Research On Microstructure Regulation And Magnetic Properties Of Dual-main-phase Nd-Ce-Fe-B Magnet

Nd-Fe-B permanent magnet is currently the most powerful and widely used rare earth permanent magnetic material.The rapid growth of its application demands has led to the consumption of a large amount of rare earth resources such as Nd,Pr,Dy,and Tb each year,which has resulted in the imbalance of rare earth resources utilization in China.Therefore,the development of Nd-Ce-Fe-B magnets rich in high-abundance rare earth Ce is not only beneficial to promote the balanced utilization of rare earth resources,but also can reduce the cost of magnets,which has important research value and engineering significance.However,the even substitution of Ce for Nd in Nd-Fe-B reduces the intrinsic magnetic properties of Nd₂Fe₁₄B tetragonal phase and seriously deteriorates the magnetic properties of the magnet.The dual-main-phase process can make Ce non-uniformly replace Nd to form multi-main-phase structure,suppressing the magnetic dilution effect produced by Ce replacing Nd and achieving the higher magnetic performance,which has become a hotspot of current research.However,when too much Ce replaces Nd,the magnetic properties of the dual-main-phase Nd-Ce-Fe-B magnet are still low.Therefore,in this paper,Nd-Ce-Fe-B magnets with both“ultrafine grain”and“multi main phase”structures were prepared by combining spark plasma sintering and dual-main-phase process.The influence law and mechanism of the sintering temperature and grain boundary modification on the microstructure,composition,and magnetic properties of the dual-main-phase Nd-Ce-Fe-B magnets were systematically investigated by the method of combining micromagnetic simulation and experimental research,aiming at further improving the magnetic properties of the magnet.The main research contents and results are as follows:?1?The effects of the microstructures and compositions of the main phase and grain boundary phase on the magnetic properties of Nd-Ce-Fe-B magnet were revealed by the micromagnetic simulations.The simulation results show that when the grain boundary phase is non-magnetic,the coercivity of the dual-main-phase magnet is higher than that of the single-main-phase magnet.By contrast,when the grain boundary phase is ferromagnetic,the coercivity of the dual-main-phase magnet is lower than that of the single-main-phase magnets.However,the coercivity of the dual-main-phase magnet can be improved,when appropriate interdiffusion between the two main phases occurs to form?Nd,Ce?₂Fe₁₄B shell.For the dual-main-phase Nd-Ce-Fe-B magnet,the thicker the nonmagnetic grain boundary phase is,the higher the coercivity is;when the thickness of the grain boundary phase is same,the smaller the grain boundary phase magnetism is,the greater the coercivity is.Moreover,the smaller the grain size of the main phase is,the higher the coercivity of the dual-main-phase magnet is.Furthermore,the formation of?Nd/Ce,Pr?₂Fe₁₄B or?Nd/Ce,Dy?₂Fe₁₄B hard magnetic shell on the surface of the main phase grains is also beneficial to improve the coercivity of the magnet.?2?The sintering temperature has a significant effect on the microstructure and magnetic properties of the dual-main-phase Nd-Ce-Fe-B magnet.It can be found from the investigation that with increasing the sintering temperature,the density of the dual-main-phase magnets gradually increases,and the magnet is almost completely dense at 700?.The trend of remanence and magnetic energy product is similar to that of density,because the remanence and magnetic energy product are directly proportional to the density of magnet.The coercivity of the magnet increases first and then decreases with increasing the sintering temperature,and it attains the maximum value of 1032.5kA/m at 700?.As the sintering temperature increases further,the coercivity of the magnet decreases gradually.When the sintering temperature increases to 800?,the coercivity decreases sharply.The microstructure study shows that the pores gradually decrease with increasing the sintering temperature and disappear at 700?,achieving complete densification.This is basically consistent with the density result.Meanwhile,the main phase grain growth phenomenon appears in the local region.At 800?,the main phase grains grow up abnormally,forming a large number of micro-scale coarse-grained structures,and destroying the original nanocrystalline structure.This is one of the important reasons leading to a sharp decrease in the coercivity of the magnet.In addition,as the sintering temperature increases,the Nd/Ce interdiffusion becomes serious,and excessive Nd?or Ce?diffuses into the Ce-rich?or Nd-rich?flakes,reducing the chemical heterogeneity of the dual-main-phase magnets.This is another important reason for the reduction in the coercivity induced by the overhigh sintering temperature,which is consistent with the the results of micromagnetic simulation.?3?The effects of sintering temperature on the microstructure and magnetic properties of the dual-main-phase Nd-Ce-Fe-B magnet with intergranular addition of DyF₃ and Cu powders were investigated for comparison.It can be found that the magnet with intergranular addition of DyF₃ and Cu powders is also completely densitified at 700?.At this time,the remanence and magnetic energy product of the magnet reach the maximum value.However,unlike the magnet without addition,the coercivtiy of the added magnet attains the maximum value of 1250.5kA/m at 750?.This shows that the sintering behavior of the magnet changes after intergranularly adding DyF₃ and Cu powders.Microstructure investigaitions show that the interface between the flake particles in the magnet is improved by adding DyF₃ and Cu powders,making the interface phase increase and distribute more continuously and uniformly.Meanwhile,the addition of DyF₃ and Cu powders increases the temperature of micro-scale coarse grains appearance to 750?,indicating that it is helpful to suppress the abnormal growth of the main phase grains at the interface.In addition,the intergranular addition also reduces the Nd/Ce interdiffusion,which is helpful to maintain the chemical heterogeneity of main phase.Except for 650?with the largest density difference,the coercivity of the added magnet is higher than that of the non-added magnet at the same sintering temperature.This is mainly because Dy and Cu diffuse along the grain boundary into the flakes to form a continuous and uniform nonmagnetic grain boundary phase and?Nd,Dy?₂Fe₁₄B and?Ce,Dy?₂Fe₁₄B hard magnetic shell with high magnetocrystalline anisotropy field during sintering.