Preparation of Coated Nanocomposite Magnetic Powders and Investigation on the Densification by SPS

　　Preparation of Coated Nanocomposite Magnetic Powders and Investigation on the Densification by SPS
　　Nanocomposite magnets consist of hard and soft magnetic phases. A high remanence and large maximum magnetic energy product would be obtained because of the exchange coupling effect of the hard and soft phases. In addition, Nanocomposite magnets have excellence in low rare-earth concentration, low cost and high use temperature. So nanocomposite magnets become a promising candidate of the fourth generation permanent magnet after the third generation magnet of Nd2Fe14B. In this thesis, the bulk nanocomposite magnets are prepared by Alnico Magnets sonochemistry, high-energy milling and spark plasma sintering. First, the preparation of the nano-iron particles was investigated. And based on the results, the coated nanocomposite magnetic powder consisted of hard and soft particles was successfully prepared by sonochemistry. The final work was focused on the densification of the coated nanocomposite magnetic powder by spark plasma sintering. The study emphasis was laid on the dependence of properties and microstructure of the magnets on the process of sintering, the content of soft phase and high-energy ball milling before sintering.The preparation of nano-iron and coated nanocomposite magnetic powder by sonochemistry and the mechanism of coating were systematically studied by means of SEM, TEM, IR, XRD and EDS. The optimum preparation condition by sonochemistry was determined. Coated nanocomposite magnetic powders were prepared through the use of sonochemistry for decomposing carbonyl iron in decane solvent at -3～0°C in Ar atmosphere http://www.999magnet.com/products/108-alnico-magnets for 2h～3h with the sonochemical power of 120W. The hard magnetic particles are well coated by nano-iron particles. In addition, hard magnetic powder used for coating needs to be carried out high-energy milling for becoming fine. On the condition of certain Ratio of grinding media to material speed of rotation, hard magnetic particles became finer with the time of milling. But after milling for 8h, the hard magnetic particles became coarser. As a result, it is determined that the time of milling for hard magnetic powder is fixed on 6h.Bulk two-phase nanocomposite magnets densified by spark plasma sintering have