Research Of Improvement For Properties Of Nd-Fe-B Magnetic Powders By Grain Boundary Diffusion Process

NdFeB permanent magnets have been widely used as the best permanent magnet materials for comprehensive magnetic performance.However,its Curie temperature is low(314?)and its thermal stability is poor.When the temperature rises,the coercivity decreases sharply with increasing temperature.Especially in recent years,NdFeB permanent magnets are required to have good thermal stability in some new applications such as electric vehicles and hybrid vehicles.It is that the high coercivity at room temperature can resist the thermal demagnetization of magnets under high temperature(above 150?)working environment.NdFeB magnetic powders are often used as a raw material for bonded magnets.Its coercivity and thermal stability are usually low.The grain size is usually less than 3 ?m and the grain boundary phase is very thin.Compared with sintered NdFeB magnet whose crystal grain size is larger than 3 ?m and the grain boundary is thicker,there is more room for improvement in coercivity and is more likely to obtain high coercivity in NdFeB magnetic powders.Therefore,the purpose of this paper is to increase the coercivity of NdFeB magnetic powders by means of grain boundary diffusion,thereby improving the coercivity and thermal stability of bonded magnets.Finally,the magnetic flux loss of the bonded magnet in a high temperature environment is weakened as much as possible,so as to meet the demand for the thermal stability of the NdFeB magnet in some high temperature applications.In this experiment,grain boundary diffusion was used to improve the magnetic properties of NdFeB magnetic powders,and diffusion agent such as NdCu,PrCu,NdAl,PrAl,NdCuAl,PrCuAl,and NdCuAlZnx were prepared by hydrogenation and rapid quenching methods.The experiment systematically explored the various factors affecting the diffusion effect and derived the final preferred process parameters.At the same time,the magnetic properties and microstructure of the magnetic powders before and after the grain boundary diffusion were systematically studied using the analysis equipment such as X-ray diffraction(XRD),vibrating sample magnetometer(VSM),transmission electron microscope(TEM),and scanning electron microscope(SEM).The behavior of the diffusion process was analyzed and the bonded magnets were finally prepared for thermal stability characterization.(1)The main factors influencing the grain boundary diffusion of magnetic powders included diffusion temperature,time,particle size distribution of diffusion agent and magnetic powders,ratio of diffusion agent,type of diffusion agent,etc.The ratio of particle size of the quenched magnetic powders to the diffusion agent during grain boundary diffusion should be 80 mesh:80 mesh.The coercivity increased as the content of diffusion agent increased and eventually saturated.Different types of diffusion agent corresponded to different preferred temperatures and times,in which NdCu and NdCuAl were 650? for 1 h,PrCu,PuCuAl were 625? for 3 h,NdAl and PrAl were 675? for 0.5 h.When the grain boundary diffusion of anisotropic(HDDR)magnetic powder was performed,the ratio of magnetic powders to the size of the diffusion agent should be 80 mesh:200 mesh.The preferred temperature and time for the diffusion of NdCuAl and NdCuAlZnx was 760? for 1 h.At the same time,the diffusion agent ratio was preferably controlled between 2%and 4%.In addition,when the atomic percentage of Zn was x=3,the diffusion effect was relatively best.(2)After the grain boundary diffusion,the coercivity of the quenched magnetic powders changed significantly.The increase of coercivity was 41%,131%and 189%respectively with the diffusion agent content of 6%,10%and 20%.The optimum coercivity was achieved when the diffusion agent content was 30%.The optimum diffusion treated powders had a coercivity of 23.14 kOe which was 211%of the initial powders.Due to the addition of a large amount of non-magnetic alloys into the magnetic powders,the magnetic energy products and the remanence would decrease to some extent.The initial magnetization curves indicated that there was the combined effect of nucleation and pinning mechanism in initial NdFeB magnetic powders.With the increase of diffusion agent content,the pinning mechanism gradually dominated during the magnetization process.The coercivity of HDDR magnetic powders increased by 1kOe?2kOe after the diffusion of 2%NdCuAl and NdCuAlZn3.And the coercivity of the diffused NdCuAlZn3 increased by 0.1 kOe?0.2kOe compared with that of the diffused NdCuAl.(3)The XRD patterns of quenched magnetic powders indicated that the initial powders were composed of Nd2Fe?4B phase and a-Fe phase.But when the content of'diffusion agent exceeded 10%,the Nd-rich phase and some undefined phases were also detected.Therefore,it was inferred that when the content of the diffusion agent was large.it could not completely diffuse into the magnetic powders and a part of it was melted and wrapped around the magnetic powders which was confirmed by the SEM image of the diffused magnetic powders.In addition,the effective diffusion of 10%and 30%NdCuAl diffused magnetic powders was approximately 44%and 50%.The TEM image of the quenched magnetic powders showed that the number,thickness,uniformity,and continuity of the grain boundary phase would gradually increase as the content of diffusion agent increased.The grain boundary thickness would increase from the initial 1 nm to the 5 nm.and the grain size would grow from the initial 30nm to the 50?70nm.The Nd-rich thin layer formed by the grain boundary diffusion could be used as a very effective pinning center to prevent the movement of the domain walls.This pinning effect was ultimately manifested in the improvement of the coercivity.The SEM images of HDDR magnetic powders showed that the number of grain boundary phases after diffusion increased significantly,but the overall distribution was inhomogeious.The 2%NdCuAlZn3 diffusion showed obvious grain boundary bands which indicated that the addition of Zn might help to increase the mobility of the diffusion agent and increase the diffusion efficiency.(4)After grain boundary diffusion.the flux loss of the bonded magnet could be reduced by about 80%at the same temperature and time,and the coercivity temperature coefficient increased from-0.39%/? to-0.37%/?.The coercivity improved from 3.1kOe without diffusion to 9.8kOe when the temperature increased to 175?,and thus the time stability and thermal stability of the bonded magnetic were significantly improved.