William L. Carter

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

Kinetics and mechanism of the (B1,Pb)2Sr2Ca2Cu3O10 formation reaction in silver-sheathed pires

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

Critical issues in the OPIT processing of high-Jc BSCCO superconductors

Abstract A detailed kinetic and mechanistic analysis of the growth of the (Bi 2− x Pb x )Sr 2 Ca 2 Cu 3 O 10 phase in silver-sheathed wires has been performed by an isothermal equilibration method. Silver tubes loaded with precursor powders were processed into wires using established metallurgical techniques. The wire specimens were immersed in a preheated equilibration apparatus, heat-treated at the desired temperature in 7.5% O 2 , for varying periods of time, then quenched in a room-temperature silicone oil bath. The results indicated that the kinetic data followed a nucleation growth mode1 derived for a reaction at the interface between thin sheets and a fine powder or a fluid. Transmission electron microscopy confirmed the two-dimensional reaction geometry and revealed the presence of an amorphous phase at grain boundaries, where rapid transport diffusion appears to occur due to the absence of the stabilizing influence of the regular lattice. A reduction in activation energy was observed at temperatures 2819°C which is tentatively attributed to the onset of a liquid-phase-controlled reaction (i.e. a phase boundary crossing). The effects of powder processing parameters and precursor particle size on the kinetic behavior and the growth rate of the (Bi 2− Pb x )Sr 2 Ca 2 Cu 3 O 10 phase are also discussed.

Thermostability and decomposition of the (Bi,Pb)2Sr2Ca2Cu3O10 phase in silver‐clad tapes

The oxide-powder-in-tube (OPIT) method has produced superconducting wires or tapes that exhibit high critical current densities in high magnetic fields when the superconducting oxide is the two-layer compound Bi2Sr2Ca1Cu2Oy or the three-layer compound Bi2−xPbxSr2Ca2Cu3Oy. For good properties to be obtained, however, numerous critical processing issues must be addressed.

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

The stability of the Bi2−xPbxSr2Ca2Cu3O10 (Pb‐2223) phase contained in silver‐sheathed oxide‐powder‐in‐tube specimens has been investigated by x‐ray diffraction, transmission electron microscopy, and energy dispersive x‐ray analysis. Silver tubes loaded with Pb‐2223 precursor powders were processed into tapes using established metallurgical techniques. The tapes were heat‐treated in a specially designed equilibration apparatus at selected temperatures (800–845 °C) for a range of times (10–5500 min) and quenched in liquid gallium held at ∼40 °C. The results showed that the Pb‐2223 phase is stable in a limited temperature interval between 810 and 830 °C in 7.5% oxygen. At 800 °C, this phase decomposes to Bi2Sr2CaCu2O8 (2212), Ca2PbO4, and CuO; while at temperatures ≥840 °C it partially melts with precipitation of Bi2Sr2CuO6 (2201) and Ca2CuO3. The effects of the silver cladding on the Pb‐2223 phase stability and microstructure are also discussed.

Influence of silver cladding on the formation and alignment of the (Bi2−xPbx)Sr2Ca2Cu3O10+δ phase

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.

Magneto‐optical imaging of flux patterns in multifilamentary (BiPb)2Sr2Ca2Cu3Ox composite conductors

Cross‐section transmission electron microscopy was used to investigate the interaction of silver with (Bi,Pb)‐Sr‐Ca‐Cu‐O phases during the formation of (Bi2−xPbx)Sr2Ca2Cu3O10+δ (Bi‐2223) in a silver‐sheathed wire containing a powder composed of (Bi,Pb)2Sr2CaCu2O8+δ (Bi‐2212) plus second phases. Observations of the interfacial regions of samples quenched at different stages of conversion revealed that (1) Bi‐2212 is initially in direct contact with silver, with the (001) planes parallel to the interface; (2) an amorphous layer between Bi‐2212 and silver appears during the induction period that precedes the conversion reaction; and (3) Bi‐2223 is detected at the silver/powder interface hundreds of minutes before it begins to appear in regions of the powder away from the interface. The implications of these results are presented and discussed.

Advances in the development of silver sheathed (Bi,Pb)2223 composite conductors

We present a study of the superconducting morphology of the transport current carrying cross section of a 19‐filament (BiPb)2Ca2Cu3Ox (Bi‐2223) composite conductor using magneto‐optical imaging of magnetic flux patterns. In conjunction with electron microscopy on the same sample this technique allows a unique correlation of superconducting and microstructural properties. Direct evidence for enhanced superconducting properties in platelike regions along the silver/Bi‐2223 interface and for weak properties near the core of the filaments is obtained. Misaligned grain colonies are found to cause an interruption of the superconducting continuity in the filaments.

Phase chemistry and microstructure evolution in silver-clad (Bi/sub 2-x/Pb/sub x/)Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ wires

Significant advances have been made in the critical current density of the technologically interesting (Bi,Pb)2223 composite conductors. Engineering current density, J/sub e/, of 9100 A/cm/sup 2/, corresponding to a filament current density, J/sub c/, of 32600 A/cm/sup 2/ (77 K, self-field) has been achieved by increasing the fraction of oxide in the conductor and improving the thermomechanical processing of the conductor. The filament microstructure may be further optimized by decreasing the volume fraction of secondary phases and porosity. The strain tolerance of the higher oxide fraction conductors remains adequate for applications by increasing the number of filaments in the conductors. Composite tapes with twisted filaments have been produced that have promising electrical properties in applied magnetic fields.<<ETX>>

Effects of oxygen pressure on phase evolution and microstructural development in silver-clad (Bi,Pb)2Sr2Ca2Cu3O10+δ composite conductors

The reaction kinetics and mechanism that control the conversion of (Bi,Pb)/sub 2/Sr/sub 2/CaCu/sub 2/O/sub z/ (Bi-2212)+alkaline earth cuprates to (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ (Bi-2223) in silver-clad wires were investigated as a function of equilibration temperature and time at a fixed oxygen partial pressure (7.5% O/sub 2/). Measured values for the fractional conversion of Bi-2223 versus time were evaluated based on the Avrami equation. SEM studies of partially and fully converted wires suggest: (1) the transformation to Bi-2223 is two-dimensional and controlled by a diffusion process; (2) liquid phases are present during part of the Bi-2212 to Bi-2223 conversion; and (3) growth of the (Sr,Ca)/sub 14/Cu/sub 24/O/sub 41/ phase accompanies Bi-2223 formation.<<ETX>>