MgB2线带材工艺及性能研究进展

MgB_2线带材的加工经历了近15年的研究与发展,生产出的线带材可以在制冷机、较低磁场下使用,另外生产MgB_2线带材的原材料相对低廉,应用前景十分广泛。本文从MgB_2传统线带材加工和后续工艺方法的优化出发,概述了MgB_2线带材研究的进展,详细介绍了内部镁扩散法,即用中心镁棒代替传统镁粉,使用铜镍材料为包套,钽或铌为阻隔层并与镁之间填充硼粉,热处理后得到致密的MgB_2相。论述了未来MgB_2线带材加工的研究重点。     

Production of MgB2 superconducting coatings by electrophoresis on metal substrates

Production of MgB₂ coatings on various metallic substrates was achieved by means of the direct electrophoretic deposition technique. An inexpensive simple heat treatment in evacuated quartz tubes was developed as an alternative to inert gas flow during the process. The films were characterized by XRD, SEM and SQUID. It resulted that the procedure led to the production of uniform, dense and well-adhesive superconducting films. Stainless steel proved to be the best substrate among the investigated metals.     

镁扩散制备Pr6O11掺杂的 MgB2块体临界电流密度和磁场性能研究

采用镁扩散方法制备了Pr₆O₁₁纳米颗粒添加的MgB₂超导块体,研究了Pr₆O₁₁掺杂对其临界电流密度(J_c),不可逆磁场(H_irr)和上临界磁场(H_c2)的影响。实验结果表明Pr₆O₁₁纳米颗粒掺杂明显提高了块体的J_c,H_irr和H_c2,但没有降低其超导转变温度T_c。在20 K自场条件下,质量比为1 wt.% Pr₆O₁₁掺杂的MgB₂块体的J_c较没掺杂样品提高了将近5倍, J_c=3.61×10₅A/cm₂。在10 K温度下,MgB₂块体H_c2 和H_irr较没掺杂样品分别提高了1.9 T and 2.6 T。同时讨论了Pr₆O₁₁纳米颗粒掺杂对MgB₂块体的电性能和磁通钉扎机制的影响。     

Ti3AlC2材料的反应合成及其烧结行为

以Ti,Al和TiC为原材料,用无压煅烧合成法制备三元化合物Ti3AlC2。详细讨论了煅烧温度和铝含量对多晶Ti3AlC2纯度的影响。利用X射线衍射仪、场发射扫描电镜和场发射透射电镜研究了粉末材料的组织结构、晶粒大小、层板厚度和选区电子衍射花样。结果表明1300℃是合成Ti3AlC2粉末的最佳煅烧温度,1:1.2:2是Ti/Al/TiC原材料的最佳摩尔比。用热压法制备了不同烧结温度下的Ti3AlC2块体试样,在1300℃热压制备的Ti3AlC2块体的相对密度可达99.9%,其维氏硬度和三点抗弯强度分别为5.7 GPa和630 MPa。通过场发射扫面电镜观察材料的断口形貌,进一步分析了Ti3AlC2块体材料的强化机理。     

As-deformed filament's density and transport currents of MgB2/Ti/Glidcop wire

Filamentary MgB₂/Ti/Glidcop wire has been manufactured by in situ process using hydrostatic extrusion, cold drawing and finally subjected to high pressure and standard densifications. Filament density of as-deformed wires was evaluated by micro-hardness measurements and related to applied deformation. It was found that filament's density and uniformity in as-deformed wire have a strong effect on the critical current density (J_c) of annealed samples. The highest J_c was measured for the filaments densified by cold isostatic pressing with 2 GPa and also for rotary swaged ones showing the best uniformity. Presented results showed the importance of filament density and homogeneity and also demonstrate suitability of hydrostatic extrusion for uniform long-length filamentary MgB₂ wires production.     

