纳米双相复合磁粉的原位制备及其热压/热变形研究

The practical obtained energy product of nano-composite magnets is far below the prediction. The uniform dispersion of soft and hard phase in nanoscale as well as the anisotropy structure are two key factors that constrain the development. In this research plan we combine the "bottom-up" synthesis method for nanomaterials with Hot Press/Hot Deformation (HP/HD) process of traditional metallurgy. The soft phase of Fe nanoparticles are in situ uniformly precipitated on the surface of NdFeB rapid quenching powders via the "seed-mediated" approach. By this way the size, dispersion and coating thickness of Fe nanoparticles can be finely controlled which is in favor of tuning the compositions between soft and hard phase and the magnetic exchange coupling. The as-prepared nano-composite powders are followed by undergoing high pressure shear rheology of HP/HD process after which the high oriented c-axis can be achieved and textured magnet can be formed. After the two steps we could get anistropic nano-composite magnets. By constructing a theoretical model we could further analyze the domain structure and interfacial effect of the exchange coupling of soft and hard magnetic phase, clarify the coercivity and magnetic exchange coupling mechanism. The optical microstructures and exchange coupling conditions for anisotropic nano-composite magnets can be established by the experiment and theoretical simulation.

目前实际得到的纳米双相复合磁体的磁能积，与理论预测相比仍有较大的差距，软硬磁相纳米尺度的均匀分散以及各向异性结构的构筑制约着其发展。本项目将"自下而上"的纳米制备手段与传统冶金的热压/热变形工艺相结合，以NdFeB快淬粉作为形核中心，借鉴"种子调控"的方法在其表面原位制备均匀包覆的软磁性Fe纳米颗粒。有利于实现对软磁性Fe纳米颗粒的尺寸、分散状态、包覆厚度的控制，从而对软硬磁相的组成及磁交换耦合作用进行调控。进一步利用热压/热变形工艺的高压剪切流变技术，实现硬磁相易磁化c轴的高取向度，形成具有取向织构的磁体，制备出各向异性的纳米双相复合磁体。通过构建理论模型分析软硬磁相的交换作用畴结构以及界面效应，阐明纳米双相复合磁体的矫顽力机制以及磁交换耦合机制，建立易于实现各向异性复合体系内纳米耦合的微观结构和条件。

本项目通过“自下而上”的纳米制备手段，利用NdFeB快淬粉作为形核中心，在其表面原位析出软磁性的Fe纳米颗粒，并对软磁性相纳米尺度分散进行控制。将这种纳米双相复合磁粉经过热压/热变形工艺制备出具有取向织构的各向异性磁体。研究发现Nd含量的减少会导致NdFeB磁体在热流变过程中所需的施加应力增大，变形时间延长，阻碍晶粒的择优取向。Fe由于熔点较高，因此Fe含量增加在热变形过程中不利于液相的形成，并影响应力的分布，造成微观结构的恶化。该结论与传统快淬工艺的结果相吻合。并在前期工作基础上对热压/热变形模型及机理进行了探讨，认为Nd含量的高低对NdFeB磁体的变形过程有很大的影响，与实验部分相互印证。因此在构筑高性能纳米双相复合磁体的过程中，需要控制稀土Nd及添加Fe的含量，既要保证有足够的液相来润滑晶粒取向，又必须控制包覆Fe纳米颗粒的尺寸及分布。通过本项目为纳米双相复合永磁材料的制备及机理研究提供了新的思路，特别是为各向异性纳米双相复合磁体的制备提供了一种可行的探索。