还原扩散法制备钐铁氮磁性材料的组织结构与性能研究

研究了还原扩散法制备Sm2Fe17Nx(x≈3)磁性材料的组织结构与性能。用DTA方法测定了Sm2Fe17Nx的热稳定性,用VSM测定了本实验所制Sm2Fe17Nx磁体的磁滞回线和退磁曲线;用TEM及高分辨电镜分析了渗氮前后Sm2Fe17合金及Sm2Fe17Nx结构。结果表明:Sm2Fe17Nx的分解温度为841 K,用还原扩散法制备的Sm2Fe17Nx磁粉做成粘结磁体后各向异性不是很强烈,性能偏低。Sm2Fe17Nx磁粉磁性的好差与渗氮层的厚度有密切的关系,渗氮后的Sm2Fe17Nx确实有体积膨胀的现象,而且属非晶结构。     

In-situ high-pressure x-ray diffraction study of zinc ferrite nanoparticles

We have studied the high-pressure structural behavior of zinc ferrite (ZnFe₂O₄) nanoparticles by powder X-ray diffraction measurements up to 47 GPa. We found that the cubic spinel structure of ZnFe₂O₄ remains up to 33 GPa and a phase transition is induced beyond this pressure. The high-pressure phase is indexed to an orthorhombic CaMn₂O₄-type structure. Upon decompression the low- and high-pressure phases coexist. The compressibility of both structures was also investigated. We have observed that the lattice parameters of the high-pressure phase behave anisotropically upon compression. Further, we predict possible phase transition around 55 GPa. For comparison, we also studied the compression behavior of magnetite (Fe₃O₄) nanoparticles by X-ray diffraction up to 23 GPa. Spinel-type ZnFe₂O₄ and Fe₃O₄ nanoparticles have a bulk modulus of 172 (20) GPa and 152 (9) GPa, respectively. This indicates that in both cases the nanoparticles do not undergo a Hall-Petch strengthening.     

Thermal behavior of ternary Sm2Fe17Nx magnets

The Mössbauer spectra of Sm₂Fe₁₇N_x, prepared by the nitrogenation of Sm₂Fe₁₇ powders in an ammonia and hydrogen atmosphere, were observed at elevated temperatures to shed light on the thermal behavior of nitrogen in the compounds Sm₂Fe₁₇N_x. It was found that there were large differences in thermal behavior between the starting Sm₂Fe₁₇, crystalline Sm₂Fe₁₇N_x (x≈1.7) and amorphous Sm₂Fe₁₇N_x(x～7). The thermal decomposition behavior of Sm₂Fe₁₇N_3.2, developed as one of the most promising hard magnetic materials, was found to be different under different atmospheres.     

Sm2Fe17合金的氢化-歧化过程演化

通过X射线衍射(XRD)、扫描电镜(SEM)和能谱分析(EDX)等手段重点研究了Sm2Fe17合金在氢化-歧化过程中的相组成、相的变化以及微观结构的演化规律。研究表明：在0．1MPa的H2气氛下，Sm2Fe17合金首先吸氢；400℃时合金出现部分脱氢现象；在T≥500℃逐渐开始歧化为SmHx和α—Fe，同时生成了大量的微晶或非晶组织；随着温度的升高，合金中的微晶非晶逐渐晶化，750℃时晶化完全，晶粒长大至20～100nm。通过对Sm2Fe17合金的氢化一歧化过程研究，建立了该过程的微观结构变化规律的物理模型。     