MgB2/Mo多层膜的制备与超导性质的研究

本文采用磁控溅射技术(MS)和混合物理化学气相沉积法(HPCVD)在单晶Al₂O₃基底上制备MgB₂/Mo多层膜。通过扫描电子显微镜(SEM)、X射线衍射(XRD)和标准四线法对样品的表面形貌、晶体结构和超导特性进行了测量研究。实验结果表明随着后续MgB₂沉积温度的增加各膜层结晶程度进一步提高,晶粒尺寸不断增大,各自保持着良好的物质稳定性。在730℃温度下生长的MgB₂薄膜的超导转变温度T_con和零电阻温度T_c0分别为39.73K~39.53K,剩余电阻率~0.77μΩcm,表明样品处于干净极限。     

Enhanced superconducting properties of bulk MgB2 prepared by in situ Powder-In-Sealed-Tube method

A systematic study on the superconducting properties of polycrystalline MgB₂ synthesized by in situ Powder-In-Sealed-Tube technique is carried out at different temperatures (750–900 °C). Both XRD and SEM results show well-crystallized MgB₂ grains in all the samples and grain size is found to be increasing with the sintering temperature. Sharp superconducting transitions are observed for all samples, irrespective of sintering temperatures, which implies the high degree phase purity and homogeneity of MgB₂ formed, while J_C(H) plot gives sample dependent critical current density. The samples heat treated at relatively low temperatures show enhanced flux pinning and hence improved J_C(H) performance. The reduced grain size and hence increased density of grain boundary pinning centers of MgB₂ bulks synthesized at low temperature are mainly responsible for the enhanced flux pinning and J_C.     

原位粉末装管法37芯MgB2线材的制备及性能研究

传统的Cu包套原位粉末装管法(in situ PIT)制备多芯MgB₂超导线材时,易于出现断芯、断线现象。针对一问题,本实验中以强度较高的梦乃尔合金(Monel 400)作为包套材料,以旋锻、拉拔、轧制及中间热处理相结合的加工手段成功的制备出直径Φ1.0 mm、37芯结构的多芯MgB₂超导长线材。微观结构分析表明多芯线材中MgB₂芯丝及替换芯丝等亚组元的分布较为规整,阻隔层未出现明显破损现象,最终线材中MgB₂超导芯丝的平均直径约80 μm。室温拉伸性能显示热处理前MgB₂线材的屈服强度为759 MPa,热处理后的线材为248 MPa。4.2 K、4 T下,线材的临界电流密度J_c达到2.31×10^₅ A.cm^_-2,工程临界电流密度达到3.16×10^₄ A.cm^_-2。     

19芯MgB2/NbCu/Monel超导线材的制备及性能研究

为了提高多芯MgB₂超导线材的强度并避免加工过程中的断芯、断线现象,实验中采用强度较高的梦乃尔合金(Monel 400)作为外包套材料,以原位法粉末装管工艺(in situ PIT)制备了19芯导体结构的多芯MgB₂超导线材。二次集束组装后的多芯复合线材通过拉拔、轧制和中间退火热处理相结合的方法从Φ25 mm加工到Φ1.0 mm。对加工过程不同阶段的多芯复合线材进行了微观结构分析,发现多芯线材中MgB₂芯丝分布较为规整,Nb阻隔层表面较为光滑,未出现明显破损现象。最终Φ1.0 mm的多芯线材中MgB₂超导芯丝的平均直径约为100 μm。热处理后MgB₂线材的抗拉强度和屈服强度分别达到396 MPa和200 MPa。MgB₂线材的临界电流密度在4.2 K、4 T时达到1.23×10₅ A.cm_-2。     

Influence of Mg particle size on the reactivity and superconducting properties of in situ MgB2

The effects of starting Mg particle size on the reactivity of Mg with B, and on remnant Mg in in situ MgB₂, and their influence on the superconducting system are studied. Samples were prepared by a powder-in-sealed tube (PIST) method, heat treated at 850 °C for 2 h in air and were characterized using XRD, SEM and magnetization measurements. As the particle size of Mg powder increases, residual Mg increases significantly. The MgB₂ prepared using smaller sized Mg powder does not have any residual Mg and show the best infield critical current density (J_C (H)).     

