Kiyoshi Yoshizaki

Y-Ba-Cu-O Superconducting Films with High Jc Values by MOCVD using Ba-Addition Products

Y-Ba-Cu-O superconducting films having high Jc of 6.3×106 A/cm2 at 77 K and Tc of 92 K were prepared by metalorganic chemical vapor deposition using a new transport technique with Ba-addition products. This CVD method could realize stable vaporization and smooth transportation of Ba source material due to the increase of the evaporation rate at low sublimation temperatures. Consequently, good reproducibility for preparation of the high Jc films (over 106 A/cm2) was achieved.

Nb 3 Sn superconducting cables processed by internal tin diffusion

Modified fabrication techniques of the internal tin diffusion process and optimization of the reaction condition are discussed. By this process, the large-scale production of practical multifilamentary Nb 3 Sn conductors has been able to be performed very easily and reliably. The critical current density without stabilizing Cu was achieved 460 A/mm2at 12 T, 4.2 K. Furthermore, the properties of the conductor fabricated from high tin composite are investigated. It was found that there is possibility to fabricate the conductor which has higher critical current density at high fields from high tin composite.

Absorption and Desorption of Oxygen in Y1Ba2Cu3O7-δ

The absorption and desorption velocities of oxygen in a Y1Ba2Cu3O7-δ sintered sample were measured by a thermogravimetric analysis when the atmospheric gas of nitrogen was exchanged for oxygen and oxygen for nitrogen. The weight change of the sample could be expressed by an exponential function of time in each process. In the absorption process of oxygen, the time constant was a complex function of the temperature, while in the desorption process, it was the monotonous temperature dependence. The diffusion constant and the activation energy were estimated at the temperature range of the tetragonal phase.

The Structure and Superconducting Properties of Multifilamentary Nb3Sn Wires Prepared by Internal Sn Diffusion Process Using Sn-Ti Cores

The multifilamentary Nb3Sn superconducting wires with the titanium addition have been fabricated by internal tin diffusion process to improve critical current density (J) at high fields (>12T). The wires fabricated have seven modules in a copper matrix. The module is composed of the Sn-2wt%Ti alloy core instead of the tin core at the center and 90 niobium filaments located by three layers around the Sn-Ti core.

Improvements in critical current densities of internal tin diffusion process Nb 3 Sn wires by additions of third elements

Improvements in critical current densities of internal tin diffusion process Nb 3 Sn multifilamentary wires have been attempted by indium and titanium additions. The Nb 3 Sn wires with the indium addition to the Sn core and the titanium addition to the Cu matrix have been fabricated. The indium addition increases critical current density (Jc) at each field compared with pure Nb 3 Sn wire, and Jc at higher fields increases with increasing filament diameter. An overall Jc of 6.2 × 104A/cm2at 12T and 4.2K is obtained for the 4.2 μm-filament indium addition wire reacted at 750 °C-50h. The titanium addition increases the Nb 3 Sn layer growth rate and Jc in fields over 13T. An overall Jc of 3.0 ×104A/cm2at 15T and 4.2K is obtained for the 11.7 μm-filament titanium addition wire. Moreover, the simultaneous indium addition to the titanium addition wire produces further improvement in Jc. The indium and titanium addition Nb 3 Sn wires are the most favorable superconductors for generating magnetic field over 10T.

Effects of titanium addition to the Nb 3 Sn wires fabricated by the internal tin diffusion process

