J. Shimoyama

Universal relationship between crystallinity and irreversibility field of MgB2

The relationship between irreversibility field, Hirr, and crystallinity of MgB2 bulks including carbon substituted samples was studied. The Hirr was found to increase with an increase of full width at half maximum (FWHM) of MgB2 (110) peak, which corresponds to distortion of honeycomb boron sheet, and their universal correlation was discovered even including carbon substituted samples. Excellent Jc characteristics under high magnetic fields were observed in samples with large FWHM of (110) due to the enhanced intraband scattering and strengthened grain boundary flux pinning. The relationship between crystallinity and Hirr can explain the large variation of Hirr for MgB2 bulks, tapes, single crystals, and thin films.

The Bi-2223 Superconducting Wires With 200A-Class Critical Current

Critical current of a silver-sheathed Bi-2223 tape using controlled-overpressure sintering has attained 201 A, which is equivalent to 493 A per 10 mm-width. The engineering current density is 21kA/cm2 , while the critical current density on the total filaments area is 56 . Critical currents are also improved noticeably at low temperatures and high magnetic fields normal to the tape plane. Typical critical currents have reached 500 A at 4.2 K in 5 T and 200 A at 20 K in 10 T. The highest critical currents of long wires currently have reached 195 A for 70 m and 191 A for 178 m. The wires with 200 A-class critical current are similar in mechanical properties to 150 A-class long wires commercially produced by Sumitomo Electric using the controlled-overpressure sintering. The microstructure study has predicted far higher potential of critical current, according to the confirmed remaining issues on the 200 A-class tapes; incomplete orientation order, aggregation of alkaline earth cuprates and inter-granular Bi-2212.

Catalytic effect of silver addition on the low temperature phase formation of MgB2

A small amount of silver addition was found to dramatically decrease reaction temperature of magnesium and boron forming the MgB2 bulk without degradations in both critical temperature and critical current properties. A sample with ~3% silver addition for magnesium heated at 550 °C for 72 h recorded a high Jc above 2 × 105 A cm−2 at 20 K in low fields reflecting its strongly grain-coupled microstructure. In addition, a sample with ~5% silver addition showed Jc of ~1 × 105 A cm−2 at 20 K in low fields even for heating at 500 °C for 72 h. Although the added silver formed Ag–Mg alloy after the heat treatment, these impurity particles mainly existed at the edge of voids without losing the effective current path. Our present results will contribute to developing low cost MgB2 conductors.

DI-BSCCO wire with Ic over 200 A at 77 K

The highest Ic of a latest DI-BSCCO® tape, which is the high performance silver sheathed Bi2223 tapes produced with CT-OP® technique, has reached 210 A (500A/cm-width) at 77K, self-field. The wide-range measurement of Ic using temperature variable system also has proved the highest performance at each temperature and external magnetic field. The combination of CT-OP® and stainless steel lamination techniques has concurrently obtained Ic higher than 200 A, rated tensile stress more than 300 MPa and rated tensile strain more than 0.4 % at 77 K. A texture analysis using synchrotron radiation has revealed explicit correlation between Jc and c-axis misalignment angle in CT-OP® processed tapes.

The Current Transport Properties of a 200 A-Class Bi-2223 Superconducting Wire at Various Temperatures and Magnetic Fields

Silver-sheathed Bi-2223 multi-filamentary tapes using controlled-overpressure sintering have attained the highest critical current of 200 A-class at 77 K, self field. The 200 A-class Bi-2223 tape has been evaluated on the critical currents at various temperatures from 4.2 K to 90 K and magnetic fields up to 12 T applied perpendicularly to the tape plane. The critical currents are 500 A at 4.2 K, 5 T, 370 A at 20 K, 3 T and 200 A at 20 K, 10 T typically. This is an improvement by a factor of 1.3-1.5 compared to current commercial tapes produced using controlled-overpressure sintering.

Bi-axial magnetic orientation in a twinned ErBa2Cu3Oy superconductor by controlling grain size

Bi-axial magnetic orientation in ErBa2Cu3Oy (Er123) superconductor with a twinned microstructure was demonstrated. A ball-milling process effectively improved the degree of the in-plane and c-axis orientation of Er123 powder samples oriented in a modulated rotating magnetic field of 10?T. From the relationship between the particle size and the degree of orientation, it is concluded that pulverization induced the appearance of the in-plane magnetic anisotropy of Er123 grains through inhomogenization of two different types of domains in their grains. The present study presents a production technique of RE123 bulk magnets and coated conductors without epitaxy technology.

