【作者】 罗阳；

【导师】 庄卫东； 李红卫；

【作者基本信息】 北京有色金属研究总院 ， 有色金属冶金， 2014， 博士

【摘要】 由于具有较高的永磁性能和丰富的物理现象,一些具有亚稳态结构的稀土永磁材料如SmFe基化合物引起了人们的关注。这些亚稳态与平衡态之间,以及亚稳态结构之间的形成和转化对磁性材料的研究具有十分重要的意义,也为新型稀土永磁材料的探索提供了新的思路。基于此,本论文中采用快淬这种非平衡凝固工艺制备SmFe基化合物,采用X射线衍射(XRD)、透射电镜(TEM)、振动样品磁强计(VSM)穆斯堡尔谱等手段研究成分、工艺等对亚稳相的结构变化及性能的影响。首先通过快淬(40m/s)及热处理(600～1000℃)制备了SmFex(3≤x≤12)化合物,分析了各条件下亚稳态结构的形成及转变规律,绘制了结构-成分-温度之间的关系图。发现SmFe2相在两种条件下稳定存在,一种为x＜4,另一种为4＜x＜8且热处理温度在900℃以下；Sm5Fe17亚稳相稳定范围为3＜x＜5且热处理温度在900℃以下；TbCu7型的SmFe9化合物在40m/s及以下难以形成单相的结构；各相的稳定程度为：Sm2Fe17-type, SmFe3-type> SmFe2-type> Sm5Fe17-type, SmFe9-type> amorphous。针对以上出现的Sm5Fe17和SmFe9两种化合物开展研究,首先分析了不同快淬速率(10～50m/s)和不同热处理工艺(600-900℃)下Sm5Fel7的结构和磁性。发现经过30m/s快淬和700℃热处理后Sm5Fe17化合物的矫顽力可达33.8kOe,结构中少量的SmFe2相使其矫顽力呈现“钉扎型”特征,但是也造成了退磁曲线上的“台阶”；添加Tb之后,Ms和Hcj下降明显,添加10%的Tb后矫顽力已经降低50%,主要原因在于加入Tb后晶粒尺寸从20-50nm长大到100nm以上,并且出现Tb6Fe23相,使矫顽力机制从“钉扎型”转向“形核型”；(Sm,Tb)5Fe17中各相之间的交换耦合作用消除了退磁曲线上的“台阶”。40m/s冷速条件下二元的SmFe9化合物中较易出现2：17和a-Fe,导致氮化后主相发生分解,磁性能难以提高,需要添加其他元素稳定亚稳态结构；对于Sm1-xZrxFe9.5+x来说,Zr具有与Sm相近的原子半径,加入后占据引起最低晶体弹性应变能的1aSm晶位,底面a轴长度缩短,同时随Zr增加Fe含量增多,c轴2e晶位上Fe的原子密度增大,c轴长度增大,这些均提高了晶体结构的长短轴比c/a,从而稳定亚稳态的TbCu7结构。研究了晶化过程对Sm1-xZrxFe95+xNy性能的影响,发现随着晶化温度的提高,矫顽力逐渐升高,剩磁在750～850℃范围内变化不大,温度大于900℃时,主相分解,剩磁出现明显下降；当Zr含量x=0.1和0.2时,综合性能较好,进一步提高Zr和Fe含量,矫顽力和磁能积出现明显下降；850℃×1h热处理,440℃×16h氮化后,Sm0.8Zr0.2Fe9.7NX的各综合性能较佳,(BH)m=6.63MGOe, Hcj=6.00kOe, Br为7.17kGs;氮化物磁粉的矫顽力具有明显的钉扎特性,随Zr和Fe的增多,结构中出现的α-Fe成为反磁化形核中心,矫顽力下降明显。在SmFe9化合物中加入Si、B形成SmFe9.5-xSixB0.1化合物,发现随着Si含量提高,c/a值增大,在x≥0.4后稳定并大于0.85,主要原因在于Si、B与Sm和Fe均具有较低的混合焓,加入后降低体系能量从而稳定TbCu7结构。在保持结构稳定的基础上,通过提高晶体结构中的Fe含量,抵消由于Si加入带来饱和磁化强度的降低,以SmFe9.3+xSi0.2B0.1(x=0,0.5,1.0)为例,增加的Fe优先占据晶格的2e晶位,带来Fe2e-Fe2e原子间距缩短,c/a值提高,在x=0.5时具有最大的超精细场29T,最高的居里温度477.68K；氮化后,居里温度相对快淬态平均提高265K,最终当x=0.5时,获得最优性能Hcj=4.31kOe,(BH)m=3.50MGOe=在TbCu7型Smo.7Zr0.1Fe9.6Bx化合物中,小原子B的加入可以提高化合物的非晶形成能力,抑制α-Fe的析出,730℃×90min处理之后仍无明显α-Fe出现,同时可与Zr一起稳定TbCu7结构,使其c/a值达到0.87；加入B后不需氮化即具有永磁特性,当x=0.8时达到2.8kOe,B对2e和3g晶位的占据可能是其具有永磁性能的主要原因。更多还原

