B magnets with high coericivty and good temperature stability were prepared successfully.Isotropic nanocrystalline Nd-Fe-B magnetswith excellent properties were prepared by SPS using the sintering condition of 700?/50MPa/5min.The remanent polarization of 0.83 T,coercivity of 1516 kA/m,and maximum energy product of 118 kJ/m³ were obtained.Due to the local high temperature field induced by spark plasma diacharge at vicinity of the particle boundaries,the distinct two-zones structure was formed in the SPSed magnets.It is found that the fine grain zones are composed of elongated grains with various aspect ratios,while the coarse grain zones mainly consist of equiaxed grains.Under the sintering condition of 650 ? /50MPa/5min,the coercivity of 1970kA/m was obtained for SPSed Nd-Fe-B magnet with 15 wt.% PrCu content.Anisotropic nanocrystalline magnets were prepared by SPS followed by hot deformation using SPS precursors.It was found thatthe use of Cu sleeve can eliminate the sawtooth appearance surrounding hotdeformed magnet,therefore,the utilization of magnets increased obviously.Also,the remanence and the maxmum energy product were imporved.Good magnetic properties with J_r=1.27 T,H_ci=752kA/m,and?BH?_max=272kJ/m³ were obtained for hot deformed magnet.Using the grain boundry diffusion process,high coercivity of 1945kA/m was acquired for hot deformed magnet,which is attributed to the reduction of Fe content in the grain boundry and the improvement of the intergranular phase distribution uniformity.The optimum magnetic propreties with H_ci=1515kA/m,J_r=1.22 T,and?BH?_max=262kJ/m³ were noticed with the GBDP condition of 500?/3h.Interestingly,it was shown that openness of recoil loops can be regulated to be nearly closed by GBDP,and a high coercivity can be acquired,which is attributed to the improvement of the intergranular phase distribution uniformity.For hot deformed magnet with GBDP,the coercivity temperature coefficient of-0.59%/K and remanence temperature coefficient of-0.13%/K were obtained,respectively.It is indicated that the temperature stability can be improved by GBDP,and promote the application in the field of high temperature.A strong domain wall pinning model was valid to interpret the coercivity mechanism.