Study On Preparation And Magnetization Mechanism Of Anisotropic Hybrid Bonded Magnets

Bonded magnets are widely used in new energy vehicles,power tools,robots,hard disk drives and other fields due to their advantages of large shape flexibility,high dimensional accuracy and high cost-effective.Driven by the domestic carbon neutrality policy,the demand for permanent magnets is accelerating.However,the density and magnetic properties of current bonded magnets is low,which can’t meet some special application requirement,which puts forward new requirements for the development and preparation of bonded magnets.In this paper,anisotropic NdFeB/SmFeN hybrid bonded magnets were prepared by warm compaction process under an orientation magnetic field.Effects of powder size,SmFeN content and coercivity of NdFeB powders on the magnetic properties of hybrid magnets were investigated.The results show that the size of powders has a great influence on the magnetic properties.Because of its bridging,there are lots of spaces between large powders,while small powders have high surfactivity and are easy to agglomerate,which makes it difficult to be pressed,both leading to a decrease in density,resulting that the magnetic properties of the magnet are the best when the size is 100-140 mesh.The addition of SmFeN powders can fill the gaps and crevices.When adding 20 wt.%SmFeN powders,compared with the magnets without SmFeN powders,the density,remanence and maximum energy product of hybrid magnets are the best,increasing from 5.6 g/cm3,8.9 kGs and 12.42 MGOe to 6.0 g/cm3,9.7 kGs and 15.43 MGOe,by the increase of 7.1%、9.0%、24.3%,respectively.Moreover,the high temperature performance of the magnet is obviously improved,and the remanence temperature coefficient increases from0.176%/℃ to-0.150%/℃.Ultra-fine SmFeN powders gather around the central NdFeB powders and evenly distribute in the magnet,acting as a magnetic buffer layer and forming a bias magnetic field,which increases the degree of alignment of NdFeB powders,thus improving the comprehensive properties of hybrid magnets.The filling effect,friction resistance and bias field produced by SmFeN powders make the（BH）max change less than 0.5 MGOe,when the amount of SmFeN varies from 20 wt.%to 40 wt.%,which means that we can regulate the coercivity of hybrid magnets in a certain range,obtaining the hybrid magnets with different properties.The magnetization process of hybrid magnets was discussed.After magnetic field orientation,the initial magnetization curve shape of anisotropic NdFeB powders changes from "sharp" type to "S" type,indicating that the magnetization process of anisotropic NdFeB powders shows distinction in different initial states.Different from single powder,an obvious step-by-step magnetization process is observed in the hybrid magnets.The magnetization curve increases rapidly at low field and then slowly,next leads to more rapid again with the increase of the applied magnetic field,and reaches the saturation magnetization finally.The interaction between powders was revealed.In the demagnetization process,the two magnetic powders with different coercivity can be regarded as the core-shell structures in grain boundary diffusion of sintered NdFeB,the powders with high coercivity have a strong"pinning effect" on the powders with low coercivity,which prevents the reversal of magnetization domain of the powders with low coercivity,thus achieving a strong ability to resist de-magnetic field.And the demagnetization curve is smooth without obvious kinks,which confirms that there is a good micron-sized long range coupling effect between powders.It provides important guiding significance for improving magnetic properties of hybrid magnets and theoretical basis for developing new hybrid bonded magnets.