还原扩散法制备稀土永磁Sm2Fe17Nx的研究

研究了还原扩散法(R/D)制备Sm2Fe17合金及通过气-固相反应法生成Sm2Fe17Nx(x≈3)化合物的整个工艺过程,并通过XRD和SEM等分析测试手段对还原扩散过程和机理作了理论分析和实验研究,通过VSM分析了Sm2Fe17Nx磁粉的磁滞回线.     

PM法制备Sm2Fe17Nx永磁材料最佳工艺的研究

采用粉末冶金工艺制备了各向异性Sm2Fe17Nx永磁粉末.重点考查了工艺参数对Sm2Fe17合金的显微组织及Sm2Fe17Nx粉末磁特性的影响.结果表明,铸态合金的均匀化、粉末的氮化以及粉碎过程是获得高性能磁粉的关键因素.采用优化工艺条件制备的磁粉的磁特性为Jr=1.24T,iHc=756kA/m,(BH)max=220kJ/m3.     

Magnetic properties of Y2Fe17?x Gax and Sm2Fe17?xGax compounds

The magnetic properties of Y₂Fe_17−x Ga_x and Sm₂Fe_17−xGa_x for 3 ≤ x ≤ 7 have been investigated using the ⁵⁷Fe M?ssbauer spectroscopy at room temperature. These compounds have the rhombohedral Th₂Zn₁₇ structure. X-ray diffraction analyses of aligned powders show that the easy direction of magnetization is parallel to the c-axis in Y₂Fe₁₀Ga₇ and Sm₂Fe₁₄Ga₃ and is perpendicular to the c-axis in Y₂Fe₁₄Ga₃, Y₂Fe₁₂Ga₅, Sm₂Fe₁₂Ga₅ and Sm₂Fe₁₀Ga₇. M?ssbauer studies indicate that all the samples studied are ferromagnetically ordered. The ⁵⁷Fe hyperfine field decreases with increasing Ga content. This decrease results from the decreased magnetic exchange interactions resulting from Ga substitution. The average isomer shift, δ, for Y₂Fe_17−xGa_x and Sm₂Fe_17−xGa_x at room temperature is positive and the magnitude of δ increases with increasing Ga content.     

HDDR处理的Sm2Fe16Ti1Nx化合物高能球磨的研究

在用HDDR法制备Sm2Fe16Ti1Nx氮化物过程中,研究了高能球磨对氮化物粉末的形貌、物相结构及磁性能的影响.发现高能球磨Sm2Fe16Ti1Nx氮化物使粉末颗粒细化的过程可描述为大粉末颗粒→压延成层片状→断裂成短棒状及球形颗粒→压延成层片状→断裂成球形小颗粒,并在球磨一定时间后使粉末中的Sm2(FeTi)17Nx主相完全非晶化,α-Fe含量增高且没有非晶化.球磨后粉末的矫顽力随着球磨时间的延长而降低,而剩磁在球磨短时间时降低,再延长球磨时间又增高,在球磨较长时间到Sm2(FeTi)17Nx主相完全非晶化后又使剩磁降低,最高磁场下的磁化强度值则随着球磨时间的延长而增加.手研磨后粉末的矫顽力随研磨时间的延长而逐渐升高而剩磁及最高场下磁化强度值变化不大.     

Simple chemical etching of Sm2TM17 magnet alloys for metallography purposes

Chemical etching for metallography of four Sm₂TM₁₇ permanent magnet alloys, typically Sm(Co/Fe/Cu/Zr)_7.3–7.5, was extensively studied. It was found that a selective etching process (SEP) with Nital is powerful for proper revealing of the complex microstructure for metallographic examinations under optical and scanning electron microscopes. The scanning electron microscopy metallographs thus obtained were comparable to those from transmission electron microscopy and complementary to those from potentiostatic etching technique. The SEP was found to be applicable to alloys with higher Zr (2.5 wt.% and above) and Fe (13 wt.% and above). The factors controlling the partitioning tendency of Fe and Cu atoms during step aging and the amount of coherent strain field around the microconstituents would affect the morphology of the etched microstructure, and are discussed in detail.     

