Kaname Matsumoto

Enhanced Jc properties in superconducting NbTi composites by introducing Nb artificial pins with a layered structure

High critical current densities (4250 A/mm2 at 5 T, 4.2 K, and 15 000 A/mm2 at 2 T, 4.2 K) were achieved by using an artificially designed pin structure composed of randomly oriented clusters of Nb/NbTi multilayer in a NbTi composite superconductor (with a round cross section). The maximum values were obtained when the Nb layer thickness was designed to be approximately twice the coherence length in NbTi, where the layer thickness is five to ten times thicker than that of the optimized α‐Ti ribbon thickness in a conventional NbTi system.

Achievement of high current density in Nb Ti superconducting multifilamentary wires by introducing designed artificial pins

Abstract It is shown theoretically that a further increase in the overall critical current density of Nb  Ti multifilamentary wire is possible by introducing artificial pinning centres into Nb  Ti filaments in a specified fashion. As the first step towards experimental confirmation of theoretical predictions of the summation of individual forces, Nb  Ti monofilamentary wires were fabricated, in each of which were embedded fine filamentary Nb rods with a given diameter and spacing, to act as artificial pinning centres. These wires show higher critical current densities than have been reported so far, at fields lower than ≈ 2 T. Although the quantitative agreement between the theoretical and observed results is not yet very satisfactory, further investigation of the pinning characteristics using various artificial pins leads to a new stage where it is possible to design the wire parameters for obtaining the required characteristics of critical current density.

Effects of artificial pins on the flux pinning force and other superconducting properties in NbTi superconductors

A marked enhancement of global pinning strength in Nb-50 wt.%Ti alloy was obtained by introducing ribbon-shaped artificial pins. Simultaneously, however, depression of B/sub c2/ was observed. This behavior is analyzed theoretically, and a satisfactory explanation of the experimental results is obtained.<<ETX>>

Pinning characteristics in multifilamentary NbTi superconducting wires with submicrometre filaments introduced artificial pinning centres

Multifilamentary NbTi superconducting wires for a.c. use were developed by introducing ultrafine Nb island-type artificial pinning centres. The maximum Jc value obtained for wire with a filament diameter of 0.4 μm reached as high as 1.07 × 1010 A m−2 at 1 T and 4.44 × 109 A m−2 at 3 T, which are more than twice the Jc values for conventional wires for a.c. use. For these wires, the constants of the scaling rule for global pinning force, Fpαbγ(1−b)δ, became γ = 0.8 and δ = 1.6. This indicates that the pinning characteristics of the present wires were shifted to the oversaturation-type by the introduction of artificial pinning centres, while they are of the saturation-type for conventional a.c. wires. As a result, the global pinning force at low and medium magnetic fields can be explained fairly well by the direct summation model which is available only for strong pins. A helpful guideline in designing Jc values for a.c. wires is also discussed.

New Fabrication Method of High-Jc YBa2Cu3O7 Superconducting Films on Flexible Metallic Substrates

In plane-aligned, c-axis-oriented YBa2Cu3O7 (YBCO) films were deposited on polycrystalline Ni tapes buffered with self-oxidized NiO. The in-plane alignment of the NiO layer was achieved by the oxidation of textured pure (00l)Ni tapes in 1 atm O2 in the furnace. Significant improvement in (00l)NiO texturing was obtained in the temperature range above 1000°C. Pulsed laser deposited YBCO films on the NiO layer were not only c-axis oriented with respect to the film surface, but also strongly in-plane textured. The full width at half maximum of the x-ray diffraction peaks from the (103)YBCO ϕ scan is approximately 11 degrees. This is similar to the values reported with other texturing techniques.

Magnetic flux pinning properties of oxide superconductors produced by melt processes

Abstract The flux pinning mechanism of YBaCuO superconductors produced using a melt process was investigated by measuring the magnetic field dependence of transport critical current densities ( J c ) at 77 K. Superconductors produced by the melt process had high transport J c values and the magnetic field dependence of the transport J c was quite small compared to that of bulk sintered materials. This suggests that, in this case, J c in a high field region is determined by flux pinning within the bulk of the material. Using procedures based on conventional flux pinning theory, the J c - B properties were analysed to estimate dominant pinning sites in solidified specimens. According to the theoretical analysis and microstructural observations, the expected site of dominant pinning is the normal precipitates dispersed in the YBaCuO grains, and the transport J c may be determined by their average size and density.

Ultrafine multifilamentary Nb Ti wires with Cu Si alloy matrix

Abstract The Cu  Si alloy has been proposed as a new matrix material for filamentary Nb  Ti wires in a.c. use. The Cu  Si alloy shows appropriate mechanical and electrical properties, and is economically more favourable than the Cu  Ni alloy matrix used currently. Moreover, the addition of Si to Cu prevents the formation of intermetallic compounds around the filaments. After extensive investigations on CuSi alloy as a matrix material, an ultrafine multifilamentary Nb  Ti/Cu − 2.5 wt% Si composite wire has been successfully fabricated. The d.c. superconducting properties of the wire were adequate for use in electrical apparatus. A preliminary study has revealed that the a.c. loss of the new wire is equivalent to that of a typical ultrafine multifilamentary Nb  Ti/Cu  Ni composite wire.

Flux-pinning characteristics of Nb artificial pins with ribbon-shape in Nb-46.5 wt.% Ti superconductors

The pinning force density, Fp, of a new type of Nb artificial pin was observed in Nb-46.5 wt.% Ti superconducting wires, where the artificial pins were made of thin Nb plates inserted into successive NbTi plates and were cold drawn to a ribbon-like shape. It is shown that the position of the peak of Fp shifts systematically from the low magnetic field side to the high-field side as the reduction rate of pin thickness increases. The flux pinning mechanism of the pins is also discussed.

Proximity coupling effect in NbTi fine‐multifilamentary superconducting composites

In order to shed light on the proximity effect in fine multifilamentary wires with NbTi filaments, measurements of magnetic susceptibility were conducted at 4.2 K for various values of electrical resistivity of the matrix between filaments, of the distances between filaments, and of the applied magnetic field strength. It was confirmed experimentally that in weak fields (10 mT or less), the critical distance for the occurrence of proximity coupling was several multiples of the characteristic leak distance of Cooper pairs in normally conductive metal, and that when the distances between filaments were large, the breakdown field decreased exponentially, irrespective of the electrical resistivity of the matrix.

Development of high-field a.c. superconducting magnet using ultrafine multifilamentary Nb-Ti superconducting wire with designed Nb artificial pins

Abstract A 2.5 T/100kVA a.c. superconducting magnet has been developed using Nb-Ti superconducting wire with artificially introduced pins. Investigations have been conducted regarding the suitability of critical current density J c design using artificial pins and the effectiveness of magnets using wires with high J c . Studies on pin design suggested that the maximum pinning force density F p would be reached at a magnetic field of 2.5 T. As anticipated, the maximum F p was achieved at 2.5T, and a high J c of 4.7 × 10 9 A m −2 was obtained. Using an a.c. magnet fabricated from this wire, current operation at 60 Hz and 4.2 K was effected, and steady state operation at a capacity of 104.8 kVA, a current of 105.8 A rms and a voltage of 991.2 V rms was achieved. At the same time, a central magnetic field of 2.5 T was obtained. In addition, it was demonstrated that the total a.c. losses were 4.8 W at a central magnetic field amplitude B 0 of 2 T, a mere 0.007% with respect to the capacity of 69 kVA and much lower than in a.c. magnets of the 100 kVA class made using conventional a.c. wires. This confirmed the effectiveness of reducing magnet size through the use of wires having high current density.