Koshichi Noto

Critical currents at 77.3 K under magnetic fields up to 27 T for an Y‐Ba‐Cu‐O film prepared by chemical vapor deposition

Critical currents of an Y1Ba2Cu3O7–δ film prepared by a chemical vapor deposition technique have been measured resistively. Excellent critical current properties in high fields up to 27 T are obtained at 77.3 K. The value of Jc is 4.1×105, 1.9×105, and 6.5×104 A/cm2 at 2, 10, and 27 T, respectively. The upper critical field defined by zero resistivity is estimated to be 35 and 180 T at 77.3 and 0 K, respectively. A peak effect in the critical current is observed at high fields.

Temperature and angular dependences of upper critical fields for the layer structure superconductor 2H-NbSe2

The temperature and angular dependences of upper critical fieldsHc2have been measured for several 2H-NbSe2 single crystals by use of an electrical conduction method in magnetic fields up to 150 kOe. As the temperature approaches the transition temperatureTc, the value ofHc2‖(parallel to the layer planes) decreases with a positive curvature, while the value ofHc2⊥(perpendicular to the layer planes) decreases almost linearly. The ratio ofHc2‖toHc2⊥increases monotonically from 2.4 nearTcwith decreasing temperature and reaches the constant value of 3.2 at the lowest temperature. It becomes clear that the simple effective mass model based on the anisotropic Ginzburg-Landau theory does not explain our experimental results. The anisotropic behavior ofHc2can be accounted for by the Takanaka theory, which includes anisotropies of both the Fermi velocity and the energy gap and the effect of nonlocality. Agreement between experimental results and the theoretical prediction is obtained by the use of values of 0.16≲ε12≲0.25 and −0.6≲ε2≲−0.3, where ε1 is the mass anisotropy parameter and ε2 the gap anisotropy parameter. The coupling strength between layers is too strong to be explained by the Josephson phase coupling model proposed for quasi-two-dimensional layer superconductors.

Simultaneous Measurement of Thermal Diffusivity and Conductivity Applied to Bi-2223 Ceramic Superconductors

A method to measure the thermal diffusivity α and the conductivity κ under an identical experimental setup has been developed and α and κ of Bi-2223 oxide superconductor have been measured quasi-simultaneously. The results are analyzed on the basis of the BRT and Tewordt-Wolkhausen theory. The simultaneous measurement makes it possible to estimate the specific heat C and the Debye temperature Θ D , as well as to separate the electron and phonon contributions to the diffusivity. The simultaneous measurement also provides a useful check on the reliability and the consistency of the analyses.

Anisotropy and hysteresis of transport critical currents in high temperature Ln-Y-Ba-Cu-O superconductors

Abstract Following the measurements of anisotropy and hysteresis in transport critical currents with changing temperature and polarity of magnetic field, a.c. magnetic measurements by the Campbell method were performed for sintered Ln-Y-Ba-Cu-O superconductors. The information derived from them indicated that hysteresis does not occur in the intragrain current but in the intergrain one. The results are interpreted in terms of flux pinning, where the anisotropy originates from the texture structure and hysteresis is associated with the superconducting weak links of the sintered oxide pellets.

Anisotropic Thermal Diffusivity and Conductivity of YBCO(123) and YBCO(211) Mixed Crystals. II

The anisotropic thermal diffusivity α of the highly c-axis-oriented Y–Ba–Cu–O bulk superconducting crystals has been measured quasi-simultaneously with the thermal conductivity κ. The estimated values of the specific heat C by use of α and κ values parallel and perpendicular to the c-direction agreed with each other. In these crystals prepared by the modified melt texture growth (MMTG) method, fine Y2BaCuO5 particles are dispersed in the YBa2Cu3O7-x superconducting matrix phase. Based on a simple model, the thermal diffusivity and the specific heat of the YBa2Cu3O7-x matrix phase are separated using independently measured data of a Y2BaCuO5 polycrystal. The influence of Y2BaCuO5 particles on the thermal properties of the mixed crystals is discussed.

