The role of annealing conditions on the equilibrium defect state of the material was investigated by computer controlled 4 probe d.c. conductivity and thermoelectric power measurements on dense homogeneous ceramics prepared from citrate precursors. At elevated temperatures, the conductivity first decreases and then increases exponentially with decreasing temperature with the conductivity minima shifting to lower temperatures with decreasing PO2. This unusual behavior is described as representing a transition from n-type to p-type conduction. The n-p transition is correlated with the corresponding stoichiometry shift upon cooling from oxygen deficient (Cu1, Cu2) to oxygen excess YBa3Cu3O6–5+x, (Cu2+, Cu3+). A preliminary defect model is proposed which provides a framework for understanding a number of the experimental observations.

A large crystal of flux-grown La2CuO4, examined through single-crystal neutron diffraction, was found to have an orthorhombic superstructure derived from the K2NiF4 structure type with a=5.359(1), b=5.363(1), c=13.127(2) Å. The pattern of systematically absent intensities indicated space group Pccn but meaningful refinement of thermal vibration parameters could not be obtained in this symmetry. Systematic examination of all possible orthorhombic subgroups of Fmmm yielded satisfactory refinement to R(F2)=2.57% only in space group Bmab. Incorporation in the crystal of Li derived from the flux results in an orthorhombic distortion smaller than found for pure La2CuO4. Refinement of the Cu site occupancy provided Cu0.95Li0.050(3), in excellent accord with a bulk chemical analysis. Reflections which violate the systematic absences of Bmab are attributed to a smaller volume fraction of domains in orientation twinned by 90° rotation about c. This interpretation is supported by transmission electron microscopy which revealed intergrown lamellae. Dark field imaging with a forbidden reflection illuminated only the twin lamellae.

The high resolution Bitter Pattern technique has been used to reveal the structure of the array of flux lines which is present when single crystal samples of the high Tc superconductor YBa2Cu3O7 are placed in a magnetic field. At 4.2K with the magnetic field parallel to the c axis, the patterns formed are strongly reminiscent of the analogous structures in ordinary type II superconductors. The structures typically consist of flux spots with short range hexagonal correlations. We interpret the patterns to consist of singly quantized vortices. Very uniform patterns are observed in thin, flat samples cooled in a constant field. Sampling the spot density yields a vortex density consistent with a flux quantum of hc/2e.

Experiments at 77K, the only other temperature studied, showed no spatially varying magnetic structure. Recent mechanical measurements which are sensitive to the bulk modulus and dissipation within the vortex lattice show evidence for a mobility transition near 75K, which would be consistent with this observation. We believe this stems from materials properties rather than some more fundamental property of the new superconductors.

This study presents two methods for preparing the High Tc superconducting YBa2Cu3O7−x compound. Aqueous nitrate solutions consisting of Yttrium, Barium and Copper were mixed, dried, blended and the resulting homogeneous mixture calcined at 700–900°C for 2–4h to develop the superconducting (1,2,3) phase. As an alternative, oxalates of Yttrium, Copper and Barium were coprecipitated from the mixed metal nitrate solution using (Et3NH)2C2O4. The resulting pale blue coprecipitate was dried and calcined in air at 700–900°C/2–4h. Both methods provided high homogeneity precursors for the High Tc YBa2Cu3O7−x superconductor. Resistance measurements, SEM, x-ray and VSM were used to characterize the microstructures and properties.

Superconducting films of Ba2RECu3O7−x were prepared from aqueous and aqueous-alcoholic mixed solutions of barium, rare earth (RE), and copper nitrates. The solutions were sprayed onto heated (100) orientated MgO, ZrO2 —9%Y2O3 and SrTiO3 substrates to form a dehydrated nitrate precursor; subsequent thermal treatment yielded the superconducting 2:1:3 compound. Earlier magnetic film studies with aqueous-alcoholic mixed solutions of rare-earth iron garnet precursors showed that alcohol improves the spreading and drying of solutions on substrates. Here, however, alcohol significantly lowers the solubility of Ba(NO3)2. Some compositions tend to phase segregate during film preparation. A key objective of this study was to understand reaction paths of the precursors in order to optimize the furnace annealing schedule. Film and substrate reactions complicate the process, especially for SrTiO3, which otherwise has a good lattice match with films. Analytical methods used were Knudsen effusion mass spectrometry, thermogravimetric analysis, x-ray diffraction, secondary ion mass spectroscopy, electron microscopy and temperature-dependent resistivity. The best samples showed a Tc of 95K and had zero resistivity below 89K. Advantages of this process include: simplicity of equipment, short deposition time; ease of making composition adjustments; and possible deposition on complex shapes.