Consolidation of Sm5Fe17 powder by spark plasma sintering method

Sm₅Fe₁₇ bulk materials, one of the newer permanent magnetic materials, were successfully produced by the spark plasma sintering method. The resultant bulk materials had high densities of 85–98%. When obtained by sintering at relatively lower temperatures, the Sm₅Fe₁₇ bulk materials consisted of the Sm₅Fe₁₇ and SmFe₃ phases, whereas they contained some -Fe phase together with the Sm₅Fe₁₇ and SmFe₃ phases when obtained by sintering at relatively high temperatures. High coercivity values, exceeding 2 MAm⁻¹, were found in the Sm₅Fe₁₇ bulk materials consisting of the Sm₅Fe₁₇ and SmFe₃ phases.     

Magnetic characterization of antiferromagnetic Ba2Fe6O11

The magnetic properties of the metastable compound Ba₂Fe₆O₁₁ present in the phase subsystem BaFe₂O₄ - BaFe₁₂O₁₉ have been analyzed by means of Mössbauer and magnetic measurements. The obtained data indicate that Ba₂Fe₆O₁₁ is a complex antiferromagnet with Neél temperature T_N = 488 K. The overall features of the Mössbauer spectra and the population of the observed iron sublattices suggest for Ba₂Fe₆O₁₁ a crystalline structure related to that of perovskite like compounds, which are often present in the chemical reactions that lead to the formation of hexaferrites. The extraneous phases present in the obtained samples, one of which turned out to be barium hexaferrite, have also been characterized.     

Multiferroic properties of Pb2Fe2O5 ceramics

Pb₂Fe₂O₅ (PFO) powders in monoclinic structure have been synthesized using lead acetate in glycerin and ferric acetylacetonate as the precursor. The powders were pressed into pellets, which were sintered into ceramics at 800 °C for 1 h. The morphology and structure have been determined by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Polarization was observed in Pb₂Fe₂O₅ ceramics at room temperature, exhibiting a clear ferroelectric hysteresis loop. The remanent polarization of Pb₂Fe₂O₅ ceramic is estimated to be Pr ∼ 0.22 μC/cm². The origin of the polarization may be attributed to the off-centers of shifted Pb²⁺ ions as well as the FeO₆ octahedra in the perovskite-based structure of Pb₂Fe₂O₅. Magnetic hysteresis loop was also observed at room temperature. The Pb₂Fe₂O₅ ceramic shows coexistence of ferroelectricity and ferromagnetism. It provides a new field of research for complex oxides with multiferroic properties.     

Magnetic properties of melt spunSmFe11Ti-Sm2TM17 pseudobinary alloys

Several alloys in the SmFe₁₁Ti-Sm₂TM₁₇ (TM=Co/Fe/Cu/Zr) pseudobinary system containing O-90 wt.% Sm₂TM₁₇ were prepared by the melt spinning technique. A maximum as-spun coercivity of 4.6 kOe was obtained for the alloy containing 10 wt.% Sm₂TM₁₇ at a substrate velocity of 15 m/s. The coercivity is enhanced to 5.3 kOe after annealing at 800°C for 1 h. XRD (X-ray diffractometry) and TEM (transmission electron microscopy) were used to investigate the microstructure of the alloys. It is suggested that the grain refinement, doping of the 1-12 phase with Sm and other elements, and reduced free iron might be the main causes coercivity enhancement     

稀土永磁材料Sm3(Fe,Ti,Co)29Nx

研究了Sm3(Fe,Ti,Co)29母合金的制备工艺,并对合金元素的选择及添加量进行了探讨.试验研究发现Ti元素可有效抑制α-Fe的生成并起到稳定Sm-Fe 329相的作用;熔炼时至少熔炼四遍可得到组织均匀的铸态试样;合理设计加料顺序可以减小Sm的挥发量,多加入20%～30%的Sm可有效的补偿Sm的挥发.成功的制备了Sm3(Fe,Ti,Co)29 329相母合金.     

Microstructure and electrical properties of Sm2O3 doped Bi2O3-based ZnO varistor ceramics

