Influences of spark plasma sintering temperature on magnetic properties of both hot pressing and hot deforming NdFeB nanocrystalline permanent magnets

Both hot-pressed and hot-deformed NdFeB magnets were prepared by spark plasma sintering technique.Effects of variable sintering temperatures on microstructures and magnetic properties of both hot-pressed and hot-deformed magnets were studied.The results showed that the density of the hot-pressed magnets increased with the sintering temperature,and reached 99.7% of theoretical density at 680℃.Besides,the grain grew up with the temperature.The remanence and the maximum energy product were influenced by the interaction of the density and grain size,which the maximum value appeared at 650℃:(BH)m=129kJ/m3,Br=0.87T,Hci=914kA/m.After hot deformation,the c-axis of the tetragonal structure gradually shifted to the direction which paralleled with pressing direction,and formed magnetocrystalline anisotropy,as a result,the remanence and the maximum energy product of the magnets increased significantly.The hot-pressed temperature has great impacts on the magnetic properties of hot-deformed magnets.With the hot-pressed temperature increasing,the remanence and the maximum energy product rose at first,and then decreased.And the hot-deformed magnets,which were hot pressed at 650℃,achieved the best magnetic properties:(BH)m=339kJ/m3,Br=1.49T,Hci=576kA/m.