Study On Preparation And Magnetic Properties Of Nd₂Fe₁₄B/Fe Nanocomposite Permanent Magnetic Materials

Permanent magnetic materials play an important role in modern technologies. The maximum energy product is one of the most critical figures of merit of permanent magnet. Theoretic calculation show that anisotropic nanocomposite magnet is a potential candidate to realize the breakthrough of maximum energy product,however, it is still a great challenge to fabricate textured bulk nanocomposite magnets. Bulk anisotropic nanocomposite permanent magnet can be made through either bottom-up or top-down methods.To make bulk anisotropic nanocomposite permanent magnet through bottom-up method, the anisotropic hard magnetic nanoparticles coated with a soft magnetic phase are prepared first, then the nanocomposite particles are compacted with field alignment, and lastly, the green part is subjected to the process of hot-pressing to get fully dense anisotropic bulk nanocomposite magnets. In this article, HDDR-treated Nd₂Fe₁₄ B powder was used as starting material, to produce nanoscale particles by the method of low energy ball milling. Homogeneously distributed submicron anisotropic Nd₂Fe₁₄ B particles were obtained, the microstructure and magnetic properties of which were investigated systematically. As the milling time is prolonged, the particles were separated along the inter-granular Nd-rich phase due to the relative mechanical weakness of which, and particle size has decreased significantly.After milling for 36 h, the fine Nd₂Fe₁₄ B particles with size below 600 nm were obtained and has no grain size changing and amorphization. After low energy ball milling for 24 hours, the powder still has a coercivity of 4.9 kOe at room temperature and possesses a strong magnetic anisotropy.Following the routine of bottom –up method, nanocomposite permanent magnets were synthesized. Anisotropic HDDR-Nd₂Fe₁₄ B particles were chosen as the hard phase, and iron was chosen as the soft phase, which was coated on the hard phase by the thermal decomposition of Fe（CO）5. The microstructure and magnetic properties were studied systematically for the nanocomposite permanent materials we prepared. With the increasement of the content of iron, a continuous and dense soft magnetic layer was finally formed on the surface of the hard phase. With the content of iron being 12 wt.%, the saturated and remnant magnetization are 150.5 emu/g and 136 emu/g, respectively, whereas the corresponding value for the original HDDR-Nd₂Fe₁₄ B particles are 141 emu/g and 131 emu/g, respectively. Obviously, the remnant magnetization enhancement effect is shown on our nanocomposite magnetic particles. The Henkel-plot shows that there is a strong exchange coupling between the hard and soft phase, which is in accordance with the remnant magnetization enhancement effect.Die-upsetting is another method for fabricating bulk anisotropic nanocomposite magnets, which is a top-down method. In this work, two kinds of commercially available isotropic nanocrystalline permanent magnetic melt-spun powders, i.e. MQP-15-7（composed of nanocrystalline α-Fe and R2Fe14B） and MQU-G(composed of Nd-rich phase and nanocrystalline Nd₂Fe₁₄B), were mixed and then subjected to the process of hot-pressing and die-upsetting to get bulk anisotropic nanocomposite magnet. The mass ratio of MQU-G and MQP-15-7 powders was chosen to be X: 1-X（X=0⁰.8）. The effects of composition variation on the magnetic properties and anisotropy of the composite magnet were examined. The degree of texture for the die-upset composite magnets increased gradually with the addition of MQU-G powders, and the maximum energy products of the magnets increase from 7.6 MGOe for X=0, to 21.6 MGOe for X=0.8.