Multifilamentary V3Ga wires and tapes with composite cores

Results on the critical current behaviour of 19 core composite superconducting wires and tapes in magnetic fields up to 9 T are reported. The composite cores used in the present studies are vanadium tubes filled with Cu-Ga matrix. Two V₃Ga layers are formed on either side of the tubular vanadium. For a given matrix volume to vanadium surface area ratio, the inner V₃Ga layer has much faster growth rate and consequently finer grains than the corresponding outer layer. An increase of about 50% in overall critical current density compared to the 19 core wires using the same starting materials but prepared with conventional geometry has been obtained. The critical current density of V₃Ga (jc) as well as the overall critical current density (J_c) of the tape specimens with composite cores is significantly higher than the corresponding wires. Enhanced surface flux pinning seems to be responsible for the higher current density in the tapes. Addition of 6 at% gallium to vanadium and 0.5 at% Mg to CuGa (19 at%) matrix leads to enhanced critical current densities in these specimens in conformity with our earlier studies.     

On the presence of voids in bronze-route multi-filamentary Nb3Sn superconducting wires

The presence of Kirkendall voids in multifilamentary Nb₃Sn superconducting wires prepared by the “bronze-route” has been thought to have an adverse effect on the ductility of the wires. The shape, density and nucleation sites of such voids in commercial Nb/Cu-Sn wires and wires of similar geometry but different scale have been investigated both experimentally and theoretically. The implications of the results for the mechanical properties are discussed.     

Filamentary MgB2 Superconductors with Titanium Barriers

Ti diffusion barrier has been applied for several Cu stabilized MgB₂ wires. Pure Ti is well formable metal allowing formation of thin barrier layers which are not reacting with MgB₂. Instead of, Ti is able to purify MgB₂ filaments by absorbing some impurities during the final heat treatment. Ti has comparable coefficient of thermal expansion with MgB₂, which allows heat treatment at higher temperatures than for Nb barrier wires. Consequently, higher critical current densities can be obtained with Ti. Higher Ti resistivity offers a depressing of coupling currents in AC regime. One disadvantage of Ti is the inter-diffusion with copper during annealing and partial contamination of Cu stabilization. Benefits of Ti diffusion barrier have been utilized for the manufacture of fine-filamentary wires with minimal filament diameter of 10 μm. High critical current densities and high resistances to tensile stress and torsion stress at twisting have been demonstrated for these wires. AC loss measurements have shown reduced losses with decreased filament size and with shortened twist pitch.     

Development of niobiumtin conductors at SLE

Superconductors Lips ECN is a recently formed company which produces NbTi and Nb₃Sn multifilament wires as well as superconducting cables. Multifilamentary Nb₃Sn wire, produced by the powder route, is developed commercially. About 30 km of strand material has been produced for the SULTAN 12 T project. These wires showed good performance. Developments are underway to reduce gradually the filament diameter at the same current performance to meet the full requirements for the NET and LHC conductors.     

SmFeAsO<Subscript>0.8</Subscript>F<Subscript>0.2</Subscript> Wires Prepared Using Different Sheaths by the Powder-in-Tube Method

SmFeAsO_0.8F_0.2 wires with composite Cu/Nb or Ta sheaths have been fabricated using an in situ powder-in-tube method. Cu diffusion into the core was observed, which led to a fuzzy boundary between the sheath and core for Cu/Nb-sheathed SmFeAsO_0.8F_0.2 wires, and destroyed the superconducting phase of SmFeAsO_0.8F_0.2. However, superconductivity has been observed in the Ta-sheathed SmFeAsO_0.8F_0.2 wires with T _c=52.5 K. A severe weak-link effect depressed the development of global supercurrent. A peak effect with a strongly temperature-dependent peak field H _peak has been observed in the J _c–H curves over the range 10–40 K. The H–T phase diagram was similar to that of a previous report.     

