Mikio Nakagawa

Fabrication and Superconducting Properties of High T c Oxide Wire

Since the discovery of high Tc superconducting oxide such as Ln-Ba-Cu-O system which shows superconductivity above liquid nitrogen temperature, intensive study has been carried out to investigate its intrinsic properties[l-4]. Superconducting wire which operates in liquid nitrogen is one of the emerging products among application, however, there are many factors to be investigated for practical use. Critical current density, Jc, is one of the most important characteristics of superconductor, although, only few report describing Jc measurement ever published[5]. In this paper, fabrication procedure of silver sheathed Y-Ba-Cu-O wire by powder metallurgical technique and its superconducting properties have been reported. Other factors which may influence superconductivity such as microstructure of the oxide, configuration of wires, etc. are also discussed.

Characteristics of high-Tc oxide wire

Abstract We have prepared an Y-Ba-Cu-O superconducting wire with silver sheath by a powder metallurgy technique. The crystal structure of the oxide core was orthorhombic and no structural change was found after cold-drawing. The wire showed superconductivity after heat-treatment at 890°C. Diffusion behavior of oxygen atoms through the sheath material is discussed. The highest critical current density of 640 A/cm2 was obtained for the sample heat-treated in oxygen flow.

Stainless steel sheathed forced internally cooled Nb 3 Sn superconductor and its coil test

Forced cooled Nb 3 Sn superconducting coil with the stainless steel sheathed conductor has been manufactured and tested in a High Field Test Facility (HFTF). Some properties of the sheath material and the conductor with stainless steel sheath have been investigated. A 316L stainless steel sheathed conductor (the outer dimension of 13 × 23 mm2) was fabricated, and one coil was completed by the wind and react method using this conductor. The coil was connected with 8 pancake coils made of copper sheathed conductor. The forced cooled magnet of 9 double pancake coils was tested under 5.9 T of the back up field in HFTF and generated 8.8 T under 7.6 kA of transport current. Experimental results show that the Nb 3 Sn strand in the copper sheathed conductor near the connector was degraded due to handling through reconnecting the conductor for several times.

Critical Currents of Unidirectionally Grown YBa2Cu3O7-x

YBa2Cu3O7-x superconductors were prepared by unidirectional solidification process. Floating zone method using electric furnace was applied. Y2BaCuO5, Ag, and Pt were added to 123 composition. Samples were grown at 1 or 3 mm/h in air and annealed in O2. Investigation was carried out by using optical microscope and X-ray diffraction. Aligned microstructure was confirmed. Superconducting properties were evaluated by dc four probe technique. The largest critical current and density were 1800A and 3.3×104A/cm2. Experiments were carried out effectively by using design of experiments. It was made clear that critical current density was enlarged by addition of Ag and Pt.

Fabrication of the Model Cable using Bi-2223 Ag Sheathed Tapes

A superconducting model cable was designed and fabricated. The cable consisted of conductor, electric insulation, thermal insulation, LN2 paths for cooling, and corrugated aluminum sheath with PVC layer. The conductor was divided into ten segments in order to improve flexibility. Laminated Bi-2223 Ag sheathed tapes were put in grooves of the segments. The thermal insulation layer using super-insulation was evacuated during the evaluation of the cable. Critical current of the cable was 1.2 kA at 77K, and did not change before and after the cable fabrication and by the heat cycles.

Nb3Sn thin films made by RF magnetron sputtering process with a reacted Nb3Sn powder target

We have developed a high field pulsed superconductor with a high elastic modulus fiber for reinforcement called FRS (fiber reinforced superconductor). These fiber reinforced superconductors are formed by magnetron sputtering techniques of two opposed planar targets. In this process for Nb3Sn compound formation, a single phase material target is desirable to shorten the deposition process. We have succeeded in making a sputtering target which consists of Nb3Sn single phase material by powder metallurgy. Fiber reinforced superconductor of Nb3Sn on Ta fiber has been successfully fabricated with this target. An attempt has been made to form Nb3Sn film as grown on a heated substrate by this single phase target.

Study for Structure of Power Cables by Bi-2223 Ag Sheathed Tapes

We have designed and constructed a prototype model of superconducting power cable (77kV, 3kA) consisted of superconductor, electric insulation, thermal insulation, going and returning LN2 paths for cooling, and corrugated aluminum sheath with PVC layer. Length of the model cable was 1m. Critical Current of the model cable, in which one of ten grooves in the copper former was filled with Bi-2223 Ag sheathed tapes, was 180A at 77K. Degradation of critical current by electric insulator winding or heat cycles up to five times was not observed.

Insulation Materials for Bi-2223 Tapes

Simulating Wind & React method, influence of insulators on superconducting properties of Bi-2223 silver sheath tape was investigated. CuO in the oxide core penetrated through the silver sheath and chemical reaction with insulation materials was observed. Resultant low Cu content in the core caused secondary phases. Critical current densities were limited by the phases. AI2O3 was found most suitable for insulation materials in our experiments.

Y-BA-CU OXIDE SUPERCONDUCTOR PREPARED BY A DIFFUSION PROCESS

YBa2Cu307-y superconducting oxide has been fabricated by a diffusion process. Starting materials are Y2BaCu05 and barium cuprate unlike conventional methods. A dense layer of YBa2Cu307-y was formed on the green Y2BaCu05 substrate after heat-treatment. Diffusion phenomenon in the system is also discussed. Critical temperature and critical current density of the heat-treated samples were 91.5K and 1900A/cm2 (77K, OT), respectively.

High Tc Superconducting Thin Film by Sputtering Process

Y-Ba-Cu-0 and Bi-Sr-Ca-Cu-0 high temperature superconductor film was prepared by a sputtering process. Film was deposited on SrTi03 or Mg0 substrate and heat-treated in oxygen. Zero resistance temperature was 88K and 81K for Y-Ba-Cu-0 and Bi-Sr-Ca-Cu-0 film, respectively. Stoichiometry was found to be important to obtain high critical temperature in Y-Ba-Cu-0 film. Diffusion of Ba was found at the interface between Y-Ba-Cu-0 film and SrTi03 substrate after the heat-treatment. Zero resistance temperature tended to lower if thickness of the film was less than 0.6um and 0.2um for Y-Ba-Cu-0 and Bi-Sr-Ca-Cu-0, respectively. Reason for such Tc degradation is discussed.