Kazuhiko Yasohama

THE UPPER CRITICAL FIELD Hc2 OF NbN FILM PREPARED BY REACTIVE SPUTTERING

The upper critical field Hc2 of about 200 kOe was obtained for niobium nitride films prepared by reactive sputtering. Electron microscopy was used to determine the crystal structure. It was shown that the electron diffraction patterns of these films were that of the NbN with the NaCl crystal structure.

Specific Heat and Superconducting Properties of Zr-Mo Alloys

The low temperature specific heat of bcc alloys of zirconium base Zr-Mo has been measured in order to investigate their superconducting properties. It was observed that the small addition of molybdenum to zirconium results in a steep rise of the electronic specific heat coefficient, γ, and a decrease of the Debye temperature, θ D . From the knowledge of γ and θ D the variation of the superconducting transition temperature, T c , of zirconium concentration was analyzed with the McMillan theory, and it is suggested that an enhancement of T c due to the small addition of molybdenum may be ascribed to the sharp rise of the electronic density of states at the Fermi surface.

Crystal growth and critical current densities of polycrystalline MgB/sub 2/ superconductor

We have investigated crystal growth and critical current densities of MgB/sub 2/ which was synthesized using stoichiometry or excess Mg and two kinds of B grain size. Excess Mg plays a role not only to promote crystal growth but also to make crystal grains fine. During crystal growth, the critical current densities decrease, although the density remains constant, approximately. Irrespective of the starting B powder, J/sub CM/ is the highest in the sample with the smallest crystal grains and highest density of MgB/sub 2/. The effects of excess Mg creates the possibility to control crystal size of MgB/sub 2/ by adjusting the quantity of Mg.

Microstructure and critical current densities of MgB/sub 2//Cu wires

Cu sheathed MgB/sub 2/ wires with a diameter of 2.50 mm and 0.93 mm have been fabricated by the In-Situ Powder-in-Tube PIT method. We have studied three subjects; the morphology of the wire core affected by the fabrication process of cold rolling, compounds in the core produced by the heat-treatment and effect of the microstructure on critical current densities Jc. The value of Jc is 10/sup 3/ A/cm/sup 2/ in 2 T at 25 K for the wire of 2.50 mm diameter. This value is three times higher than that of the wires of 0.93 mm diameter. EDS analysis and X-ray diffraction patterns have shown that in the wire with a diameter of 0.93 mm Cu and Mg diffusion on heat-treatment process, leading to formation of Cu-Mg compounds. We have paid attention to texture change of the wire core from granular to filamentary by the cold rolling process. It has been supposed that this filamentary texture gives an effect on the formation of MgB/sub 2/ and Jc value.

Development of an Nb/sub 3/Sn AC coil with react & wind method

The Nb/sub 3/Sn superconductor possesses a higher critical temperature than the NbTi strand. Therefore, it is now possible to make highly stable superconducting strands. We examined the manufacturing process of AC Nb/sub 3/Sn strands. The internal Sn diffusion process showed a higher critical current density than the conventional process. Nb/sub 3/Sn strands for AC use have a high Jc with low temperature reaction heat treatment, because they have fine filaments to decrease AC loss. We made a 400 kVA class superconducting coil using the developed Nb/sub 3/Sn cable with the React and Wind method. The loss density of this coil was 25.7 MW/m/sup 3/ at the point just before the quench. In this case, the temperature of the cable increases about 3.39 K. This means that the coil using Nb/sub 3/Sn cables has a very high stability in AC use.

Fibrous Structure and Critical Current Density of a MgB_2 Superconducting Wire

The authors have fabricated a novel MgB2 superconducting wire using the in-situ powder-in-tube method with fine boron powder (0.25 μm) and coarse magnesium powder (150 μm). The core of the wire has a fibrous structure consisting of numerous discontinuous MgB2 filaments. The critical current density Jc increases as wire size decreases, and the Jc of a 1.00 mm wire is 1.4×104 A/cm2 at a temperature of 20 K in a magnetic field of 4 T.