Takayuki Miyatake

Nuclear spin-lattice relaxation and Knight shift in YBa2Cu4O8

Abstract The temperature dependences of the 63Cu nuclear spin-lattice relaxation rate 1/T1 and Knight shift K in YBa2Cu4O8 have been measured by nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) techniques. In the normal state, both the 1 T 1 for the Cu(2) sites and the in-plane component of the Knight shift K2⊥ increase rapidly with temperature from Tc up to about 250 K. Approximate T-linear temperature dependence of 1 T 1 was observed in a wide temperature range between 250 and 680 K, that is in general accord with previous analyses.

Preparation and superconducting properties of YBa2Cu4O8

Abstract Samples of YBa2Cu4O8 are prepared by means of a new high oxygen pressure technique employing oxygen-HIP. Both magnetization and resistivity measurements show the superconductivity transition at 82.5 K. The lower and upper critical fields of YBa2Cu4O8 are obtained from the magnetization measurements. The Ginzburg-Landau parameters, ζ(0)=17 A , λ(0)=2400 A , are estimated from these results.

Pressure dependence of superconducting transition temperature in Ca-doped YBa2Cu4O8

Abstract Measurements of the superconducting transition temperature T c in Y 1− x Ca x Ba 2 Cu 4 O 8 ( x =0.0, 0.025 and 0.1) are carried out under hydrostatic pressure up to 1.5 GPa. As in other hole-doped superconductors, T c increases with pressure, and the pressure coefficients of T c get lower for higher T c compounds. These variations of d T c /d p seems to be correlated with a difference between the hole concentration in the Cu-O plane and that in the Cu-O chain.

Single crystal growth and characterization of Bi2Sr2Ca1-xYxCu2Oy by TSFZ method

Abstract Single crystals of Bi2Sr2Ca1−xYxCu2Oy were grown by means of the Traveling Solvent Floating Zone (TSFZ) method. Several single crystals with an average size of 0.5mm×2mm× 10μm were obtained by cleaving the boules. Observation by EPMA shows that the ab-plane of the samples are smooth and of single phase. X-ray diffraction data show that the lattice constant along the c-axis decreases with increasing the Y content as reported for sintered materials.

Critical current density and microstructure of YBa2Cu4O8

We investigated the intragranular critical current density Jc in YBa2Cu4O8 prepared by two routes with different oxygen pressures at various temperatures and applied fields. A relationship between the Jc’s and the microstructures observed by transmission electron microscopy is discussed. The magnitude of Jc for the low oxygen processed sample is higher by a factor of 5 than that for the high oxygen pressure sample. Transmission electron microscopy reveals that the sample prepared at high oxygen pressure shows an almost perfect stacking sequence, but the low oxygen pressure prepared one has a severely distorted structure. We conclude that the difference in Jc(H) for the samples must be related to the different microstructures. Jc at a fixed magnetic field shows an exponential temperature dependence similar to that seen in other high‐Tc superconductors. The field dependence is Jc∝H−n with n between 0.5 and 1.5, and with the n value becoming larger at higher temperatures. This type of the behavior may arise ...

Critical current densities of Y1-xCaxBa2Cu4O8 (x = 0 and 0.1) superconducting ceramics

From magnetization measurements of Y1-xCaxBa2Cu4O8 superconducting ceramics with critical temperatures Tc of 83 K for x=0.0 and 91 K for x=0.1, the critical current densities, Jc(H), at 5 K and 77 K are obtained by employing the Bean critical-state model. The Jc at 77 K is evaluated to be 1.5×103 A/cm3 at 2 kOe for the sample of x=0.1. Compared with YBa2Cu3O7, the Jc(H) values in Y1-xCaxBa2Cu4O8 are lower and more sensitive to the magnetic field. These Jc(H) properties are discussed on the basis of the superconducting properties, i.e., critical fields and Ginzburg-Landau parameters.

High-resolution transmission electron microscopy of defects in YBa2Cu4O8

Microscopic structures of the high-Tc superconductor YBa2Cu4O8 (Tc=80 K) are examined by high-resolution transmission electron microscopy. Planar defects are observed as white spot lines in the micrograph of the single-phase YBa2Cu4O8 and they have a structure with an absence of one Cu-O chain in the double Cu-O chain of YBa2Cu4O8. YBa2Cu3O7 and Y2Ba4Cu7O15-like local structures exist around the planar defects.

Effect of heat treatment on critical current density and n-value of (Nb,Ti)/sub 3/Sn multifilamentary superconducting wire

Studies on the relationship between n-value and heat treatment conditions of Nb/sub 3/Sn multifilamentary wires in addition to those on be are presented. The wire was manufactured in bronze process. It was heated in various conditions, then voltage-current (V-I) characteristics of each specimen were measured. In the range of the heat treatment temperature, n-value increases with temperature but J/sub c/ decreases. The properties were discussed based on the results of microscopic observation and were investigated with upper critical field (Bc/sub 2/). The results from these investigations suggest the dependence of n-value on Bc/sub 2/.<<ETX>>

Development of Nb 3 Sn Superconducting Wires for High-field Magnets

Synopsis: Research and development activities and some recent results related to Nb3Sn superconducting wires produced by Kobe Steel, Ltd. and Japan Superconductor Technology Inc. (JASTEC) are introduced. An outline of the activities is described from a historical point of view. Improvements in the characteristics (i.e., critical current density (Jc), n-value and mechanical properties) of bronze-processed Nb3Sn wires are reviewed. Finally, the status of development for the Ta-Sn powder-in-tube (TS-PIT) process newly proposed by Tachikawa is described.

Improvement of Critical Current Density in the Bronze-Processed Nb3Sn Superconductor

The superconductors with bronze matrix of Cu-15wt%Sn-0.3wt%Ti were fabricated on a commercial scale. The number of filaments and Cu to non Cu ratio of the conductors were 25099 and 0.3, respectively. A non Cu Jc of 980 A/mm2 at 12 T, 4.2 K was obtained for a conductor with a round cross section in diameter of 0.7 mm. A rectangular shaped conductor, i. e. 1.75 mm × 3.50 mm, showed a non Cu Je of 133 A/mm2 at 21 T and 1.8 K, corresponding to a critical current of 620 A. The dependence of non Cu Jc on heat treatment conditions is also reported. These characteristics are considered with results from microscopic observations and relations with upper critical fields based on the pinning properties.