In situ electrical conductivity measurements were employed to study the kinetics of decomposition of YBa2Cu3O7−x in flowing 5% CO2/95% O2 atmosphere at 815 °C. Three regimes could be observed in the decay of the conductivity with time, interpreted as corresponding to nucleation over the initial 5–10 min, rapid formation of Y2Cu2O5, BaCO3, and CuO between 10 min and 1 h, and finally transition to the formation of Y2BaCuO5, BaCO3, and CuO at longer times. The absence of dispersion in impedance measurements as a function of frequency together with the gradual decay of conductivity (roughly corresponding to the remaining volume fraction of YBa2Cu3O7−x) indicates that not all grain boundaries are involved in the decomposition process.

Variations in lattice parameters for materials that form very thin lamellae are analyzed using a thermodynamic model that incorporates surface stress effects. It is predicted that lattice spacing variations should be proportional to the reciprocal of the lamella thickness, in agreement with experimental data for polyethylene, n-paraffins, and a copolymer of tetrafluoroethylene and hexafluoropropylene. The model is then used to calculate the surface stress associated with lamella interfaces in these crystalline materials. The calculated surface stress has the same order of magnitude as a surface tension, but is negative. The model is extended so that surface stresses associated with grain boundaries can be measured in very fine-grained metals and ceramics.

Metal-organic precursors can facilitate the production of superconducting thin films on large area substrates for microelectronic device applications. This paper describes the chemistry that occurs during the thermal processing of yttrium-, barium-, and copper-neodecanoate mixtures into a superconducting material, YBa2Cu3O7−x. Intermediates such as metal carbonates and oxides are identified by combining thermogravimetry with infrared spectral and x-ray diffraction analysis. Changing from argon to oxygen atmospheres increased decomposition rates and metal oxide formation. We found that (1) a low temperature (500–600 °C) decomposition in an oxygen-poor atmosphere, (2) a high temperature (>925 °C) annealing step in an oxygen-rich atmosphere, and (3) slow cooling to 50 °C are all required to produce uniform superconducting films.

We present measurements on the superconductor compound Bi–Sr–Ca–Cu–O, obtained using a scanning Auger microscope (SAM), which indicate that this material is extremely unstable under an electron beam with energy of several KeV, Using relatively high current densities for the SAM, it is possible to melt small regions of the material and to form spheres several micrometers in diameter in a few seconds. We characterize the changes in the elemental concentration of the surface of these spheres as a function of time after the beam exposure.

The phase diagram of the Y2O3–CuO–Cu2O system was constructed as a function of temperature in air by means of thermal analysis and x-ray diffraction. The phase Y2Cu2O5 was found to be stoichiometric and melted peritectically at about 1110 °C. A new intermediate phase with the composition YCu2O2.5 was discovered. It exists between 990 and 1105 °C in air and quenching did not preserve it to room temperature. High temperature x-ray diffraction established its pattern and confirmed its peritectic decomposition. Above 1110 °C YCu2O2.5 consists of Y2O3 and liquid. The results of this system are shown on a ternary system as well as on a pseudo-binary phase diagram. The solid state reactions were established. Melting occurs with isothermal oxygen loss or gain, depending on the initial composition.

The microstructure and microwave properties of YBa2Cu3O7−δ thin films grown on SrTiO3 (100) and MgO (100) substrates by chemical vapor deposition have been studied. Both 100 GHz cavity measurement of surface resistance and ground plane substitution in a 5 GHz microstrip resonator show that films on SrTiO3 have better microwave properties than those on MgO. Although there are some a-axis grains and secondary phases on the surface, a large fraction of each film on SrTiO3 is epitaxial with its c axis normal to the substrate. The 100 GHz surface resistance of these films is less than copper for temperature ⋚ 82 K, and approaches the detection limit at 10–20 K. For the films on MgO, c-axis grains of different in-plane rotation are found together with some a-axis needle-like grains. The grain boundaries are detrimental to the microwave properties, and the resulting surface resistance at 100 GHz and 10–20 K is about 20 mΩ higher than that of films on SrTiO3.

The evolution of the size and shape of molybdenum nanocrystals fabricated by sputtering in a thermal gradient has been studied as a function of the argon gas pressure, p. For 4 < p < 100 mTorr, continuous Mo films are deposited. At p = 150 mTorr, isolated and well-faceted Mo nanocrystals of two sizes (20 and 5 nm average size) are formed. For 200 ≤ p ≤ 400 mTorr, the particle size decreases with increasing pressure and is about 7 nm at 400 mTorr. On increasing p further, larger particles start to form and at p = 700 mTorr, particle agglomerates are observed. Possible mechanisms leading to these results are suggested.