Development of design for large scale conductors and coils using MgB2 for superconducting magnetic energy storage device

MgB₂ wires are commercially available, and their superconducting characteristics have been continuously developed in the last decade. The relatively high critical temperature of these wires has attracted the attention of researchers, especially in the field of superconducting magnetic energy storage (SMES) coil applications in terms of its relatively high critical temperature, as it enables the use of liquid hydrogen for cooling the coils. The sensitivity of multi-filament MgB₂ wires to bending strain makes the design of large-scale conductors and coils for an SMES system technologically difficult, and the careful investigation of the applied strains during manufacturing is required. Two-conventional methods have been introduced for the fabrication of the coils: wind-and-react (W&R) and react-and-wind (R&W). These methods have been demonstrated to be suitable for the production of large-scale MgB₂ magnets to maximize the coil performance. The W&R and R&W methods have been successfully applied to the designs of conductors and coils, and small W&R test coil fabrication, as well as stability demonstrations are performed in this study. Our study is the first to demonstrate the feeding of hundreds of amperes of transport current using multifilamentary MgB₂ wires at around liquid hydrogen temperature in the practical background magnetic field of 2 T. The minimum quench energy and normal zone propagation velocity are also experimentally investigated for the protection of the actual coils for SMES application.     

The Effect of Crossed Configuration on Giant Magneto-Impedance Properties of Cobalt-Based Amorphous Wires

In this work, the impedance properties of Co_68.15Fe_4.35Si_12.5B₁₅ cobalt-based amorphous wires connected in a crossed configuration have been studied. For this purpose, at first, the magnetic properties of a single wire and also two wires perpendicular to each other were investigated. The obtained results for a single wire present a decrease and increase, respectively, in the transverse and longitudinal saturation magnetization by increasing the angle of the wire and the applied magnetic field. In the same time, the anisotropy field of a single wire shows the sinusoidal variations by increasing the angle from 0 to 90^° whereas coercivity field has the lowest value at the angle of 45^°. Furthermore, the saturation magnetization and the coercivity of two crossed wires get the highest and lowest values at the angle of 45^°. Besides, the magneto-impedance and sensitivity of two crossed wires increase from 193 % and 4.95 (1/Oe) for an angle of 0^° to 528 % and 20.05 (1/Oe) for an angle of 45^°, which is in agreement with the magnetic properties of wires. Altogether, our results show that crossed configuration of wires at an appropriate angle of magnetic field can improve the GMI response and make the samples more suitable for magnetic vector sensors.     

Effect of elastic stresses on the differential magnetic permeability of amorphous ferromagnetic wires in a broad frequency range

The effect of elastic mechanical stresses on the field dependence of the differential magnetic permeability (DMP) of low-magnetostriction amorphous wires of Co₆₆Fe₄Nb_2.5Si_12.5B₁₅ and Co₆₆Fe₄Ta_2.5Si_12.5B₁₅ compositions has been studied in the range of magnetic field frequencies from 1 kHz to 10 MHz. The influence of elastic stresses on the DMP of amorphous ferromagnetic wires is frequency-dependent, the most significant effects being observed above 100 kHz.     

Fabrication of 6-filament MgB2 wires enhanced by high strength 91-filament Cu–Nb composite

We chose high strength and high conductive Cu–Nb composite as strengthening core to improve the mechanical properties of 6-filament MgB₂ wires. The Cu–Nb core become partially dispersion strengthened during the fabrications of the MgB₂ wires. It has been found that this Cu–Nb composite offers good promise of increased strength while maintaining the superconducting properties of the MgB₂ wire. The Young’s modulus of the best wire samples increased significantly to about 130 GPa, which is comparable to those of high strength ferromagnetic materials sheathed wires but without negative ferromagnetic effects. Those mechanical properties were enough to satisfy the low field application needs. The critical current I _c also achieves 200 A (engineering critical current density, J _ce above 1.30 × 10⁴ A/cm²) at 20 K 1 T field. The 91-filament Cu–Nb composite core reinforced wires were fabricated by in situ Powder In Tube method.     

