烧结(Nd,Tb)-Fe-B磁体的元素扩散、反磁化和矫顽力

Effect of atomic diffusion on magnetization reversal and coercivity in sintered (Nd,Tb)-Fe-B magnets

摘要:

采用双合金法将两种粉末混合制备烧结永磁体可提高磁体磁性能;但在烧结过程中两种粉末之间存在元素扩散,元素扩散对磁性能的影响程度需要进一步研究。本文将Nd13Fe81B6和TbHx粉末混合制备烧结磁体,Nd13Fe81B6磁体矫顽力为4.5 kOe。当TbHx混合量为3 wt.%,烧结磁体的矫顽力增加至20.0 kOe。通过热激活研究认为,磁畴壁的形核是反磁化需要经过的过程。由于热力学的原因Tb元素更容易扩散进入Nd2Fe14B主相而不是富集在晶间富稀土相。Tb元素进入主相替代Nd可形成具有更高各向异性场的(Nd,Tb)-Fe-B表层,在反磁化过程中晶粒表层磁畴壁的形核场会增加,因此矫顽力增加程度显著。但是,TbHx混合量超过5 wt.%,矫顽力增加幅度降低。对于TbHx混合量7 wt.%的磁体,元素分布显示在主相晶粒内部贫Tb区域明显增少,证实在烧结过程中更多Tb原子从晶粒表层扩散入晶粒内部,这样晶粒表层反磁化形核场的提高程度会减弱,因而磁体矫顽力增加幅度降低。本研究说明要提高双合金Nd-Fe-B磁体磁性能需进一步控制元素扩散并优化磁体的元素分布。

Abstract:

The magnetic properties could be improved in rare-earth magnets prepared by the dual-alloy method via mixing two different types of powders. The atoms would diffuse in the sintering, and it is necessary to make clear the effect of atomic diffusion on the magnetic properties. In this paper, the sintered magnets were prepared by mixing Nd13Fe81B6 with TbHx powders. The coercivity is 4.5 kOe in Nd13Fe81B6 magnets, and it increases to 20.0 kOe with 3 wt.% TbHx addition in the sintered magnets. The investigation of thermal activation indicates that the magnetization reversal undergoes the nucleation of reversed domain wall at grain outer-layer. Owing to the thermodynamical origin Tb atoms prefer to diffuse into the grain interior of Nd2Fe14B phase rather than aggregate in the intergranular rare-earth-rich phase. Tb atoms could substitute for Nd and form (Nd,Tb)-Fe-B shell layer with higher anisotropy in the main phase, and so the nucleation field of reversed domain is improved at grain outer-layer, leading to the significant increase of coercivity. However, for 5 wt.% TbHx addition the increase amplitude of coercivity decreases in the sintered magnets. The elemental distribution shows that for 7 wt.% TbHx addition the area of Tb-lean regions decreases, indicating that more amount of Tb atoms diffuse into the grain interior of Nd2Fe14B phase. The enhancement effect of improving the nucleation field is weakened at grain outer-layer, resulting in the reduction of the increase amplitude of coercivity. Regulating the atomic diffusion and optimizing the elemental distribution are necessary to further improve the magnetic properties in the sintered magnets prepared by the dual-alloy method.