晶界扩散Tb对烧结Nd-Fe-B组织与磁性能的影响

用磁控溅射法在烧结Nd-Fe-B磁体表面沉积Tb金属薄膜并进行晶界扩散处理,对比经不同热扩散温度及时间处理后的磁体组织和磁性能变化。结果表明,925℃×10 h+500℃×2 h为最佳晶界扩散工艺,可将磁体矫顽力提高到1630.9 kA·m^-1,较原始磁体提升50%,同时剩磁和磁能积无明显下降,磁体仍具有较高的退磁曲线方形度。晶界扩散处理后磁体取向度有所提高,主相晶粒表面形成了明显的富Tb壳层结构,其厚度随离开磁体表面距离的增加逐渐变薄,随热扩散温度升高和时间延长逐渐增厚。长时间热扩散处理使磁体内形成沿晶界分布的连续薄层富Nd相,将主相晶粒彼此分隔,有效降低磁性相颗粒间交换耦合作用。能谱(EDS)分析表明,适当的热扩散工艺可使Tb元素扩散至磁体芯部,渗透厚度4 mm的磁体。

Microstructure and Magnetic Properties of Sintered Nd-Fe-B Magnets with Grain Boundary Diffusion Tb

Diffusing Tb element through the grain boundary diffusion process(GBDP)to improve the coercivity had gradually be-come the main production process of commercial high-performance sintered Nd-Fe-B.However,the optimum diffusion conditions andthe distribution of the Tb element in the Tb-deposited magnets prepared by the GBDP with the magnetron sputtering method still needfurther investigation.Understanding the relationship between thermal treatment processes and the magnetic properties of the GBDPmagnet will help us to optimize the GBDP and provide the reference basis for practical production.A commercial sintered Nd-Fe-B per-manent magnet without Dy or Tb was used as the diffusion substrate.The initial(annealed)magnet was cut into small pieces along theorientation of the magnet.The thin film of Tb metal was deposited on the surface by the magnetron sputtering technology.Subsequent-ly,Tb-deposited samples were exposed to heat treatment in a vacuum at 825~1025 ℃ for 2~10 h and then annealed at 500 ℃ for 2 h.The magnetic properties of the magnet were measured by high field hysteresis meter Hy MPulse(Metis Instruments).Back scatteredelectron(BSE)images were obtained by JSM-7800 F field emission scanning electron microscope(FE-SEM)equipped with energy dis-persive X-ray spectroscopy(EDX)detector for the constituent analysis.The microstructure and magnetic properties of the magnet treat-ed with different diffusion temperatures and time were compared.The higher the diffusion treatment temperature was,the deeper theTb element could enter into the magnet.So that the coercivity enhanced gradually as the concentration of Tb element diffused into themagnet increased.The shell structure was formed on the surface of main phase grains.Energy dispersive spectrometer(EDS)analysisindicated that Tb element apparently gathered at the shell.That revealed Tb did not diffuse into the interior,but formed the(Nd,Tb)2Fe14 Bshell in the outer regions of the main phase grain.The thickness of the Tb-rich shell gradually became thinner with the increase of thedistance from the magnet surface but gradually thickens with the increase of the diffusion temperature and the extension of time.The av-erage atomic mass of the surface layer was higher than other regions in the diffusion samples.As the surface layer with a thickness of ~40 μm was polished from the diffusion magnet,the remanence gradually recovered to that of the initial magnet,while the coercivitywas almost still constant.The squareness of demagnetization curves of the Tb-deposited sample after 2 h diffusion treatments conspicu-ous lower than that of the untreated one.The magnetic properties of Tb-diffused magnet maintained an upward trend with the extensionof diffusion time,but long-time diffusion treatment over 6 h did not contribute significantly to improve the coercivity.The remanenceand magnetic energy product of the Tb-deposited sample after diffusion treatment for 10 h was enhanced significantly compared withthose of the magnet diffused for 2 h.In addition,the squareness of demagnetization curve was gradually recovered with the extending ofthe diffusion time,which almost consistent to that of the initial sample.The Nd-rich thin layer distributed continuously among grainboundary was formed in the magnet by long time diffusion treatment.The more continuous and homogeneous distribution of Nd-richphase reduced the defects in the grain boundary and inhibited the nucleation of reverse magnetization domain.The main phase grainswere separated from each other,which effectively reduced the magnetic phase exchange coupling effect between magnetic phase parti-cles.The thickness of Tb-rich shells became thinner with the distance from the deposited surface increasing.EDS analysis showed thatthe thin shell structures could still be observed at the grain boundary of the center away from the magnet surface.The results confirmedcore-shell structures exists on the entire magnet,which means Tb element penetrated the magnet with a thickness of 4 mm.That was themain reason for the increase of remanence and magnetic energy product after diffusion for 10 h.The results showed that 925 ℃×10 h+500 ℃×2 h was the best diffusion treatment process.The coercivity could be increased to 1630.9 kA·m^-1,that of 50% higher than that ofthe untreated magnet.At the same time,the remanence and magnetic energy product of the magnet had no decrease obviously and kepthigh squareness of demagnetization curves.The orientation of magnets was improved after grain boundary diffusion treatment.The micro-structure and properties of the GBDP magnets as a function of the diffusion temperature and time were systematically investigated.Tb el-ement diffused via the grain boundary to the inner and formed the distinct core-shell structures near the external of the Nd2Fe14B grains.The removal of the surface layer increased the remanent magnetic polarization and had no effect on the coercivity of the GBDP magnet.Tb element could be diffused to the center of the magnet by the appropriate diffusion treatment process and gathered in the grain bound-aries.The Tb-rich shell structure existing within the magnet was the reason why the GBDP magnet holds the high squareness of demagne-tization curves and the high remanence.Thereby,the magnetron sputtering method deposited Tb film combined with GBDP was a practi-cable process to prepare Nd-Fe-B sintered magnets with high coercivity,remanence,and magnetic energy product.