Noritsugu Enomoto

The High Field Magnetic Dependence of Critical Current Density at 4.2 K for Ag-Sheathed Bi2Sr2CaCu2Oy Superconducting Tape

Critical current density (Jc) at 4.2 K and its dependence on applied magnetic field was measured for Ag-sheathed Bi2Sr2CaCu2Oy superconducting tape. The specimen showed a strong grain alignment which could be attained by adjusting the conditions of heat treatment. The degree of crystal orientation (F) was evaluated to be up to 95% by XRD for this specimen. In the case of magnetic field applied in a direction parallel to the tape, the Jc initially decreased with the magnetic field from 1.2×105 A/cm2 at 1 T to 5×104 A/cm2 at 10T and then remained almost constant up to 23 T. As the F value decreased from 95%, Jc also decreased. The pinning force (Fp) of the 95%-F-value tape was estimated to be about 1×1010 N/m3 at 23 T. This value is larger than those of conventional low-Tc superconductors.

Synthesis of Superconductive Oxides by Vacuum Calcination Method

The vacuum calcination method (VCM) was developed to synthesize Ln-Ba-Cu-O group superconductive oxides represented by YBa2Cu3O7-x through solid phase reaction. When Y2O3, BaCO3 and CuO are used as raw materials in the form of powder in this method, the carbon content of the resultant calcinated product will be 1/10 that of the normal methods or lower. VCM powder is a mixture of Y2BaCuO5 and BaCuxOy, an unknown phase, and has extremely high activity because of oxygen content that is lower than the stoichiometric composition of YBa2Cu3O7-x. For this reason, it absorbs oxygen during sintering under an oxidizing atmosphere causing transition to the perovskite phase of YBa2Cu3O7-x, thus indicating excellent sinterability and high Jc and magnetization.

The Transport Critical Current Properties of BSCCO Superconducting Wires

The critical transport current density (Jc) for BSCCO wires were measured at 77K and 4.2K in magnetic field up to 30T. The Jc of wire specimen was significantly improved by this grain orientation of low-Tc phase. Although the maximum Jc at 77K and OT were enhanced up to 3.5×104 A/cm2 consequently, the magnetic field dependence of Jc is not sufficient for a magnet coil and showed large anisotoropy with respect to the magnetic field for pursuing crystal orientation up to over 95% F value. While the Jc at 4.2K and 10T was 2.1×105 A/cm2, which is almost kept constant up to 30T parallel to the tape surface. The Jc anisotropy was dependent on the angle between the magnetic field direction and the tape surface and the Jc reduction at 4.2K in the field normal to the surface remained in about one-half up to 10T. By using this transport property at 4.2K, traial coils of BSCCO superconductor were prepared and tested. Magnetic field of 1220 gauss and 370 gauss were generated by BSCCO coil in the external magnetic field of 0T and 10T, respectively. This indicates the applicability of BSCCO wire for high field superconducting apparatuses.

The Transport Critical Current Property of High Tc Superconducting Wires

The Ag-sheathed high Tc superconducting wires were prepared by the powder- in-tube method. The transport critical current density (Jc) was measured by the four-probe method with 1 μ v/cm criterion. And the magnetic field dependence of transport Jc at 77K and 4.2K were evaluated. In YBCO system, we obtained the Jc of 103 A/cm2 order, but we could not obtain the higher Jc because of the weak link problem. In case of BSCCO system, the Ag-sheathed tapes showed strong grain alignment which could be attained by adjusting the process parameters. So we obtained the maximum Jc of 3.5x104A/cm2 at 77K and 0T. But this Jc decreased drastically under high magnetic field over 0.1T, because of the weak pinning force and resulting flux creep. At 4.2 K, the Jc of BSCCO Ag-sheathed tape under 10T showed 1.7x 105 A/cm2.

Synthesis of Bi-Sr-Ca-Cu-O Superconductor by Vacuum Calcination Method

The vacuum calcination method (VCM) was developed to synthesize Bi-Sr-Ca-Cu-O superconductive oxides through solid phase reaction. When Bi2O3, SrCO3, CaCO3, and CuO are used as raw materials in powder form in this method, the carbon content of the resultant calcined product was lower than that of the conventional method. The VCM process gave a higher ratio of high-Tc phase (110 K class) in comparison with the conventional method.

Fracture origins of optical fibers fabricated by hybridized process

Fracture origins of single-mode optical fibers that were produced by the CIP hybridized process consisting of overcladding a VAD-derived core rod with silica glass made from commercial silica powder were studied. It was found that there were many inclusions that caused failure of the fiber at the screening test. To identify the inclusions, the EPMA technique was successfully used and new internal defects such as a Cr, a Cr-Fe, and a Zr inclusions were found. By investigating the manufacturing process precisely, stainless steel particles and Cr/sub 2/O/sub 3/ particles in the silica powder were found to become origins of the fractures. Further, the removal of Cr/sub 2/O/sub 3/ particles by the reaction with chlorine gas at high temperature was studied, and its dependence on the reaction temperature and on the chlorine content was clarified. It was also suggested that the initial particle size of a Cr/sub 2/O/sub 3/ particle was an important factor for its elimination. The results obtained by this study should be used to remove both the Cr and the Cr-Fe inclusions.

Studies on the strength of optical fiber fabricated by a hybridized process

Strength of single-mode optical fibers that were produced by a hybridized process consisting of overcladding a VAD-derived core rod with silica glass made from commercial silica powder was studied. There were such defects as a Zr-containing inclusion (Zr-inclusion), a bubble and two kinds of Cr-containing inclusions (Cr-inclusions) found in the fiber. These defects caused failure of the fiber at a proof test. In order to make a strong fiber, removal of the defects were studied and the following results were obtained as follows. 1) The Zr-inclusion could be removed by decanting an aqueous suspension of silica powder having an average diameter of 10 /spl mu/m (Silica A). 2) The bubble could be removed by using the decanted Silica A only or by purifying at 1000/spl deg/C. 3). The Cr-inclusions could be removed by purifying at 1000/spl deg/C in helium gas containing more than 30 vol.% chlorine. The strongest fiber fabricated had a break probability as low as 0.01/km in a proof test of 0.7% elongation for 1 s.

Transport Property and Microstructure of Silver-Sheathed Bi-Based Superconductors with High Degree of Crystal Orientation

Critical current density (Jc) and its dependence on applied magnetic field was improved by alignment and homogenization of the low-Tc phase (90K). It was found that transport Jc of wire specimens was strongly and positively related to degree of c-axis orientation (F) measured by XRD and that pursuing higher F value up to around 99%, Jc of 3.5x104A/cm2 was achieved at 77K and 0T. This result shows that grain alignment can supress weak intergranular coupling.