| Since 1984,Nd-Fe-B permanent magnets have been widely used in electric drive,electronic information,space flight and aviation,national defense and military industry,because of their excellent magnetic properties such as high coercivity(Hc),remanence(Mr)and magnetic energy product.With the expansion of sustainable development industries,the market requirements of high-performance Nd-Fe-B have encountered unprecedented opportunities and challenges.Due to the weak thermal stability of NdFe-B permanent magnet material,the coercivity decreases rapidly as temperature increases,which resulting in limitations of the applications.Therefore,the preparation of Nd-Fe-B magnets with excellent properties is still emphases and difficulties of the research until we find a new material to replace it.Nd-Fe-B magnet is a multiphase alloy with the major hard magnetic phase of Nd2Fe14B and the minor Nd-rich phase.As the main grain phase,the Nd2Fe14B mainly determines the high coercivity and remanence properties of Nd-Fe-B magnets.The grain boundary of Nd-rich phase wraps the grains uniformly and continuously,which plays a very important role in the magnetization reversal.Based on the structure of the Nd-Fe-B magnets,the theoretical and experimental explorations of their magnetic properties are mainly divided into two aspects:the regulation of main phase composition and the optimization of microstructure.As the basic theory of applied magnetism,micromagnetics plays an important role in theoretical research,which can analyze the energy variations and magnetization processes combined with the microstructure of materials.In this paper,a multi-grain model for nanocrystalline NdFe-B is presented.Effects of grain boundaries and grains on coercivity,remanence,and magnetic energy product of nanocrystalline Nd-Fe-B permanent magnet materials are studied by micromagnetic simulation.The main contents and conclusions are summarized as follows:Firstly,a valid theoretical model of nanoscale poly crystalline is established for simulation.By labeling the grain centers and using the Voronoi method to divide mesh of the three-dimensional cubic,a multi-grain model of Nd-Fe-B with adjustable grain size or grain boundary width is obtained.The reasonable operation parameters of the models are selected by referring to relevant experiments to ensure the reliability of the results.Secondly,the accuracy of simulation methods and models are verified.For the simulation method,the domain wall width of Permalloy is calculated.The error between the simulated result and the theoretical value is only 1%.Furthermore,demagnetization curves of nano-single crystal Nd-Fe-B permanent magnet with a triangular prism,cube,hexagonal prism,and cylinder shape are studied,respectively.The results show that for the cubic nano single-crystal Nd-Fe-B,the coercivity and remanence decrease with the increase of grain boundary width(WGB).The Nd-Fe-B model with cylinder shape has the largest coercivity and remanence,followed by hexagonal prism and cube,while the triangular prism model has the smallest coercivity and remanence,those conclusions are consistent with the known results.This work is the foundation for the subsequent calculation.Thirdly,the influences of grain boundary width,grain boundary saturation magnetization(M's),magnetocrystalline anisotropy constant(K')and exchange constant(A2*)on the coercivity and remanence of nanocrystalline Nd-Fe-B materials are studied.It is found that the coercivity and remanence of poly crystalline Nd-Fe-B material decrease with the increase of WGB.In addition,nucleation forms at the grain boundary during magnetization reversal.The remanence of the material incresses as M's increases,while the exchange coupling between grain boundary and grains increases,resulting in the decrease of coercivity.The coercivity increases,whereas the remanence remains unchanged as K'increases.With increases of A2*.the exchange between grain boundary and grain increases to lower the coercivity.Fourthly,the effects of grain size(SG)and grain shape on coercivity and remanence are analyzed,respectively.The effects of grain size on coercivity and remanence of nanoscale poly crystalline Nd-Fe-B magnet are simulated for both grain boundary(GB)model and direct contact(DC)model.The simulated grain size dependence of Hc with grain boundary(GB)model agrees with the experimental data well.Grain boundaries function as both nucleation and pinning.In the DC model,Hc increases with grain refinement,while Mr remains unchanged.However,due to the lack of weak magnetic grain boundaries,the nucleation of materials is difficult and expands rapidly once reversal domains are formed.When the grain shape is changing,the results show that the columnar crystal material has the largest coercivity and the hot-deformed shape material has the largest remanence.Finally,the hysteresis loops of nanocrystalline Nd-Fe-B material are simulated from 300 K to 450 K.When the temperature increases from 300 K to 350 K,the coercivity decreases by 42%,and the remanence decreases by 13%,which is in good agreement with the experiments.The decreasing trend of coercivity is more obvious than that of remanence,mainly because the reduction of anisotropy energy at high temperatures is significantly greater than that of saturation magnetization.In this thesis,an effective multi-grain Nd-Fe-B model is established.The effect of grain boundary width,grain boundary magnetic parameters,grain sizes,grains shapes and temperature on magnetic properties of Nd-Fe-B magnets are analyzed by choosing the reasonable parameters.The results are in good agreement with the experiments.It can provide guidance for related experiments of high-performance Nd-Fe-B permanent magnets. |
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