| Nanocomposite magnets consist of hard magnetic phase with strong magnetic anisotropy and soft magnetic phase with high saturation magnetization, which have a potential to get excellent magnetic performance due to exchange coupling between hard and soft phases within a nano-sized range. In addition, this type of magnet has advantages such as low rare earth content, cheap price, good corrosion resistance and high thermal stability. Nanocomposite magnets were considered as a promising candidate of the fourth generation permanent magnet after Nd2Fe14B. However, the experimentally magnetic performance of the prepared nanocomposite magnets was much lower than the expected, because it was difficult to prevent grains from coarsening during sintering by traditional techniques. In order to gain the well-dispersed Nd2Fe14B/Fe nano-composite powders, in this thesis, Fe nanoparticles prepared by sonochemical process of carbonyl iron were applied to coat on the surface of micrometer NdFeB permanent magnetic particles. The composite powders were then sintered into bulk nanocomposite Nd2Fe14B/α-Fe magnets by spark plasma sintering (SPS) technique. The study emphasis was laid on the optimum conditions of the sonochemical process, the microstructure of the Nd2Fe14B/Fe powders, and the dependence of properties and microstructure of the magnets on the process of sintering, the particle size of hard phase, the content of soft phase and its oxygen content. The effect of soft magtic phase on the exchange-coupling interaction of mangets was also investigated. When adoping graphite die, the effect of SPS cycle on properties and microstructure of the magnets was done and an optimized sintering process was obtained.The coated nanocomposite magnetic powders with the nominal Fe content of 1~25wt% could be obtained by sonochemical treatment for 2h, applying 180w power. The content of the nano-iron increased obviously when more Fe(CO)5 added, so the thickness of the coated Fe can be adjusted by controlling the amount of Fe(CO)5. It shows that the soft phase is highly dispersed and uniform with 15wt% Fe coating. It is indicated that fine hard phase powders are much easier dispersing in the solution than the big ones during the ultrasonic process. On the other hand, the fine powders are prone to oxidation, which make the magnetic properties declined. The experimentally optimized particle size of NdFeB powders is 50~75μm.The fully dense signal-phase magnets with a magnetic properties of (BH)max=131.52kJ/m3,Br=0.90T were prepared when sintered at 903K, under the pressure of 400MPa. The results show that the oxygen impurity were picked up during the sonochemistry process and the oxygen impurity increased when increasing soft magnetic phase. The SEM observation shows that the iron phase with about 100~300nm in thick was distributed at the interface of hard magnet phase. The Fe content is much less than the theoretical value, which is considered that not all resultant nano-Fe coated on the surface of NdFeB particles during sonochemistry and the uncoated Fe particles lost when washing the solution. It is found that the content of soft magnetic phaseɑ-Fe plays an important role in the magnetic properties of the composite magnet. The nanocomposite magnet with a good magnetic properties of (BH)m=128.2 kJ/m3 (16.1MGOe),Br=0.92T,Hcj=607.35kA/m were prepared with nominal 15wt% Fe coating.The optimization of SPS sintering process was carried out by the use of graphite die, because graphite process advantages such as good thermal conductivity, better temperature controllability and low cost. The research showed that 120K/min was the best heating rate. NdFeB magnet with the best magnetic properties of (BH)max= 122.90kJ/m3 (15.44MGOe) ,Br=0.86T,Hcj=1188.43kA/m was SPS sintered at 943K, and Nd2Fe14B/α-Fe nanocomposite magnet with the best properties of (BH)max=82.94 kJ/m3 (10.42MGOe), Br=0.79T,Hcj=378.10 kA/m was sintered at 993. Compared with one SPS cycle, magnetic performance of Nd2Fe14B/α-Fe magnet had been evidently improved by 4MGOe with two SPS cycles, and the corresponding properties are (BH)max=114.39kJ/m3 (14.37MGOe), Br=0.90T,Hcj=406.76 kA/m. The downtrend of coercivity was well mitigated during the process, and the enhancing of the remanence was still the dominant factor to the magnetic properties. It was two-SPS-cycle process that cause the fully sintered sample and uniform distribution of the soft magnetic phase and hard magnetic phase, which decreased demagnetization factor Neff of the magnet grain, avoided sharp decline of the coercivity and hence led to better magnetic properties.
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