【作者】 李玉平；

【导师】 胡连喜；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2010， 博士

【摘要】 纳米晶永磁材料作为一种新型永磁材料,由于具有潜在的高剩磁、高矫顽力与高磁能积,近年来受到了人们的广泛关注。目前纳米晶磁粉可以通过快淬法和机械合金化法来制造,这类磁粉主要用来制备粘结磁体。粘结磁体具有成型工艺简单、尺寸精度高及力学性能好等优点,但是粘结磁体密度很低而且磁体中存在粘结剂,影响了磁体的磁性能。为此,本文提出了“机械力驱动歧化与脱氢-再结合烧结法”制备高致密纳米晶NdFeB磁体的新工艺。本文系统地研究了机械力驱动下铸态NdFeB合金的氢化-歧化反应,歧化态NdFeB合金粉末的冷压致密工艺,歧化态NdFeB合金粉末坯的脱氢-再结合原位烧结工艺,探讨了工艺条件对纳米晶NdFeB磁体组织结构与性能的影响。同时,还研究了纳米晶NdFeB各向异性磁体的制备工艺,探讨了工艺条件对磁体取向程度与磁性能的影响,并对纳米晶NdFeB磁体的矫顽力机制进行了理论分析。利用X射线衍射(XRD)、穆斯堡尔谱(M?ssbauer)以及透射电镜(TEM)等分析手段,研究了铸态NdFeB合金在充氢球磨过程中组织与结构的变化。结果表明,由于Nd2Fe14B相的歧化反应激活能远高于氢化反应激活能,NdFeB合金在机械力驱动作用下与氢气反应的特点为:氢化与歧化反应并非同时或交替发生,而是按先氢化、后歧化的顺序进行;合金氢化后必须再经过适度延时球磨使原子被激活到相对高能状态,才能使歧化反应在随后的球磨过程中发生;氢化反应速度很快,歧化反应速度很慢。对机械力驱动下NdFeB合金氢化–歧化反应的热力学条件及动力学特征进行了研究,并建立了描述机械力驱动下NdFeB合金歧化反应特征的动力学公式。结果发现,在充氢球磨的过程中,合金的歧化反应速率受到球磨机转速、球料质量比、氢压以及合金成分这几个因素的影响。对纳米晶歧化态NdFeB合金粉末的室温压制性能进行了研究,结果表明,歧化态NdFeB合金粉末具有良好的压制性能,易于采用常规模压的方法压制成高致密冷压坯。在1400MPa的压制压力下,歧化态Nd16Fe76B8合金粉末可获得致密度0.9左右的冷压粉末坯。这种歧化态粉末坯中含有大量的α-Fe相,可以在室温下对之进行镦锻塑性变形,变形后的坯料组织中具有明显的α-Fe相{110}织构。利用XRD、M?ssbauer谱以及TEM等分析手段研究了纳米晶歧化态NdFeB合金粉末坯在烧结过程中的脱氢–再结合反应,分析了烧结工艺参数对材料组织的影响。研究表明,歧化态NdFeB合金粉末坯经过780℃真空烧结30min后,获得了平均晶粒尺寸为50nm左右的纳米晶NdFeB致密磁体。利用振动样品磁强计(VSM)对在不同工艺条件下所获磁体的磁性能进行了分析,探讨了烧结温度与烧结时间对磁体磁性能的影响。实验研究获得的最佳制备工艺参数为:780℃×30min,在该工艺条件下,合金成分为Nd16Fe76B8的纳米晶各向同性致密磁体的磁能积达到106.3kJ/m3,抗压与抗弯强度分别达到301MPa与116.5MPa。与相同成分的各向同性磁体相比,各向异性纳米晶致密磁体的磁性能更高,达到135.2kJ/m3。对纳米晶NdFeB致密磁体的各向异性形成机理进行了研究,发现磁体的各向异性实际上源于歧化态NdFeB合金粉末变形坯中α-Fe相的{110}晶面织构。对所获得的纳米晶NdFeB磁体的磁化与反磁化机制进行了探讨,指出了再结合不完全的纳米晶磁体磁化与反磁化是由形核型和钉扎型两种机制共同作用的结果,而再结合完全的纳米晶磁体磁化与反磁化是由钉扎型机制作用的结果。更多还原