Vickers hardness properties of YBCO bulk superconductor at cryogenic temperatures

Abstract Vickers hardness properties of YBCO bulk superconductor have been investigated from 40 to 293 K in this study. Hardness increased up to 18, 000±2500 MPa with decreasing temperature. Furthermore, cracks caused by the indentation process were observed with SEM, and the increase in crack length with decreasing of temperature was obtained. Fracture toughness KIC at cryogenic temperatures was attempted to estimate with Vickers hardness and crack length.

Nb/sub 3/Sn multifilamentary wires with CuNb reinforcing stabilizer

A bronze processed multifilamentary Nb/sub 3/Sn superconducting wire with CuNb reinforcing stabilizer (CuNb/Nb/sub 3/Sn), which exhibits a residual resistance ratio of 20 and a magnetoresistance of 0.2 mu Omega -cm at 23 T and 4.2 K, has been developed. The yield stress at 4.2 K, was noticeably improved; a value of 22 kg/mm/sup 2/ was obtained even after heat treatment of Nb/sub 3/Sn formation at 700 degrees C for 200 h. The effect of strain on critical current density, J/sub c/, was described well by using the upper critical field in the prestrain state. It was verified that the CuNb/Nb/sub 3/Sn wire mechanical and superconducting properties need to perform under an enormous electromagnetic force in high magnetic fields.<<ETX>>

Highly strengthened multifilamentary (Nb,Ti)/sub 3/Sn wires stabilized with CuNb composite

Multifilamentary (Nb,Ti)/sub 3/Sn superconducting wires with reinforcing stabilizer of CuNb composite, CuNb/(Nb,Ti)/sub 3/Sn, were developed. The outstanding point is that the 0.2% proof stress at 4.2 K in CuNb/(Nb,Ti)/sub 3/Sn was extremely improved to 320 MPa even after the heat treatment at 670/spl deg/C for 192 hours, which is 1.8 times higher than that in ordinary Cu/(Nb,Ti)/sub 3/Sn. The overall J/sub c/ value in CuNb/(Nb,Ti)/sub 3/Sn was obtained to be 110 A/mm/sup 2/ at 16 T and 4.2 K. These results show that the bronze processed multifilamentary (Nb,Ti)/sub 3/Sn superconducting wire with CuNb reinforcing stabilizer is actually employable for a high field superconducting magnet. >

CHARACTERISTICS OF AG-AU ALLOY SHEATHED BI-PB-SR-CA-CU-O SUPERCONDUCTING TAPES FOR CURRENT LEADS

Oxide superconductor tape with Ag‐Au sheath was studied for current leads. A 100‐cm‐long current lead of 1000 A with 0.2 W/kA heat leakage was designed using Ag‐11 at. % Au alloy sheath with low thermal conductivity. The overall current density of the superconducting tape is required to be 350–770 A/cm2 at 77 K when the superconductor cross‐section ratio in the tape is 0.35–0.7. Bi‐Pb‐Sr‐Ca‐Cu‐O superconducting tape with Ag‐11 at. % Au alloy sheath was fabricated and the critical current density Jc was measured. The overall‐Jc was 1700 A/cm2 at 77 K under 0 T. The possibility of the current lead of 1000 A under 0.28 T with 0.2 W/kA was made clear.

Time evolution and spatial distribution of temperature in YBCO bulk superconductor after pulse field magnetizing

The time evolutions of temperature rise ?T(t) after pulse field magnetizing have been measured on the surface of a cryo-cooled YBaCuO bulk superconductor. The temperature rise ?T increases with increasing pulse field strength, and a quite large temperature rise (?Tmax ~ 18.8 K) is observed just before the trapped magnetic flux reaches the central region of the bulk. The epoxy resin, impregnated and coated to the bulk crystal for mechanical reinforcement, prevents the heat exhaust towards the cold stage of the refrigerator and it needs much time to recover its initial temperature. The spatial distribution of ?T(t) has indicated that the magnetic fluxes tend to enter into the bulk, avoiding the 4-fold growth sector boundaries of the bulk crystal.