【作者】 李浩嘉；

【导师】 张久兴； 李永利；

【作者基本信息】 北京工业大学 ， 材料学， 2009， 硕士

【摘要】 纳米双相复合永磁材料是利用硬磁相的高磁晶各向异性和软磁相的高饱和磁化强度的优点,通过纳米尺度下两相晶粒间的铁磁交换耦合作用来获得优异的磁性能。由于其稀土含量少,价格便宜,抗蚀性好,使用温度高等特点,具有潜在的开发应用前景,有望发展为新一代高性能稀土永磁材料。采用传统烧结方法难以克服在烧结致密化的基础上晶粒粗化的问题,因而制得的复合磁体的性能较低。为了获得两相均匀分布的纳米结构,提高铁磁交换耦合作用,本文采用超声化学-非均相沉淀法,制备出Nd2Fe14B/Fe纳米复合磁粉,然后采用放电等离子烧结技术进行快速致密化制备出块状纳米双相复合磁体。主要研究了超声化学制备包覆磁粉的最佳工艺条件、Nd2Fe14B/Fe纳米复合磁粉微观结构和形貌,以及烧结工艺、硬磁相粒径、氧含量、软磁相含量对烧结磁体的组织和性能的影响。针对不同烧结模具的特点来进一步优化调整SPS烧结工艺,分析研究了二次烧结相比一次烧结对磁体结构性能的影响。采用SEM、TEM、XRD等分析方法对超声化学制备纳米Fe颗粒以及制备包覆型纳米复合磁粉和超声化学包覆机理进行了研究。采用超声化学反应时间为2h,反应功率为180w条件下制备出多组软硬磁性相均匀分布的Nd2Fe14B/Fe双相纳米复合磁粉,Fe在复合磁粉中的名义质量分数分别为:1~25wt%。通过改变Fe(CO)5的加入量可以调整包覆层的厚度和均匀性。随着Fe(CO)5的加入量的增加,即名义质量分数的提高,NdFeB颗粒表面包覆上的Fe的量明显增加。在名义质量分数为20wt% Fe反应后包覆层较为密实、均匀。实验发现包覆过程中硬磁颗粒越小其表面积越大,有利于其在溶液中均匀分散,就越有利于软磁相在其表面的规则均匀性包覆;但是同时表面积越大,颗粒表面的氧化就越严重,这对磁体磁性能将会产生较大危害。综合硬磁相颗粒大小及氧含量对包覆效果和磁体性能的影响,硬磁相的颗粒度选择在50~75μm范围内。采用SEM、XRD、B-H回线仪,IRO-I定氧仪对Nd-Fe-B单相磁体和包覆后Nd-Fe-B/ -Fe双相复合磁体进行了研究。研究表明在烧结温度为903K,压力400MPa条件下,单相磁体能够获得高的致密度,磁体密度为7.60g/m3(相对密度98.4%) ,该条件下烧结单相磁体具有最佳综合磁性能,(BH)max=131.52kJ/m3(16.44MGOe),Br=0.90T。对原料磁粉、单相烧结磁体及双相复合磁体中的氧含量测定表明,发现磁体中的氧一部分是由原料磁粉所带入的,另外一部分主要是在超声化学包覆过程中带入的,超声包覆过程中纳米Fe颗粒的生成量的增加会造成复合磁粉中吸附的氧的增加。同时研究发现双相复合磁体中纳米Fe大部分分布在NdFeB颗粒间界,其厚度大约在100~300nm,但磁体中的Fe含量与理论计算的质量分数相差较远,其主要原因是Fe(CO)5分解时溶液中产生的Fe有一部分未能有效的包覆在NdFeB颗粒表面,这部分悬浮在介质中的纳米Fe颗粒在反复清洗溶液时会流失。不同名义Fe质量分数包覆的双相复合磁体的最大磁能积和剩磁随着软磁性相含量的增加呈现出先减小后增大再减小变化规律,而矫顽力则总体上趋于下降,在名义质量分数为15wt% Fe包覆时双相复合磁体取得最佳磁性能,(BH)m=128.2 kJ/m3 (16.1MGOe),Br=0.92T,Hcj=607.35kA/m。利用石墨模具导热性能好,温度可控性强,造价低等特点进行SPS烧结工艺优化调整。研究表明石墨模具最佳烧结速率为120K/min。在943K温度下SPS烧结单相NdFeB磁体的烧结温度磁体综合磁性能最佳:(BH)max= 122.90kJ/m3 (15.44MGOe) ,Br=0.86T,Hcj=1188.43kA/m。993K温度下烧结Nd2Fe14B/α-Fe复合双相磁体获得了最佳磁性能:(BH)max=82.94 kJ/m3 (10.42MGOe), Br=0.79T,Hcj=378.10 kA/m。Nd2Fe14B/α-Fe复合双相磁体二次烧结的整体磁性能较一次烧结所得磁体有了较大的提升,最大磁能积提高了近4MGOe,最佳磁性能为(BH)max=114.39kJ/m3 (14.37MGOe), Br=0.90T,Hcj=406.76 kA/m。在二次烧结工艺中矫顽力的下降趋势得到了很好的缓解,而剩磁的提高对磁性能的影响仍然是主导因素。二次烧结工艺确保样品完全烧结充分、致密,均匀合理的分布了内部软磁相硬磁相晶粒,降低了磁体晶粒的退磁因子Neff,避免了矫顽力的大幅下降,确保获得较为理想的磁性能。更多还原

【Abstract】 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.更多还原