改进型镁扩散法制备高临界电流密度MgB_2超导体性能研究（英文）

采用一种改进型镁扩散法成功制备出密度达到1.95g/cm~3的MgB_2超导块材。研究了不同的热处理条件对MgB_2块材的超导转变温度和临界电流密度性能的影响。采用最佳热处理条件制备的MgB_2超导体T_c和J_c分别达到了38.1K和0.53MA/cm~2(10K,自场)。为了改进镁扩散法MgB_2超导体中弱的高场磁通钉扎性能,还研究了nanao-Pr_6O_(11)和C掺杂对MgB_2超导体的临界电流密度和不可逆场的影响。结果表明C掺杂的MgB_2超导体临界电流密度在10K,6T下达到了104A/cm~2,该结果比未掺杂MgB_2超导体在同样条件下性能提高了2个量级,甚至比固态反应法制备的nano-C掺杂MgB_2超导体性能更好。利用该方法制备的nanao-Pr_6O_(11)掺杂的MgB_2超导体在10K,2T下也比未掺杂样品Jc提高达9.4倍。根据大量的实验结果和理论分析作者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MgB_2超导体非常有效的途径。     

多芯MgB_2超导线材的挤压技术及其微观结构研究

为了提高多芯MgB_2超导线材中芯丝相互之间的结合强度和超导芯丝的致密度,将传统的热挤压技术引入到MgB_2线材制备过程中。采用挤压工艺制备180芯导体结构的多芯MgB_2/Nb/Cu超导线材,Φ64mm的复合包套通过单道次挤压工艺加工到Φ20 mm。挤压后的线材通过冷拉拔和中间退火热处理最终加工到Φ0.81 mm。对加工不同阶段的复合线材进行了微观结构分析,发现多芯线材中MgB_2超导芯丝分布良好,Nb阻隔层厚度分布较为均匀,无破损现象。通过该工艺已成功制备出百米量级长度的多芯MgB_2超导线材。该技术为MgB_2超导长线的制备提供了新途径。     

石墨烯掺杂和烧结温度对MgB_2块体微观结构和超导性能的影响（英文）

采用未掺杂石墨烯的粉末制备MgB_2块体作对比,研究了石墨烯掺杂对MgB_2块材微观结构和超导性能的影响,以及退火温度对石墨烯掺杂MgB_2块材微观结构和超导性能的影响。对烧结后的样品采用XRD,SEM,SQUID进行相组成,微观结构和超导性能等的分析检测。研究发现,石墨烯掺杂明显提高了MgB_2超导材料的临界电流密度,在20 K和1 T磁场下,最大的临界电流密度达到1.8×105A/cm~2。     

C和SiC掺杂对分步反应法制备MgB_2块体和线材微观结构的影响（英文）

通过两步反应法制备了C和SiC掺杂的MgB_2块材和线材。首先,按一定比例均匀混合Mg粉、B粉和C或SiC粉,压块后在900℃密闭氩气条件下烧结2 h,得到C或SiC掺杂的MgB 4块体;将一次烧结后的MgB_4块体磨碎过筛,和补充的Mg粉混合作为前驱粉末制备MgB_2,MgB_2块体在密闭氩气条件下750℃烧结2 h而MgB_2线材在密闭氩气条件下680℃烧结2 h。同时采用传统的固态烧结法制备了C或SiC掺杂的Mg B2块体以作对比。对烧结后的样品进行了微观结构和相组成等分析检测。分步反应法与以往的固态烧结法相比,不仅因降低了Mg元素的影响而提高样品组织致密性,更因其采用了分步混合粉末而大大地提高了元素掺杂的均匀性。和SiC掺杂相比,C掺杂能更有效地进入MgB_2晶粒和晶界。     

晶粒微细化使强度提高20%以上的镁合金丝材

日本住友电气工业公司有效地利用其拉丝加工技术与热处理工艺 ,开发成功了通过晶粒组织细化来生产高强度轻质镁合金丝材的新技术。该公司生产的镁丝材分为高韧性型和高强度型两个类型 ,产品直径包括 0 2～ 7ｍｍ ,较比传统镁丝抗拉强度提高了 2 0 %以上 ,一般为 370ＭＰａ ,屈     

