B. Seebacher

The high-temperature reactions of YBa2Cu3O7-δ

Abstract Real-time observations of the YBa 2 Cu 3 O 7− δ (123) melting sequence in air and oxygen were made using high-temperature XRD techniques and high-temperature optical microscopy. Results obtained from these analyses show a surprising surface peritectic reaction before the melting peritectic reported from quench studies. The present data indicate the probable existence of Y 2 Cu 2 O 5 between about 700 and 940°C, in air, but not oxygen, due to reaction between the 123 and CO 2 . The first liquid phase reaction beginning at about 940°C, is a slow surface reaction producing BaCuO 2 . This reaction has been misinterpreted as an impurity effect by most other workers. BaCuO 2 continues to grow in concentration until about 1020°C where it and the remaining 123 rapidly disappear during the peritectic reaction forming Y 2 BaCuO 5 (211) and liquid. At approximately 1150°C, Y 2 O 3 begins to form from the 211 phase. The kinetics of this reaction are much slower than the 1020°C peritectic melting.

Investigation of formation reactions in the system Bi-Pb-Sr-Ca-Cu-oxide starting from oxides and carbonates

Abstract We were able to follow the formation reaction of high-temperature superconductors in the system Bi-Pb-Sr-Ca-Cu-O by using in-situ high-temperature X-ray diffraction. We first observed reactions in the confined system Bi-Sr-Cu-O, and then investigated modifications in the reaction path introduced by lead and calcium. We found that the addition of Ca does not modify reactions below 850°C but lowers the melting point of Bi2Sr2CuO6+x, Calcium also extends the stability range of the latter phase which, in contrast to Ca-free samples, does not decompose before melting. The addition of lead lowers the formation temperature of some intermediate phases, and introduces strontium lead oxide as a new phase stable up to 880°C. The addition of calcium and lead leads to the formation of very stable Ca2PbO4 early in the process (550°C). This reaction consumes, depending on stoichiometry, most of the lead. We found that Ca2PbO4 decomposes when the high-temperature superconductor Bi2Sr2CaCu2O8+x is formed above 830°C. Up to this temperature, Ca2PbO4 acts therefore as a reservoir for calcium and keeps the lead oxide from acting as a flux.

High-temperature X-ray diffraction study of the peritectic reactions of Bi-2212 with and without Ag additions

Abstract High-temperature X-ray diffraction (HTXRD) was used to determine the peritectic melting sequence of Bi2Sr2CaCu2O8 (Bi-2212) and Bi-2212 + 20 wt.% Ag thick films on MgO substrates. The residual carbon concentration of all samples was reduced to 1600 ppm by careful thermal treatment before the HTXRD analyses. Analysis of samples quenched in oil using XRD and EMPA supports the HTXRD results. Lattice-parameter analyses were used to determine the compositions of solid solutions present in the reaction sequence. In the case of pure Bi-2212, two phases form immediately on melting at 870°C, namely (Cax, Sr1−x)CuO2 (1:1) and an unidentified phase. The unidentified phase decomposes by 900°C, while the 1:1 phase is present up to 920°C. (Ca1.4Sr0.6)CuO3 (2:1) forms by 880°C, immediately after the 1:1 phase, and decomposes by 930°C, while (Ca0.96Sr0.04)O appears at 900°C, and coexists with the liquid up to at least 1350°C. Only slight changes in the solid-solution compositions were found with 20 wt.% Ag added to the system, with the phases and stability regions remaining the same.

The valence of copper in YBa2Cu3O6.9 — an X-ray absorption study

Abstract The Cu K absorption spectra of YBa 2 Cu 3 O 6.9 and of a number of mono-, di- and trivalent copper model compounds have been measured at 77 K and at room temperature. YBa 2 Cu 3 O 6.9 is found to contain 2 divalent Cu ions with a K-edge similar to that in Y 2 BaCuO 5 (but dissimilar to that in CuO), with 0.84 chemically trivalent Cu ions as in KCuO 2 and with 0.16 monovalent Cu ions as in Cu 2 O. The Cu K-edge of the superconductor cannot be fitted with only divalent copper. The nature of chemically trivalent copper in KCuO 2 and in the superconductor is discussed.

Electrical characterisation of single grain boundaries in DyBa2Cu3Ox ceramics

Abstract Single grain boundaries in polycrystalline DyBa 2 Cu 3 O x bulk were isolated. Measurements of their electrical properties show, that they act as Josephson junctions and that their properties can be described by the RSJ model. The current distribution inside the junction is inhomogeneous and there is evidence for small areas with high current density. A model for calculating I c ( T ) taking into account a depression of the order parameter Δ at the superconductor insulator interface is introduced. Calculations using a distinct depression of Δ are fitting the experimental data very well. Finally, it is shown that the grain boundaries also exhibit the AC-Josephson effect.

X-ray absorption spectroscopy of YBa2Cu3O6.90

Abstract The Cu K-absorption spectra of YBa 2 Cu 3 O 6.90 and a number of mono-, di- and trivalent copper model compounds have been measured. YBa 2 Cu 3 O 6.90 is found to contain 2 divalent Cu ions as in Y 2 BaCuO 5 (but dissimilar to CuO), 0.84 chemically trivalent Cu ions as in KCuO 2 and 0.16 monovalent Cu ions as in Cu 2 O. The nature of chemically trivalent copper in KCuO 2 is discussed.

a−b anisotropy properties of millimeter-sized monodomain YBa2Cu3O7-x single crystals

Abstract Millimeter-sized monodomain YBa 2 Cu 3 O 7- x single crystals have been obtained both by growth from self-flux of the oxides (under the naturally occurring uniaxial strain in the solidifie d flux) as well as with a strain-annealing procedure. In the a − b plane, these crystals show a small B cl -anisotropy and large pinning-anisotropies. At low temperatures the B cl values are enhanced by surface pinning effects.

Superconductivity in the system Bi-Sr-Ca-Cu-O

Ceramic specimens of nominal composition BiSrCaCu 2 O x and BiSrCaCu 3 O x show a sharp decrease of electrical resistance and inductance with zero resistance temperature between 50 to 75 K. For samples with the nominal composition BiSrCaCu 2 O x a two-step drop is observed for certain sintering conditions. By extrapolation one gets a hypothetical zero resistance temperature of 100–105 K. Samples with the nominal composition Bi 2 Sr 2 CaCu 2 O x show a decrease of resistance and inductance with extrapolated values for hypothetical zero resistance temperatures of 73–90 K. But there remains a finite value of resistance down to 15 K.