S. E. Dorris

Synthesis of highly pure bismuth-2223 by a two-powder process

Abstract Use of a so-called two-powder process results in rapid formation of a highly pure superconducting bismuth-2223 phase during powder-in-tube processing. In this process, Sr χ Ca 1−χ CuO 2 and Bi 1.8 Pb 0.4 Sr 2−χ Ca 1+χ Cu 2 O 8 are formed as separate intermediate phases, and mixed, then processed together within a silver sheath. Mixtures with χ values of 0.0, 0.25, 0.50, 0.75, and 1.0 are studied in order to probe the relationship between 2223 formation and the melting behavior of 2212. The best results to date have been obtained with the χ=0.0 mixture containing high-melting, strontium-rich 2212 and calcium cuprate. The process is very reproducible and produces nearly pure 2223 in as little as 50 h.

Use of mixed conducting membranes to produce hydrogen by water dissociation

Abstract We have studied the production of hydrogen by water dissociation at moderate temperatures (700–900°C) with novel mixed-conducting membranes. Hydrogen production rates were investigated as a function of temperature, water partial pressure, membrane thickness, and oxygen chemical potential gradient across the membranes. The hydrogen production rate increased with both increasing moisture concentration and oxygen chemical potential gradient. A hydrogen production rate of 6 cm 3 ( STP )/ min cm 2 was measured with a 0.10-mm-thick membrane at 900°C and 50 vol % water vapor on one side of the membrane and 80% hydrogen (balance helium) on the other side. Hydrogen was used as a model gas on one side of the membrane to establish a high oxygen potential gradient; however, another reducing gas, methane, was substituted in one experiment to maintain the high oxygen potential gradient. The hydrogen production rate increased with decreasing membrane thickness, but surface kinetics played an important role as membrane thickness decreased.

On the role of Vickers and Knoop microhardness as a guide to developing high critical current density Ag-clad BSCCO-2223 tapes

Abstract Because there are many parameters influencing the critical current density (Jc) of Ag sheathed (Bi, Pb)2Sr2Ca2Cu3Ox tapes, good and easily measured correlation variables to Jc are highly desirable. Yamada et al. have recently shown that very high Jc values can be obtained when the BSCCO core is rolled to a high Vickers microhardness (Hv) prior to heat treatment. The present experiment extends the correlation by showing that there is a linear relation between the Hv of reacted tapes and Jc. Measurement of the Knoop microhardness (HK) shows that the hardness is highly anisotropic. HK measured with the long indentor axis parallel to the a-b planes shows no dependence on reaction time or Jc, while HK measured parallel to the c-axis is strongly correlated. A high microhardness appears to be directly correlated to a high-density BSCCO core with a small crack density. Such a core has good connectivity which directly translates to a high Jc value.

Mode of lead addition and its effects on phase formation and microstructure development in composite conductors

An extensive examination has been made of silver-sheathed (Bi-2223) composite conductors produced by the two-powder processing method, wherein single-phase (Bi-2212) is combined with a mix of Pb-Ca-Cu-O phases to give an overall stoichiometry of . The principal variable is the proportioning of lead between the Bi-2212 phase and the Pb-Ca-Cu-O phases. X-ray diffraction measurements and scanning electron microscopy analyses have revealed that the length of the induction period prior to the onset of Bi-2223 formation decreases monotonically as the initial lead fraction in the Bi-2212 is increased up to a value of 0.3. The observed decrease in critical current density of fully processed composites with decreasing lead fraction in the Bi-2212 phase appears to be more closely related to the quantity of unconverted Bi-2212 than to the quantity of residual alkaline earth cuprate secondary phases. Transmission electron microscopy shows that the frequency of Bi-2212 intergrowths declines with increasing lead fraction in the starting Bi-2212 component.

Development of coated conductors by inclined substrate deposition

Abstract Inclined substrate deposition (ISD) offers the potential for rapid production of high-quality biaxially textured buffer layers suitable for YBa 2 Cu 3 O 7− δ (YBCO)-coated conductors. We have grown biaxially textured magnesium oxide (MgO) films on Hastelloy C276 (HC) substrates by ISD at deposition rates of 20–100 A/s. Scanning electron microscopy of the ISD MgO films showed columnar grain structures with a roof-tile-shaped surface. X-ray pole figure analysis revealed that the c -axis of the ISD MgO films is titled at an angle ≈32° from the substrate normal. A small full-width at half maximum of ≈9° was observed for the φ -scan of MgO films. YBCO films were grown on ISD MgO buffered HC substrates by pulsed laser deposition and were determined to be biaxially aligned with the c -axis parallel to the substrate normal. The orientation relationship between the ISD template and the top YBCO film was investigated by X-ray pole figure analysis and transmission electron microscopy. A transport critical current density of J c =5.5×10 5 A/cm 2 at 77 K in self-field was measured on a YBCO film that was 0.46-μm thick, 4-mm wide, 10-mm long.

Pulsed laser deposition of YBCO films on ISD MgO buffered metal tapes

Biaxially textured magnesium oxide (MgO) films deposited by inclined-substrate deposition (ISD) are desirable for rapid production of high-quality template layers for YBCO-coated conductors. High-quality YBCO films were grown on ISD MgO buffered metallic substrates by pulsed laser deposition (PLD). Columnar grains with a roof-tile surface structure were observed in the ISD MgO films. X-ray pole figure analysis revealed that the (002) planes of the ISD MgO films are tilted at an angle from the substrate normal. A small full-width at half maximum (FWHM) of ≈9° was observed in the -scan for ISD MgO films deposited at an inclination angle of 55°. In-plane texture in the ISD MgO films developed in the first ≈0.5 μm from the substrate surface, and then stabilized with further increases in film thickness. Yttria-stabilized zirconia and ceria buffer layers were deposited on the ISD MgO grown on metallic substrates prior to the deposition of YBCO by PLD. YBCO films with the c-axis parallel to the substrate normal have a unique orientation relationship with the ISD MgO films. An orientation relationship of YBCO100∥MgO111 and YBCO010∥MgO110 was measured by x-ray pole figure analyses and confirmed by transmission electron microscopy. A Tc of 91 K with a sharp transition and transport Jc of 5.5 × 105 A cm−2 at 77 K in self-field were measured on a YBCO film that was 0.46 μm thick, 4 mm wide and 10 mm long.

