Yoshihide Wadayama

Phase Compatibility and Superconductivity of Y-Ba-Cu-O Compounds

Superconducting properties of Y-Ba-Cu-O compounds were investigated in relation to precipitation of second phases. Contour lines of Jc, Tc and diamagnetic signal intensities were presented in the YO1.5-BaO-CuO system. These superconducting properties decreased steeply with BaCuO2 precipitation, and the maximum Jc was obtained in the yttrium-rich composition of Y1.2Ba2Cu3O7-y. It is considered, therefore, that the BaCuO2 precipitating around the YBa2Cu3O7-y grains interrupt the superconducting current pass at the grain boundary. Yttrium-rich composition, which inhibits BaCuO2 precipitation, is effective for the preparation of the high-Jc sample.

Microstructure control of YBa2Cu3O7−x by high temperature compression

The development of texture during high temperature deformation was studied using uniaxial compression and compression with grooved dies. The analysis of the orientation distribution for compressed YBa2Cu3O7−x polycrystals was performed using electron channelling patterns (ECPs). It was found that the (001) plane of grains in the compressed sample was approximately parallel to the compressed plane. The angle of deviation of the (001) plane in each grain from the sample surface was within 30° for both types of sample. Furthermore, an a- or b-plane texture was developed by compression with grooved dies. The transport critical current density Jc (trans) at 77 K was 450 A cm−2 for the sample compressed 50% uniaxially and 500 A cm−2 for the sample compressed 50% with grooved dies. The Jc values from magnetization measurements at 4.5 K were 2.4 × 105 A cm−2 and 3.8 × 105 A cm−2 respectively in a magnetic field parallel to the c axis. The low Jc(trans) value is attributed to microcracking, induced by anisotropic contraction of the grains during reoxidation.

Microstructural characterization of Nb3Al ultrafine multifilamentary superconducting wire by analytical electron microscopy

Microstructural characterization of the longitudinal cross section of Nb3Al wires fabricated by an Nb tube composite process is presented. The wire was produced by cold reduction and a subsequent reaction between Nb matrix and Al‐Mg core (final size: ∼φ 80 nm) at 950 °C×5 min+750 °C×72 h. The A15 superconducting compound (grain size: 100 nm) and unreacted fibrous Nb were observed in the microstructure. Nb2Al or other phases rarely appeared. The average Al concentration at the A15 grain was 22–23 at. % from x‐ray microanalysis. The critical current density Jc of composite processed wire depended on the grain size of A15, with fine grains providing a high Jc value.

Development of Single-Stacked Internal-Tin Nb

The internal-tin technique is an excellent method of fabricating Nb3Sn superconducting wires with high critical current densities. We have developed an internal-tin Nb3Sn wire in which the multi-filamentary wire is fabricated by assembling mono-filamentary Nb elements and mono-filamentary Sn elements. The simple fabrication process allows a reduction in the fabrication cost of the wires. By optimizing the spacing of the Nb3Sn filaments, a 600 m long wire with a high critical current density of 400 A/mm2 at 18 T and no flux jumping was fabricated.

_{3}

High-temperature compression tests were performed in air for YBa2Cu3O7−x polycrystals with grain sizes of 3 and 7 Μm at various strain rates between 1.3×10−5 and 4×10−4s−1 and at temperatures between 1136 and 1253 K. Steady state deformation appeared above 1203 K for both samples. A stress exponent of 1.3 and an activation energy of 150 kJ mol−1 were evaluated. The compression tests and microstructural observations revealed that there was a difference in deformation mechanism above and below 1203 K. The dominant mechanism was diffusional creep associated with grain-boundary sliding above 1203 K, and dislocation glide accompanied with grain-boundary sliding below 1203 K. The growth of anisotropic grains and their preferred arrangement were enhanced by deformation.

Sn Superconducting Wire

Chemical composition pore volume and superconducting properties of plasma sprayed film was investigated in ralation to the spraying conditions. Y1Ba2Cu3O7-y powder was sprayed onto a Ni-Cr alloy substrate. The superconducting film formed was about 100pm thick. The amount of Cu decreases with excessive melting of the powder, related to the powder particle size and power supply. The pore volume could be controlled to less than 5% in area fraction by using fine powder and low atomospheric gas pressure. The Tc and Jc reached values of 91K and 1120A/cm2(at 77K, OT)respectively for Y1Ba2Cu3O7-y film after post-annealing

High-temperature deformation behaviour of YBa2Cu3O7-x

The critical current density, Jc, of Er0.5Y0.5Ba2Cu3O7-y was investigated in relation to its microstructure. An increase was observed in the Jc of a sintered sample prepared from a mixture of YBa2Cu3O7-y and ErBa2Cu3O7-y with sintering time of up to 10 h at 930°C. The insufficiently sintered sample had a BaCuO2 network in the grain boundaries possibly caused by partial melting of the 1:2:3 compounds. This phase disappeared with increased sintering time, which relates to an endothermic reaction. It is considered that the increase in Jc is due to an increase in the degree of sintering and to the disappearance of the highly resistive phase from the grain boundary.