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.     

Graphene micro-substrate-induced π gap expansion in MgB2

The lattice effect induced by tensile strain on the superconductivity of graphene–MgB₂ composites was studied systematically to deduce the electron–phonon coupling (EPC) and the multiple superconducting gap behavior. Compared with nano-carbon doped MgB₂, graphene–MgB₂ composites show larger lattice parameters and higher critical superconducting transition temperatures (T_c). The EPC strength of MgB₂ with ∼2 wt.% graphene addition is even higher than that of the pure reference sample, as estimated from the Sommerfeld constant. The π gap was found to be expanded by graphene addition through the analysis of heat capacity data, and it is responsible for both the enhanced EPC strength and the weak dependence of T_c on the graphene content.     

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.     

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。     

Deformation processing of high-Tc superconducting oxides

Plastic deformation and texture development in polycrystalline YBa₂Cu₃O_{7? δ} has been studied to expedite the process development of high-critical-temperature (high-T_c) superconducting wires and tapes. It is anticipated that deformation texture will be a major processing consideration in terms of maximizing critical current density, assessing conductor-fabrication options in light of critical current density, and developing such mechanical properties as strength, toughness and thermal fatigue. The intrinsic texture development in YBa₂Cu₃O_{7? δ} deformation processing should be highly beneficial, insofar as the c axes of the crystals tend to become oriented along the compression axis. This means that conducting tapes and wires formed by rolling, extrusion and drawing can develop textures with the c axis in the transverse or radial direction, thus maximizing the flow of current along the length of the conductor.     

Structure,superconductivity and magnetic properties of mechanically alloyed Mg1 − xFexB2 powders with x=0–0.4

Mechanical milling of Mg_1 − xFe_xB₂ with x=0–0.4 led to the formation of amorphous phase. For x=0, the hexagonal MgB₂ phase was formed after a heat treatment at 450 °C or above with critical temperature T_c=38–40 K. Fe-substituted MgB₂ phase was formed after annealing at 450 °C or above when x=0.05, 0.1 and 0.2. Fe solubility in the MgB₂ phase decreased with increasing annealing temperature, while lattice parameters (a and c) decreased. It has been found that critical temperature T_c increased with increasing annealing temperature. For Mg_0.6Fe_0.4B₂ powder, MgB₂ phase could not be formed after crystallization. Single solid-solution MgB₂ phase could be formed in Mg_0.95Fe_0.05B₂ after annealing at 450 °C. This sample exhibited paramagnetic between 30 and 290 K with a high magnetic moment of μ_Fe=5.5–6.0μ_B. An anomaly (minimum in reciprocal magnetic susceptibility) was observed below 30 K, accompanied by magnetic splitting in Mössbauer spectroscopy.     

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.     

High-pressure synthesized nanostructural magnesium diboride-based materials for superconductive electromotors,generators and pumps

Additions of Zr can increase critical current density (j_c) of high-pressure synthesized MgB₂ (HPS-MgB₂) in the same manner as additions of Ta or Ti, i.e. due to the absorption of impurity hydrogen (to form ZrH₂). The formation in HPS-MgB₂ of ZrB₂ phase at higher synthesis temperatures (about 950 °C) does not result in the j_c increase. Some increase in j_c of HPS-MgB₂ at 10 K in the fields higher than 8 T was observed when nano-SiC was added. The additions of Zr, Ta or Ti can prevent the harmful MgH₂ impurity phase from appearing and may prevent hydrogen from being introduced into the material structure and besides, their presence in HPS-MgB₂ promotes the formation of a higher amount of Mg–B (most likely MgB₂) inclusions in the Mg–B–O material “matrix” that in turn leads to the increase of j_c in magnetic fields. The high level of superconductive (SC) and mechanical characteristics attained for HPS-MgB₂ and the possibility to manufacture large samples make its application in the superconductive electromotors, generators, pumps, etc., very promising.     

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.     

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.     

Synthesis of nanocrystalline magnesium diboride (MgB2) metallic superconductor by mechano-chemical reaction and post-annealing

MgB₂ is recently discovered superconducting compound with a record-breaking transition temperature (T_c = 39 K) for a conventional metallic superconductor. Nanocrystallinity can improve its electrical properties by strong pinning. In the present work we report the results of the synthesis of nanocrystalline MgB₂ superconducting compound by mechano-chemical reaction followed by post-annealing. The first stage of synthesis was carried out from elemental crystalline Mg and amorphous B powders by controlled mechanical alloying (CMA) in the magneto-mill Uni-Ball-Mill 5. X-ray diffraction studies reveal that the nucleation of small amount of MgB₂ is initiated after two-step milling for combined 100 h under protective helium gas. Further reaction to form MgB₂ is accelerated by subsequent annealing of the milled powder at various temperatures. X-ray diffraction shows formation of a well-developed nanocrystalline MgB₂ after annealing of the pre-nucleated powder at the 630–650 °C range for a few hours.     