Modification of TiO2 nanorods by Bi2MoO6 nanoparticles for high performance visible-light photocatalysis

In this work, TiO₂ nanorods were prepared by a hydrothermal process and then Bi₂MoO₆ nanoparticles were deposited onto the TiO₂ nanorods by a solvothermal process. The nanostructured Bi₂MoO₆/TiO₂ composites were extensively characterized by X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy and UV-vis diffuse reflectance spectroscopy. The photocatalytic activity of the Bi₂MoO₆/TiO₂ composites was evaluated by degradation of methylene blue. The Bi₂MoO₆/TiO₂ composites exhibit higher catalytic activity than pure Bi₂MoO₆ and TiO₂ for degradation of methylene blue under visible light irradiation (λ > 420 nm). Further investigation revealed that the ratio of Bi₂MoO₆ to TiO₂ in the composites greatly influenced their photocatalytic activity. The experimental results indicated that the composite with Bi₂MoO₆:TiO₂ = 1:3 exhibited the highest photocatalytic activity. The enhancement mechanism of the composite catalysts was also discussed.     

Complete flux jumping in nano-structured MgB2 superconductors prepared by mechanical alloying

High density nano-crystalline MgB₂ bulk superconductors with induced pinning centres were prepared from elemental precursors by a sequence of ball milling, heat treatment, and final pressing. The XRD results revealed the main phase was MgB₂ with a minor component of MgO. The magnetic moment versus temperature indicated that the transition temperature of MgB₂ samples was around 34 K, which is less than the typical transition temperature of commercial powders and also the transition temperature strongly depended on the milling time. It is well known that introduction of defects, grain boundaries and impurities act as effective flux pinning centres in MgB₂ and results in increased critical current density, J_c and decreased the transition temperature, T_c. The magnetization measurements were performed using VSM at 10 K, 20 K and 30 K, and the M–H curves indicated a complete flux jump effect, which is a severe problem for the application of superconductors. It was determined that a noticeable amount of heating (0.3 K jumps at 10 K) occurs at these jumps. In addition, it was found that the sweeping rate of magnetic field and the size of bulk sample are very effective to promote the flux jumping and whereas a low sweeping rate (12 Oe/s) avoids these “avalanches”, consistent with a kind of supercritical phenomenon: going slower allows the field gradients to stay close enough to equilibrium so that the avalanche effect is not triggered. In contrast, the sweeping rate of 100 Oe/s leads to numerous jumps.     

改进型镁扩散法制备高临界电流密度MgB2超导体性能研究

本文采用一种改进型镁扩散法成功制备出密度达到1.95g/cm₃的MgB₂超导块材。论文研究了不同的热处理条件对MgB₂块材的超导转变温度(T_c)和临界电流密度(J_c)性能的影响。采用最佳热处理条件制备的MgB₂超导体T_c和J_c分别达到了38.1K和0.53MA/cm₂(10K,自场)。为了改进镁扩散法MgB₂超导体中弱的高场磁通钉扎性能,本文还研究了nano-Pr₆O₁₁和C掺杂对MgB₂超导体的临界电流密度和不可逆场(H_irr)的影响。结果表明C掺杂的MgB₂超导体临界电流密度在10K,6T下达到了10₄A/cm₂,该结果比未掺杂MgB₂超导体在同样条件下性能提高了两个量级,甚至比固态反应法制备的nano-C掺杂MgB₂超导体性能更好。利用该方法制备的nano-Pr₆O₁₁掺杂的MgB₂超导体在10K,2T下也比未掺杂样品J_c提高达9.4倍。根据大量的实验结果和理论分析我们提出基于改进型镁扩散法和化学掺杂,包括纳米粒子和C掺杂,很有可能是一种制备高性能MgB₂超导体非常有效的途径。     

Catalytic effect of Ni nanoparticles on the desorption kinetics of MgH2 nanoparticles

Mg and Ni nanoparticles were prepared by hydrogen plasma-metal reaction (HPMR). MgH₂ nanoparticles were obtained by hydriding the Mg nanoparticles. Hydrogen storage kinetics of the MgH₂ nanoparticles doped with different amount of Ni nanoparticles was investigated by differential scanning calorimetry (DSC) and hydrogen desorption rate measurements. The obtained samples show superior hydrogen storage kinetics. 6.1 wt% hydrogen is desorbed in 10 min at 523 K under an initial pressure of 0.01 bar of H₂ when the proportion of Ni nanoparticles is 10 wt%. The desorption rate increases when enhancing the amount of catalyst. However, the activation energy of desorption does not decrease any more when the amount of Ni exceeds a value. The enhanced desorption kinetics are mainly attributed to the accelerated combination process of hydrogen atoms by the Ni nanoparticles on the surface of MgH₂.     

