YBa2Cu3O6+δ microcrystals are synthesized by extraction of a nonstoichiometric melt similar to that employed for crystal growth of this compound. This method uses, first, initial compositions corresponding to the field YBa2Cu3O6+δ plus liquid, and second, the interaction properties of the flux with an appropriate substrate, i.e., Y2O3. Using other substrates like alumina produces 123 microcrystals, but embedded in the residual flux; in contrast yttrium oxide reacts with this one, and it is essential for its extraction. Choosing proper conditions makes it possible to obtain relatively pure YBa2Cu3O6+δ samples made of regular pellets (size: 10 to 20 μm thickness: 3 to 6 μm). The process is discussed using the phase diagram and the concept of dissolution-reprecipitation often involved in liquid phase sintering.

Bi1.84Pb0.34Sr2.03Ca1.9Cu3.06Oy, bulk superconductors were prepared by hot press sintering at 800°C at a pressure range from 50 kgf/cm2 to 200 kgf/cm2 in air for 2 h. The relative density increased with increasing the hot-pressing pressure, and specimens with various relative densities of 70%–90% were prepared. The flexural strength (σ) at room temperature was related to the porosity (p) as follows: σ = 138 exp(−2.83 p) (MPa). The flexural strengths of the specimen with p = 0.1 were 103 MPa and 113 MPa at room temperature and at 83 K, respectively. The flexural strength tended to increase with decreasing temperature. Anisotropy in flexural strength was observed, which seemed to be due to the grain-oriented microstructure.

Although a NbAl3 alloy containing Cr, W, and Y has excellent oxidation resistance above 1440 K, it suffers from severe environmental attack during deformation at intermediate temperatures between 900 and 1100 K. Specimens tested in constant velocity and constant load direct compression tests showed varying degrees of degradation depending on environment (i.e., air or argon), surface finish, stress, and temperature. As a result, there were corresponding differences in mechanical behavior and in the observed microstructures. At high stresses and strain rates, specimens with as-machined surfaces were brittle at and below 1100 K when tested in air but showed fracture strains above 4% when deformed in argon. However, reproducible compressive ductility of 2–3% was attained on polished specimens tested in air. At intermediate stresses, the creep curves showed sudden and periodic increases in strain before the specimens failed catastrophically after about 80 h. Microstructural examination of these specimens revealed extensive oxidation within cracks. Constant load tests conducted at lower stresses below 100 MPa showed an apparent incubation period where the change in the length of the specimen was immeasurably small. Following the incubation period, which typically lasted between 10 and 110 h depending on stress, temperature, and surface condition, specimens increased significantly in length due to oxide growth. In this case, considerable oxide spalling occurred during the course of the test, often leading to a substantial decrease in the cross-sectional area of the specimen. Microstructural observations revealed extensive cracking in the oxide layer and in the matrix, where the cracks had originated at the oxide-metal interface. The effects of environment on the mechanical properties are rationalized with the help of a schematic environmental-deformation mechanism map.

We have developed a novel pressurized partial melt growth process for producing large pieces of bulk Y–Ba–Cu–O superconductors. During long-time partial melt growth stage, an additional driving force for solidification is obtained by using pressurized oxygen gas. The microstructure and superconducting properties of the resulting samples were investigated. It was found that this new technique can eliminate porosity and inhomogeneity, promote large-scale grain-texturing, and improve interdomain coupling as well.

The precipitation behavior, especially the early nucleation stages, of the industrial strip-cast Al3003 alloys was investigated by using transmission electron microscopy (TEM). An icosahedral quasicrystalline phase was found as secondary particles in these strip-cast alloys after heat treatment for a few seconds. Three different nucleation paths are proposed based on the TEM observations. They have the same origin, viz. (Mn, Fe)-containing Mackay icosahedra, and are governed by the composition of alloys, especially the Mn and Si content.

Young's modulus of ceramic YBa2Cu3O7−δ was measured between room temperature and 1000 °C, E(T), by an ultrasonic pulse-echo technique. Experimental results are presented for nonaligned ceramics with average grain sizes from 2 to 10 μm and densities from 80% to 95% of the theoretical value. Young's modulus is shown to be strongly sensitive to oxygen content with the orthorhombic phase being significantly stiffer than the tetragonal phase. In addition, the phase transition is denoted by a pronounced minimum in E(T) relating to softening of certain bonds in the unit cell. At high temperature (>900 °C) melting of the second phase gives a steep drop in E(T) while subsequent densification increases E. Finally, cooling of large-grained ceramic below 450 °C opens microcracks due to anisotropic volume changes. This results in hysteresis between the heating and cooling curves for E(T). Hence the technique is used to show that ceramic, with an average grain size of 2 μm and 85% of theoretical density, is well oxygenated and undamaged by its thermal treatment.