This paper reports the findings of a phase-equilibrium study of the Y-Ba-Cu-O quaternary system in which the oxygen activity was monitored and controlled by a solid-state ionic technique. A stabilized zirconia electrolyte was used to titrate oxygen in or out of the sample and to monitor the oxygen activity inside a sealed sample chamber.

Compositions at or near YBa2Cu3Ox were studied at elevated temperatures in a wide range of oxygen pressures. All compositions indicated the presence of an invariant polyphase reaction- e.g., at 1 × 10-3 atm of oxygen pressure at 850 °C-- below which the superconducting phase becomes thermodynamically unstable and undergoes a decomposition reaction. Complementary x-ray diffraction analysis of samples prepared under well-defined oxygen pressures identified the phases present and, along with the oxygen coulometric titration data, allowed the construction of a portion of the relevant quarternary phase diagram.

The question of valence of Cu and rare-earth atoms in the newly discovered oxide superconductors with high transition temperatures is crucial to the understanding of their electronic structure. We have measured the X-Ray Absorption Near Edge Structure (XANES) of Cu K transition and Pr Ljjj transition to obtain information on the valence of Cu, and Pr in La2−xSrxCuO4 and Y1−xPrxBa2Cu3O7-δ. We will present the experimental results, compare them with calculations of absorption cross-section for model atomic clusters, and discuss the valence of Cu as a function of oxygen concentration.

The resistivity, Hall, and thermopower tensors are calculated for the normal state of oxide superconductors on the assumption of band quasiparticle behavior. The shape of the resistivity ραβin the metallic a-b plane is consistent with ordinary impurity and electron-phonon scattering, but the magnitude is larger than predicted. The Hall tensor is predicted to be hole-like for orbits in the a-b plane but electron-like for a-c or b-c orbits, while the thermopower is predicted to be electron-like in the a-b plane and hole-like along the c-axis. Single crystal experiments have confirmed some of these predictions for the Hall tensor.

The application of superconducting material to a magnet requires the winding tape or wire conductor with sufficient flexibility into a solenoid. Tape conductors of Ba2YCu3O7-y were successfully prepared by a new technique of surface reaction. Cu tape or Cu-electroplated Ag tape was coated with a fine mixture of Ba2CO3, and Y2O3 and then heat treated at about 900°C to form Ba2YCu3O7-y oxide layer with high density on both surfaces of the tape. Zero resistance state has been achieved at the temperature above liquid nitrogen temperature for Ag-substrate tape. The critical current density is about 40A/cm at 77K and zero magnetic field. Although, at present, the superconducting properties of these tapes are somewhat inferior to those of conventional powder metallurgy processed conductors, the surface reaction process has the potential feasibility of processing Ba2YCu3O7-y into a long-length ductile tape desirable for practical application.

In the first section of this paper the sputter deposition of thin films of YBa2Cu3O7−x is considered. In the second section low and high temperature heat treatments are discussed in the light of the thermodynamics of the material and in the third the development, fabrication and current-voltage (IV) characteristics of a variety of device structures are described, including the hysterctic characteristics of certain YBa2Cu3O7−x / YBa2Cu3O7−x devices that reveal Josephson supcrcurrcnt tunnelling.

Sintered compacts of magnetically aligned single-crystal particles have been studied by x-ray, microscopy, and magnetic measurements. Though significant alignment and very anisotropie magnetic hysteresis were obtained, the magnitude of the hysteresis indicates, through the critical state model, that the bulk critical current density remains low.

Defect structures in YBa2Cu3O7−x produced by electron irradiation at 300 K were investigated by transmission electron microscopy. Threshold energies for the production of visible defects were determined to be 152 keV and 131 keV (+ 7 keV) in directions near the a and b (b>a) axes (both perpendicular to c, the long axis in the orthorhombic structure), respectively. During above threshold irradiations in an electron flux of 3×1018 cm-2 s-1, extended defects were observed to form and grow to sizes of 10–50 nm over 1000 s in material thicknesses 20–200 nm. Such low electron threshold energies suggest oxygen atom displacements with recoil energies near 20 eV. The observation of movement of twin boundaries during irradiation just above threshold suggests movement of the basal plane oxygen atoms by direct displacement or defect migration processes. Crystals irradiated above threshold were observed after about 24 hours to have transformed to a structure heavily faulted on planes perpendicular to the c axis.