The grain growth and microstructure development of YBa2Cu3O7−δ have been investigated utilizing two different starting particle size distributions (normal and bimodal). The grain growth exponent, n, was found to be about 0.21 for both normal and bimodal samples. An activation energy of 125 kJ/mole was calculated. The low value of n might be attributed to the high anisotropy of grain boundary energy in this system. Samples made from the bimodal powder were found to accelerate grain growth without introducing abnormal grain growth. Although most of the samples attained fractional densities greater than 0.95, the presence of various amounts of porosity (particularly in the case of the bimodal starting powder) did not affect the growth kinetics. The measured aspect ratio of grains did not significantly change during growth. A significant difference in aspect ratio was measured between samples made from the two different starting powders. Critical currents ranged from 10 to 120 A/cm2, but no concrete relationship with grain size was established. This implies that the grains produced by this experiment were in the size range where other factors, presumably microcracking, severely limited the current carrying capacity by the weak link effect.

The transport critical current density Jc has been measured in high quality polycrystalline YBa2Cu3O7 samples made from extruded preforms. The hysteretic response of Jc to applied magnetic fields was studied as a function of temperature (77–87 K) and sample morphology for various field sweeps in the 0–5 KOe range. A variety of samples show a basic Jc ∼ H−n behavior where n depends on temperature, but is independent of the other variables. A simple model of the local magnetic field distribution is presented and compared to the data, and the theoretical implications of the power law behavior are considered.

GdBa2Cu3O7 crystals were irradiated at room temperature with 200 keV Ne ions and 300 keV Xe ions. In situ standard TEM and further HREM studies show two types of extended defects: (i) mobile extended defects, which account for the preferential defect pinning to twin boundaries reported earlier. These defects are rapidly recovered and difficult to observe by HREM investigations; (ii) stable amorphous areas which are clearly identified by HREM observations. Their overlapping and aggregation conceivably lead to amorphization of the sample.

In sintering and annealing of ceramic superconductors, diffusion of oxygen and sample weight changes are two important parameters to understand. In this work, we have undertaken to make weight loss measurements at different temperatures. At 400–600 °C, the weight loss data can be simply interpreted via one diffusion process. The Arrhenius activation energy is found to be ca. 23 kcal/mole. At 500 °C, the isotropic diffusion constant was found to be 9.5 × 10−12 cm2/s. Above 600 °C, the weight loss curves become complex and indicate the occurrence of two diffusion processes. The oxygen loss and the oxygen uptake data are interpreted via our present knowledge of this system.

Oxygenation characteristics in high-density YBa2Cu3Ox ceramics with a relative sintered density ≥97%, which were produced by hot-pressing to have various oxygen contents as-hot-pressed, have been investigated. The results indicate that the oxygenation characteristic in the materials was strongly affected by their oxygen content before oxygenation; that is, the attainable oxygen content (directly connected with the superconducting temperature Tc) of the materials after oxygenation increases with a decrease in their oxygen content before oxygenation. Oxygen annealing at 500 °C for 24 h dramatically increased the oxygen content of an YBa2Cu3Ox with x = 6.08 before the oxygen annealing up to 6.90, while the oxygen content of an YBa2Cu3Ox ceramic with x = 6.32 before oxygenation rose only to a value of 6.61 after the same oxygen annealing. This newly observed phenomenon on the oxygenation characteristic in the present materials may provide an idea for improving the superconducting properties in high-density YBa2Cu3Ox ceramics.

The microstructures of YBa2Cu3O7−x thin films deposited by laser ablation on single crystal (001) LaAlO3 substrates have been investigated. The orientation of the YBa2Cu3O7−x layer next to the interface is found to be completely c-perpendicular, with a high degree of epitaxy between the film and the substrate. Misfit dislocations, with a periodic spacing of around 13 nm, are present at the interface. Two distinct interfacial structures are seen in these films. At a film thickness of around 400 nm, nucleation of c-parallel grains occurs, leading to a switchover from a c, and, and-perpendicular to a c-parallel microstructure. Amorphous particulates, ejected from the target during processing, lead to the formation of misoriented grains, giving rise to high-angle grain boundaries in the film.

An aqueous sol-gel procedure to obtain fine Y–Ba–Cu particles with an approximately 1-2-3 stoichiometric ratio is described. This involves precipitating an aqueous solution of the oxides in HC1 by neutralization with Ba(OH)2, and precipitation of BaCO3 from solution with CO2. A novel treatment with H2O2 decreased the weight loss while sintering. The average particle size and shape was characterized by rheology, light scattering, and scanning electron microscope. Particle composition was studied with x-ray diffraction, electron microprobe, and infrared spectroscopy. The suspension was formed into a fiber and sintered, and measurements were made indicating a superconducting transition temperature near 60 K.