The dependence of the bulk density, microstructure and dc electrical properties of bismuth oxide (Bi₂O₃)-based zinc oxide (ZnO) varistor ceramics for various samarium oxide (Sm₂O₃) contents was investigated. The value of bulk density was found to 5.43-5.50 g cm⁻³ with Sm₂O₃ (mol%) content. The maximum value of bulk density is observed to be 5.50 for 0.30 mol% Sm₂O₃ containing varistor ceramics. The grain sizes for all the samples calculated from the scanning electron micrographs were found to decrease as Sm₂O₃ increases. The presence of ZnO phases, Bi-rich phases, spinel phases and Sm₂O₃ phases were observed in the samples by the energy dispersive X-ray analysis and X-ray diffraction analysis. As the Sm₂O₃ amount increased in the Bi₂O₃-based ZnO varistor ceramics, the nonlinear coefficient, α increased up to 0.10 mol%, reaching a maximum value of 58 and then decreased. The breakdown electric field, E_b, increased with the increase of Sm₂O₃ content with a maximum value of 3220 V cm⁻¹ for the 0.75 mol% Sm₂O₃ doped ZnO varistor ceramics. The leakage current, I_L, showed a minimum value of 1.10 μA for the composition of 0.30 mol% Sm₂O₃ doped Bi₂O₃-based ZnO varistor ceramics. The 0.30 mol% Sm₂O₃-doped Bi₂O₃-based ZnO varistor ceramics sintered at 1200 °C exhibited a good stability for dc accelerated aging stress of 0.90 V_1 mA/90 °C/12 h.     

Influence of Sm2O3 on the crystallization and luminescence properties of boroaluminosilicate glasses

Sm₂O₃-doped SiO₂-B₂O₃-Al₂O₃-BaO glasses were prepared by melting method in order to study the influence of Sm₂O₃ on the crystallization behavior and luminescence properties. Differential scanning calorimetry (DSC), X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy were used to characterize the rare earth glasses and the crystalline phases after heat-treatment. The course of phase separation and devitrification of the glasses were also investigated. The results show that the thermal stability of the glasses decreases with the increase of content of Sm₂O₃. The crystalline phase changed from SmAl_2.07(B₄O₁₀)O_0.6 to SmBO₃. Divalent Sm²⁺ ions were detected in the crystallization product after heat-treatment. The valence transformation from Sm³⁺ to Sm²⁺ in the crystal suggests the samarium atoms entering the barium sites. The charge carried in vacancy defect induced by the substitution led to the partial reduction process. The reduction of Sm³⁺ ions was promoted by the increasing of Sm₂O₃ content or the extending of heat-treated holding time in boroaluminosilicate glass.     

Catalytic decomposition of NO over brownmillerite-like compounds,Ca2Fe2O5 and Sr2Fe2O5

A high-temperature form of a brownmillerite-like compound Sr₂Fe₂O₅, which has a perovskite structure with the oxygen vacancies randomly distributed, showed a remarkable NO decomposition at 700–900°C, whereas another brownmillerite-like ferrate (III) of composition Ca₂Fe₂O₅ did not exhibit any significant catalytic activity for NO decomposition over the temperature range from room temperature to 900°C. This difference in their catalytic activities has been explained on the basis of their crystallographic properties, i.e., ordering or disordering of the oxygen deficiencies.     

机械球磨制备纳米双相Nd2Fe14B/α-Fe永磁材料及晶化行为研究

机械球磨氲气保护下的铸态Nd8Fe86B6合金,并进行晶化处理,制备了纳米双相Nd2Fe14B/α-Fe永磁材料.用XRD、TEM和DSC等手段研究了不同球磨工艺和晶化处理工艺对纳米双相Nd8Fe86B6材料组织结构影响.同时研究了非晶态Nd8Fe86B6材料的晶化行为.结果表明延长球磨时间,Nd2Fe14B相迅速细化形成非晶,α-Fe的晶粒尺寸逐渐减小,25h后趋于定值(约为7nm).球磨时间越长,所需完全晶化的温度越高,晶化后粉末的晶粒越小且越均匀.在晶化过程中,非晶态Nd8Fe86B6首先形成Nd4 4Fe77.8B17.8、Nd1.1Fe4B4、Nd2Fe14B和α-Fe四相混合物.升高温度,最后得到Nd2Fe14B相和α-Fe相,但最终产物中仍有少量未分解的Nd1.1Fe4B4相.     

Sur quelques nouveaux oxydes mixtes de formule BaLn2Fe2O7 et SrLn2Fe2O7

Several new compounds have been prepared, with the general formula : BaLn₂Fe₂O₇ (Ln³⁺ = La to Gd) and SrLn₂Fe₂O₇ (Ln³⁺ = Nd to Tb). The experimental conditions are given, along with some crystal data. The diffraction patterns show a close similarity with that of Sr₃Ti₂O₇.     