【Abstract】 The permanent magnetic materials with metastable structure like SmFe based compounds have attracted much attention due to the high magnetic properties and rich physical phenomenon. The research of the formation and transformation between these metastable and equilibrium state has vital significance on the understanding of crystal structures relationship and thermodynamic stability.So, SmFe based compounds are prepared by melt spun and subsequent heat treating. The influences of composition and preparing method to the phase transition, microstructure and magnetic properties are tested by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM) and Mossbauer Spectroscopy.Firstly the SmFex(3≤x≤12) compounds were prepared by melt spinning (40m/s) and subsequent heat treatment (600-1000℃×1h). The formation and transition of the metastable phases are mainly indexed by XRD. The relationship diagram of Phase-Composition-Temperature (PCT diagram) is ploted. The glass forming ability can be enhanced by the increase of Sm content to x<5. Metastable Sm5Fe17-type structure exists when3<x<5and temperature lower than800℃. SmFe2-type structure is stable when x<4or temperature lower than900℃when4<x<8. The SmFeg compounds with TbCu7-type structure are very sensitive to composition and preparation process. The stability of all the phase in this experiment can be listed as:Sm2Fe17-type, SmFe3-type> SmFe2-type> Sm5Fe17-type, SmFe9-type> amorphous.The Sm5Fe17and SmFeg based compounds according to the PCT diagram are studied in detail. The structure and magnetic properties of Sm5Fe17under different quenching velocity (10-50m/s) and different heat treatment (600-900℃) process are analysed. The highest coercivity of33.8kOe are prepared under30m/s and700℃×1h. The coervicity is increased by the "domain wall pinning" mechanism due to SmFe2phase, which also caused the "kink" in the demagnetization curve. Saturation magnetization and coercivity are dropping obviously after the adding of Terbium (decreased by50%, from30kOe to17kOe after adding10%Tb). The main reason is due to the increase of crystal size from20-50nm to100nm and emerging of soft magnetic phase Tb6Fe23in the700℃×1h heat treated samples, which caused the coercivity mechanism changed from domain wall pinning to nucleation gradually. The exchange coupling effect in the (Sm,Tb)5Fe17compounds smoothed the demagnetization curve.It’s different to elevate the final magnetic properties of SmFe9Nx compounds due to the unstable TbCu7type structure under40m/s. The similar atomic radius of Zr to Sm leads to the occupation of1α crystal site to avoid increasing of crystal elastic strain energy. The TbCu7type structure is stabilized as the c/a value increase to higher than0.86.The coercivity of Sm1-xZrxFe9.5+xNxmagnetic powders after crystalized and nitrided is increasing with the heat treating temperature. As to saturation magnetization, no obvious change can be observed from750to850℃while falling rapidly after900℃. Considering about composition, better properties are those Sm1-xZrxFe9.5+xNy samples with x=0.1and0.2. The coercivity and saturation magnetization dropped steeply when the content of Zr and Fe increase, especially when x>0.4and the magnetic properties fall to about zero. The best properties can be obtained when x=0.2and the procedure of heat treated at850℃×1h and nitrided at440℃×16h, which can be (BH)m=6.63MGOe, Hcj=6.00kOe, Br=7.17kGs. The nitride powders have obvious domain wall pinning mechnism to explain high coercivity while more reverse magnetization nucleation center formed in the higher a-Fe content samples which cause the decrease of coercivity.Besides of Zr, the combined adding of Si and B can stabilized the metastable TbCu7-type structure because of low mixing enthalpy with Sm and Fe. The samples’ saturation magnetization decreases with the added non-magnetic elements Si. As to SmFe93+xSi0.2B0.1(x=0,0.5,1.0), the extra Fe atoms prefer to take2e site, which bring the shrink of atom distance between Fe2e-Fe2e sites and increase of the value of c/a. The highest hyperfine field of29T and Curie temperature of477.68K can be obtained when x=0.5. After nitridation, the Curie temperature increase about265K. The best magnetic properties of Hcj=4.31kOe,(BH)m=3.5MGOe can be found when x=0.5.B in the melt spun Sm0.7Zr0.1Fe9.6Bx samples can elevated the glass forming ability and inhibited the precipitation of a-Fe. No a-Fe can be found in the730℃×90min treated samples. The high c/a value of0.87means the stability of this SmZrFeB compounds. After crystallization, the Smo.7Zr0.1Fe9.6Bx samples have obvious permanent magnetic properties. The coercivity increase with the Boron content to highest of2.8kOe at x=0.8. The boron rich phase emerged as the Boron content continue to increase. The preferential occupation of2e or3g sites may the main reason to have these permanent magnetic properties.更多还原