Study On The Effect Of Al And Rare Earth Grain Boundary Co-Diffusion On The Microstructure And Coercivity Of Sintered Nd-fe-B Magnets

Sintered Nd-Fe-B materials are the key basic functional materials for the economy.However,their coercivity deteriorates with the increasing temperature.To meet the demand in high-temperature application,it is of great importance to enhance their coercivity.At present,the enhancement of coercivity needs to be realized by the substitution of heavy rare earth for Nd,and the utilization of heavy rare earth is more energy-efficient.The need for emission-reduction and new-energy areas.The heavy rare earth grain boundary diffusion technology greatly improves the utilization of heavy rare earth by improving the distribution of heavy rare earth,and provides a technical route for further improving the utilization.In this dissertation,the diffusion of low-melting-point metals and rare earth elements is studied to study the variation of microstructure and magnetic properties with various diffusion sources and heat treatment.The mechanism of coercivity is discussed.A grain boundary diffusion technique for the coordinately controlling the distribution of heavy rare earth distribution and grain boundary phase to optimize the coercivity and a technique for optimizing the coercivity of the magnet without heavy rare earth and continuous boundary phase are developed,and the compatibility is explored.It is of significance for the industrialization and application of grain boundary diffusion technology for coordinately controlling the microstructure and magnetic properties of magnets.The main results are as follows:?1?The co-diffusion element Al is selected and preparation method of diffusion sources with rare earth elements was determined through experiments.The diffusion source?PrNd-H?_1-xAl_x and?Tb-H?_1-xAl_x were prepared by mixing Al and rare earth elements.The composition of the diffusion source is limited to the current state of the low melting point alloy composition.The influence of Al and Tb/PrNd co-diffusion on the microstructure and coercivity of the sintered Nd-Fe-B magnet is systematically studied.In the heating process of grain boundary diffusion,Al first reacts with Tb/PrNd to form Tb₂Al₉₈/?PrNd?₈Al₉₂,and melts immediately,which is beneficial to the contact of rare earth elements with the high-diffusivity path grain boundary,and diffuses into interior of the magnet.During the grain boundary diffusion process,Al promotes the grain boundary diffusion of Tb/PrNd.?2?The study on the influence of processes of heat treatment,such as diffusion source composition,diffusion time,diffusion temperature and low-temperuature annealing on the microstructure and coercivity of the magnet are conducted.It can be found that continuous grain boundaries are formed by grain boundary co-diffusion of Al and rare earth elements.The formation of the grain boundary phase needs conditions sufficient Al in the diffusion source,a high enough diffusion temperature and a heat treatment at 500°C.In addition,the co-diffusion rate of Al and Tb/PrNd depends on the temperature.The higher the temperature,the larger the diffusion rate;the effect of diffusion at a certain temperature depends on the diffusion time,and the longer the diffusion time,the more sufficient the diffusion of Tb/PrNd and Al,the more uniform the element distribution.?3?Al and Tb enter the interior of the magnet during the co-diffusion process,forming?Nd,Tb?₂?Fe,Al?₁₄B,which is the Tb-rich main phase grain shell,and Al is also enriched in the grain shells.When a annealing at 500°C for a diffusion source with a Al content higher than 0.7,a continuous grain boundary phase is formed,which contains Fe and Al compared to the conventional grain boundary phase and can Good demagnetization coupling is an important reason for the enhancement of coercivity of the magnet by grain boudnary diffusion of the high Al content diffusion source.?4?In the magnets by Al and Tb grain boundary co-diffusion treatment,as the content of Al increases,the thickness of the shell layer of the magnet gradually decreases,the coercivity first is enhanced and then deteriorates,and reaches the largest when the Al content is 0.4,from the initial 13.73 kOe to 23.68 kOe.When the Al content is 0.7,the coercivity is 23.29 kOe,the coercivity is 12.3 kOe higher than 1.03 kOe when the diffusing Tb-H,and the coercivity temperature coefficient of 25¹50°C is-0.53%/°C,and The heavy rare earth content of the magnet with an Al content of 0.7 is only 0.47 wt.%,and the heavy rare earth content of the magnet is 1.85 wt.%,which reduces 74.6%of the rare earth.?5?Al and PrNd migrate in the interior of the magnet during the co-diffusion process.When the grain boundary is diffused at low temperature?700°C and 800°C?,the Al content is too high?x=1?,which leads to the formation of Nd₂?Fe,Al?₁₇.The formation of phase deteriorates the performance of the magnet;the low content of rare earth in the matrix also leads to the formation of Nd₂?Fe,Al?₁₇,which can be eliminated by treatment at a high temperature of 900°C.This indicates that the addition of PrNd can inhibit the formation of Nd₂?Fe,Al?₁₇ within a certain range,and it is beneficial to form a continuous boundary phase and enhance the demagnetization coupling between grains.?6?In the magnets of Al and PrNd eutectic diffusion treatment,the coercivity of the diffusion treated magnet reaches a maximum at x=0.6,from 13.7 kOe to 16.77 kOe,an increase of 3.07 kOe,in low rare earth content magnets.The medium coercivity increased from 12.66 kOe to 15.61 kOe.In the magnets containing Y and Ce,the coercivity increased from 10.31 kOe to 12.73 kOe,from 12.43 kOe to 14.51 kOe,and the increase was 2.95kOe,2.42 kOe,2.08 kOe,equivalent to the coericivity level via 1²%Dy addition in traditional route.In addition,by comparing the effect of Al and Sm,Er,Tb,and PrNd on the co-grain boundary diffusion,the enhancement of the relative demagnetization coupling of the continuous boundary is illustrated.This study systematically studies the mechanism of performance and microstructure,discusses the coercivity mechanism,provides theoretical guidance for microstructure control and coercivity,and achieves efficient use of rare earths.The completion of this project explores and develops high-performance,high-rare earth utilization sintered Nd-Fe-B grain boundary diffusion technology,which is of scientific significance and application value for the development of Nd-Fe-B materials.