Magnetron Sputtering Of Sintered NdFeB Magnets For Heavy Rare Earth And Alloy And Grain Boundary Diffusion Research

At room temperature,sintered Nd-Fe-B magnets show excellent magnetic properties,so they are one of the most widely used magnetic materials.However,with the increase of working temperature,the magnetic properties of sintered Nd-FeB magnets will decrease greatly,which limits its application in high temperature field.It is a reliable method to improve the temperature stability of sintered Nd-Fe-B magnets by adding heavy rare earth elements(HREE)to enhance the coercivity.The grain boundary diffusion process(GBDP)is the most efficient way to improve the coercivity of sintered Nd-Fe-B magnets.However,the heavy rare earth elements Tb and Dy,which are mainly added at present,are strategic resources with high application value.In this paper,a new method of magnetron sputtering vacuum deposition of multi-layer heavy rare earth and alloy thin films is adopted.The films of heavy rare earth and alloy elements with different compositions are deposited on the surface of the magnet.After vacuum heat treatment,the coercivity of the diffusion magnet is greatly improved.In this paper,the effects of diffusion time and the addition amount of auxiliary element Al on the magnetic properties,microstructure and diffusion depth of heavy rare earth elements in the process of grain boundary diffusion were systematically studied.The equivalent formula of auxiliary alloying element Al for heavy rare earth elements terbium was proposed to further characterize the partial substitution of auxiliary elements for heavy rare earth elements.The influence of other auxiliary elements on the magnetic properties of the diffusion magnet is also studied.Compared with the traditional co deposited thin films,the films obtained by magnetron sputtering have better film substrate adhesion and sputtering efficiency.The coercivity of the sputtered magnets can be increased to 20.0kOe,which is higher than that of the co deposited magnets by 18.8kOe.The squareness of the magnet with multilayer films is 0.95,which is also higher than that of the magnet with multilayer films.This is due to the fact that the heavy metal element Tb does not accumulate near the surface of the magnet as the diffusion co deposition film does,but diffuses deeper into the magnet,and has finer and continuous grain boundary phase.When the Tb content in the film accounts for 0.24% of the mass of the magnet,the coercivity of the sintered Nd-Fe-B magnet can be further increased to 21.36 kOe by adding Al as the auxiliary alloying element on the basis of increasing the coercivity of the sintered Nd-Fe-B magnet to 20.56 kOe.The coercivity of the diffusion magnet increases from 20.78 to 21.36 kOe when the diffusion time increases from 3h to 5h,and decreases to 19.43 kOe when the diffusion time continues to extend to 10 h.The microstructure observation shows that the grain size of the sintered NdFe-B magnet increases significantly when the diffusion time is extended to 10 h.It is this grain growth that has a negative effect on the coercivity of the sintered Nd-Fe-B magnet.The coercivity of the sintered Nd-Fe-B magnet increases from 19.43 kOe to20.81 kOe when the content of Tb in the film is 0.24% of the mass of the magnet and the content of Al is increased from 0.065% to 0.52%.However,the coercivity of the sintered Nd-Fe-B magnet decreases to 20.73 kOe when Al is added to 1.04%,which hinders the further improvement of the sintered Nd-Fe-B magnet.The addition of Al hinders the diffusion of HREE from the grain boundary phase to the main phase in the process of grain boundary diffusion.Therefore,with the increase of Al addition,the size of the core in the core-shell structure of the magnet increases,while the size of the shell decreases.However,the effect of Al on the overall grain size is less than that of Tb and diffusion time.Al can inhibit the formation of abnormal core-shell structure,which is harmful to the coercivity,by preventing the diffusion of Tb from the grain boundary phase to the main phase,and thus plays a positive role in the coercivity of sintered Nd-Fe-B magnets.In the diffusion pure Tb magnets,Tb is consumed rapidly in the range of 0-12 ?m,while the consumption rate of the heavy rare earth decreases obviously with the addition of Al,and Tb can diffuse deeper into the magnet.The promotion effect on the diffusion depth of HREE increases with the increase of the addition of Al.The addition of Al increases the diffusion depth of heavy rare earth and improves the utilization rate of heavy rare earth,which can partly replace expensive resources and save production cost.The equivalent value of Tb increased from 0.072 to 0.26 when the amount of Tb increased from 0.12% to 0.48% and the amount of Al remained unchanged at 0.26%.Al can effectively replace the heavy rare earth elements partially and improve the coercivity of sintered Nd-Fe-B magnets.At the same time,the proposed equivalent formula and the calculation of equivalent value are also helpful to optimize the production process and save production cost.For remanence,maximum energy product and squareness,the same auxiliary elements have different effects on them in different HREE diffusion magnets.For example,in Tb diffused magnets,the squareness increases from 0.94 to 0.95 with the increase of Al content from 0.065% to 0.26%,while in Dy diffsued magnets,the same amount of Al is added,The squareness decreased from 0.93 to 0.92.However,in the magnets with the same heavy rare earths,for example,0.24% Tb and 0.52% Al can increase the coercivity to 20.81 kOe,while the same amount of Cu can only increase the coercivity to 19.00 kOe.The contribution of Al to the coercivity is greater than that of Cu.