The severity of degradation of YBa2Cu3O7−x from exposure to an aqueous phase has the following dependence: wet steam (condensed thin film) > bulk fluid phase > dry steam, other variables being constant. Also, the surface reactivity increases with oxygen deficiency. It is found that the early stages of degradation are associated with surface hydroxylation and preferential extraction of Ba. Subsequently, dissolution takes place, followed by solution saturation and sequential precipitation, nucleation, and growth of secondary phases. These observations form the basis of a model which incorporates surface hydroxylation and OH− penetration of the bulk, formation of planar defects and extraction of Ba, lattice dissolution, and mobility and solubility constraints.

A novel process for the continuous densification of YBa2Cu3O7−δ wires by infrared zone melting is presented. Superconducting ceramic wires with moderate porosity (20–40%) were passed through a high temperature infrared zone at a speed of 1–2 cm/min, which resulted in zone melting followed by fast solidification to give near zero porosity wires. The atmospheric conditions during zone melting had a significant effect upon the resultant properties and microstructure of the wires. The best results were obtained for infrared zone melting under a vacuum that resulted in the decomposition of the superconducting phase into four phases without significant macroscopic chemical segregation across the wire. Subsequent conventional sintering and annealing treatment of such wires fully restored them to polycrystalline YBa2Cu3O7−δ wires having 96% of the theoretical density. The maximum transport critical current density obtained for such wires at 77 K in zero magnetic field was 2.65 × 103 A cm−2. Normalized critical current values as a function of magnetic field indicated that the quality of the intergrain connections for vacuum infrared zone melted wires was significantly better than that for conventionally processed wires and tapes.

Microstructures of two MPMG processed YBaCuO materials with and without Y2BaCuO5 (211) inclusions were investigated by transmission electron microscopy. Using the MPMG process, it is possible to change the quantity of the 211 inclusions in the YBa2Cu3O7 (123) matrix. We prepared two YBaCuO samples with 0 and 30 vol. % 211 and with respective critical current density values of 2000 and 30 000 A/cm2 at 77 K and 1 T (magnetic field parallel to the c-axis). As possible pinning centers, we found stacking faults in the 123 matrix. However, we observed no appreciable change in their number and structure by introducing the 211 inclusions. Therefore, the difference in Jc values can be attributed to the 211 inclusion itself.

We introduce a novel containerless melt-spin processing technique for YBa2Cu3O7−x using prereacted oxide powders that are rapidly melted and subsequentially quenched in a controlled atmosphere. This results in flakes with typical dimensions of 3000 × 150 × 15 μm3. Powder XRD indicates that all samples contain Y2O3, but that the Ba–Cu–O phases present depend on the processing parameters. SEM and TEM studies show these phases are finely dispersed: the typical grain size for Y2O3 is 1–2 μm, and for the Ba–Cu–O phases it is <0.5 μm.

Superconducting YBa2Cu3O7−x fibers were fabricated by the sol-gel method using modified metal alkoxides. To prepare a heterometallic alkoxide as a precursor of a homogeneous and viscous sol, Y and Cu alkoxides were modified with ethyl acetoacetate (EAA) and ethylenediamine (en), respectively. Their structures were determined by FT-IR and EXAFS. The heterometallic alkoxide was synthesized by partial hydrolysis of the modified alkoxides and was further hydrolyzed and concentrated to obtain a viscous sol. This sol had good spinability and was spun into gel fibers by a spinning apparatus. The gel fibers had a high oxide content of 61 wt.%. The fiber, fired at 950 °C for 10 h in O2, had only closed pores and a dense structure. Tc (onset) of this fiber was 94 K.

We have examined the atomic structure of growth interfaces in thin films of Y–Ba–Cu–O grown on [001] perovskite or cubic substrates. At substrate heater temperatures in the range of 780–820 °C c-axis oriented growth is observed on these substrates. On SrTiO3, the first layer appears to be either a BaO or a CuO2 plane while on LaAlO3 the first layer appears to be a CuO chain layer. The mismatch on the a-b plane is accommodated by the formation of interface dislocations. Defects on the substrate surface propagate as defects in the film. These defects are primarily translational boundaries and in some cases second phases. At lower substrate heater temperatures, i.e., 650–700 °C, a, b-axis growth dominates. Defects and steps on the substrate surface are more detrimental in the growth of a, b-axis oriented films, since they tend to favor the nucleation of c-axis oriented domains. This is ascribed to the ledge mechanism of c-axis film growth, for which the surface steps are good nucleation sites.

YBa2Cu3O7−δ has an internal friction peak near 210 °C measured at 1.18 Hz. The shift in the temperature of this peak with frequency as measured with different internal friction techniques indicates that the relaxation process has an average activation energy of 1.14 ± 0.05 eV and a pre-exponential factor, τ0 = 1.5 × 10−13s (log τ0 = −12.82 ± 0.23). The peak height decreases during annealing at above 375 °C in helium and increases when annealed in oxygen. The mechanism for the relaxation is believed to be the stress-induced motion of oxygen atoms in the Cu–O plane. Detailed analysis shows that the internal friction peak is determined by a spectrum of activation energies; this spectrum is a symmetrical single peak 0.12 eV wide at half-height. Models for the relaxation are considered and the resulting diffusivities are compared to tracer diffusion results.

