Motomune Kodama

Mechanism for high critical current density in in situ MgB2 wire with large area-reduction ratio

A comparative study of in situ MgB2 wire and MgB2 bulk was carried out to clarify the mechanism for the high critical current density, Jc, in the practical in situ MgB2 wires. The in situ MgB2 wire was manufactured with an area-reduction ratio of 99.93%, which was one of the highest values in MgB2 superconducting wires previously reported. The electrical connectivity, K, and the flux pinning strength, Fp, which are important factors in explaining the behavior of Jc, could be determined in the same manner as those for the bulk sample; K was well understood with the three-dimensional percolation model, and Fp was effectively explained by the electron scattering mechanism by grain boundaries. On the other hand, the area-reduction process dramatically enhanced the value of K, leading to an increase in the value of Jc. The respective values of K and Jc(20 K, ~0 T) reached 0.24–0.34 and 6.0 × 103−8.4 × 103 A mm−2, which were twice or three times higher than those of typical in situ bulks. This is because the plastic deformation of magnesium particles increased the packing factor of raw powders through a repetitive drawing process.

Electromagnetic properties and microstructures of in situ MgB2 wires made from three types of boron powders

In powder-in-tube processed MgB2 wires, the choice of boron powder as a starting material crucially affects their performance. In this paper, we prepared in situ MgB2 wires from three types of boron powders in various heat-treatment conditions and investigated their electromagnetic properties and microstructures. Their critical current density, J c, varied over a wide range from sample to sample. The difference in J c is understood to be caused by the effect of changes in the electrical connectivity, K, and intrinsic residual resistivity, ρ 0. Here, K represents the effective cross-sectional area for current, and ρ 0 reflects the degree of the charge carrier scattering caused by lattice defects. It was found that the use of boron powder with a large specific surface area leads to a large degree of lattice defects in MgB2 grains and enhances ρ 0, resulting in improving J c. The boron powder produced by thermal decomposition of B2H6 has a large specific surface area. Hence, this boron powder is the most suitable as a starting material for MgB2. Meanwhile, dry pulverization of low-cost boron powder, which is largely produced by active-metal reduction of B2O3, is also effective to increase its specific surface area without introducing impurities, resulting in the enhancement of J c in the entire magnetic field region. This finding broadens the choice of boron powder and contributes to realizing superconducting applications with excellent balance between performance and cost.

Superior Jc-B-T Characteristics of 10-μm-Thick MgB2 Film for Tape Application

Superior <italic>Jc</italic>-<italic>B</italic>-<italic>T</italic> characteristics of 10-μm-thick MgB₂ films on copper plates were obtained. The <italic>Jc</italic>-<italic>B</italic>-<italic>T</italic> properties were measured at a temperature range from 5 to 20 K and at an external magnetic field range from 0 to 17 T. At <italic>B </italic> = 6xa0T, <italic>Jc</italic> were 54xa0200, 30xa0840, 9900, and 820xa0A/mm² at <italic>T</italic> = 5, 10, 15, and 20xa0K, respectively. The high <italic>Jc</italic> suggest that MgB ₂ film is a promising candidate for next-generation superconducting tape conductors and coils.

Conduction-Cooled MgB2 Coil in Maximum Self-Magnetic Flux Density 2.3 Tesla Made With 300-Meter-Long Multifilamentary MgB2 Wire

We fabricated a 300-m-long multifilamentary MgB₂ wire as a powder in tube wire by using an <italic>in situ</italic> method. The <italic>J</italic>_e values were approximately 250 A/mm² from 2 to 3 T at 20 K and 3 to 4 T at 15 K. These <italic>J</italic>_e values were interpolated by <italic>J</italic> _c values on monofilament wire because <italic>I</italic>_c measurements over 100 A were difficult due to joule heat on solders. By using this 300 m wire, a coil was designed to be driven in the maximum magnetic flux density in MgB₂ filaments (<italic>B</italic>_max) over 2 T. The coil was made by using a wind-and-react method and cooled by using conduction cooling. The coil was successfully driven at 24 K and 27 K as temperatures of coil winding, and calculated <italic>B</italic>_max was 2.3 T in 24xa0K operation. The coil <italic>I</italic>_c values measured at 24xa0K and 27 K agreed well with the estimated values based on <italic>I</italic>_c values on short-length wire. This means that the 300-m-long multifilamentary wire has sufficient longitudinal homogeneity for designing MgB₂ coils and can be used at a maximum magnetic flux density over 2 T.

Analysis for formation of current path in the superconducting joint between Nb-Ti wires with the solder matrix replacement method

We investigated a mechanism for formation of a current path in a superconducting joint between Nb-Ti wires with the solder matrix replacement method, where Pb-Bi connects Nb-Ti filaments. Superconducting joints were formed in the following steps. Firstly, the ends of wires were soaked in molten tin. Secondly, the ends of wires were soaked in molten Pb-Bi. Thirdly, the two ends of wires were set into the container filled with molten Pb-Bi and the Pb-Bi was solidified. One factor in the current path is the interface between Nb-Ti and Pb-Bi, where there was an intermediate layer composed of Nb3Sn and Ti-S. The intermediate layer appeared to be formed in the first step; niobium in Nb-Ti reacted with tin, and leftover titanium reacted with sulfur which may be contained as an impurity in the copper matrix of the wires. Another factor is the microstructure of Pb-Bi, which consisted of a superconducting phase and a bismuth phase. The specific surface area of the bismuth particles, which was affected by the composition and solidification rate of Pb-Bi in the third step, was positively correlated with the critical current density of Pb-Bi. This is because the bismuth particles are dominant flux pinning centers, as shown by Campbell. The present superconducting joints had excellent properties; the resistances were lower than 10−13 Ω.