Generic positive effects of low level impurity doping on flux pinning properties of HTSC and MgB/sub 2/

Low level impurity doping effects on flux pinning properties have systematically studied for Bi(Pb)2212 single crystals, Y123 melt-solidified bulk and MgB/sub 2/ polycrystalline bulks. In all these systems, moderately dilute impurity doping was found to be effective for improving critical current properties without losing their original T/sub c/. For Bi(Pb)2212, iron, cobalt, nickel and yttrium were effective dopant for enhancement of J/sub c/ particularly under low fields. Second peak effect in J/sub c/-H properties of Y123 were observed in Sr doped Y123 bulk. These positive effects were happened only when mean distance of impurity ions was longer than the vortex core size, 2/spl xi//sub ab/. In the case of MgB/sub 2/, both aluminum doping for magnesium and carbon doping for boron improved J/sub c/ with their low doping levels less than 1%.

Recent Progress in High Performance Ag-Sheathed Bi2223 Wire (DI-BSCCO®)

Sumitomo Electric has been developing the silver-sheathed Bi2223 multi-filamentary wires since the discovery of Bi-based superconductors. DI-BSCCO is the high performance wires produced using the controlled-overpressure (CT-OP) sintering technique. The present commercial DI-BSCCO can provide the uniform high critical current, Ic, up to 180 A with length over 2000 m, and recently 200 A were succeeded to be obtained by the same kind of 1000 m length wires, resulting from the improvement and control of the microstructure in Bi2223 multi-filaments. The short trial wires of several meters have exhibited the highest Ic over 240 A at 77K in self-field (corresponding to 580 A per 1 cm-width). Besides, the optimization of carrier density after CT-OP led to further enhancement of Ic, reached 250 A. All the derivative products also have uniform critical current properties over entire length even after lamination with the reinforcing metals. The performances of DI-BSCCO can meet the growing needs for various application of high temperature superconductor like high in-field applications, such as magnets and motors. The recent progress in transport properties of commercial DI-BSCCO and R&D short trial wires is shown.

Switching current distributions and subgap structures of underdoped (Hg,Re)Ba2Ca2Cu3O8+δ intrinsic Josephson junctions

We have investigated the intrinsic Josephson properties in slightly underdoped (Hg,Re)Ba2Ca2Cu3Oy [Hg(Re)1223] intrinsic Josephson junctions (IJJs) with a dimension of 1.0×1.5×0.11 μm3. The current-voltage characteristics of the IJJs exhibit clear multiple branches with subgap structures similar to those of other cuprate superconductors. The switching current distributions P(I) from the zero-voltage to the nonzero-voltage state in the current-biased IJJs agree well with the theoretical curves of the thermally assisted escape model at temperatures above ≈5 K. The plasma frequency fp of the IJJs is estimated to be 1.3 THz from the fluctuation-free critical current density of 2.0×105 A/cm2, which is one of the highest among cuprate superconductors, reflecting the high Tc and a relatively low anisotropy of the Re doped Hg system. The P(I) gradually becomes independent of temperature below ≈5 K, which suggests a crossover of the escape process from thermal activation to quantum tunneling at such a high tempera...

Dramatic effects of Ag addition on low temperature synthesis of MgB2

We have recently discovered that small amount Ag addition is effective in lowering formation temperature of MgB2 from a powder mixture Mg and B. In the present paper, synthesis and superconducting properties of Ag-added and Ag- and SiC-added MgB2 bulks, Ag-added MgB2 tapes and a dense Ag-added MgB2 bulk are summarized. C-substitution for the MgB2 phase was confirmed to occur even for a sample reacted at 550°C by Ag-addition. For in-situ Fe-sheathed MgB2 tapes, reaction temperature to obtain strongly grain coupled MgB2 core was decreased down to 500°C by Ag-addition. Moreover, a dense MgB2 bulk was synthesized by the PICT-diffusion method performed at 700°C for 72 h. In all these studies, Ag-addition was found to be effective to decrease the processing temperatures without serious degradation of Tcand critical current properties. Our results suggest that small amount of Ag-addition is a promising way to develop low cost and high performance MgB2materials.