Magnetic Properties And Microstructure Of Nanocrystalline Misch-metal Magnets

Nd-Fe-B is one of the most widely used permanent magnets,and its preparation cost has been widely concerned.The price fluctuation of praseodymium and neodymium affects the preparation cost of the magnets.Therefore,the reasonable application of high abundance rare earth elements in rare earth permanent magnets has become an urgent problem to be solved.Misch-metal（MM）,as the initial separation product of the rare earth ore,its price much lower than praseodymium and neodymium.Therefore,it is considered to have high application potential.In this paper,the hot-pressed and hot-deformed magnets of misch-metal magnets were prepared by spark plasma sintering（SPS）and low-melting alloys grain boundary addition technology.The mechanism of magnetic propertie was explained in detail by the characterization of magnetic properties and microstructure.The high La and Ce contented magnetic powder were used in this paper,and its composition is MM_28.1F_ebal.Zr_1.5B_1.1（MM=23.8 wt.%La,47.7 wt.%Ce,7.1 wt.%Pr,21.3 wt.%Nd）.First of all,the effect of SPS process on the microstructure and magnetic properties of MM-Fe-B hot-pressed magnet was studied,and the optimal preparation process was obtained.Subsequently,the properties of the magnets were further optimized by Nd Al grain boundaries addition.The results show that the performance of SPSed MM-Fe-B hot-pressed magnet is better than that of ferrite.The magnetic properties are J_r=6.99 k Gs,H_cj=4.23 k Oe,（BH）_max=9.1 MGOe.After 10 wt.%Nd Al addition,the comprehensive magnetic performance is further optimized.The magnetic properties are J_r=6.32 k G,H_cj=9.14 k Oe,（BH）_max=8.51 MGOe.The study shows that the improvement of magnetic properties is due to the optimization of microstructure and the improvement of intrinsic magnetic properties.Secondly,the properties of magnets are improved by annealing without changing the composition of the magnet.Two kinds of annealing methods including post-sintering annealing and pre-sintering annealing were adopted respectively.The results show that the magnetic properties can be optimized by pre-annealing,but the magnetic properties are obviously reduced by annealing after sintering.It is found that the volume fraction of coarse grain in powder pre-annealed magnets decreases obviously,hence the average grain size decreases.Moreover,the average Nd content in the main phase is higher than that of other magnets,indicating that the Nd from the diffusion source is better absorbed.The magnetic hardening effect is more effective.In addition,the service performance of the magnet has also been improved.Finally,the preparation of MM-Fe-B hot-deformed magnet is discussed.The Nd₇₀Al₃₀-added and Nd₈₅Al₁₅-added MM-Fe-B hot-pressed magnets as the precursors.Then the anisotropic magnets were fabricated by hot deforming at different temperatures and pressures.The experimental results show that the completely different effects on the magnetic properties by Nd₇₀Al₃₀ and Nd₈₅Al₁₅addition.The magnetic properties are obviously improved by Nd₇₀Al₃₀ addition.The optimal magnetic properties are J_r=8.38 k Gs,H_cj=6.03 k Oe,（BH）_max=9.7 MGOe.However,the performance of the Nd₈₅Al₁₅-added magnets is seriously deteriorated.The analysis shows that the distribution of Re-rich phase and the volume fraction of coarse grain area are the main reasons.After Nd₈₅Al₁₅addition,the Re-rich phase aggregate between powders,and the coarse grain area leading the decrease of the average grain orientation of the magnet,which deteriorates the magnetic properties.In summary,the magnetic properties of the low-melting alloy added MM-Fe-B magnets were optimized,and the relationship between the phase composition,microstructure and magnetic properties of the magnet was systematically studied.This work provides the theoretical guidance for the practical application of the misch-metal permanent magnet.