Strong Magnetic Mechanism and Microstructure Regulation of Soft/Hard Dual-Phase Nano-Permanent Magnets
Zhang Yikun Wang Shuhuan Liu Kun Zhao Dingguo Song Chunyan
College of Metallurgy and Energy,North China University of Science and Technology
Tangshan Key Laboratory of Special Metallurgy and Material Preparation,North China University of Science and Technology
Abstract:
Considering the bottleneck of single-phase nano rare earth permanent magnetic research and preparation,this paper introduced the necessity of research on nano dual-phase rare earth permenant magnetic materials by introducing the superior magnetic properties. The change coupling mechanism and remanence enhancement principle of nano-biphase rare earth permanent magnet materials were briefly described. The corresponding grain size and volume fraction range of soft and hard magnetic phases that intended to obtain magnetic properties were determined. The soft magnetic phase grain size was ~10 nm,and that of the hard magnetic grain was about30 nm,and the soft magnetic phase volume fraction should be about 30%~70% when the coercive force reached the maximum value.The nucleation mechanism and pinning mechanism of coercivity and the reasons of low coercivity were analyzed in depth. The grain size and volume fraction of soft and hard magnetic phases,the effects of microstructure defects,interfacial composition and structure on coercivity were pointed. In order to overcome the relationship between coercivity and remanence,the theoretical basis was established. Finally,the microstructure regulation method of soft/hard dual-phase nano-permanent magnets was summarized,and the coreshell structure was proposed to improve coercivity and remanence simultaneously. The advantages of the large plastic deforation method in the microstructure control were stated,and finally the outlook of nano dual-phase permanent magnet materials was given.
Keyword:
nano dual-phase;permanent magnet material;intergranular exchange coupling;coercive force mechanism;
Jones
[14]认为矫顽力机制与形核机制和钉扎机制均有关系。这一观点也得到了大多数学者的认可。Sun
[15]在两种矫顽力机制的基础上发现,双相纳米永磁材料中软/硬磁性相间的交换耦合作用对畴壁钉扎机制的影响很大。硬磁相晶粒间的交换耦合作用越大,晶粒的有效各向异性场越低,矫顽力越小。因此矫顽力是相的组成及分布、晶粒尺寸及界面成分及结构等因素综合作用的结果,要想透彻地理解矫顽力机制,还需要大量的实验研究与理论分析。
Li等
[18]利用物理方法制备出三维自组类核壳结构高性能纳米永磁体,核壳的结构模型如图4(a)所示。这种核壳结构使Nd-Fe-B永磁体的磁能积(BH)max超过之前研究的各向同性永磁体最大值159.2 k J·m-3,达到199 k J·m-3,如图4(b)所示,其中Br为剩磁,Hcj为矫顽力,(BH)max为二者乘积。实验通过控制凝固过程中软/硬晶粒的谐调增长,纳米晶软磁性相α-Fe(~13 nm)以类壳结构的形式分布于硬磁相Nd2Fe14B晶粒(~45 nm)周围,且软磁性相α-Fe的体积分数高达28%,这种分布规律不但没有因为软磁相α-Fe的存在使矫顽力降低,反而由于其特殊的存在位置使矫顽力、剩磁及磁能积显著增加,这取决于饱和磁化强度高的软磁相(α-Fe)和各向异性场高的硬磁相(Nd2Fe14B)之间的界面对畴壁的钉扎作用,抑制了畴壁的运动,并且纳米级别尺寸的软硬磁相间的交换耦合作用加强,硬磁相间的交换耦合作用降低,有效各向异性增强,这与之前对永磁材料中软磁相α-Fe降低矫顽力的认识大相径庭。可见晶粒的尺寸、相的含量及分布对材料性能的影响起到关键的作用。但由于Nd-Fe-B永磁体中硬磁相晶粒的易轴随机排列,得到的各向同性永磁体的极限磁能积理论值仅相当于硬磁晶粒易轴线性排列的Nd-Fe-B磁体磁能积的40%
[19],若能在该核壳结构的基础上制备出硬磁晶粒易轴线性排列的磁体,这种纳米复合永磁体材料应用前景将会更加广阔。
图4 软硬磁晶粒核壳结构模型及磁性能
Fig.4 Core shell structure model of soft and hard magnetic grains and magnetic properties
[18]
此外非晶晶化法制备复合双相纳米永磁材料也取得了长足的进步。研究者采用非晶直接晶化方法对Nd2Fe14B/α-Fe纳米复合永磁材料进行制备,得到的固体均以粉末或薄带的形式存在,并且磁性能的提高也受到限制。主要原因在于直接晶化过程中在形成硬磁性晶粒前会有亚稳相Fe3B和Nd2Fe23B3的生成
[20],这两种物质中Fe的含量很高,这就抑制了α-Fe相的形成,导致剩磁的减少。如果能抑制亚稳相的产生或促进其在低温条件下分解,就能获得高体积分数的α-Fe相,这对纳米复合永磁材料磁性能的提高有很大的帮助。为了解决这个问题,在先前研究的基础上,Li等
[21]采用对非晶薄带进行室温大塑性变形(HPT)和后续退火处理相结合的方法,在非晶基体上诱导出Nd2Fe14B和α-Fe晶粒,并且有效地抑制了退火过程中亚稳相Fe3B和Nd2Fe23B3的生成。通过实验得出随应变量的增加Nd2Fe14B和α-Fe晶粒尺寸减小,α-Fe的体积分数增加,在应变ε=6.2时,α-Fe和Nd2Fe14B的晶粒尺寸分别为12.7和22.4 nm,明显低于直接退火后相应的晶粒尺寸23和53 nm。得到的α-Fe体积分数为35%高于直接退火后体积分数(14%)。除了组织的改变,磁性能也得到大幅度提高,在最佳应变(ε=6.2)下,变形后磁体的矫顽力Hcj=573120A·m-1;最大磁能积(BH)max=141.688 k J·m-3;而直接退火非晶相得到的矫顽力Hcj=493520 A·m-1;最大磁能积(BH)max=93.818 k J·m-3。可见高压扭转有效地控制了后续退火过程中非晶基体上纳米晶的产生,使退火后晶粒尺寸显著减小,界面(面缺陷)体积分数增加,充当钉扎点,阻碍畴壁的运动,提高矫顽力。此外,在大塑性变形的作用下,非磁性原子在界面自由体积处富集,导致界面原子结构和化学环境的变化,界面结构的改变增加了畴壁的钉扎强度,有助于矫顽力的增加
[22]。大塑性变形法(SPD)克服了球磨法、非晶晶化法等传统方法所制备材料小尺寸的缺点,SPD法可以使制备出的纳米结构试样尺寸达到十几毫米,使纳米功能材料的应用成为可能
[23]。