Preparation And Properties Of Sintered NdFeB Magnets With High Stability

As the third generation rare earth permanent magnet materials,sintered NdFeB possesses excellent comprehensive magnetic properties and high cost performance,which is known as the contemporary"magnet king",and is an important basic functional material for social development.However,with the continuous development of wind power generation,new energy vehicles and other emerging industries,higher requirements are put forward for the stability of sintered NdFeB magnets in service.In view of the disadvantages of sintered NdFeB magnets,such as low coercivity,poor heat resistance,easy corrosion and high brittleness,the stability of magnets is improved by optimizing alloy composition,adjusting grain boundary structure and adding protective coating.The relationships between magnetic field stability,temperature stability,mechanical stability,chemical stability and microstructure of the magnet were systematically studied.The protection mechanism of nano-CeO₂ modified Zn-Al coating was also studied,which provides a reference for development and application of sintered NdFeB magnets with high stability.The main contents are summarized as follows:（1）Sintered NdFeB magnets without heavy rare earths elements were prepared by grain refinement with jet milling.The magnetic properties and stability of magnets without heavy rare earth elements and commercial magnets containing heavy rare earth elements were compared.The microstructure and images of the two magnets were characterized by X-ray diffractometer and scanning electron microscope.The temperature stability of the two magnets was studied by measuring the magnetic properties and irreversible flux loss at different temperatures.And the difference of temperature coefficient was compared.The mechanical stability of the two magnets was compared by measuring the microhardness,bending strength and compressive strength.The chemical stability of the two magnets was studied by testing corrosion weight loss,static full immersion corrosion,potentiodynamic polarization curve and AC impedance spectrum.Results show that the magnetic properties of magnets without heavy rare earth elements are almost the same as those of commercial magnets containing heavy rare earth elements at room temperature,and they possess good mechanical properties.However,the high temperature magnetic properties and chemical stability of the magnet are poor.Under the same experimental conditions,the weight loss of the magnets without and containing heavy rare earth elements are 3.3 mg/cm² and 1.9 mg/cm² respectively.（2）（Ho,Nd）FeB magnets with different Ho contents were prepared by Ho substitution for part of Nd.Effects of Ho content on the stability and microstructure of NdFeB magnets were studied.The microstructures and images of（Ho,Nd）FeB magnets with different Ho contents were characterized by X-ray diffractometer and scanning electron microscope.The optimum magnetic properties of（Ho,Nd）FeB magnets with different Ho contents were obtained by optimizing the fabrication process.Effects of Ho content on the temperature stability,mechanical stability and chemical stability of the magnet were studied.Results show that the addition of Ho can increase the H_cj and reduce the B_r of（Ho,Nd）FeB magnets to a certain extent.At the same time,the addition of Ho improves the temperature stability,mechanical stability and chemical stability of the magnets.The corrosion weight loss of（Ho,Nd）FeB magnets with 21.0 mass%Ho decreases from 2.7mg/cm² of NdFeB magnets to 0.9 mg/cm².（3）（Dy,Nd）FeB magnets were prepared by gas phase grain boundary diffusion treatment of sintered NdFeB magnets in Dy vapor ambience.The stabilities of NdFeB magnets and（Dy,Nd）FeB magnets were studied.The microstructures and images of NdFeB and（Dy,Nd）FeB magnets were characterized by scanning electron microscope and electron probe analyzer.The temperature stability,mechanical stability and chemical stability of the magnets before and after Dy grain boundaries diffusion were studied.Results show that the coercivity of（Dy,Nd）FeB magnets is significantly improved,and the H_cj increases from 16.08 to 22.98 k Oe.Compared with NdFeB magnets,（Dy,Nd）FeB magnets possess better magnetic field stability,temperature stability and mechanical stability.However,the chemical stability of（Dy,Nd）FeB magnets decreases obviously with Dy addition.（4）（Tb,Nd）FeB magnets were prepared by grain boundary diffusion method based on magnetron sputtering.The microstructure and stability of magnets before and after Tb grain boundary diffusion were studied.The microstructures and images of NdFeB magnets and（Tb,Nd）FeB magnets were characterized by scanning electron microscope and electron probe analyzer.The temperature stability,mechanical stability and chemical stability of the magnets before and after Tb grain boundary diffusion were studied.The results show that the coercivity of（Tb,Nd）FeB magnets is significantly improved.Compared with NdFeB magnets,（Tb,Nd）FeB magnets possess better magnetic field stability,temperature stability and mechanical stability.However,the chemical stability of（Tb,Nd）FeB magnets decreases significantly.Under the same test conditions,the corrosion weight loss of（Tb,Nd）FeB magnets and（Tb,Nd）FeB magnets are 3.4 and 6.3mg/cm² respectively.（5）CeO₂/Zn-Al composite coatings with different nano-CeO₂ contents were prepared on surface of sintered NdFeB magnets by spraying process.Effect of CeO₂ content on the properties of the composite coatings was studied.The microstructures of the composite coatings were observed by scanning electron microscope.The content and distribution of nano-CeO₂ were analyzed by an energy dispersive spectrometer.Effect of CeO₂ addition on the mechanical properties and corrosion resistance of Zn-Al coating was studied.The corrosion process of the composite coatings was analyzed by neutral salt spray corrosion test and potentiodynamic polarization curve measurement for studying the corrosion protection mechanism of the composite coating.Results show that CeO₂ nanoparticles are uniformly distributed in the Zn-Al coating,which can not only increase the hardness of Zn-Al coating,but also improve the density of Zn-Al coating.Hence,the anticorrosion performance of CeO₂/Zn-Al coated NdFeB magnets in evidently enhanced with NSS time of 720 h.