The problem of separating YBCO crystals from solidified residual melts could be solved by systematic growth experiments. Free crystals up to 7×7 mm2 area and about 0.1 mm thickness could by obtained from a BaO-CuO solvent and small alumina crucibles. Superconductivity was revealed by levitation and Meissner effect.

Optical microscopy and transmission electron microscopy show that most of the crystalline grains in Y1Ba2Cu3O7−x superconducting specimens have large number of twins. These bands of different image contrast have similar microdiffraction patterns. However the diffraction conditions are different and correspond to different tilts of the crystalline grains along one of the zone axis. HREM images of these twin boundaries indicate that the boundary area has appreciable dimensions and both matrix and twin crystals show the existence of a superstructure with double of the common periodicity obtained under [001] diffraction conditions.

A quinary-compound system consisting of Y, Ba, Cu, O, and S has been prepared in our laboratory. The nominal composition of Y, Ba, Cu, and O is similar to the well known material YBa2Cu3O7-y; the addition of S is intended to partially substitute the O sites. X-ray absorption measurements show that the lattice is distorted with the presence of S atoms. The transition temperature is found to decrease from 93K to about 77K with an increasing S content up to a S/Cu concentration ratio near 0.25; further increase of S results in unstable superconducting behavior and very broad resistive transition.

The local structure around Ba atoms in the compound superconductor Y-Ba-Cu-O has been studied by measuring the extended x-ray absorption fine structure (EXAFS) near the Ba K edge. This technique is used to investigate the changes in the short range order structure when the system is doped with impurities.

The results of Bednorz and Muller [1] and Wu and Chu [2] have stimulated a tremendous volume of research on ceramic superconductors [3–7]. Despite a number of other synthetic techniques, the solid state reaction remains the choice of most workers due to its simplicity [4,5,7]: CuO, Y2O3 and BaCO3 are often used in the powder synthesis of Ba2YCu3O7−x. BaO2 is known to be a useful reagent in the synthesis of compounds containing higher oxidation states of metals [8]. Therefore, in our study of the possible role of Cu3+ in superconducting perovskites, we also use BaO2 in the solid state reaction. This paper compares reaction chemistry, material processing and material characterization using BaCO3or BaO2 in the solid state reaction. We obtain greater mass density, increased sample homogeneity, lower resistance, and improved reproducibility for material prepared using BaO2. We would like to point out that other groups have produced Ba2YCu3O7−x using BaO2 [4,7]; Calestani and Rizzoli have reported a one-step process for producing single crystals of Ba2YCu3O7−x using BaO2 [9].

One method for the synthesis of superconducting oxides is high temperature oxidation of a metallic alloy (a metallic precursor) that contains the metallic constituents of the oxide. In addition, a noble metal can be added to the precursor such that, after oxidation, a two phase, finely divided oxide/metal composite results. Superconducting oxides have been produced from Eu-Ba-Cu and Yb-Ba-Cu alloys by oxidizing the alloys at 300°C followed by an “oxygen anneal” at elevated temperatures. Similarly, a Eu1,Ba2Cu3O7−x /Au composite has been formed from a quaternary metallic precursor. each case an onset of superconductivity occurred at about 92K. This approach to producing superconducting oxides is felt to have a number of advantages associated with it, not the least of which is the improvement of the mechanical properties of the superconducting product.

Superconductivity of YBa2Cu3FxOy system prepared by using a mixed solid state and gel method. A consistent and non-zero resistance drop near 280K was achieved in the nominal YBa2Cu3FxOy system for x=l and 2, with complete loss of resistance at 90K. The metal-semiconductor transition occurs at x value between 2 and 3. Temporal room temperature superconductivity with zero resistance was observed in a YBa2Cu3F2Oy sample at 308K. The phenomenon lasted for three minutes and was not reproducible.

Samples of YBa2Cu3O7−x were prepared by solid state reaction of Y2O3, BaO2, and CuO powders mixed in stoichiometric proportions. Powders reacted at 900°C-925°C in air were pressed at 35 MPa into rectangular bars and sintered at 950°C in air for six hours. Slow cooling at 50°C/hour promoted complete transformation to the orthorhombic structure. The effects of subsequent oxidation processing on superconducting properties were also investigated. Oxidation anneals at one atmosphere pressure in flowing O2over the temperature range from 300 to 550°C were performed. Results indicate that oxidation at 450°C optimized the superconducting transition temperature at 91K with a 2-degree transition width. Subsequent high-pressure oxidation (2000 psi O2) at 200°C for one week significantly degraded the superconducting characteristics. Results based on i udometric titration, thermogravimetric analysis, and x-ray photoelectron spectroscopy suggest that the copper valency is about 2.3 and oxygen stoi chiometry is 6.94 (x = 0.06) in samples processed to optimize superconducting properties.

Raman spectra of several samples of YBa2Cu3O7 presented, correlated with resistivity and structural data, and compared with previous results. Laser heating to ∼1100°C in air is shown to produce a surface layer of Y2O3 and Y2BaCuO5. Similar heating of La2−x(Sr,Ba)xCuO4 produces a layer of La2O3. All of these compounds are readily identified by their Raman spectra.