Improved superconducting properties of the composite-processed V3Ga wires with changed configuration

Single core V₃Ga superconducting wires have been fabricated into two configurations following the composite process. The first configuration is a normal one in which the composite consists of a pure vanadium core in a Cu-Ga (19 at%) matrix. In the second configuration the same matrix has been used as a core inside a pure vanadium sleeve. Wires have also been fabricated in these two configurations by adding gallium and magnesium elements to pure vanadium and Cu-Ga matrix respectively. A simultaneous addition of gallium to vanadium and magnesium to the Cu-Ga (19 at %) matrix has been found to raise the critical current density, J_c of the V₃Ga appreciably in conformity with earlier results reported on the composite processed V₃Ga tapes using identical contents of additional elements. The configuration-2 leads to a more uniform V₃Ga layer formation and a faster growth rate thus increasing the overall critical current density still further. A value of J_c of ～ 1.6 × 10⁶ at 4.2 K and 9T has been obtained for single core wires prepared in configuration-2 and using magnesium and gallium additional elements.     

Thermal stability of reinforced Nb3Sn composite superconductor under cryocooled conditions

Several types of reinforced Nb₃Sn wires have been developed to prevent reduction of superconducting properties by applying a strong electromagnetic force. To fabricate a cryocooled magnet using those reinforced wires, we experimentally measured the minimum quench energy (MQE) under cryocooled conditions of some reinforced Nb₃Sn wires. As a result, it became clear that thermal stability expressed as MQE was controlled by the temperature margin between the temperature of the operating condition and the transition temperature from superconductivity to normal. Using the FEM analysis, it was realized that the cause of the decline in thermal stability for the reinforced wires was the low thermal conductivity of the reinforced materials.     

Synthesis and characterization of SiO2 nano- and microwires by a non-catalytic technique

In this paper, we report on the synthesis of pure silica (SiO₂) nano- and microwires by a non-catalytic process. The structure, morphology and properties of the products were examined by scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. The microstructural analysis shows that the SiO₂ wires are high purity silica and are completely amorphous; their diameter ranges from 70 nm to 2 μm. The fiber length varies from 100 μm to several mm. The wire manufacturing process involves reduced temperatures and times (1000 °C, 30 min) compared to other processes, without needing any catalyst and has been carried out on different kinds of substrates (SiC and Si). Successful growth of SiO₂ wires on the surface of silicon microcantilevers has been demonstrated; they could be used as an alternatively way to enhance the sensor performances.     

In2O3 nanowires for gas sensors: morphology and sensing characterisation

Thin and densely packed In₂O₃ nanowires have been synthesised on alumina substrates via transport and condensation method, starting from nanoparticles of indium or palladium as catalysts for the condensation process. Indium catalyst promoted wires growth according to vapour-solid (VS) mechanism, while palladium catalyst leads to wires formation based on vapour-liquid-solid (VLS) condensation. Electron microscopy and related diffraction analysis demonstrated that the wires are monocrystalline, with atomically sharp termination of the lateral sides, and are free from extended defects. The sensing properties of nanowires bundles have been tested to acetone using the flow through technique in the temperature range between 100 and 500 °C.     

Stability of multi-core MgB2/Ti/Cu/SS wires

Stability of 19-filaments MgB₂/Ti/Cu/SS wires carrying the transport currents has been examined. Properties of two identical wires annealed at 600 °C/2.5 h and 800 °C/0.5 h were compared. It was found that annealing conditions influence not only the current carrying capacity of MgB₂ filaments but also affect the inter-diffusion at Ti/Cu and Cu/SS interfaces, which is worsening the electrical and thermal conductivities of composite elements. Consequently, differences in wire’s resistances and I–V quenches were measured, which are correlating well with experimentally estimated minimum quench energy and normal zone propagation velocity.     

Monofilamentary In Situ Fe/MgB<Subscript>2</Subscript> Superconducting Wires Fabricated by Pellet-in-Tube Method

Thin monofilamentary Fe/MgB₂ superconducting wires without barriers are investigated by means of electrical transport measurements and surface and structural analysis methods. Small diameter wires are fabricated by pellet-in-tube method (PeIT) to obtain a high uniform initial filling density and heat treated as a function of various sintering temperatures and times. The results are discussed in terms of the grain connectivity, Fe₂B phase formation, and the relation between wire diameter and sintering conditions. We suggest that PeIT has a crucial importance to achieve homogeneous initial filling density, which leads to the fabrication of uniform long-length MgB₂ wires.     

