Ronald D. Parrella

HTS wire at commercial performance levels

Short rolled multifilamentary BSCCO-2223 oxide-powder-in-tube (OPIT) wire has reached a core critical current density J/sub c/ over 73,000 A/cm/sup 2/ (77 K, self-field, 1 /spl mu/V/cm) in multiple samples, with engineering (full-cross-section) current density J/sub c/ of 22,800 A/cm/sup 2/ (77 K, self-field, 1 /spl mu/V/cm). Regular production wires several hundred meters long show average engineering current density over 10,000 A/cm/sup 2/ (77 K, self-field, 1 /spl mu/V/cm), a benchmark for commercial electric power applications such as cables and motors. Cost studies indicate that cost-performance below

Enhancement of the 77 K irreversibility field and critical current density of (Bi,Pb)2Sr2Ca 2Cu3Ox tapes by manipulation of the final cooling rate

10/kA-m is attainable for full-scale production levels, Next-generation YBCO-123 coated conductor technology offers further potential cost-performance improvements.

Progress in superconducting performance of rolled multifilamentary Bi-2223 HTS composite conductors

By manipulating the cooling rate from the final heat treatment, we have raised the 77 K, self‐field critical current density (Jc) of multifilament (Bi,Pb)2Sr2Ca2Cu3Ox (2223) tapes by a factor of 3, and the irreversibility field (H*) by more than 50%. The Jc of samples cooled in 7.5% O2 from their reaction temperature of 825 °C increased from ∼8 to ∼24 kA/cm2 and H*(77 K) increased from ∼120 to ∼200 mT as the cooling rate was decreased from 5 to 0.016 °C/min. The results unambiguously show that the flux pinning properties of 2223 tapes can be improved by simple changes in wire processing.

Through-process study of factors controlling the critical current density of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox tapes

Significant enhancements in critical current densities in rolled multifilamentary Bi-2223 HTS composite conductors have been achieved using the powder-in-tube (PIT) technique. At 77 K and self field, oxide critical current densities (J/sub c/) of 55 kA/cm/sup 2/, overall or engineering critical current densities (J/sub e/) of 15 kA/cm/sup 2/, and critical currents (I/sub c/) of 125 A have been achieved in different rolled multifilamentary composites. Progress in achieving such high electrical performance is believed to stem in part from an improvement of grain connectivity by reducing weak links. The J/sub c/ dependence on magnetic field (B) and the degree of c-axis texture of these high quality conductors have been investigated at various temperatures. Our results also demonstrate that the critical current retention in magnetic field can be independently controlled from the self field critical current density, suggesting that flux pinning improvements and weak link reductions can be separately engineered into Bi-2223 composites fabricated using manufacturable processes.

Connectivity and flux pinning improvements in Ag-clad BSCCO-2223 tapes produced by changes in the cooling rate

The connectivity and flux pinning components of the critical current density Jc in a high-Jc Ag-clad, 19-filament (Bi,Pb)2Sr2Ca2Cu3Ox(Bi-2223) composite throughout important steps in its thermomechanical treatment were investigated by mass density measurement, microstructural observation and extensive superconducting property characterization. The mechanical integrity of the filaments was greatest after the first heat treatment, because damage produced by intermediate rolling was not completely erased by subsequent heat treatment. The final relative filament mass density was less than 75%, even though Jc more than tripled to reach ~50 kA cm-2 (77 K, 0 T) after final heat treatment. Filament resistance and Jc data showed that the electrical connectivity improved significantly throughout the process, although ultrasonic fracturing, magneto-optical images and density measurements all showed that the mechanical and physical connectivity degraded. Our study suggests that the attainable Jc of Bi-2223 is determined by the electrical connectivity of each filament which is itself an uncertain compromise between minimizing the significant porosity produced by the Bi-2223 formation reaction and the extensive crack network that porosity reduction by intermediate rolling produces.