A precipitation process is described for the preparation of powders that can be thermally decomposed to form high critical temperature superconductors such as YBa2Cu3O7. In the process, a cationic solution (a concentrated chloride or nitrate solution) is instantaneously mixed with an anionie solution (a mixture of tetramethylammonium hydroxide and carbonate) to produce a metal-hydroxycarbonate precipitate having the metal stoichiometry of the desired superconducting oxide. The calcining and sintering of the precipitates is critical in controlling the structural integrity and morphology of the superconducting ceramics made from the chem-prep powders, as well as controlling superconducting properties. Under appropriate conditions, high density (>95%) materials that exhibit good superconducting characteristics can be prepared with the chem-prep powders.

Air sintering and subsequent oxygen annealing at high temperatures are integral to the production of ceramic superconducting YBa2Cu3O7-δ. We present results directed toward optimizing the temperature for these two process steps, using static magnetization to determine the intragranular properties. Surprisingly, pure oxygen stabilizes the material against decomposition at temperatures near the melting point, allowing the production of significantly higher quality single phase material.

The effects of implanted fluorine on the resistivity of high temperature superconductors and its diffusion within the superconductors have been studied in an attempt to duplicate a recent report of enhanced critical temperatures. Implanted fluorine was found to have no effect on the critical temperature and, upon heating, the fluorine diffuses out of the near surface.

YBa2Cu3O7-δ single crystals and epitaxial films are grown and characterized. In flux growth for bulk crystals, effects of growth conditions on yield and electric resistivity of crystals are examined. The yield of flaky crystals depends on the formation of cavities. The transition temperature is 86 K after annealing in an oxygen atmosphere. Films are prepared on SrTiO3 by sputtering and epitaxial growth is confirmed by high resolution electron microscopy. Periodic lattice defects are observed near the interface between the substrate and the film. It seems that these defects result from the diffusion of impurities from the substrate.

Since their discovery last yearf[1], the high Tc super-conductors have been the subject of intense investigation. Most researchers have studied the material as ceramics--sintered masses of randomly oriented grains of the material. The materials, however, are anisotropie, so measurements made on ceramic pellets only result in averages over the crystallographic directions.

High Tc superconducting YBa2Cu3O7−x ceramics were prepared from the appropriate oxide constituents and heat treated under various conditions to determine the effects of annealing on superconducting properties. Sintered samples (900–920°C) were air cooled at 5°/min followed by annealing in oxygen or air at 500°C/12 hr. Resistance measurements, SQUID, SEM, and x-ray analysis were used to characterize the samples, which were of 38% porosity. Results showed all samples to be superconducting with TC(O) 84K. Superconducting properties of the air and oxygen annealed samples were equivalent but showed lower overall resistance and 30% higher superconducting phase (30 to 70 K) compared to the air cooled samples.

A STUDY OF THE Y-Ba-Cu-Ag-O SYSTEM: The specimens studied were produced using the standard ceramic technique. High purity source materials Y2O3, BaCO3CuO, and Ag2O were used. Solid state synthesis was carried out at 900 C for six hours in oxygen ambient. Pressed samples of 16 mm diameter and 2 mm thick were processed in an atmosphere of pure oxygen at 950 C for 12 hours. The transition temperature to the superconducting state was measured by the four point probe resistance technique. The ac magnetic susceptibility was determined by the method outlined in [1].

Sintering at temperatures in excess of the melting point for followed by soaking at 980 C for extended periods results in domain-type microstructure and grain orientation in samples that demonstrate zero resistivity and significant Meissner effect.

The structural, magnetic and electrical properties of EuBa2Cu3O7-δ have been investigated both for bulk ceramics and screen printed thick films. The bulk ceramic reaches zero resistance at 92 K, and has large intragranular critical current density inferred from magnetization loops; the directly measured critical current density is substantially lower because of the weak links (Josephson junctions) between grains. Screen printed thick films on strontium titanate have poor magnetic and resistive transitions with evidence for two phases. High temperature oxygen anneals produce a bulk ceramic with significantly enhanced magnetic properties.

Thick (20–300 /im) freestanding films of ceramic YBa2Cu3O0–9 have been obtained by resintering YBa2Cu3O0–9 powder in air after screen printing onto fused silica substrates. An interfacial reaction occurs between the powder and the silica substrate which efficiently debonds the resintered film upon cooling. The magnetization and transport properties of these films are comparable to those for high quality bulk ceramic samples.

Superconducting thin films of high Tc cuprates have been prepared using wet chemical precursors, which are spun on a substrate, pyrolyzed and heat-treated. This new method is simple and inexpensive, and is easily amenable to scale-up to cover large areas. No vacuum equipment is required. The technique allows easy manipulation of film stoichiometry and gives films having excellent compositional uniformity. A 1600 A film of Ba2YCu3O7 prepared by this method had a room temperature resistivity of 500/xfzcm, Tc/(onset) of 90K, and R=0 at 58K. The preparation and electrical and microstructural characterization of thin films of Ba2YCu3O7 and other high TC cuprates will be discussed.