Bi2Sr2CaCu2O8+x films were prepared by mixing 2-ethylhexanoate precursors with heptafluorobutyric acid and annealing at 820 °C in dry ambient. The films underwent substantial mass transport and exhibited abnormal grain growth, indicating that some liquid phases might be involved in the process. It was found that the high atomic mobilities during annealing significantly improved in-plane orientations and consequently increased critical current densities.

Effects of an excitation frequency on on-axis rf magnetron sputtering have been studied for fabrication of YBa2Cu3O7−δ thin films on MgO (100). The excitation frequencies examined are 13.56, 40.68, 67.80, and 94.92 MHz. Sputtering was performed in a pure argon gas in order to distinguish excitation frequency effects of oxygen emitted from a target from those of oxygen in a mixture atmosphere. Excitation frequencies higher than 40.68 MHz result in both a low self-bias voltage and a narrow distribution of the energy of sputtering ions, which reduce resputtering of a film surface caused by negatively charged oxygen from the target. Thin films of a-axis oriented YBa2Cu3O7−δ deposited at 94.92 MHz have shown better morphology and crystallinity than those at lower excitation frequencies under the same conditions. Furthermore, a-axis oriented superconductive YBa2Cu3O7−δ films have successfully sputtered in 200 mTorr atmosphere of argon and oxygen on SrTiO3 (100) by using an excitation frequency of 94.92 MHz, showing a full width at half maximum of 0.1° in the rocking curve through the (100) peak and a very smooth surface. The zero resistance temperature is, at most, 33 K.

The effect of atmosphere on the formation of the 110 K (Bi2Sr2Ca2Cu3O10, 2223) superconductor phase was studied at different oxygen partial pressures and temperatures. Step heating at low oxygen partial pressures with intermediate grinding promoted 110 K phase formation. With this synergy the shortest time for single-phase 110 K superconductor formation was found to be 12 h between 845 and 865°C at Po2 = 0.078 atmosphere. The activation energy of 110 K phase formation is 1513 KJ/mole in air.

The CeO2-added Y–Ba–Cu–O oxides were prepared by two different processes, the conventional solid-state reaction process and the partial melt process using powders, to examine the effect of the dopant on microstructure, microhardness, and superconductivity. In the solid-state reacted sample, most of the added CeO2 was converted to a form of BaCeO3, but some might enter into the 1-2-3 phase, resulting in the orthorhombic-to-tetragonal phase transition that accompanied the disappearance of twin structure in 1-2-3 grains. In the partially melted sample, however, the phase change was not observed up to 5 wt. % of CeO2. All the added CeO2 in these samples was consumed to form only BaCeO3 which was finely dispersed in large 1-2-3 grains during the peritectic reaction stage. The zero resistance temperature (Tc) of the solid-state reacted sample gradually decreased with increasing CeO2 content due to the phase change and the formation of BaCeO3, whereas the Tc of the partially melted sample was nearly constant regardless of CeO2 content up to 5 wt. %, owing to the separation of the second phase from the 1-2-3 superconducting phase. Microhardness of the partially melted sample increased with increasing CeO2 content. The strengthening effect appears to come from the composite system where the fine BaCeO3 particles are dispersed in a 1-2-3 matrix.

A sort of composite material can be produced from a single kind of natural organic fiber. This has been observed with bovine leather. During the formation process in a die particulated leather fibers were subjected for a period of about 6 min to a pressure of over 100 bar at a processing temperature of between 60 and 200 °C. In this way a portion of these collagenous fibers was plastically deformed and converted into continuous matrix material in which unconverted fibers act as matrix reinforcement. Round, collagenous fibers assumed an angular cross section and became the building blocks of the continuous matrix. This is clearly visible on pictures made with a scanning electron microscope. At a processing temperature of 70 °C and a pressure of 650 bar, applied for 6 min, the maximum compression strength at room temperature is found to be 185 MPa. At these processing parameters Young's modulus is about 2.4 GPa and the bulk density is 1350 kg/m3. Other natural fibers, to be used as organic raw material for the production of composites, are currently under investigation.