Synthesis of CeO2/TiO2 nanoparticles by laser ablation of Ti target in cerium (III) nitrate hexahydrate (Ce(NO3)3·6H2O) aqueous solution

A new synthesis process, laser ablation in an aqueous solution of target material, was applied to synthesize nanostructured CeO₂/TiO₂ catalyst particles. Reactivity within the laser plume (plasma) can be used to synthesize CeO₂ from an aqueous solution, 2 M cerium (III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O) aqueous solution, and to fabricate TiO₂ from Ti target. CeO₂/TiO₂ nanoparticles were successfully synthesized by the laser ablation of Ti target in 2 M cerium (III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O) aqueous solution. Laser ablation of Ti in a liquid environment and chemical reactions of the solution within a plasma plume are discussed.     

Densification and mechanical properties of fine-grained Al2O3-ZrO2 composites consolidated by spark plasma sintering

Alumina matrix composites containing 5 and 10 wt% of ZrO₂ were sintered under 100 MPa pressure by spark plasma sintering process. Alumina powder with an average particle size of 600 nm and yttria-stabilized zirconia with 16 at% of Y₂O₃ and with a particle size of 40 nm were used as starting materials. The influence of ZrO₂ content and sintering temperature on microstructures and mechanical properties of the composites were investigated. All samples could be fully densified at a temperature lower than 1400 °C. The microstructure analysis indicated that the alumina grains had no significant growth (alumina size controlled in submicron level 0.66-0.79 μm), indicating that the zirconia particles provided a hindering effect on the grain growth of alumina. Vickers hardness and fracture toughness of composites increased with increasing ZrO₂ content, and the samples containing 10 wt% of ZrO₂ had the highest Vickers hardness of 18 GPa (5 kg load) and fracture toughness of 5.1 MPa m^1/2.     

Photocatalytic performance of nano-photocatalyst from TiO2 and Fe2O3 by mechanochemical synthesis

Nano-particles of homogeneous solid solution between TiO₂ and Fe₂O₃ (up to 10 mol%) have been prepared by mechanochemical milling of TiO₂ and yellow Fe₂O₃/red Fe₂O₃/precipitated Fe (OH)₃ using a planetary ball mill. Such novel solid solution cannot be prepared by conventional co-precipitation technique. A preliminary investigation of photocatalytic activity of mixed oxide (TiO₂/Fe₂O₃) on photo-oxidation of different organic dyes like Rhodamine B (RB), Methyl orange (MO), Thymol blue (TB) and Bromocresol green (BG) under visible light (300-W Xe lamp; λ > 420 nm) showed that TiO₂ having 5 mol% of Fe₂O₃ (YFT1) is 3-5 times higher photoactive than that of P25 TiO₂. The XRD result did not show the peaks assigned to the Fe components (for example Fe₂O₃, Fe₃O₄, FeO₃, and Fe metal) on the external surface of the anatase structure in the Fe₂O₃/TiO₂ attained through mechanochemical treatment. This meant that Fe components were well incorporated into the TiO₂ anatase structure. The average crystallite size and particle size of YFT1 were found to be 12 nm and 30 ± 5 nm respectively measured from XRD and TEM conforming to nanodimensions. Together with the Fe component, they absorbed wavelength of above 387 nm. The band slightly shifted to the right without tail broadness, which was the UV absorption of Fe oxide in the Fe₂O₃/TiO₂ particle attained through mechanochemical method. This meant that Fe components were well inserted into the framework of the TiO₂ anatase structure. EPR and magnetic susceptibility show that Fe³⁺ is in low spin state corresponding to μ_B = 1.8 BM. The temperature variation of μ_B shows that Fe³⁺ is well separated from each other and does not have any antiferromagnetic or ferromagnetic interaction. The evidence of Fe³⁺ in TiO₂/Fe₂O₃ alloy is also proved by a new method that is redox titration which is again support by the XPS spectrum.     

