Development Of Heavy Rare Earth Thermal Diffusion Coatings For Fabricarion Of High-Performance And Low-cost NdFeB Sintered Magnets And Its Performance

Sintered NdFeB magnets have excellent magnetic properties,and have been applied in a large variety of applications,including energy,aerospace,military,electromechanical,information,robotics,intelligent manufacturing,wind power generation,and electric vehicles.However,the coercivity of sintered NdFeB magnets decreases sharply with increasing temperature due to the thermal demagnetization effect,which limits its application seriously.In order to improve the operating temperature of the magnet,an effective way is to increase its coercivity.Adding heavy rare earth element Dy into the magnet is the technique commonly used to increase the coercivity.However,excessive Dy will cause a significant reduction in remanence and magnetic energy product.Moreover,the large consumption of heavy rare earths leads to high manufacturing costs.Recently,a technology of heavy rare earth thermal diffusion was proposed,in which Dy or Dy compound powder is coated on the surface of magnets,and subsequent heat treatment allows Dy to diffuse into the magnet along the grain boundary,which greatly improves the coercivity and reduces the usage of heavy rare-earth.For thermal diffusion technology,the coating method of heavy rare earth is a key problem.Existing Dy coatings are usually prepared by mixing alcohol and DyF3 powder.After the coatings are deposited on the magnets,the DyF3 coating layer formed on the surface is uneven and easy to peel off,resulting in unstable thermal diffusion effect,uneven performance of the magnet,and increased cost.Therefore,the development of new coatings for thermal diffusion is an urgent problem to be solved in the preparation of high-performance,low-cost NdFeB magnets.In view of the above background,a new type of coating used for heavy rare earth thermal diffusion is developed in this work.The coating layers obtained with the new coatings perform the advantages of uniform and flat,good adhesion,etc.,and the difficulties in the existing coating thermal diffusion technology are solved effectively,which is of great applied value in the promotion of the thermal diffusion technology.Additionally,the thermal diffusion process of DyF3 was analyzed,and the effects of particle size and addition amount of DyF3 on the thermal diffusion mechanism of Dy were discussed,so it has certain theoretical significance.The new coating is mainly composed of main solids(DyF3)and organic components(resin,dispersant,and wetting dispersant,etc.).The organic component formula of the coating was determined firstly.When the amount of resin is 3 wt.%,the coating layer has good adhesion and its surface is dense.A uniform and smooth coating layer is obtained when the addition amount of dispersant,wetting dispersant and leveling agent is 0.5 wt.%,0.4 wt.%,and 0.6 wt.%,respectively.Under this condition,the drying time of the coating liquid is 25 s,the optimal level of leveling property is 10,and the adhesion of coating layer reaches level 4.What’s more,the thickness of coating layer can be controlled by adjusting the viscosity of the coating,thereby,the coating layer with different DyF3 coating amounts can be obtained.In addition,the effects of coating amount and particle size of main solids DyF3 on the properties of the magnet after thermal diffusion were studied.With the increase of DyF3 coating amount,the coercivity of the magnet increases first and then decreases.When the coating amount is 1.21 wt.%,the coercivity increases from 14.60 kOe to 19.78 kOe,an increase of 35.5%is achieved.With the decrease of DyF3 particle size,the coercivity of the magnet presents a similar trend,namely,it increases first and then decreases.More in-depth,the mechanism of the performance modified by the thermal diffusion was analyzed by applying an electronic probe analysis.According to the results,the more DyF3 is applied,the more Dy diffuses into the magnet,so the coercivity is significantly improved.However,when it is too much,Dy enters the main phase grains,which deteriorates the performance.The smaller the particle size of DyF3,the better the thermal diffusion effect.However,when the particle size is too small,the oxygen content in the DyF3 fine powder increases.During thermal diffusion process,O element enters the NdFeB magnet through the grain boundary,forming a large number of nonmagnetic Nd2O3 phases in the NdFeB magnet,resulting in a reduction of the coercivity.A multi-functional thermal diffusion coatings was also prepared by adding Al powder to the heavy rare earth thermal diffusion coating.During the thermal diffusion process,Al diffuses into the magnet through the grain boundary,which can improve the grain boundary structure of NdFeB magnets,reduce the electrode potential difference between the grain boundary and the main phase grains,ensuring the good thermal diffusion effect while improving the corrosion resistance of the magnet effectively.Undoubtedly,the multi-functional thermal diffusion technology may be a new route for preparing high-performance,low-cost sintered NdFeB magnets in the future.