N. Merchant

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.

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.

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).

Phase formation and microstructure development in silver-clad Bi-2223 multifilament composite conductors

Phase evolution and microstructure development in silver-clad Bi-2223 composite conductors have been investigated as a function of filament count (mono, 19, 85 and 361), precursor powder type (metallic powder or oxide powder), oxygen partial pressure (<10-3 to 0.13 atm), temperature (790-855 °C) and time (0-6000 min). Favourable conditions for rapid Bi-2223 phase formation and stability were achieved with an oxygen partial pressure around 0.08 atm, regardless of precursor type or filament count. In general, the rate of the Bi-2223 formation reaction increased and the width of the temperature window for rapid kinetics became broader as the filament count increased. The most rapid kinetics and the broadest heat treatment window were observed for the 361-filament metallic powder composite. These trends in Bi-2223 formation rate and stability are believed to be associated with the increased proximity of powder to silver as the filament count increases and the filament transverse dimensions become smaller. For each conductor type (in terms of precursor form and filament type) there was an optimum processing temperature in ca 0.08 atm O2 that produced a well-developed Bi-2223 grain colony microstructure with the least amount of second-phase. This optimum temperature was near the peak of the reaction rate versus temperature profile.

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.

Epitaxial growth of YbBa 2 Cu 3 O 7−δ films on (100)-oriented MgO and SrTiO 3 substrates by oxidation of a liquid alloy precursor

Textured superconducting films of YbBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} were grown on single crystals of MgO (100) and SrTiO{sub 3} (100) by oxidation of a liquid alloy precursor. The substrates were coated by dipping them in molten YbBa{sub 2}Cu{sub 3} (m.p. {similar to}870 {degree}C). After removal from the melt, the liquid layers on the substrates were oxidized in pure oxygen to form the tetragonal oxide phase, i.e., YbBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, then annealed at 500 {degree}C to obtain the superconducting orthorhombic phase of the same compound. The microstructure of the films obtained in this way was found to be related to the nature of the substrate as well as to processing variables that included oxidation temperature and oxidation time. Films grown on MgO (100) showed {ital c}-axis texture as well as a random growth structure. Films prepared on SrTiO{sub 3} (100) showed either a {ital c}-axis texture or a mixture of {ital c}-axis and {ital a}-axis texture. The superconducting properties of the as-prepared films and the effects of key process parameters on film quality and microstructure are presented and discussed.

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

The time evolution of the phases present in the ceramic cores of silver-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox (Ag/Bi-2223) multifilament superconducting tapes heat-treated at selected oxygen partial pressure/temperature (pO2/T) set points was investigated using scanning electron microscopy and energy dispersive spectroscopy coupled with computer-based image processing methods. The numerical values for individual phase contents and non-superconducting second phase (NSP) size distributions generated in this way were used to quantify the temporal evolution of composition in the variably treated Ag/Bi-2223 tapes. Results for the three pO2/T set points investigated (21.0% O2/835 °C, 7.5% O2/825 °C and 4.0% O2/815 °C) revealed characteristic patterns of recurring maxima and minima in the time evolution of the NSPs. (Ca, Sr)14Cu24O41 was found to be the stable phase at 21.0% O2/835 °C, possibly evolving as a co-product of the Bi-2223 formation reaction, while (Ca, Sr)2CuO3 and CuO were stable at 4.0% O2/815 °C, presumably as a consequence of competitive secondary reactions proceeding in parallel with Bi-2223 formation. The best-aligned grains were formed in Ag/Bi-2223 tapes treated at 21.0% O2/835 °C, while the best conversion to Bi-2223, together with the least amount of NSP particles >0.5 μm in major dimension, was produced by the 7.5% O2/825 °C treatment. A forward-looking conclusion of the study is that it appears possible to minimize the amount and size of NSP particles during Ag/Bi-2223 heat treatment by using a combinatorial sequence of appropriately timed pO2/T set points.

Growth of c-axis-oriented films of YbBa2Cu3O7−δ on single and polycrystalline MgO substrates by oxidation of a liquid alloy precursor

Abstract Textured superconducting films of YbBa 2 Cu 3 O 7–δ supported on both single and polycrystalline MgO substrates were prepared by oxidation of a liquid precursor alloy. The substrates were coated by dipping them into the molten alloy (YbBa 2 Cu 3 , melting point ≈870°C). After withdrawal from the melt, the adhering metal was oxidized to the corresponding oxide phase, i.e., Y YbBa 2 Cu 3 O 7−δ , which exhibited a superconducting transition at ≈80 K following annealing in pure O 2 at 500°C. With MgO(1 0 0) substrates, evidence was seen for the epitaxial growth of YbBa 2 Cu 3 O 7−δ crystals having their c -axis parallel to the [1 0 0] direction of the substrate. For polycrystalline MgO, X-ray diffraction and microstructural examination showed that the high- T c crystallites in the films were also oriented with their c -axis perpendicular to the substrate surface, but the directions of the a and b axes were disordered rather than epitaxial. The superconducting properties of as-prepared films are discussed together with the effects of key processing parameters on composition and microstructure.