Examination of current limiting mechanisms in monocore Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ tape with high critical current density

The rate at which Ag-clad (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x tapes are cooled from their final reaction heat treatment influences both the intergranular connectivity and intragranular flux pinning strength of the polycrystalline filaments. As the cooling rate from 825 °C to 730 °C in 7.5% O 2 was decreased over a range of 5 °C/min to 0.005 °C/min, J c (77 K, 0 T) increased from ∼8 to ∼24 kA/cm 2 , and the irreversibility field increased from, ∼120 to, ∼200 mT. The J c (4.2 K, 0 T) increased in a similar fashion. Cooling slowly also sharpened the critical temperature transition and increased the critical onset temperature from 107 K to 109 K. These improvements in the superconducting properties occurred despite partial decomposition of the (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x phase into non-superconducting impurity phases during the slow cooling. A microstructural basis for these multiple effects is described.

Spatial variations in composition in high-critical-current-density Bi-2223 tapes

Magneto-optical imaging, electromagnetic measurements and SEM have been used for the characterization of a monocore BSCCO tape made by oxide powder-in-tube process which had a transport J/sub c/ (77 K, 0 T) of 37 kA/cm/sup 2/. Our study evaluates the connectivity between superconducting grains and reconstructs the local current distribution at <10 /spl mu/m scale. Some local regions are found to carry up to 180 kA/cm/sup 2/ at 77 K, a new record for BSCCO wires.

Investigation of a multi-set-point first heat treatment methodology for the silver-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox composite conductor

A detailed compositional analysis of high-critical-current-density (J{sub c}) (55 and 65 kA/cm2 at 77 K) (Bi, Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub y} (Bi-2223) tapes was undertaken by energy dispersive spectroscopy in the transmission electron microscope. Structural features were coupled with characteristic compositions of the Bi-2223 phase. The average of all compositional measurements of the Bi-2223 phase was determined to be Bi{sub 1.88}Pb{sub 0.23}Sr{sub 1.96}Ca{sub 1.95}Cu{sub 2.98}O{sub y}. However, spatial variations in the Bi-2223 composition and differing phase equilibria were found throughout the filament structure. In particular, a considerable range of Bi-2223 compositions can be found within a single tape, and the lead content of the Bi-2223 phase is significantly depressed in the vicinity of lead-rich phases. The depletion of lead in the Bi-2223 phase around the 3221 phases may be a current-limiting microstructure in these tapes. (c) 2000 Materials Research Society.

Improving the critical current density in Bi-2223 wires via a reduction of the secondary phase content

A new type of first heat treatment procedure for the silver-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox composite conductor (Ag/Bi-2223) has been investigated. This new procedure consists of using multiple oxygen partial pressure/temperature set point conditions that are systematically varied during the course of the first heat treatment. The present embodiment of this procedure (referred to as thermal slide heat treatment) involves the use of three specific set point conditions (21.0% O2/835 °C, 7.5% O2/825 °C and 4.0% O2/815 °C) appropriately sequenced and tuned to specific time intervals. Application of this procedure as a first heat treatment for powder-in-tube-type Ag/Bi-2223 composite wire specimens was found to produce a minimal amount (<8 area% as seen in cross section views of transverse mounted specimens) of finely divided nonsuperconducting secondary phases in the Bi-2223 filament cores. Also, a direct correlation was observed between the amount of nonsuperconducting secondary phase contained in the Ag/Bi-2223 specimens at the end of the first heat treatment and the performance (in terms of JC) of the fully processed specimen.

Time evolution of phase composition and microstructure in the Ag/Bi-2223 composite superconductor heat-treated at specific pO2/temperature set points

Progress in the performance of multifilamentary composite Bi-2223 wire fabricated at American Superconductor is reviewed. Critical current (I/sub c/) performance of 175 A at 77 K and self-field and critical current density (J/sub c/) performance of 22.1 kA/cm/sup 2/ (77 K, 0.1 T perpendicular to the tape plane) have been measured for short samples of Bi-2223 wire (standard size is 0.21/spl times/4.0/spl times/100 mm). Microstructure analysis by SQUID magnetometry, transmission XRD and TEM shows a large amount of residual 2212 intergrowth remaining in Bi-2223 grains. Residual 2212 intergrowth is seen to correlate with self-field J/sub c/ at 77 K across a range of experimental samples, suggesting that higher levels of performance will be achieved by further reducing residual 2212.