西北有色金属研究院MgB2新型超导材料研究获得新突破

2002年,西北有色金属研究院在高温MgB2和铋系线带材性能及实用化研究方面取得了新突破;在Ta-W系功能材料加工方面取得了新进展。     

西北有色金属研究院高温超导带材研制性能再上新台阶

2002年,西北有色金属研究院在高温MgB2和铋系线带材性能及实用化研究方面取得了新突破;在Ta-W系功能材料加工方面取得了新进展。     

西北有色金属研究院承担的国家重点材料研制项目取得突破性进展

2002年,西北有色金属研究院在高温MgB2和铋系线带材性能及实用化研究方面取得了新突破;在Ta-W系功能材料加工方面取得了新进展。     

石墨烯掺杂对MgB_2块材微观结构和超导性能的影响

通过直接掺杂石墨烯、石墨烯包覆硼掺杂、石墨烯丙酮溶液掺杂3种方法,系统研究了石墨烯采用不同方法掺杂时MgB_2块材的晶体结构、临界电流密度(J_c)、磁通钉扎性能(F_p)。通过直接掺杂,MgB_2在低场下临界电流密度值得到了明显提高,而包覆法和溶液法掺杂石墨烯由于在空气中氧化严重并没有改善MgB_2超导性能。     

7芯复合包套Ba1-xKxFe2As2线带材的显微结构与超导性能研究

传统Ag包套与铁基超导材料具有良好的化学相容性以及塑性加工特性,但是其机械强度低;而采用复合包套来制备高强度Ba1-xKxFe2As2铁基多芯线带材是一种直接高效且成本低廉的方法。本研究选取高强度高延展性的Ag/Nb/Cu三层复合包套,从线带材包套尺寸、冷加工工艺和中间退火等方面进行优化,获得了结构稳定且易于加工的Ba1-xKxFe2As2多芯线带材。Ag/Nb/Cu复合包套Ba1-xKxFe2As2线带材的多芯孔型完整并且分布均匀,超导填充因子约为25.3-28.0%,并且具有较好的力学性能。Ba1-xKxFe2As2多芯带材的转变Tc为37.5 K,M-H曲线表明样品具有较强的内在磁通钉扎能力。在4.2 K和2 T下其传输Jc约为1.0×104 A/cm2,在10 T时传输Jc仍然保持为8.7×103 A/cm2。     

EDX and ion beam treatment studies of filamentary in situ MgB2 wires with Ti barrier

In situ SiC-doped filamentary MgB₂ wires (with the diameter of 0.860 and 0.375 mm) with Cu stabilization separated by Ti barrier layers supported by outer SS sheath and annealed at 800 °C/0.5 h have been studied by combination of EDX analysis and ion beam selective etching. It was found that several Ti-Cu inter-metallic compounds were created by Cu-Ti interdiffusion and thus the barrier protection against Cu penetration into the superconducting filaments is limited. We showed an advantage of Ti use as the barrier material in our wires. Ti getters silicon out from the superconducting filament, what purges superconducting MgB₂ from Si and creates an additional Si-rich layer in inner part of Ti barrier which prevents Cu diffusion more effectively.     

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.     

Mechanical properties of superconducting MgB2 wire

This study aims at examining the mechanical properties of MgB₂ wires fabricated with PIT method by studying the effect of tensile and bending stresses on their current carrying capacity. Wires are mounted on a tensile machine and are subjected to different load increments within both the elastic and the plastic regions. The current carrying capacity is measured for each load and the behavior of I_c versus stress/strain is studied. Microstructures of MgB₂ core are studied for different loads by using SEM. For bending tests, two cases are examined. The first case is to anneal MgB₂ wires then wind them on mandrels with different diameters, while the second case is to wind un-annealed wires on the same mandrels with different diameters then anneal the winded wires. A comparison between both cases is made using SEM for all different diameters and measuring the corresponding I_c. The behavior of I_c versus bending strain is studied. This whole study aims at giving a clear picture of the optimum loading, bending and processing conditions at which MgB₂ wires will possess a high current carrying capacity for practical applications.     

Structure and stability of superconducting core of single-core MgB2/Cu,Nb tube composite with a high critical current

The core of a single-core MgB₂/Cu,Nb composite, which has been prepared by the ex-situ technique and exhibits a high critical current equal to 427 A (at 0 T and 4.2 K, j _c ≥ 10⁵ A/cm²), has been studied using various structural methods. Two kinds of MgB₂ crystals were observed; those of the first kind is large, highly dense crystals characterized by a low oxygen content (2–8 at %) and the others are fine, weakly coupled crystallites characterized by high oxygen content (4–21 at %). To perform a comparative analysis of the structures, we have also studied an MgB₂ bulk sample synthesized at 1000°C. It was found that two phases with the same lattice are formed; they differ in the magnesium and boron contents (within the homogeneity range), impurity oxygen content and microstructure as well but differ slightly in the lattice parameters. The two-phase state of MgB₂ bulk sample is due to the mechanism of its formation, which includes the melting of magnesium, the dissolution of solid boron in it, and the crystallization of MgB₂ from the melt with the formation of dendrite-like structure characterized by corresponding redistribution of components and impurities. To a certain degree, the two-phase structure of MgB₂ bulk sample is inherited by the MgB₂/Cu,Nd composite prepared by ex-situ technique (annealing of composite at 700°C). It was shown that oxygen in the MgB₂ compound is the destabilizing factor and leads to the transformation of the superconductor into MgO.     