Enhancement of bending strain tolerance and current carrying property of MgB2 based multifilamentary wires

The effect of bending strain on current carrying capacity of MgB₂ multifilamentary wires was studied with 4, 8 and 16 multifilamentary wires. The critical current density (J_C) of straight wires and bent wires with 5, 10, and 15 cm diameter was measured. Both annealed & bent and bent & annealed wires were used for measurement. The J_C of annealed & bent wires were found to decrease with decrease in bent diameter and the rate of degradation of J_C decreased with increasing number of filaments, while bent & annealed wires almost retained its J_C at all diameters studied.     

Fabrication and Microstructure of BSCCO Superconductive Wires Made by LHPG Method

Laser heated pedestal growth technique (LHPG) has been used to fabricate BSCCO (BiSrCaCuO)superconductive wires.The critical current density of the wires has surpassed 5000A/cm~2 (78K,OT;cross area=1.04 mm~2).The fiber rods obtained directly from LHPG method are semiconductiveand after heat treatment at temperature range from 780 to 870℃,they transformed intosuperconductive wires.Triple junctions and sharp grain boundary exist in the sample.HREM studiesshowed the intergrowth of (010) and (110),the occasional missing of CaCuO_2 layer in the 2212phase.     

Evaluation of processing parameters effects on the formation of Si3N4 wires synthesized by means of ball milling and nitridation route

Crystalline silicon nitride (Si₃N₄) wires have been synthesized by means of ball milling and nitridation route. The influence of temperature of reaction and starting condition of the powder (milled or unmilled) on the synthesis of Si₃N₄ wires were studied. The reduced size of silicon particle during the milling process led to an increased degree of nitridation.Silicon powders with higher surface energy can react incessantly with nitrogen to form silicon nitride wires. The results show that the Si₃N₄ was fully formed with two kinds morphologies including globular and wire with a width of 100–300 nm and a length of several microns at temperature of 1300 °C for 1 h by employing the milled silicon powder. The infrared adsorption of wires exhibit absorption bands related to the absorption peaks of Si–N band of Si₃N₄.     

Tensile strain dependence of critical current of RHQ-Nb3Al wires

Nb₃Al superconducting wires produced by rapid heating and quenching (RHQ) have been developed for high-field accelerator magnets. It is known that critical currents of A15 superconductors (e.g., Nb₃Al and Nb₃Sn) have a dependence on stress/strain. It is thus important to clarify the stress/strain behavior of the RHQ-Nb₃Al wires for the development of high-field accelerator magnets. We recently started experimentally investigating the strain dependence of the critical current of the RHQ-Nb₃Al wires with a Ta interfilament by measuring their critical currents under longitudinal tensile strains. To evaluate the effect of residual strain in Nb₃Al filaments induced by thermal contraction of the materials in the wire, neutron diffraction measurements were performed at room temperature.     

Boundary resistance in SC composite wires and cryogenic stability

The measurement of transverse resistivity of NbTi composite wires has shown already the existence of a resistive barrier between SC filaments and the copper matrix. The electric and thermal resistances of this barrier are respectively much higher than those of copper matrix and this barrier is expected to have an influence on the cryogenic stability of composite wires. The transverse and longitudinal resistivities are measured for NbTi composite wires which were heat-treated at different temperatures from 300°C to 600°C. These measurements show that the barrier grows with the heat-treatment temperature. From the experimental results, the effect of the barrier on cryogenic stability is estimated to be negligibly small for the composite wire which is heat-treated under the normal condition. As for Nb₃Sn composite wires, two different structures of composite wires, each of which has a tantalum or niobium diffusion barrier, are studied and the same measurements as on NbTi composite wires are carried out. The results obtained indicate that the transverse resistivity depends appreciably on the structure of composite wires and that the larger transverse resistivity reduces not only the cryogenic stability, but also requires a larger transfer length at a current lead junction.