Effect Of The Surface Layer Magnetic Hardening Structure On Properties Of Sintered Nd–Fe–B Magnets

In recent years,as the global actively promotes energy conservation and environmental protection,green energy fields such as wind power develop rapidly,looking forward to high performance sintered Nd–Fe–B with high temperature stability.However,the magnets prepared by existing technology can not meet the application needs of high temperature environment.To solve this problem,the common solution is adding heavy rare earth（HRE）elements through grain boundary diffusion（GBD）to increase the coercivity at room temperature,in order to obtain high-temperature coercivity values.To improve the magnetic properties of sintered Nd–Fe–B magnet,the chemical bath deposition（CBD）method and electrophoretic deposition（EPD）method were used,further exploring the GBD process.Using CBD method,the magnetic powders were wrapped uniformly by HRE compound,after sintering and annealing treatments,the macroscopic surface magnetic hardening structure was formed,breaking through the thickness limitation of the conventional diffusion magnets;Using EPD method,HRE low-cost low-melting point alloy powders were coated homogeneously on the surface of the magnet.In the diffusing process,low-melting point metal decreased the melting point of grain boundary（GB）phase,increased the diffusion coefficient of elements in GB phase,assisting HRE elements to penetrate along GB to the interior of the magnet,achieved the microscopic surface magnetic hardening structure.By regulating the distribution of HRE elements Dy and Tb at grain boundaries,the phase composition of GB and the microstructure of the magnet were optimized.The effect of HRE elements concentration gradient distribution on the properties of sintered Nd–Fe–B magnets from grain surface to core was systematically studied.The research of the surface magnetic hardening structure is of great significance for the preparation of high performance and high temperature stability magnets.1.Adopting CBD method,mixed HRE organic compound Dy（CH₃CHOHCH₃）₃（Dy-ipa）with alcohol and ultrasonic dispersed forming HRE organic suspension,then soaked magnetic powders in the alcohol suspension,sintered Nd–Fe–B magnetic particles were surrounded by HRE element homogeneously,breaking through the thickness limitation of diffused magnets.By controlling the parameters like the addition amount of Dy-ipa and sintering temperature of CBD magnets,obtaining the best comprehensive performance.The results of EDS analysis showed that the concentration of Dy in the grain boundaries was higher than that in grains,forming the surface magnetic hardening structure.The magnetic flux loss and the temperature coefficient of coercivity and remanence of the CBD magnets were all decreased,the thermal stability had promoted.The CBD method not only effectively penetrates the limitation of the slice magnets,but also provides guidance for working out sintered Nd–Fe–B magnet with high coercivity.2.Taking advantage of EPD technique,prepared uniform and thickness could be controlled HRE alloy coatings on the surface of the magnets,confirming the optimal diffusing temperature and diffusing time of Tb₈₅Fe₁₅5 and Tb₈₅Cu₁₅5 alloy powders diffused magnets.Through optimizing the process parameters,the coercivity of two diffused magnets were increased from 14 kOe to 22 kOe,rising by 8.0 kOe.XRD results declared that Tb partly replaced Nd in matrix phases,further microstructure analysis indicated（Nd,Tb）₂Fe₁₄B core-shell structure formed in grain surface.Under the same process condition,compared with Tb₈₅Fe₁₅,Tb₈₅Cu₁₅5 diffused magnet developed thicker core-shell structure in the diffused magnets surface layer,higher concentration of Tb in shell,deeper diffusion level and grain boundaries were more continuous.Explained the relationship between the thickness of shell layer,the concentration and distribution of Tb and the distance to the magnet surface,clarifying the coercivity strengthening mechanism;Discussed the employed properties of the diffused magnets,revealing that at the same temperature,the magnetic flux loss of the diffused magnets were much lower than that of the original magnet.The irreversible flux loss of the untreated magnet at 80℃was more than 3%,while the diffused magnets still met the demand at 150℃,exhibiting the improved temperature stability dramatically.Using EPD method,Tb was mainly distributed in grain boundaries and grain surface,strengthening the coercivity significantly,improving the employed property at high temperature remarkably as well.It is instructive and meaningful to prepare both high temperature stability and high performance sintered Nd–Fe–B magnets at a lower HRE concentration.