Phase formation of MgB2 superconducting materials fabricated by spark plasma sintering

Much research on MgB₂ has been carried out because MgB₂ has a higher transition temperature (T_c) of 39 K than that of other metallic superconductors and because the bulk form of MgB₂ has exhibited high current density. In this study, Mg powder of less than 10 μm and B powder of less than 3 μm with equivalent MgB₂ composition were mixed simply under argon atmosphere. In order to consider the effect of a pinning element on the superconducting properties, activated carbon of 5 at.% was added to mixed powders. The MgB₂ bulk was fabricated with mixed powders in graphite molds at the various temperatures by spark plasma sintering. The formation of the MgB₂ phase was confirmed with Differential Thermal Analysis (DTA) at 550 °C. The relative density of sintered MgB₂ was 97 %, which increased as the sintering temperature increased. The sintering proceeded initially in the solid state and then by liquid phase sintering with increasing temperature without abnormal grain growth. In the Physical Property Measurement System (PPMS) result, the Tc was about 37 K in the carbon-added sintered sample. The 300 nm size MgB₂ grains of hexagonal shape were formed after spark plasma sintering, but the MgB₄ phase did not produce precise T_c.     

高能球磨法制备MgB2材料的研究进展

MgB₂超导体临界温度为39 K,具有价格低廉和临界转变温度相对较高等优点,具有工程应用前景,然而其大尺度应用还依赖于超导性能的改善。经过系统的研究发现高能球磨和元素掺杂是提高MgB₂磁场下J_c性能最有效的方法。本文介绍了采用高能球磨法制备MgB₂的研究现状,采用高能球磨能有效细化晶粒,有利于提高超导芯丝的致密度,强化MgB₂晶粒的连接性,同时晶粒细化形成的更多晶界能形成钉扎中心,进一步提高线/带材在高磁场下的临界电流密度。我们还介绍了通过分步反应法和高能球磨在常压条件下合成MgB₂,高能球磨法可以减少MgB₂长线中的孔洞并提高粉体密度。     

Stress analysis of ceramic insulation coating on Cu/MgB2 wires for W&R MgB2 coils

Ceramic insulation coatings were produced on Cu/MgB₂ wires, which were fabricated by Hyper Tech Research Inc., using Continuous Tube Forming and Filling (CTFF) process, from the solution of Zr, and Y based organometalic compounds, solvent and chelating agent using reel-to-reel sol–gel technique for MgB₂ coils. Y₂O₃–ZrO₂/Cu/MgB₂ wires were annealed at 700 °C for 30 min with 5.8 °C/min heating rate under 4% H₂–Ar gas flow. Residual stresses were examined for Cu/MgB₂ wire and YSZ coatings with varying thicknesses. It was observed that displacement values are independent from YSZ thicknesses and the maximum effective stress value is in the Cu region. The surface morphologies and microstructure of samples were characterized using SEM. SEM micrographs of the insulation coatings revealed cracks, pinholes and mosaic structure.     

Mechanisms and kinetics of MgB2 synthesis from boron fibers

Superconducting MgB₂ fibers were synthesized through the reaction of liquid magnesium with 140 μm diameter boron fibers. Fiber reaction occurs by two concurrent mechanisms. First, concentric MgB₂, MgB₄ and MgB₇ shells grow radially into the B fibers. Diffusion modeling provides rate constants and effective diffusion coefficients for the borides and their activation energies. Second, radial cracks form in the MgB₄ or MgB₇ shells, allowing for Mg ingress into the fibers and diffusional growth of MgB₂ wedges in the radial and circumferential directions. Combining both shell and wedge MgB₂ growth mechanisms into a single model provides predictions of overall MgB₂ reaction kinetics as a function of time and temperature, which are in good agreement with in situ X-ray diffraction measurements performed at 885–1025 °C.     

Processing high critical current density Bi-2223 wires and tapes

Considerable progress has been made in fabricating (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ in high-T_c superconductor wires or tapes with high critical current densities that are attractive for electric power and high-field magnet applications. The powder-in-tube technique appears to be useful for making silver-clad Bi-2223 composites. This article discusses the processing and the excellent superconducting properties of the resulting wires and tapes.     

Superplastic compressive flow in MgB2

Macroplasticity in the brittle, superconducting ceramic MgB₂ would allow for the mechanical drawing of thin, dense superconducting wires, as done for metallic superconductors. Here, we report very large uniaxial compressive deformation (engineering strain of 67% or true strain of −1.1) without fracture at 1000 °C for specimens densified from commercially available MgB₂ powders with MgO and MgB₄ second phases. Plastic flow occurs under a diffusion-controlled mechanism with activation energies of 255–447 kJ mol⁻¹ and stress exponents of 1.4–2.0, indicative of superplastic behavior.