Research On Thermal Stability Of TbCu₇-type Isotropic SmFeN

With excellent magnetic property and isotropic characteristic,Rare earth permanent magnetic materials for complex bonded magnets are widely used in various fields such as miniature motors,actuators,and sensors etc.Magnetic property of TbCu7-type SmFeN magnetic materials is comparable to NdFeB bonded magnetic materials,moreover,TbCu7-type SmFeN has advantages of high Currie temperature,better corrosion resistance,and low raw material cost.However TbCu7-type SmFeN bonded magnets show large flux loss when working at high temperatures.The thermal stability has become an important factor that limits the commercial application of isotropic SmFeN.In this paper,some factors that affect magnetic property and thermal stability of SmFeN bonded magnets,effects of the second phase and Nb addition on SmFeN microstructure and magnetic property and thermal stability,have been investigated systematically.The investigation was carried out by means of vibrating sample magnetometer(VSM),X-ray diffraction(XRD),magnetic force microscope(MFM),transmission electron microscope(TEM),et al.The following are summaries:After 300 hours' exposure at 120?,the irreversible flux loss of SmFeN bonded magnets increased to over 10%,while MQ powders NdFeB 1412 with high coercivity showed less than 2%flux loss.There is a certain amount of magnetic soft phase a-Fe in SmFeN compounds,and the ?-Fe phase will augment after a long period of exposure at high temperature,which increases the nucleation of reversed magnetic domain.The inhomogeneous microstructure,coarse grain structure and magnetic soft phase of TbCu7-type SmFeN directly deteriorate its intransic coercivity,and this leads to serious irreversible flux loss of magnets.The Sm8.5Fe85.8Co4.5Zr1.2 ribbons were prepared through melt spinning at 50m/s surface velocity.By the adjustment of annealing process,the phase structure of ribbons was modified to promote the TbCu7-type metastable SmFe9 transforms to Th2Zn17-type second phase.The Rietveld analysis(XRD)and TEM photographs indicated,the phase structure of SmFeCoZr alloy annealed at 825? comprised 89.0%TbCu7-type metastable phase and about 9.4wt%Th2Zn17 type second phase,in contrast to TbCu7-type dominated SmFeCoZr alloy annealed at 775?.The grain size was increased from 50?100nm for 775?to more than 200nm for 825 ?.In comparison with simple TbCu7-type structure SmFeN,the coercivity of SmFeN containing both type structures was enhanced from 9.6kOe to 11.5kOe,and its magnetic energy product and squareness were decreased slightly.The two hour irreversible flux at 120?of SmFeN bonded magnets was reduced from 9.8%to 6.1%respectively.The optimum magnetic property of SmFeN nanocomposite was achieved:Br=8.5kGs,Hcj= 11.5kOe,(BH)max=13.4MGOe.The compositions of Sm8.5Feg5.8-XCo4.5Zr1.2Nbx were prepared by the method of melt-spun.Nb addition caused the lattice expansion as Nb atoms normally occupy the position of Fe atoms,so the lattice parameter ratio c/a of TbCu7-type structure was increased up to 0.871.Nb addititon makes the TbCu7-type crystal structure more stable.Nb addition impedes the formation of ?-Fe and Sm2Fe17 in the as-quenched alloy and stabilizes the TbCu7-type metastable SmFeg phase.When the content of Nb element x>1.5,the Sm8.5Fe85.8-xCo4.5Zr1.2Nbx ribbons contains much more amorphous structure.Nb addition refines the grain structure of SmFe ribbons.TEM photographs show that the average grain size of SmFeCoZr ribbons is 50nm,while the average grain size of specimen with 1.2at%Nb addition is reduced to 35nm.According to the result of magnetic force microscope,magnetic domains of SmFe nanocrystalline are much larger than most grains on the scale,thus the reversed magnetic domains' propagations suffer from pinning effect of grain boundaries.Nb addition enhances the pining effect on magnetic domain movement by promoting the homogeneous microstructure with more grain boundaries.The hysteresis loops of SmFeN illustrated the tendency of pining field Hp as Nb content increases,Hp first increased and achieved the highest value 11.9kOe and then decreased when Nb content exceed 1.2.The optimum magnetic property of SmFeN with 1.2at%Nb addition was achieved:Br=8.2kGs,Hcj=9.7kOe,(BH)max=11.4MGOe.The two-hour irreversible flux loss at 120? of SmFeN bonded magnets decreased notably from 8.3%to 6.3%,with 1.2at%Nb addition.The refined microstructure enhances the pining effect,reduces the irreversible flux loss and improves the thermal stability of TbCu7-type SmFeN magnets.