Nitrogen dioxide was used to oxidize nonsuperconducting, powdered YBa2Cu3O6.0 to form a material that exhibits the Meissner effect at liquid nitrogen temperature. Nitrogen dioxide treatment at 200 °C for 70 h results in a mixed-phase material containing superconducting YBa2Cu3O6.9 that exhibits onset of superconductivity at 91.5 K and a midpoint at 88 K. During a 70-h exposure of YBa2Cu3O6 to NO2 at 200 °C, a mixture of oxidation and degradation products is formed, including YBa2Cu3O6.9 (61% by weight) and Ba(NO3)2 (23% by weight). Heating the NO2-treated product to 900 °C results in the evolution of NO, NO2, and O2; cooling the sample and annealing at 400 °C in an O2 stream generates the superconducting oxide.

The misorientations of over 200 pairs of adjacent grains separated by grain boundaries in textured YBa2Cu3O7−δ were measured using a transmission electron microscopy technique. The results indicate that there exist discrete preferred rotation angles and rotation axes. The existence of low-energy boundaries is inferred. The results are analyzed based on the Constrained Coincidence Site Lattice (CCSL) and O2-lattice theories and imply the applicability of such theories for the case of large-angle grain boundaries in a complex crystal structure such as YBa2Cu3O7−δ. The results of analysis also show that some boundaries are likely to be reduced in oxygen near the boundary to satisfy the constraint of the coincidence site lattice.

Working principles are developed as guidelines for the selection and/or design of organometallic polymers for processing fiber precursors to metal oxide fibers. These principles form the basis for the selection of metal carboxylate preceramics as an optimal approach to processing yttrium barium cuprate (123) ceramic superconducting fibers. A variety of candidate yttrium, barium, calcium, strontium, bismuth, and copper metal carboxylates were synthesized. Solubility and empirical rheology tests were conducted to screen these compounds to choose spinnable precursor systems. Simple extrusion studies confirmed that THF solutions of mixtures of yttrium, barium, and copper isobutyrates with some quantity of barium 2-ethyl-hexanoates can be used to successfully form 60–70 μm diameter 123 precursor fibers.

Using a highly-spatially-resolved mechanical properties microprobe, the Young's modulus and hardness of bulk YBa2Cu3O7−x (1:2:3) and YBa2Cu4O8 (1:2:4) have been determined. The Young's modulus of a superconductor is an important parameter in determining critical grain sizes above which microcracking will occur due to anisotropic thermal stresses that arise during processing. This phenomenon of microcracking has been determined to cause a decrease in the attainable critical current densities in bulk superconductors. The mechanical properties data for these two materials show that the Young's modulus of 1:2:3 is approximately 35% greater than the modulus of 1:2:4. This along with available anisotropic thermal expansion data for 1:2:3 and 1:2:4 suggests that the critical grain size for 1:2:4 is about 7 times greater than the critical grain size for microcracking in 1:2:3.

The effect of silver additions and excess copper on the sinterability and weak link behavior has been studied in the nominal composition Bi1.72Pb0.32Sr2Ca2Cu3Ox (Ag = 1−4 wt. %) and Bi1.72Pb0.32Sr2Cu3+yOx where y = 0.2−0.5, using magnetic susceptibility, XRD, and SEM methods. Ag additions enhance the weak link behavior, giving a magnetic susceptibility shoulder at 103 K. This shoulder became more prominent at high fields (35 Oe), and a low Tc phase became evident with 4 wt. % Ag additions. In contrast to stoichiometric compositions, samples containing excess copper demonstrated an enhancement of the sinterability and a large volume of 2223 phase even in calcined samples. XRD data and SEM analysis failed to detect any Cu-rich phase up to copper contents y = 0.4. However, in copper samples with y = 0.5, a small x-ray peak at 2θ = 48.5°has been observed corresponding to CuO.

Growth conditions for YBaCuO thin films are investigated. Films have been made by reactive e-beam coevaporation using three metal sources. In the best cases, as-made films are superconducting with Tc's (R = 0) up to 90 K and Jc's (at 4.2 K) above 107 A/cm2. Oxygen pressure, deposition temperature, as well as compositional dependencies of the films are presented. It is found that in conditions of lower oxygen, pressure films with average composition off the 1–2–3 stoichiometry have higher Tc's. For pressure <10 mTorr, the highest Tc obtained is for Ba/Y deposition ratio ⋚1.4. The morphology and impurity phases of these films are examined. The Ba-deficient films have oriented CuYO2 and CuO as the dominant impurity phases. C-axis lattice parameters (c0) are also examined. It is found that for a given Tc, films made at lower pressure have c0's which are expanded compared to the films made at higher pressures (>100 mTorr). The expanded c0's for these films cannot be reduced by a low temperature oxygen anneal. We suggest that metal-atom point-like defects are quenched into these films and we discuss a particular Ba-for-Y substitution model.