Research on the Grain Boundary Diffusion of Sintered Nd-Fe-B Magnets with Low Melting Eutectic Alloys

Author:Chen Fu Gang

Supervisor:zhang lan ting


Degree Year:2018





As the third generation rare earth permanent magnet Nd-Fe-B magnet is famous for its high magnetic flux density,and it has been widely used in industry.The high magnetic flux density of Nd-Fe-B magnets satisfies the miniaturization development of equipments.In recent years the rapid developments of green transportation vehicles(electric/hybrid vehicles),wind power energy industry,high efficiency frequency-controlled compressors etc.increase the demand of Nd-Fe-B magnets with high coercivity greatly.At present the production of Nd-Fe-B magnets with high coercivity is usually realized via the substitution of light rare earth element Nd by heavy rare earth element Dy.But two drawbacks are inevitable,on the one hand,substituting Nd by Dy increases the cost of raw materials;On the other hand,the antiparallel coupling of magnetic moments between Dy and Nd decreases the remanence,leading to the decrease of magnetic flux density.Developing the high coercivity Nd-Fe-B magnets without Dy or with low Dy has been a hot research topic in recent years.The grain boundary diffusion process(GBDP)is a new way to fabricate high coercivity Nd-Fe-B magnets with the minimum usage of heavy rare earth element Dy.This technique can increase the coercivity of Nd-Fe-B magnets while keeping a high remanennce.This thesis investigated the effect of GBDP on sintered Nd-Fe-B magnets with two different kinds of rare earth eutectic alloy diffusion sources which have low melting points:one is light rare earth element eutectic alloy Nd70Cu30,the other is heavy rare earth element eutectic alloys Dy70Cu30 and Dy60Co40.In addition,the thesis investigated the effect of cooling rate after annealing on the magnetic properties of sintered Nd-Fe-B magnets.And this thesis also studied the magnetic properties of the surface damaged layer of sintered Nd-Fe-B magnets.The main conclusions are as follows:The diffusion driving force of Nd70Cu30 eutectic alloy in sintered Nd-Fe-B magnet is low.The low diffusion depth and inhomogenous distribution of Nd rich phases near grain boundaries restrict the enhancement of coercivity after GBDP,and the squareness factor of demagnetization curve is low.Exerting uniaxial pressure during the GBDP can effectively increase the diffusion depth and distribution homogeneity of Nd rich phases,which leads to a large coercivity enhancement and a high squareness factor.Microstructure analysis suggests that the micro cracks induced by pressure act as the good channels for diffusion.Moreover,the pressure also increases the capillary force of GBDP.Under the same pressure GBDP parallel to the c-axis is much better than that perpendicular to the c-axis.This is because at the diffusion temperature 750℃the elastic stain energy genenrated along the c-axis is much larger than that perpendicular to the c-axis under the same pressure,and large strain energy is much more beneficial for the generating of micro cracks acting as diffusion channels.Due to the low formation energy of Dy2Fe14B(-7.721 eV/unit)than that of Nd2Fe14B(-3.486 eV/unit)eutectic alloys Dy70Cu30 and Dy60Co40 have large diffusion depths in sintered Nd-Fe-B magnets.The squareness of demagnetization curve of the sintered Nd-Fe-B magnet after GBDP with Dy70Cu300 along the diffusion direction parallel to the c-axis(0.92)is higher than that with the diffusion direction perpendicular to the c-axis(0.83).Composite Nd-Fe-B magnets made of N50 and N40 magnets with different coercivity distribution gradient were fabricated by vaccum diffusion bonding.The different demagnetization behaviors of composite Nd-Fe-B magnets correspond to Nd-Fe-B magnets after GBDP with Dy70Cu30 along different diffusion directions.This indicates that the squareness of demagnetization curves depends on the orientation relatiosnship between the gradient of anisotropy field distribution and c-axis for magnets after GBDP with Dy rich diffusion sources.The result was further demonstrated theoretically via micomagnetic simulation software OOMMF.The demagnetization increases the self-magnetostatic energy for the magnet with the coercivity distribution gradient parallel to the c-axis while it decreases the self-magnetostatic energy for the magnet with the coercivity distribution gradient perpendicular to the c-axis.Thus,a much higher energy barrier should be overcome for the magnet with coercivity distribution gradient parallel to the c-axis,which makes the critical demagnetization field of the magnet with the diffusion parallel to the c-axis larger than that with the diffusion perpendicular to the c-axis.This result provides a new explaination for the different demagnetization curve squareness factors of the GBDPed Nd-Fe-B magnets along different diffusion directions reported in literature.This further suggests that GBDP parallel to the c-axis is the much more efficient to make full use of heavy rare earth Dy than that with the diffusion direction perpendicular to the c-axis.This result provides a theoretical guidance for producing high coercivity sintered Nd-Fe-B magnets with limited heavy rare earth elements using GBDP.Duplex shell structures(Dy enriched shell and Co enriched shell)were formed simultaneously near the Nd2Fe14B grain boundaries for the sintered Nd-Fe-B magnet after GBDP with Dy60Co40.The Dy enriched shell near grain boundaries increases the nucleation field of reversal domains,thus increasing the coercivity.The Co enriched shell coupled with the core enhances the remanence stability of the Nd-Fe-B magnet.The temperature coefficient of remanence improves greatly from-0.140%/°C to-0.095%/°C at the temperature range 25℃to 177℃,which makes the remanence of the GBDPed magnet higher than the annealed one at high temperatures.However,compared with the initial untreated Nd-Fe-B magnet only 2°C increase of the Curie temperature was detected(from 318℃to 320℃),which suggests that the Curie temperature is not the only determinant factor for the temperature stability of remanence.This experiment provides a new idea for fabricating Nd-Fe-B magnets with high remanence and coercivity simulatenously at high temperatures.Rapid cooling after annealing at high temperatures increases the stress in the sintered Nd-Fe-B magnet,which leads to the decrease of the coercivity.However,the cooling rate after annealing at low temperatures does not affect the coercivity.Because of surface damage and oxidation of sintered Nd-Fe-B magnets the coercivity of surface grains is much lower than that in the inner part.For sintered Nd-Fe-B magnets with small sizes the increase of specific surface area decreases the magnetic flux density,but does not affect the coercivity and remanence.This paper measures the surface coercivity of Nd-Fe-B magnets directly via magnetic optic Kerr effect.Under a low cycling magnetic field far below the coercivity of the Nd-Fe-B magnet the hysteresis loop of the surface grains can be obtained.For sintered Nd-Fe-B magnets the surface coercivity is about one tenth of the overall coercivity.