Degradation mechanism of long service life Ti/IrO2-Ta2O5 oxide anodes in sulphuric acid

Ageing performances of long service life Ti/70%IrO₂-30%Ta₂O₅ (at mole fraction) anodes prepared at 450 °C over the whole electrolysis time in H₂SO₄ solution have been investigated. It is showed that, the whole electrolysis processes of these anodes can be divided into three stages consisting of `active', `stable' and `de-active' regions. In the first two stages, the dissolution of coated oxides is dominated (with IrO<sub>2</sub> component preferential loss) which results in decreasing of apparent current for oxygen evolution and voltammetric charge. It is found that, in these two stages, the preferential orientations of (1 1 0) and (1 0 1) planes in IrO<sub>2</sub> rutile decreases with electrolysis time, and that of (0 0 2) plane increases, while the preferential orientations remain to be stabilized in the `de-active' region. Therefore, the loss mechanism of catalyst coatings in this region is changed. Electrochemical impedance spectroscopy measurement shows a slight increase in reaction resistance (R_ct) of oxide catalysts for oxygen evolution in the `de-active' region, while a sharp rise in totally physical impedance of the whole anode. According to the experimental results, a degradation mechanism of the Ti based anodes has been proposed by degradation of Ti/catalyst layer interface resulting from the dissolution and anodic oxidation of metal base alternatively.     

Growth, crystal structure and optical properties of layered dibarium cadmium diborate, Ba2Cd(BO3)2

A novel dibarium cadmium diborate, Ba₂Cd(BO₃)₂, has been successfully synthesized by standard solid-state reaction. Large sheet-like crystal with size up to 20 mm × 15 mm × 0.7 mm has been obtained using top-seed solution growth method. Ba₂Cd(BO₃)₂ crystallizes in the monoclinic space group C2/m with a = 9.6305(4) Å, b = 5.3626(3) Å, c = 6.5236(2) Å, β = 118.079(3)°, Z = 2. The crystal structure is composed of isolated [BO₃] triangles, [CdO₆] octahedra and [BaO₉] polyhedra. CdO₆ are vertex-connected with six BO₃ to form infinite _∞[Cd(BO₃)₂] layers extending in (0 0 1) plane, and two rows of Ba atoms closely occupy two side of _∞[Cd(BO₃)₂] layers to forming stoichiometric sheets. IR and transmittance spectrum of Ba₂Cd(BO₃)₂ were reported.     

Ultrasonic-assisted in situ synthesis and characterization of superparamagnetic Fe3O4 nanoparticles

Superparamagnetic Fe₃O₄ nanoparticles were synthesized via a modified coprecipitation method, and were characterized with X-ray diffraction (XRD), vibrating sample magnetometer (VSM), Zeta potential and FT-IR, respectively. The influences of different kinds of surfactants (sodium dodecyl benzene sulfonate, polyethyleneglycol, oleic acid and dextran), temperatures and pH values on the grain size and properties were also investigated. In this method, Fe³⁺ was used as the only Fe source and partially reduced to Fe²⁺ by the reducing agent with precise content. The following reaction between Fe³⁺, Fe²⁺ and hydroxide radical brought pure Fe₃O₄ nanoparticles. The tiny fresh nanoparticles were coated in situ with surfactant under the action of sonication. Comparing with uncoated sample, the mean grain size and saturation magnetization of coated Fe₃O₄ nanoparticles decrease from 18.4 nm to 5.9-9.0 nm, and from 63.89 emu g⁻¹ to 52-58 emu g⁻¹ respectively. When oleic was used as the surfactant, the mean grain size of Fe₃O₄ nanoparticles firstly decreases with the increase of reaction temperature, but when the temperature is exceed to 80 °C, the continuous increase of temperature resulted in larger nanoparticles. the grain size decreases gradually with the increasing of pH values, and it remains unchanged when the PH value is up to 11. The saturation magnetization of as-prepared Fe₃O₄ nanoparticles always decreases with the fall of grain size.