Sm2Fe17-xSbx的化学合成与磁性质

稀土与铁形成的 L n2 Fe17金属间化合物的居里温度较低 ,不能用于永磁材料 .近年来的研究表明 ,当 L n2 Fe17金属间化合物的间隙格位被氮占据时可大大提高材料的居里温度和饱和磁化强度 [1,2 ] ,但由于形成的金属氮化物属于热力学亚稳相 ,因而只能用于粘结磁体 .为得到性能更好、更加稳定的稀土永磁材料 ,人们对其它元素取代的 Ln2 Fe17进行了大量的研究 ,发现一些主族和过渡金属元素 (如 Al,Ga,Cr或 Si等 )取代 Sm2 Fe17中的部分铁可以改善材料的磁性能 [3] ;从以往对铁系金属间化合物的结构与磁性的研究可以知道 ,部分取代使稀土 -铁…     

Ca-Sm2O3-Fe体系生成Sm2Fe17的研究

对由Ca-Sm2O3-Fe体系通过还原扩散反应生成Sm2Fe17合金的过程进行热力学和动力学分析; 研究了产物Sm2Fe17合金的收率及Sm和残余CaO的含量. 结果表明 还原扩散法制备Sm2Fe17合金的收率较高, 均大于80%, 且残余CaO的含量极低, 均小于0.55%; 随着温度的上升和反应时间的延长, Sm2Fe17合金的收率和Sm2Fe17合金中Sm的含量均增大. 收缩核模型处理结果表明 液态Sm和固态Fe之间的包晶反应为整个还原扩散反应过程的控速步骤; 还原扩散反应的表观活化能为73.74 kJ·mol-1, 指前因子为7.79×10-3.     

Comparison of 17beta-estradiol structures from x-ray diffraction and solution NMR

The NMR-derived structure of estrogen (17beta-estradiol, E2), the drug of choice for postmenopausal women, was compared with a recent literature crystal x-ray structure of Fab-bound E2. 1H and 13C NMR spectra of E2 were acquired in DMSO-d6. Assignments were obtained from an analysis of DQF-COSY, TOCSY, HETCOR, HMQC and HMBC 2D NMR spectra. The 1H and 13C NMR assignments are the first reported for E2 in DMSO-d6. Two solution structures, S1 and S2, were obtained with molecular modeling using NOE constraints. S1 overlaps with the crystal structure for all rings. S2 shows prominent differences in the C-ring (C9--C11--C12--C13) segment, which deviates from a chair conformation, and excellent overlap in the A-, B- and D-rings of E2. The C-ring in S2 adopts a boat conformation as opposed to a chair conformation in the x-ray and S1 structures. The S2 structure is about 6 degrees more twisted than the bound x-ray and S1 models. The S1, S2 and x-ray structures had ring bowing values of 10.1 +/- 0.3, 11 +/- 1 and 10.37 degrees , respectively. Of the 100 solution conformers generated, 83 had S1 conformation and 17 had S2 conformation, with average internal energies of 112 +/- 2 and 141 +/- 2 kcal mol(-1), respectively. The 100 S1- and S2- derived conformers showed a r.m.s.d. of 0.72 A for all atoms. The x-ray, S1 and S2 C18--O17 distances were 2.93, 2.92 +/- 0.01 and 2.93 +/- 0.01 A, respectively, and the O3--O17 distances were 11.06, 11.18 +/- 0.12, and 10.89 +/- 0.05 A, respectively.     

The detection of variations on the lro parameter of V3Ga by means of x-ray and neutron diffraction

The A15 phase field in the system V-Ga was investigated by DTA and photomicroscopy, in order to determine the appropriate annealing conditions for inducing a maximum disorder in this compound by argon-jet quenching. The conditions of formation of a single-phase V₃Ga alloy were studied.The long range order (LRO) parameter, S, of V₃Ga has been refined from X-ray and neutron diffraction data obtained on the same sample after two greatly different heat treatments. To the values of the superconducting transition temperature, T_c = 14.9 Kand 13.8 K, respectively, corresponding to these heat treatments, the following LRO parameters were determined: S = 0.982 ± 0.010, and S = 0.955 ± 0.010 (neutron diffraction), S = 0.972 ± 0.03 and S = 0.924 ± 0.03, respectively (X-ray diffraction, MoKα radiation).A comparison between the temperature factor of V₃Ga and that of other A15 type compounds shows a substantial difference between the values of high T_c superconductors such as V₃Ga, V₃Si, Nb₃Al and Nb₃Sn, and those of non super-conductors, such as V₃Co and Cr₃Si.     