【Abstract】 Nanocrystalline magnetic materials have attracted considerable attention in recent years because of their potential high remanence, coercivity, and maximum energy product. Nanocrystalline magnetic powders are usually synthesized by melt-spinning and mechanical alloying, and these powders are generally used for making bonded magnets. The bonded magnets have the advantage of easy processing, high dimension precision and good mechanical properties. However, the low density and the existence of polymer binder for the bonded magnets reduce the magnetic properties, leading to the low values of (BH)max. To solve this problem, a new technique, which combines mechanically activated disproportionation and desorption-recombination sintering, has been proposed to obtain nanocrystalline highly-densified NdFeB magnets. The mechanically activated hydrogenation– disproportionation of as-cast NdFeB alloy, the cold compaction process of as-disproportionated NdFeB alloy powders, and the desorption- recombination in situ sintering of as-disproportionated NdFeB alloy powder compacts were investigated systematically. The effect of processing parameters on the microstructures and magnetic properties of the nanocrystalline NdFeB magnets was studied. The production process of nanocrystalline anisotropic NdFeB magnets was investigated as well. The effect of processing parameters on the microstructure anisotropy and magnetic properties of the anisotropic magnets was studied, and the coercivity mechanism of such nanocrystalline NdFeB magnets was also analyzedMicrostructural changes of as-cast NdFeB alloy during mechanical milling in hydrogen were characterized by XRD, M?ssbauer spectra and TEM analysis. The results showed that,since the disproportionation of Nd2Fe14B phase needs much higher activation energy than that for hydrogenation, the characteristics of the reaction between NdFeB alloy and hydrogen during milling could be interpreted as follows: the hydrogenation occurred much earlier than disproportionation; after hydrogenation, a further milling was needed to promote the energy level of the hydrogenated alloy so that the disproportionation reaction was activated; the kinetics of hydrogenation is much faster than that of disproportionation.The aspects of the thermodynamics and kinetics for the mechanically activated disproportionation were investigated, and the kinetic equation was derived. The results revealed that the disproportionation kinetics was affected by the rotation speed, ball to powder mass ratio, hydrogen pressure, and composition of the alloy.The cold compactability of the as-disproportionated NdFeB powder was investigated, and the results showed that the nano-structured as-disproportionated NdFeB alloy powder had good cold compactability, they could be cold pressed into highly-densified compacts by conventional die pressing. After cold pressing at 1400MPa, the relative density of the compacts can achieve above 0.9. The cold deformation process of the compacts was also studied, and it was shown that a strong {110} crystallographic texture of theα-Fe was formed after cold upsetting.The desorption–recombination behavior of as-disproportionated NdFeB alloy during vacuum sintering was investigated by the means of XRD, M?ssbauer spectra, and TEM analysis, and the effect of processing parameters on the microstructure of magnets was also analyzed. The results showed that nanocrystalline NdFeB magnets with average grain size of about 50nm was obtained by vacuum sintering at 780℃for 30 min.The magnetic properties of the nanocrystalline NdFeB magnets obtained at various parameters were measured by VSM. The influences of processing parameters on the microstructure and properties of the magnets were investigated. In the present study, the highest magnetic energy product of the nanocrystalline isotropic Nd16Fe76B8 magnets, was 106.3kJ/m3, the compressive strength and flexural strength of the magnets achieved 301MPa and 126.3MPa, respectively. The magnetic properties of the nanocrystalline anisotropic NdFeB magnet were higher than that of the isotropic NdFeB magnet, which achieved 135.2kJ/m3.The formation of magnetic anisotropy in nanocrystalline anisotropic NdFeB magnets was investigated by means of XRD, TEM and VSM. It was found that the magnetic anisotropy of the magnets was attributed to the seeding effect of the {110} crystal texture of theα-Fe phase in the as-deformed alloy. The mechanism of magnetization and demagnetization for the nanocrystalline NdFeB magnets was discussed. It was found that the coercivity was controlled by the domain wall pinning for the magnets obtained by completed desorption-recombination reaction and controlled by the nucleation-pinning co-working magnetization mechanism for the magnets obtained by uncompleted desorption-recombination reaction.更多还原