An alternative synthetic route for obtaining bulk forms of the high temperature superconductor YBa2Cu3O7−x has been investigated. The approach is based on first preparing the phases Y2Cu2O5 and BaCuO2, followed by a single sintering of an appropriate mixture of these intermediate compounds to produce the superconducting phase. The resulting materials are largely single-phase as shown by x-ray diffraction, and have densities as high as 86% of the theoretical value, and superconducting onset temperatures of 93 K with magnetic shielding factors ranging from 0.85 to 1.02 (±0.05). Metallography and scanning electron microscopy data were also obtained on the best (high Tc, high shielding factors) of the samples. This synthesis approach is believed to be simpler, more reproducible, and has the potential of producing better materials than previously used bulk synthesis methods.

Superconducting properties are studied for glass-ceramics which were prepared by reheating glass rods and the glass powder compacts in the BiSrCaCu2Al0.5Ox system, respectively. The glass-ceramic rod specimens obtained by reheating rod glass at 800–830 °C for 50 h have a Tc (R = 0) of 85 K, while the disk specimens obtained by reheating the powdered glass compacts in the same way do not exhibit superconductivity above 77 K. This difference in superconductivity between the specimens is discussed in terms of the crystallization process and the amount of oxygen absorption of the specimens during heating.

Electrical properties of Y–Ba–Cu–O superconductors, such as the critical current density and the zero-resistance temperature, are systematically improved by changing the compression pressure during the formation process of the specimens. Also, by changing the silver powder content of the superconducting Y–Ba–Cu–O powder, the critical current density and the zero-resistance temperature are remarkably improved. The resistance-drop temperature is insensitive to changes in the compression pressure, silver content, and magnetic field, if the field is applied perpendicular to the specimens. The critical current density of the specimens with and without silver decreases exponentially with the perpendicular magnetic field. The critical current densities of specimens without silver showed much lower sensitivity to perpendicularly applied magnetic fields than those of the specimens having a silver content. Along with the above improvements, the present paper also examines several factors that affect the superconducting characteristics. These factors include impurities in the air and strong magnetic fields.

Powder x-ray diffraction, d.c. and a.c. susceptibilities, and SEM have been used to study (Bi1−xPbx)2Sr2Ca2Cu3O10−δ fibers grown by the Laser Floating Zone method. The well-oriented, long-grained superconductor fiber properties are shown to be highly dependent on the partial pressure of oxygen in the growth atmosphere, as well as on fiber pulling rate. Slowly grown fibers contain initially the 2212 (80 K) phase; the 2223 (110 K) phase also appears upon annealing in air. Faster growth rates result in fibers that contain a mixture of the 2212 and 2201 phases and, in this case, long annealing procedures are necessary to observe the 2223 phase.

Electron spectroscopies were used to study the reaction of Si and SiO2 layers with high-Tc superconductors at room temperature and under annealing conditions. The superconductor samples include YBa2Cu3O7 ceramic and thin film samples as well as GdBa2Cu3O7 ceramic samples. The results show that the Si overlayers withdraw oxygen from the superconductor and form an interfacial layer of Si oxide at room temperature. Annealing increases the reaction rate so that films as thick as 30 Å become completely oxidized following annealing above approximately 100 °C. Ba segregates to the surface from the bulk after Si oxidation when annealed at temperatures higher than 200 °C, while the rare earth element (Gd) does not segregate.

Fifty-two faceted grain boundary segments ([001] tilt boundaries) in clusters of bulk-scale YBa2Cu3O7−x crystals having coincident c-axes were characterized by optical microscopy techniques. Grain boundary orientations were widely distributed. All grain boundaries, except those far from the symmetric condition and those with {110} facets, exhibited well-developed matching of the twin domains across the boundary. It is suggested that this newly reported phenomenon of twin pattern matching occurs due to a local coordination of the tetragonal → orthorhombic transformation strain across grain boundaries and may be beneficial with regard to electrical transport in highly textured polycrystalline material.

The purpose of this study was to (1) examine the spatial (within-sample) uniformity of superconducting behavior and microstructure in YBa2Cu3O7−x specimens over the pore fraction range 0.10–0.25 and (2) determine the viability of using a room-temperature, nondestructive characterization method (ultrasonic velocity imaging) to predict spatial variability. Spatial variations in a.c. susceptibility were observed for specimens containing 0.10 pore fraction. An ultrasonic velocity image constructed from measurements at 1 mm increments across one such specimen revealed microstructural variation between edge and center locations that correlated with variations in a.c. shielding and loss behavior. Optical quantitative image analysis on sample cross sections revealed pore fraction to be the varying microstructural feature.