A new approach to the problem of kaolinite interference in the determination of chrysotile asbestos by means of x-ray diffraction

In the determination of chrysotile asbestos by means of powder x-ray diffraction, a mathematical approach is used to overcome the problem of kaolinite interference. The overlapped peaks are deconvoluted by using least-squares and simplex optimization procedures.     

151Eu and57Fe Mössbauer and X-ray diffraction study of high temperature Tl-containing superconductor

⁵⁷Fe and¹⁵¹Eu Mössbauer spectroscopy and X-ray diffractometry measurements were performed with europium and iron containing Tl₂(Ca_0.8Eu_0.2)Ba₂(Cu_0.98Fe_0.02)₂O₈ and EuBa₂Cu₃O_7-d high temperature superconductor samples in order to study the effect of europium incorporated in a Tl-containing high temperature superconductor. Significant differences have been found between the⁵⁷Fe Mössbauer spectra of Tl-containing samples with and without Eu or between the¹⁵¹Eu Mössbauer spectra of Tl-containing and 1-2-3 type superconductors. The results can be interpreted by assuming that Eu can be localized at the Ca site in the Tl-containing superconductor.     

Characterization of phase transition of Ba2-xSrxIn2O5 by thermal analysis and high temperature X-ray diffraction

The phase transitions of Ba_2-xSr_xIn₂O₅ were investigated with various thermal analyses and high-temperature X-ray diffraction. It was clarified that crystal structure of Ba_2-xSr_xIn₂O₅ with x=0.0~0.4 varies from brownmillerite through distorted perovskite to another distorted perovskite with increase of temperature. The phase transition from brownmillerite to distorted perovskite was revealed to be first order, whereas transition from distorted perovskite to another one was second order. The specimen with x≥0.5 showed only one first order phase transition from brownmillerite to distorted perovskite. The phase diagram of Ba_2-xSr_xIn₂O₅ was established and existence of tricritical point at ~1100°C with x=0.4~0.5 was suggested. This revised version was published online in August 2006 with corrections to the Cover Date.     

Direct observation of phase transitions of polyethylene under high pressure by a PSPC x-ray system

A position-sensitive proportional counter (PSPC) x-ray measuring system is employed to observe directly phase transition processes of polyethylene at high temperature and high pressure. X-ray diffraction measurements reveal important new experimental data. First, an irreversible crystal transition from the hexagonal to the orthorhombic structures occurs in the critical region where the hexagonal structure begins to appear at a pressure of 350 MPa. That is, the (100) hexagonal reflection is observed only on cooling at 350 MPa. At pressures above about 400 MPa, however, the hexagonal phase is stable and the phase transitions melt ? hexagonal ? orthorhombic occur reversibly. Second, during cooling at pressures above 400 MPa, the (100) hexagonal reflection can be observed at temperatures below the hexagonal ? orthorhombic transition temperature. This behavior suggests that all the crystal morphologies of polyethylene, from “highly-extended-chain” crystals to crystals with a low melting point, are formed by the transitions melt → hexagonal → orthorhombic. Third, in heating at elevated pressures above 500 MPa, a shoulder in the peak intensity versus temperature plot for the (100) hexagonal reflection is observed at a higher temperature than the large maximum which occurs immediately after the crystal transition. This behavior indicates melting in two stages of hexagonal structures with different thermal stabilities, and the shoulder at higher temperature may be due to the fusion of the hexagonal phase annealed either below or above the transition point.     

