低稀土纳米晶热变形NdFeB永磁晶界改性及性能调控机制

Hot deformed NdFeB magnets have excellent magnetic properties, good corrosion resistance, and high thermal stability etc. For hot deformed single phase NdFeB and nanocomposite NdFeB/a-Fe magnets prepared by normal processes, it has the weakness of high production cost and low magnetic properies. Hot deformed NdFeB magnets with low rare earth content, high coercivity, and high hot workability could be prepared by grain boundary modification implemented by the addition of intergranular phase with low melting point and good wettability. At present, the price of rare earth elements is expensive, especially for heavy rare earth elements. Hot deformed magnets, which have low heavy rare earth content and high coercivity, could be prepared by the addition of heavy rare earth compounds using co-precipitation processes and grain boundary diffusion processes (GBDP). Effects of different intergranular phase on hot workability, c-axis texture,and magnetic properties for hot deformed NdFeB magnets will be investigated in details. The characteristics of recoil loops and their relationships with the microstructure, ingredient, and distribution for intergranular phases will be studied in details, and to get the aim of coercivity improvement by reducing the openness of recoil loops. The above studies have a certain practical significance with broadening the field of use and enhancement of magnet utilization.

热变形纳米晶NdFeB磁体具有磁性能好、抗腐蚀性优及热稳定性高等特点。直接制备热变形单相NdFeB和纳米双相NdFeB/a-Fe磁体生产成本高昂，且性能较低。通过在单相NdFeB和纳米双相NdFeB/a-Fe合金中添加低熔点、与主相润湿性好、扩散反应性的晶间相,可制备出低稀土含量、高塑性变形能力、高矫顽力的各向异性磁体。目前稀土材料特别是重稀土材料价格昂贵，通过化学共沉淀技术和晶界扩散技术在快淬富钕NdFeB合金晶界中实施重稀土化合物的添加，后进行致密化和热变形，可制备出低重稀土含量、高矫顽力的热变形磁体。研究不同晶间相对NdFeB磁体热塑性变形能力、C轴织构和磁性能的影响。阐明回复曲线张开特征及其与晶间相所呈现的结构、成分和分布的关系，并能通过减小回复曲线张开程度达到增加矫顽力的目的。这对拓宽热变形NdFeB磁体的使用领域和提高其利用率具有一定的实际意义。

热变形各向异性NdFeB磁体由于具备了细小的晶粒组织结构，因而具有磁性能好，且热稳定性和断裂韧性均好于烧结磁体。开发出具有优异磁性能的热变形NdFeB磁体具有一定的实际意义。. 本项目主要研究了通过放电等离子烧结技术（SPS）制备纳米晶Nd-Fe-B永磁材料。优化了烧结工艺并阐明了其烧结机制，放电等离子体在颗粒边界所产生的暂态高温场是磁体形成粗晶区的原因。针对SPS磁体原始颗粒边界晶粒粗大的特点，通过调整磁粉的粒度，达到了消除粗晶区、调控微观结构和磁性能的目的。700oC/50MPa/5min烧结条件下，磁体的综合磁性能最佳，其剩磁、矫顽力和磁能积分别为Jr=0.82T、jHc=1516kA/m和(BH)max=116kJ/m3，剩磁和矫顽力温度系数分别为α=－0.107%K-1和β=－0.483%K-1，且致密度高达99.5%。PrCu晶间相的添加可有效提高磁体的矫顽力，最大提高可达25%。 . 以SPS磁体为前驱体，通过热变形技术研究制备了各向异性Nd-Fe-B磁体，揭示了微观结构演化、磁性能和磁体温度稳定性的内在关系。阐明其矫顽力机制为钉扎机制。制备出了磁性能为Jr=1.35T、jHc=829kA/m、(BH)max=336kJ/m3和矫顽力温度系数为β=－0.682%K-1的热变形磁体。在此基础之上，通过晶界扩散PrCu合金，有效提高了热变形磁体的磁性能，最佳综合磁性能可达：jHc=1336kA/m、Jr=1.26T、（BH）max=286kJ/m3。. 研究了SPS磁体和热变形磁体的回复曲线特点及其与微观结构和磁性能的关系。发现SPS磁体中的孔洞会导致后续热变形过程中富Nd相的聚集，尺寸大小为几微米到十几微米的条形富钕相，不仅难以作为畴壁的钉扎点，而且会产生不可忽略的且不均匀分布的退磁场，进而引起磁各向异性的不均匀性，最终导致回复曲线的张开。. 扩展研究了各向异性CeFeB磁体的制备及磁性能，发现低熔点PrCu合金的添加可有效改善CeFeB磁体的塑性变形能力，可制得高各向异性CeFeB磁体。. 本项目瞄准纳米晶热变形稀土永磁体，针对其尚未厘清和阐明的科学和工艺问题进行探索和研究，项目成果有利于促进热变形NdFeB磁体的工业化生产应用，具有一定程度的实际意义。