Magnetic Domain Imaging of Nd2Fe14B Single Crystals With Unmodified Scanning Electron Microscopy

The stray flux manifestations of surface magnetic domains found in as-grown Nd₂Fe₁₄B single crystals were observed by conventional scanning electron microscopy (SEM) without instrumental modifications. Kerr optical microscopy was employed to confirm the results obtained by SEM. Spike domains were observed on the (001) plane, while a lozenge-type domain pattern was observed on (223) plane of Nd₂Fe₁₄B. A modified image-distortion mode was applied to image the three-dimensional stray flux emanating from the sample. The optimum scanning electron microscope imaging conditions are attained with an incident-electron energy set at 5 to 6kV, which produced images with resolution on the order of 1μm. The simplicity of the technique and the ready adaptability of the SEM to such modifications as in situ current and magnetic field application suggest the extension of these to investigations of other materials of technological interest, such as perpendicular media disks.     

纳米Dy0.15Fe1.85O3磁颗粒的低温磁特性研究

采用共沉淀–相转化法制备了纳米Dy0.15Fe1.85O3磁颗粒,研究了低温条件下温度变化对其磁学特性的影响.采用XRF、TEM、XRD和超导量子磁强计(SQUID)对样品的化学成份、形貌、晶型结构和磁学性能进行了表征.结果表明:样品中的金属离子主要是Dy3+、Fe3+,含量分别为12.17wt%、51.88wt%.形貌为类球形,平均粒径约15 nm,且结晶性完全,晶型为面心立方尖晶石结构.在56.5～200.2 K范围内,样品的饱和磁化强度(MS)、矫顽力(HC)均随温度的降低呈现出增大的变化规律,当温度T≥142.8 K时,呈超顺磁性状态;当T<142.8 K时,由超顺磁性状态转化为铁磁性状态.     

Magnetic properties of metallic La2CuO4

X-ray, magnetic susceptibility, and electron paramagnetic resonance measurements have been made on La₂CuO₄. Magnetic measurements in the range 4.2 – 300 K are consistent with the metallic nature of La₂CuO₄, and the itinerant - electron susceptibility is estimated to be 239 × 10^?6 emu mol^?1. The high susceptibility and density of states indicate that the conduction band is narrow. The results are discussed in terms of previous models of the level structure of La₂CuO₄, and it is concluded that conduction occurs via a $\text{σ1}{}_{\mathrm{<mtext>x</mtext>}}\text{2}{}_{\mathrm{<mtext>?</mtext><mtext>y</mtext>}}\text{2}$ band. Comparison of La₂CuO₄ with semiconducting lanthanide cuprates suggests that this compound is on the verge of a semiconductor-to-metal transition.     

Magnetic properties and magnetoelectric effect in Ni0.8Co0.1Cu0.1Fe2O4 + PbZr0.2Ti0.8O3 composites

Composites with compositions x (Ni_0.8Co_0.1Cu_0.1Fe₂O₄) + (1−x) PbZr_0.2Ti_0.8O₃ (x-mole fraction varies as 0.15, 0.25, 0.35 and 0.45) were prepared by standard ceramic method. The presence of constituent phases, namely ferrite and the ferroelectric were confirmed by X-ray diffraction. The structural analyses were carried out using the obtained powder pattern X-ray data. The porosity of the samples was calculated and the values obtained lie between 10% to 19%. To date, the variations in the magnetic properties with variation in ferrite phase in composites and thereby its influence on magnetoelectric effect is not yet reported. The saturation magnetization (Ms) and magnetic moment (η_B) in Bohr magneton were calculated for all the composites. The static value of magnetoelectric voltage coefficient (dE/dH) was measured as a function of intensity of magnetic field. The maximum value of ME coefficient was observed for a composite with 35% ferrite + 65% ferroelectric phase.     

Magnetic properties of nano-crystalline Li0.35Cd0.3Fe2.35O4 ferrite prepared by modified citrate precursor method

Single phase nano-crystalline lithium cadmium ferrite Li_0.35Cd_0.3Fe_2.35O₄ is synthesized by a modified citrate gel precursor technique in different pH media. The modified citrate precursor technique reduces the formation of the impurity phase α-Fe₂O₃ in the inverse spinel phase of lithium ferrite. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the average crystallite size. As-prepared sample shows the paramagnetic behaviour of M–H curve measured by vibrating sample magnetometer (VSM). The coercivity (H_c) and magnetization (M) both increase with decrease in temperature from 300 K to 80 K. Temperature dependent magnetic properties below the Curie point are defined by the Bloch's law and Neel relaxation relations. The effect of annealing on magnetic properties at different temperature is studied.