G. N. Riley

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

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.

Enhancement of transport critical current densities at 75 K in (Bi,Pb)2Sr2Ca2Cu3Oy/Ag tapes by means of fission tracks from irradiation by 0.8 GeV protons

The transport critical current density Jc of oxide‐powder‐in‐tube mono‐ and multifilamentary Bi‐2223/Ag tapes has been determined before and after irradiation by 0.8 GeV protons at fluences up to 7.0×1016 protons/cm2. Proton‐induced fission of the Bi nuclei produced up to 8.6×1013 fissions/cm3, creating long tracks at densities equivalent to matching fields up to 1.1 T. Relative to unirradiated tapes, Jc values at 75 and 64 K show no decrease in self field, indicating no breakdown of intergranular coupling, and show large, dose‐dependent enhancements in magnetic fields oriented parallel to the tape normal.

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

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.

Development of twisted high‐temperature superconductor composite conductors

Multifilamentary high‐temperature superconductor (HTS) composite conductors have been developed for alternating current (ac) applications. A twisted HTS conductor containing the Bi‐2223 phase fabricated using a modified powder‐in‐tube technique is reported. Transport critical current densities of 13 800 and 10 900 A/cm 2 (77 K, self‐field, 1 μV/cm) have been achieved for twisted tape and wire conductors with twist pitches of 3.7 and 3.6 mm, respectively. These conductors are strongly linked and are thus suitable for use in ac applications.

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

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.

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

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>>

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

Abstract The kinetics of phase evolution and the development of microstructure in silver-clad composite conductors containing a Bi-Pb-Sr-Ca-Cu-O powder with a nominal stoichiometry of Bi 1.7 Pb 0.4 Sr 2 Ca 2 -Cu 3 O 10+δ were studied as a function of oxygen partial pressure ( pO 2 ), temperature, powder stoichiometry, and powder pre-treatment. The optimum temperature for growth kinetics of the Bi-2223 phase increased as the oxygen partial pressure was increased from 0.02 to 0.21 atm. The width of the phase stability region was largest for pO 2 = 0.075 atm and smallest for pO 2 = 0.21 atm. It was found that for a 25-h heat treatment, the reaction kinetics were the fastest in 0.075 atm of oxygen at temperatures between 825° and 830°C. The secondary phase particles (observed by scanning electron microscopy) were smaller for lower pO 2 (0.02 and 0.075 atm) than for higher pO 2 (0.13 and 0.21 atm).

Advances in the design and fabrication of high-Tc composite conductors

Abstract Advances in the performance of high-temperature superconductor (HTS) multifilamentary composite conductors have been dramatic. Using the power in tube technique, conductor and filament critical current densities of 6700 and 26,500 A/cm 2 (77 K, self field) have been achieved, respectively, for these technologically interesting conductors. Moreover, the powder in tube approach provides a cost-effective method for the manufacture of high-performance HTS composite conductors. Using oxide precursor powders, average engineering, or conductor, critical current densities of 3100 A/cm 2 (77 K, self field) have been achieved over long (∼ 100 m) lengths with a high measure of performance uniformity.

Stability and growth of the (Bi,Pb)2Sr2Ca2Cu3Ox phase in a silver sheath

Abstract The chemical stability and growth kinetics of the (Bi,Pb)2Sr2Ca2Cu3Ox (2223) phase have been investigated as a function of temperature and composition using x-ray diffraction, scanning electron microscopy and wavelength dispersive x-ray spectroscopy. Silver tubes were filled with precursor 2223 powders and processed into tapes using conventional deformation techniques. The tapes were processed at temperatures ranging from 810 to 830°C for times ranging from 10 to 12,000 minutes in an oxygen partial pressure of 0.075 atm. The 2223 phase shows limited non-stoichiometry of Cu, (Ca+Sr), and (Bi+Pb), but relatively broad solid solubility within the alkaline earths and heavy metals. The kinetics of transformation from Bi2Sr2Ca1Cu2Ox to the 2223 phase appear to be controlled by a two dimensional diffusion process.