We studied grain boundaries in calcined and sintered YBa2Cu3Ox transmission electron microscopy (TEM) and found complex Ba-Cu oxides. The complex oxide was polycrystalline and composed of grains smaller than 100 nm in diameter and it often included minor phases rich in Y. Some stoichiomet-ric oxides such as BaCuO2, CuO and BaO were also observed along grain boundaries.

We report a strong field-dependence (from 0.1% to 71%) of flux expulsion (Meissner effect) in a single crystal of YBa2Cu3Ox in magnetic fields from 36 kOe down to 50 mOe. The data is discussed in the context of flux pinning forces and glassy behavior.

Polycrystalline samples of YBa2Cu2O7-δ with a variety of surface treatments show differences in absolute reflectance and width of phonon lines. Samples that are not polished and are measured immediately after annealing have largely grains with the c axis normal to the surface. Such oriented samples show a gap-like depression of conductivity in the far infrared that sets in below the superconducting transition temperature but no true gap. Phonon lines at 195 cm−1 and at 155 cm−1 narrow in the superconducting state, in analogy with the effect of the electron phonon interaction in BCS superconductors. In the normal state the background conductivity is Drude like with a plasma frequency of 0.75 eV and a relaxation rate of 200 cm−1. The extrapolated far-infrared conductivity agrees with the measured dc conductivity.

Transmission and scanning electron microscopy were used to study some features of twins in the orthorhombic phase of Y1Ba2Cu3O7-Y compounds. A model that explains the formation and proliferation of thene defectr, is proposed. Some conclusions about the kinetics of formation of these defects are stated, which agree with observed changes in twin densities produced by different cooling rates. Electron energy loss spectrometry (EELS) was ucod to measure relative changes in oxygen, which give some information about the relative value and the stability of oxygen content in twinned regions. A Braggs-Williams model of the order-disorder transition, for oxygen occupancies in the Cu-O2 planes, predicts an enrichment of vacancies at the boundaries in these oxygen deficient perovskites. This could explain the observed deficiency in oxygen content with respect to the perfect Y1Ba2Cu3O7 stoichio-metry.

Barium yttrium cuprate powders were prepared by a sol-gel emulsion process in which an aqueous nitrate solution containing Ba, Y, and Cu in the correct stoichiometry was emulsified in heptane. The resulting micrometer-sized droplets were gelled by adding a high-molecular-weight primary amine that extracts nitric acid from the aqueous phase and raises the pH of the droplets. Thermal gravimetric analysis revealed that the correct overall stoichiometry was obtained at temperatures lower than 400 °C during calcination, but X-ray diffraction showed that calcination at temperatures greater than 800° may be required to produce the correct phase. One possible advantage of this technique is the control of powder morphology on a micrometer scale so that powders can be prepared for advanced ceramics processing methods. Also, segregation of components, if any, will be restricted to distances less than one micrometer.

The recent discovery of high temperature superconductivity in YBa2Cu3O7 has led to intense activity in the processing of these oxides. The earliest attempts to synthesize these materials were based on solid state routes. These routes do not produce highly sinterable powders. Solution routes can potentially produce much more sinterable powders. We have investigated several synthesis routes - both solid state and solution. The details of these will be presented elsewhere(1). Here, we contrast the physical properties and the densification behavior of powders made via solid state and solution routes. We also present preliminary results on the microstructural characterization and electrical properties of the sintered samples.

Polycrystalline black coloured Ba2Cu3PrO7−x is prepared from a mixture of analytical grade BaO2, CuO, Cu metal, and Pr 2O3 (2 : 2 : 1 : 0.5), which are ground, pelletized, and heated in evacuated quartz ampoules at 1273 K for 4 days, and then slowly cooled.

X-ray powder diffraction data show that Ba2Cu3PrO7−x is orthorhombic, a = 3.823(2), b = 3.877(2), c= 11.793(5) Å, space group Pmmm (no. 47), Z = 1. The structure of Ba2Cu3PrO7−x is refined from 37 reflections by using a least-squares method up to Rc = 0.044 and Rw = 0.041. Structural data and main interatomic distances are given.

La-Ba-Cu-O and Y-Ba-Cu-O superconducting systems have been successfully prepared by gel techniques with high degree of chemical homogeneity. The precursor gel was synthesized from mixture of the corresponding metallic nitrates and di-carboxylic acids. The sintered oxides prepared from the different dicarboxylic acids were all Tc = 90K perovskite superconductors. However differences in electrical and magnetic properties were also observed. Effects due to the different acids elucidated by magnetic susceptibility measurements and X-ray diffraction analysis will be discussed.