By controlling the pH of mixed nitrate solutions, we have produced single-phase YBa2Cu3O7−x platelet particles with an average size of 2.5 μm. The kinetics of superconducting phase formation and the thermal properties of the related phases were systematically studied. Through this understanding, we have developed sintering techniques which produce YBa2Cu3O7−x with wide areas of highly aligned grains at 1010 °C. By incorporating aligned microsized particles into the matrix of this reactive precursor powder as nucleation sites, samples with large randomly oriented domains can be formed at 980 °C.

Both YBa2Cu4O8 and (Y0.95Ca0.05)Ba2Cu4O8 were successfully prepared by firing for 160 h at 850 °C and at oxygen partial pressure of 3 atm without using any catalysts. These samples were characterized in terms of the crystallographic structure and thermal and superconducting properties. The x-ray powder diffraction patterns and superconducting properties for these samples were little changed after post-annealing for 6 h at 1070 °C and at oxygen partial pressure of 400 atm. However, when heat-treated at 700 °C in air and then quenched into liquid nitrogen, samples without post-annealing showed broader superconducting transitions than those post-annealed. Actually, the sharpness of the superconducting transition for a post-annealed sample was little affected by quenching. These observations were in agreement with the results of both transmission electron microscopy and thermal analyses.

In this paper the mechanism of phase transition and structural relaxation are elucidated by means of model experiments in the differential scanning calorimeter. Materials for this study were varied in structure and oxygen content by the introduction of Ni for Cu, through co-precipitation of oxolate gels, calcination, and sintering in ambient atmosphere. Both relaxation and phase transition are revealed by scrutinizing the heating rate dependence of heat capacity, and they seem to occur contemporaneously. Two higher order transitions are observed and are characterized by activation energies of 186 kJ/mole and 550 kJ/mole at the low and high temperatures, respectively. Perhaps the first transition is of a displacive type and occurs in anticipation of a reconstructive structural change from a fourfold chain to a sixfold sheet-like configuration. It is delineated by a change in relaxation kinetics from a localized type to a cooperative type with activation energies of 36 and 10 kJ/mole, respectively. The cooperative relaxation occurs at higher temperatures and is kinetically continuous across the reconstructive transition region. The structure characteristic exponent has a value of 0.70 for both localized and cooperative relaxation and resembles the two-dimensional relaxation often observed in depolymerized glasses. With kinetic impediment the transition region seems to approach a lambda-like behavior. In the subtransitional region, impediments in relaxation are useful in delineating oxygen stoichiometric differences between samples. The above discussion is supported by the structural and chemical characteristics that were gathered by x-ray powder diffraction, Raman spectroscopy, scanning electron microscopy and microprobe, and low temperature resistance measurements on Y123 samples containing 5 and 10 at. % Ni, respectively.

Thin films of YBCO and thallium-based superconductor compounds have been deposited with very similar polycrystalline structures. The critical current densities carried by the YBCO films are much lower than measured in the thallium films (>103, as opposed to >104 A/cm2). Grain boundaries in these films have been studied to correlate microstructure with the measured electrical properties. High defect densities and frequent microcracking have been observed at and around the boundaries in the YBCO films, but these defects are not seen in the thallium-based films. We suppose that this difference is because of the higher differential thermal expansion stresses set up in YBCO during cooling. Our observations imply that eventual application of polycrystalline superconducting films prepared by an ex situ process is more likely for the thallium-based materials than for YBCO.

A metallo-organic deposition technique was employed to grow Bi2Sr2CaCu2O8+x flexible films on unpolished metal foils (stainless steel and silver). Since strong interactions occurred at elevated temperatures between stainless steel and superconducting oxides, HfO2 buffer layers prepared by ion-assisted deposition were used to stabilize the system and Na additions were used to reduce processing temperature and enhance grain growth. The transition of superconducting films on stainless steel was complete around 70–73 K, and the critical current density was about 1000 A/cm2 at 10 K. Silver appears to be an attractive metal support due to its chemical stability and mechanical ductility. The film on silver showed the onset to superconductivity at 77 K, attaining zero resistance at 73 K. The critical current density in zero magnetic field was approximately 27000 A/cm2 at 20 K, comparable to that obtained from MOD films on MgO (100).

Semiconducting samples of a Bi–Ca–Sr–Cu oxide, having a nominal composition 4:3:3:4, were prepared by melting high purity oxides of the various elements. Superconducting samples were obtained by annealing the semiconducting samples at a temperature close to the melting temperature. Electrical resistivity measurements indicated the presence of two superconducting phases, one with the onset temperature at about 110 K, the other with a significantly lower onset temperature. The presence of this second phase is proven by the x-ray powder diffraction patterns. The crystalline structures of both the semiconductor and superconductor samples have almost equal lattice parameters and are orthorhombic, but belong to different space groups: Fmmm and Bbmm, respectively. No significant amount of the 110 K superconducting phase was present in the diffraction pattern of the superconducting compound. XPS and XANES data on the Ca atom indicate that a different electronic state of Ca is present in the superconducting compound, which still contains an appreciable amount of the semiconducting phase.