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)).     

Micro-indentation study of Mg-addition MgB2 superconducting wires

The Mg-added (0%, 5%, and 10%) MgB₂/Cu superconducting wires were prepared by the powder-in-tube (PIT) method. Vickers micro-indentation tests were performed on the samples with different peak loads at room temperature. The loading-unloading (P-h) curves were analyzed by the displacement approach to indentation. It was found that hardness (H) and the effective elastic modulus (E) values increased with Mg-added. In addition, these values showed peak load dependence (i.e. indentation size effect (ISE)).     

Solid solution strengthening and damping capacity of Mg−Ga binary alloys

The mechanical behaviors and damping capacities of the binary Mg?Ga alloys with the Ga content ranging from 1 to 5 wt.% were investigated by means of optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD), hardness test, tensile test and dynamic mechanical analyzer (DMA). The hardness (HV_0.5) increases with the increase of Ga content, which can be described as HV_0.5=41.61+10.35c, and the solid solution strengthening effect Δσ_s of the alloy has a linear relationship with cⁿ, where c is the molar fraction of solute atoms and n=1/2 or 2/3. Ga exhibits a stronger solid solution strengthening effect than Al, Zn or Sn due to the large atomic radius difference and the modulus mismatch between Ga and Mg atoms. The addition of Ga makes the Mg?Ga alloys have better damping capacity, and this phenomenon can be explained by the Granato?Lücke dislocation model. The lattice distortion and the modulus mismatch generated because of the addition of Ga increase the resistance to motion of the dislocation in the process of swinging or moving, and thus the better damping capacity is acquired.     

STRAIN EFFECT ON TRANSPORT PROPERTIES OF Nb3Sn MULTIFILAMENT STRANDS PREPARED BY INTERNAL TIN ROUTE

采用Pacman弹簧装置测量了4.2 K, 12 T条件下Nb₃Sn多芯股线的I_c--ε特征曲线. 材料轴向应变变化范围在-0.9%---+0.6%. 通过偏量比例模型模拟了材料临界电流I_c随应变状态的变化规律. 研究了轴向应变条件下超导材料载流能力(I_c)的衰退和lg V--lg I曲线斜率n值的变化, 通过临界电流随轴向应变的变化规律比较了不同制备工艺Nb₃Sn股线的轴向应变敏感性.     

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。     

Characteristic and synthesis mechanism of MgB2 nanoparticles in solid-state reactive sintering

MgB₂ nanoparticles were synthesized by a one-step reactive sintering method. The sample was heated from room temperature to 994 K, and then directly cooled down at a rate of 40 K/min. The results of X-ray diffraction indicate that MgB₂ superconductors with high quality were successfully prepared, and only a few impurities were detected on it. The sample consisted of two distinguishable structures by the TEM observation: nanoparticles and single crystals, which is a result of nonequilibrium conversion under the direct cooling condition. The nanoparticles with a diameter of about 10–20 nm are considered to be at the onset for the formation of MgB₂ phase. Transition temperature (T_c) and critical current density (J_c) of the compound structures were determined to be 38.5 K and 1.8 × 10⁵ A cm⁻² by means of SQUID measurement, which indicate good superconducting properties. The inter-diffusion and dislocation incorporation mechanisms for the formation and growth of MgB₂ nanoparticles are also proposed, and the further growth will be accelerated due to an adequate holding time at 994 K.     

The effect of absorbed hydrogen on the corrosion behavior of sintered NdFeB magnet

The absorption of hydrogen by NdFeB magnet has been investigated by using the electrochemical charging technique at constant cathodic current density I_c ranging from 0 to 4 mA/cm². Open circuit potential measurements (OCP) and polarization curves were carried out to study the corrosion behavior of the charged NdFeB magnet in 0.01 mol/L NaCl solution. The results showed that hydrogen had a strong influence on the corrosion of NdFeB magnet. The open circuit potential became gradually negative due to the hydrogen incorporation into the NdFeB magnet. The corrosion resistance was reduced gradually with the increasing cathodic current density I_c. The surface structure and the morphology of the charged NdFeB magnet were examined by XRD and SEM. The results revealed that the effect of the absorbed hydrogen focused mostly on accelerating the exfoliation corrosion of Nd₂Fe₁₄B matrix grain.