Non-isothermal kinetics of high temperature oxidation studied by a fast X-ray diffraction method

Temperature resolved X-ray diffraction using a diffractometer equipped with a high temperature device was applied to the study of the oxidation of Fe-ARMCO below 570°C. The oxidation products are identified in situ and the difference method yields curvesY(T) showing the global changes. These curves were calibrated to the microscopically measured end thickness of the oxide layer obtaining the growthx(T). A calculated curve including the Arrhenius relation was fitted to thex(T) curve yielding activation energy and values of the oxidation rate constant as a function of temperature.

---

Zusammenfassung Unter Anwendung eines Diffraktometers mit einer Hochtemperatureinheit wurde unterhalb 570°C mittels Röntgendiffraktion die Oxidation von Fe-ARMCO untersucht. Die Oxidationsprodukte wurden in situ bestimmt und das Differenzverfahren ergabY(T)-Kurven, die die globalen Änderungen zeigen. Diese Kurven wurden anhand der mikroskopisch bestimmten resultierenden Dicken der Oxidschicht kalibriert, wobei man die Funktionx(T) erhält. Dieser Kurve wurde eine berechnete, die Arrheniussche Beziehung enthaltende Funktion angepaßt, wodurch man die Aktivierungsenergie und die Oxidations Geschwindigkeitskonstante als Funktion der Temperatur erhält.

     

Application of time and temperature resolved x-ray diffraction (TRXRD) to thermal analysis

The application of TRXRD for thermal analysis is demonstrated using examples of phase transitions, solid-state reactions and high-temperature corrosion. The measuring system produces a series of diffraction patterns, which are evaluated by a difference procedure that a reduces the data to curves comparable to DSC and TG curves and thus suited to kinetic evaluation.     

Sm2Fe17N3晶体的价电子结构分析和结合能、最强键能计算

由EET理论直接建立了Sm2Fe17N3晶体的价电子结构,同时计算并筛选了晶体的结合能和最强键能,分别为EC^0=14716.8±13.7kJ·mol^-1和Eα=110.1311 kJ·mol^-1。分析计算结果表明:Sm2Fe17N3晶体内共价电子数主要分布在12对由Fe(c1),Fe(c2)和Fe(c3)参与形成的最强键能的键上,由这3种Fe晶位原子形成的共价键键距普遍小于0.3 nm,共价键较强对晶体结合能作主要贡献;并且其结合能相比Sm2Fe17晶体的小得多,解释了Sm2Fe17合金在低温和非真空状态条件下易氧化而经过渗氮后得到的Sm2Fe17N3则表现出常温下结构稳定、化学性能好的特性;计算出N原子参与形成的成键原子对的理论键能值普遍在1 kJ·mol^-1左右,反映出Sm2Fe17N3化合物内在渗氮特性,分析了制备钐铁氮永磁材料过程中Sm2Fe17合金较低温渗氮难、渗氮不稳定和渗氮不均匀的缺陷。     

Picosecond calorimetry: time-resolved x-ray diffraction studies of liquid CH2Cl2

Liquid phase time-resolved x-ray diffraction with 100 ps resolution has recently emerged as a powerful technique for probing the structural dynamics of transient photochemical species in solution. It is intrinsic to the method, however, that a structural signal is observed not only from the photochemical of interest but also from the embedding solvent matrix. To experimentally characterize the x-ray diffraction signal deriving from the solvent alone we performed time-resolved diffraction studies of a pure liquid sample over a time domain from -250 ps to 2.5 micros. Multiphoton excitation was used to rapidly heat liquid CH(2)Cl(2) using UV pulses of 100 fs duration. A significant x-ray diffraction signal is visible prior to the onset of thermal expansion, which characterizes a highly compressed superheated liquid. Liquid CH(2)Cl(2) then expands as a shock wave propagates through the sample and the temporal dependence of this phenomenon is in good agreement with theory. An unexpectedly slow initial release of energy into the liquid as heat is observed from multiphoton excited CH(2)Cl(2), revealing the presence of a metastable state of multiphoton excited CH(2)Cl(2).