湿化学法制备NdFeB微粉及其生长机制、烧结NdFeB磁体的微观结构设计和晶界优化

We are proposing a novel method, in which chemical synthesis of NdFeB submicron powders combines sintering process to produce NdFeB sintered magnet. In this proposal, firstly we are going to synthesize NdFeB submicron powders with controllable size and morphology, and then growth mechanism of powder will be discussed. A SiO2 layer with controllable thickness is coated on surface of NdFeB powder to form core-shell NdFeB@SiO2 particle,which can enhance oxidation resistance of NdFeB core. Distribution of SiO2 on grain boundary will be studied.The relationship between the distribution and eletrode potential for grain boundary will be investigated in order to improve and optimize grain boundary phase. Based on the experimental analysis,a foundmental reason of oxidation resistance will be revealed by this work. There is an inheritable performance from mophology and size of powder to sintered magnet microstructure. Therefore, the microstructure of magnet can be designed according to the inheritable performance. The microstructure of magnet can be optimized by using controllable NdFeB powders and accurately sintering process. It’s expected to pioneer a new route to produce sintered NdFeB magnet with high coercivity and low weight-loss. The route evades Japanese patents of Nd-Fe-B sintering magnets,and decreases cost of NdFeB magnet production since it discards melting, hydrogen decrepitation treatment,and jet milling processes to save process period and power.

本项目申请拟提出化学法制备NdFeB亚微米级磁粉，并将这亚微米磁粉与烧结工艺相结合的新思路去制备烧结NdFeB磁体。通过研究NdFeB粒子的生长机制，合成出粒度、形貌可控的NdFeB亚微米磁粉。在磁粉表面沉积上一层SiO2，制备出类似壳核结构的NdFeB@SiO2磁粉，从而提高磁粉的抗氧化性能，将该磁粉通过烧结后得到NdFeB烧结磁体。研究SiO2在磁体晶界上的分布，以及对晶界电极电位的影响，改善和优化晶界相，提高晶界相的电极电位，从机理上解决磁体的耐蚀性差问题。利用磁粉对磁体结构的遗传性来设计磁体微结构，使用可控的亚微米NdFeB磁粉以及控制烧结等工艺实现磁体微结构的优化，为制备高矫顽力、低失重烧结磁体奠定理论基础。该方法突破了日本烧结NdFeB的专利，不使用熔炼、氢爆破碎、气流磨制粉等工艺，节约大量时间和能源，从而降低了磁体制造成本。

项目采用湿化学与还原处理相结合的新工艺，成功合成出Nd2Fe14B主相的亚微米磁粉。该磁粉分散性好，尺寸和形貌均一，并且具有良好的磁性能。研究表明，Nd化学活性高，直接还原困难、且不稳定，因此项目设计了湿化学—还原处理工艺。先用湿化学法合成出由Fe3O4粒子和Nd、B有机物配位体组成的NdFeB中间体；然后通过一级还原处理将Nd、B从有机配位体中还原分离，形成Nd、B的氧化物；再通过二级还原处理，借助强还原剂CaH2把Fe、Nd和B元素从其氧化物中还原，获得成分可控的NdFeB磁粉。项目揭示了反应工艺对中间体粒子、磁粉形貌、尺寸以及成分的影响，明晰了各步反应的机理，获得了制备Nd2Fe14B磁粉的最佳工艺。项目制备的亚微米磁粉，矫顽力达到了5.4 kOe，与商业磁粉性能接近，尺寸均一，形貌一致，明显优于商业磁粉。项目通过热扩渗工艺将Dy、Al元素沿晶界通道渗入至磁体表层深2 mm处，Dy和Al包覆在主相晶粒周围，优化了磁体晶界结构，提高矫顽力14.7%；研究了富Ce稀土永磁的腐蚀机理，发现Ce的少量加入可以提高磁体晶界的电极电位，改善磁体耐蚀性，并在此基础上开发了Zn、Al鳞片和硅烷偶联剂等构成的新型绿色环保防腐涂层。项目的执行突破了目前气流磨制粉不均一的瓶颈，给稀土永磁制粉提供了一个新的途径，有望替代现有气流磨技术制备低成本、高性能稀土永磁，基本完成了项目研究内容，达到了预期研究目的。