Synthesis of ultrastrong nanocomposite-permanent magnet with magnetic anisotropy

纳米复合永磁体作为一种新型磁性材料越来越受到人们的关注。近年来，机械合金化（MA）和熔融纺丝法制备了Nd_2Fe_<14>B+α-Fe（Fe_3B）、Sm_2Fe_<17>Nx+α-Fe、SmCo_5+α-Fe等纳米复合磁体。然而，这些非复合磁体具有各向同性的磁性，因此需要具有各向异性的复合磁体。我们尝试在Sm_2Fe_<17>Nx+α-Fe中合成磁性各向异性粉末。将粗粒Sm-Fe母合金粉末机械研磨至1 μm左右，再进行氮化处理，可得到各向异性的Sm_2Fe_<17>Nx单相硬相。将Sm-Fe母合金粉末和α-Fe工业粉末的混合物进行机械研磨和氮化处理。由于α-Fe粉末难以粉碎成纳米级颗粒，因此无法实现软硬磁相之间的交换耦合。根据…More Sm-Fe二元图可知，Sm_2Fe_<17>组成的富铁侧存在Fe相和Sm_2Fe_<17>相。在此基础上，将Sm_2Fe_<17>合金粉末机械研磨后进行氮化处理，在1273 K真空条件下，采用Sm蒸发法制备Sm_2Fe_<17>Nx+α-Fe复合合金粉末。然而，由于在样品制备过程中这些细粉末被氧化，因此很难获得永磁体的性能。另一方面，我们利用旋转球磨机在NH_3中用MG短时间制备了Sm_2Fe_<17>Nx单硬相的各向异性粉末，晶粒尺寸约为300 nm。在此基础上，对Sm- fe母合金在1227 K真空蒸发Sm， NH_3中MG，氮化，制备磁性各向异性的Sm_2Fe_<17>Nx+α-Fe复合合金粉末进行了试验。粉末由Sm_2Fe_<17>Nx+α-Fe相组成，具有磁性各向异性，可逆磁化率达66.8%。结果表明，软相和硬相之间具有良好的交换耦合。这些粉末还显示出磁各向异性，这可以使每个硬颗粒在外加磁场中的c轴方向对齐。包括上述永久性纳米复合磁铁在内，所有的强力永磁体都是用稀土合金制造的。然而，稀土磁体的耐腐蚀性差是出了名的。本研究选择HDDR（氢分解解吸复合）工艺生产的Nd-Fe-B磁铁作为代表性稀土磁铁，通过电化学阻抗谱的动电位极化曲线和Nyquist图研究其腐蚀行为。结果表明，在试样表面强阴极极化区形成了优异的耐腐蚀膜。少

Nanocomposite permanent magnets have become more attractive as new magnetic materials. Recently, mechanical alloying (MA) and the melt-spinning method have been applied to the production of nanocomposite magnets such as Nd_2Fe_<14>B+α-Fe(Fe_3B),Sm_2Fe_<17>Nx+α-Fe,SmCo_5+α-Fe, etc. These nonocomposite magnets, however, are magnetically isotropic, and a magnetically anisotropic composite magnet is significantly required. We have tried to synthesize magnetically anisotropic powders in the Sm_2Fe_<17>Nx+α-Fe. Generally, an anisotropic Sm_2Fe_<17>Nx single hard phase was produced by the mechanical grinding (MG) of coarse Sm-Fe mother alloy powders into about 1 μm and subsequent nitriding. Mixtures of the Sm-Fe mother alloy powder and the commercial α-Fe powder were mechanically ground and subsequently nitrided. An exchange coupling between the hard and soft magnetic phases could not be obtained, because it was difficult to pulverize the α-Fe powder into nanoscale particles. According to the … More Sm-Fe binary diagram, the Fe phase and Sm_2Fe_<17> phase coexist on the Fe-rich side from the composition of Sm_2Fe_<17>. Based on this fact, we tried to prepare the Sm_2Fe_<17>Nx+α-Fe composite alloy powders by Sm evaporation during heat treatment at 1273 K in a vacuum from the mechanically ground Sm_2Fe_<17> alloy powder and subsequent nitriding. However, it was found that the properties as a permanent magnet were hardly obtained because of the significant oxidation of these fine powders during sample preparation. On the other hand, we have prepared the anisotropic powder of the Sm_2Fe_<17>Nx single hard phase, which has a grain size of about 300 nm, by MG in NH_3 for a short time using a rotary ball mill with subsequent nitriding. Based on these findings, the synthesis of magnetically anisotropic Sm_2Fe_<17>Nx+α-Fe composite alloy powders was tried by Sm evaporation from the Sm-Fe mother alloy at 1227 K in a vacuum, MG in NH_3 and nitriding. The powders were composed of the Sm_2Fe_<17>Nx+α-Fe phase possessed a magnetic anisotropy, and showed a reversible magnetization reaching 66.8 %. This result indicates that a satisfactory exchange coupling between the soft and hard phases. These powders also showed a magnetic anisotropy, which can align the directions of the c-axis of each hard particle in an applied magnetic field. Including the above mentioned permanent nanoccomposite magnets, all of the strong permanent magnets are produced by using rare-earth alloys. However, the rare-earth magnets are notorious for their poor corrosion resistance. In this study, the Nd-Fe-B magnet produced by HDDR (hydrogen decomposition desorption recombination) process was selected as the representative rare-earth magnet, and its corrosion behavior was investigated through the potentiodynamic polarization curve and Nyquist plot by electrochemical impedance spectroscopy. As the result, it was confirmed that the excellent corrosion resistant film was formed on the specimen surface polarized strongly to a cathodic region. Less