The effect of lead content on the critical current density, irreversibility field, and microstructure of Ag-clad Bi 1.8 Pb x Sr 2 Ca 2 Cu 3 O y tapes

We studied the effect of lead content ( x = 0.20−0.40) on the critical current density J c (0 T, 77 K), irreversibility field H * (77 K), and microstructure of monocore, Ag-clad Bi 1.8 Pb x Sr 2 Ca 2 Cu 3 O y (2223) tapes, finding that tapes with lower lead contents ( x = 0.20–0.25) required higher processing temperatures (840 and 832 °C, respectively) to complete 2223 formation, as compared to the optimum 820 °C reaction temperature of the x = 0.30–0.40 tapes. We found that both the zero-field and the in-field properties correlated strongly to the phase purity with J c (0 T, 77 K) reaching a maximum of ˜20 kA/cm 2 for x = 0.30, and then decreasing with increasing lead content to ˜12 kA/cm 2 for x = 0.40. H * (77 K) increased from ˜165 mT at x = 0.20 to ˜265 mT at x = 0.30, then declined to 195 mT at x = 0.40. Optimizing the lead content at x = 0.30 maximized both the connectivity and the flux pinning contributions to the critical current density.

Growth and properties of YBCO-coated conductors fabricated by inclined-substrate deposition

YBCO-coated conductors with high current-carrying capability are desirable for electric power transmission applications. Inclined-substrate deposition (ISD) is capable of producing high-quality biaxially textured template films, which are important for fabrication of YBCO-coated conductors. We have grown biaxially textured ISD-MgO template films on flexible metallic substrates at deposition rates of 2-10 nm/sec. Columnar grains with a roof-tile-shaped surface structure were observed on the ISD-MgO films. X-ray pole figure analysis revealed that the ISD-MgO film is biaxially textured and its c-axis is titled at an angle from the substrate normal. Strontium ruthenium oxide (SRO) buffer films were epitaxially grown on ISD-MgO by pulsed laser deposition prior to the deposition of YBCO. Low /spl phi/-scan full-width at half maximum (FWHM) values of 6/spl deg/ and 7/spl deg/ were observed for YBCO and SRO, respectively. T/sub c/ of 91 K with a sharp transition and transport J/sub c/ over 1.4 MA/cm/sup 2/ at 77 K in self-field were measured on YBCO coated conductors grown with ISD MgO architectures using a SRO buffer.

Controlled decomposition and reformation of the 2223 phase in Ag-clad (Bi,Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub {ital x}} tapes and its influence on the microstructure and critical current density

The decomposition of almost fully reacted (Bi,Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub {ital x}} (BSCCO-2223) tapes caused by heating in 1 atm of pure O{sub 2} at 825{degree}C has been studied. It was found that partially decomposing 2223 tapes to a mixture of Bi{sub 2}Sr{sub 2}Ca{sub 1}Cu{sub 2}O{sub {ital y}}, (Ca,Sr){sub 2}PbO{sub 4}, and other secondary phases reduced the critical current density (77 K, 0 T) from {approximately}20 kA/cm{sup 2} to nearly zero. Reheating the tapes in 7.5{percent} O{sub 2} restored the 2223 phase and, while there was some degradation of the 2223 grain alignment due to residual secondary phase growth, the critical current density was also restored to nearly its original value. We hypothesize that such a decomposition/reformation process can be useful in increasing the connectivity of polycrystalline 2223, by encouraging the formation of a liquid phase which heals residual cracks in the BSCCO core. {copyright} {ital 1996 Materials Research Society.}

Growth and properties of YBCO-coated conductors on biaxially textured MgO films prepared by inclined substrate deposition

YBa2Cu3O7?? (YBCO) films were fabricated on SrRuO3 (SRO)-buffered MgO templates grown on Hastelloy C276 metallic substrates, on which the MgO layers had been deposited by inclined substrate deposition (ISD) using electron beam evaporation. YBCO and SRO films were deposited by pulsed laser deposition (PLD). ISD-MgO substrates fabricated with two different substrate inclination angles (? = 35? and 55?) were used to grow YBCO films. High transport critical current density, Jc = 1.4 ? 106?A?cm?2, has been measured at 77?K in self-field for YBCO film grown on ISD-MgO with ? = 35?, whereas YBCO film grown on ISD-MgO with ? = 55? had a lower Jc = 0.5 ? 106?A?cm?2. X-ray pole figure patterns revealed a cube-on-cube orientation relationship among YBCO, SRO, and ISD-MgO films, with the c-axis of the YBCO film being tilted. X-ray ? and -scans revealed good in-plane and out-of-plane textures of YBCO film grown on ISD-MgO substrate. The YBCO film grown on ISD-MgO with substrate inclination angle ? = 35? had a YBCO(005) -scan full width at half maximum (FWHM) = 5.8? and a YBCO(007) ?-scan FWHM = 2.8?, while YBCO film with ? = 55? had YBCO(005) -scan FWHM = 5.4? and YBCO(007) ?-scan FWHM = 2.6?.