The titanium-doped Nb 3 Sn wires have been successfuly fabricated by the internal tin diffusion process (IDP). These wires consist of a Sn-4.8 at%Ti alloy core at the center and 99 pure niobium and/or Nb-1.9at%Ti alloy filaments placed in three layers around the Sn-Ti core. These wires were easily drawn to the final sizes without any intermediate annealing, and were heat treated at 650-750°C for 30 - 300 hr. The Electron Probe Micro Analysis analysis (EPMA) has revealed that, when the titanium is added only to the tin core, the titanium content in Nb 3 Sn filaments decreases with increasing distance from the center of the wire. On the other hand, the simultaneous titanium addition to the tin core and two outer layers of niobium filaments has attained the uniformity of the titanium content; ∼ 0.8at%Ti is incorporated in all Nb 3 Sn filaments. The overall J c of \sim 4 \times 10^{4} A/cm2at 15 T and the improved B c2 have been obtained for the Nb 3 Sn wires with the simultaneous titanium addition. These results indicate that the IDP Nb 3 Sn wires with simultaneous titanium addition to the tin core and niobium filaments are the most promising candidate materials for high field magnets.

Superconducting Properties of Chevrel-Phase PbMo6S8 Wires by an Improved Powder Process

A Chevrel-phase PbMo6S8 superconductor is one of the most promising superconducting wires for very high field magnets. In this study, PbMo6S8 superconducting wires fabricated by using a Ta barrier in a powder process have been heat-treated under high hydrostatic pressure for producing PbMo6S8 compound. And then superconductivity measurements and microstructure observations have been performed. The wires are very easily drawn to a final size at room temperature, and have a critical current density (Jc) for the compound of 1.7×104A/cm2 at 12T. The typical transition temperature (Tc) and its width, ΔTc are 14.5K and 0.6K, respectively. Furthermore, EPMA and PSPC-type X-ray Microdiffractometer observations indicate Mo, MoO2 precipitates and unreacted MoS2 in the compound core inside the Ta barrier. Consequently, the PbMo6S8 wires with excellent Jc at high fields can be fabricated by using a Ta barrier and heat-treating under hydrostatic pressure.

The Superconducting Properties for La–Sr–Cu–O Systems

Superconducting oxides, La–Sr–Cu–O and Y–Ba–Cu–O were prepared using nitrate mixture by spray dry method and/or oxide and carbonate powder mixtures followed by pressing and sintering. The superconducting properties such as critical temperature (Tc) and critical current density (Jc) were measured by resistive and inductive methods, and transport current and magnetization measurements, respectively. The Tcs(the resistive offset) were 32, 91 K for La and Y system, respectively. The Jc(77K) of 1,840 A/cm2 was observed for Y system by transport current measurement.

Nb/sub 3/Sn superconducting coil for AC use

Nb/sub 3/Sn superconductors were developed for AC use, and a coil was fabricated. The Nb/sub 3/Sn superconductors were manufactured using the internal diffusion process. To reduce AC losses, the spacing between Nb filaments was designed to be 0.5 mu m; consequently, the space factor of Nb filaments was 6%. The diameter of a strand was 0.2 mm, and the diameter of a Nb filament was 0.4 mu m. AC losses in the strand were 180 kW/m/sup 3/ at 0.5 T (60 Hz, peak value). A coil was made using the wind-and-react method using conductors composed of 7*7 strands. The Specifications of the coil were an inner diameter of 156 mm, an outer diameter of 188 mm, a height of 34 mm, and a number of turns of 17 turns*4 layers. To reduce wire motion, the coil was impregnated with epoxy resin. The quench current for DC operation was 1280 A, and the maximum magnetic field of the conductors was 1.6 T. Coil degradations were not observed. The magnet was tested under AC 60-Hz operation. The quench current was 340 A (r.m.s.). The cause of quenching is thought to be the temperature rise of the conductors due to coupling losses among the strands.

Axial anisotropy of intergrain critical current in Bi-Pb-Sr-Ca-Cu-O

The intergrain critical current densities, Jc, parallel and perpendicular to c-axis were measured as a function of the magnetic field at 4.2 and 77.3K by an ac inductive method for the fairly oriented Bi1.6Pb0.4Sr2Ca2Cu3Oy specimens. The Jcs perpendicular to c-axis were higher than those parallel c-axis, about ten times within 5T at 4.2K, and about three times within 0.1T at 77.3K. Our results suggect the orientation of each grain to be especially worthwhile for performing the high Jc at the low temperature.