A series of Cu/Pd superlattices with composition modulation wavelengths (Λ's) ranging from 1.6 to 3.5 nm and a strong [111] growth texture were prepared by electron beam evaporation. The elastic properties of the films were examined using the methods of uniaxial tension tests [a Young's modulus (1/s11), where sij is an elastic compliance] with the applied load parallel to the plane of the Cu/Pd interface and Brillouin scattering [a shear modulus (1/s44) with the shear waves parallel to the plane of the Cu/Pd interface]. Also, the films were characterized using both x-ray diffraction and high-resolution electron microscopy; this was done to assess the effect of the nanostructure on a possible “supermodulus effect.” The films are nanostructurally very similar to the superlattice films employed in previous studies at Northwestern in which a supermodulus effect was reported. But, contrary to previous studies, no anomalous behavior was observed for the measured elastic properties of the thin films. Therefore the present results negate the earlier results and cast a serious doubt on the existence of a supermodulus effect.

Textured superconducting films of YbBa2Cu3O7−δ were grown on single crystals of MgO (100) and SrTiO3 (100) by oxidation of a liquid alloy precursor. The substrates were coated by dipping them in molten YbBa2Cu3 (m.p. ~870 °C). After removal from the melt, the liquid layers on the substrates were oxidized in pure oxygen to form the tetragonal oxide phase, i.e., YbBa2Cu3O7−δ, then annealed at 500 °C to obtain the superconducting orthorhombic phase of the same compound. The microstructure of the films obtained in this way was found to be related to the nature of the substrate as well as to processing variables that included oxidation temperature and oxidation time. Films grown on MgO (100) showed c-axis texture as well as a random growth structure. Films prepared on SrTiO3 (100) showed either a c-axis texture or a mixture of c-axis and a-axis texture. The superconducting properties of the as-prepared films and the effects of key process parameters on film quality and microstructure are presented and discussed.

Al films deposited on Pt layers developed voids after annealing at 250 °C. The amount of Al in the area surrounding the voids increased relative to the as-deposited film. The addition of 1 or 4% Cu to the Al suppressed the void formation and lateral Al migration. The void formation is related to compound formation with the Pt. The addition of Cu to the Al did not modify the Pt2Al3 formation.

Electron paramagnetic resonance (EPR) studies of pure Y2BaCuO5 in powder and single crystal forms and of YBa2Cu3O7−δ in powder and single crystal forms provide further evidence that it is Y2BaCuO5 that is the common green impurity found in many preparations of YBa2Cu3O7−δ as a powder or in pellet forms. Y2BaCuO5 tends to be excluded in the growth of YBa2Cu3O7−δ single crystals. A method is presented for the growth of Y2BaCuO5 crystals from a flux. An apparent discrepancy between the observed single crystal EPR anisotropy and the reported crystal structure is resolved in three independent ways from the Y2BaCuO5 Powder and single crystal EPR data. These results show that the EPR spectrum is a superposition of the spectra of the two differently oriented Cu sites in the unit cell and is not a spectral average of them. The temperature independence of the EPR spectrum between 150 K and 300 K is also consistent with there being no temperature dependent exchange averaging of the EPR spectra of the two sites in this range. The orientations of the Cu crystal field axes, as indicated by the g axes, are in agreement with the crystal structure. Crystal field splittings of the Cu d-orbitals are estimated from the measured g values and indicate an appreciable covalency in the Cu–O bonds. The linewidth and its anisotropy indicate a minor degree of exchange narrowing.

The valencies of the Bi and Cu ions in Bi2Sr2(Ca1−x Yx)Cu2Oy were determined separately by a coulometric titration technique. The Cu valency decreased monotonically from 2.16 to 2.04 with increasing Y content over the range from 0 to 1. Superconductivity occurred for Cu valency greater than 2.1. The Bi valency was also found to decrease from +3.12 to +2.99 as the Y content increased. It leveled off at ∼ +3.0 for the Y content larger than 0.8. This change in the Bi valency was correlated with the change in the structural modulation period and with its transformation from an incommensurate to a commensurate state.

YBa2Cu3O7−x superconducting films have been prepared on a YSZ ceramic substrate through the sol-gel method using metal alkoxides of Y, Ba, and Cu. A homogeneous solution containing the three metal alkoxides was prepared by dissolving the copper alkoxide, which is otherwise insoluble, in diaminoalcohol. The gel film, formed by dipping the substrate into the sol of 80 °C and withdrawing it up, was converted to a ceramic film consisting of YBa2Cu3O7−x phase by sintering at 850 °C for 30 min. The resulting film of about 10 μm in thickness showed zero resistivity (Tc end) at 54 K. Cold isostatic pressing of the film before sintering remarkably improved the Tc end to 85 K.

The fine structure of dislocations in lightly deformed samples of several cubic ordered alloys with composition based on Al3Ti has been examined by weak beam electron microscopy. For all the materials examined the dislocations tend to dissociate into two 1/2(110) partials separated by APB. Dislocation dissociation is not complete at very small strains and the strain required to dissociate, as well as the dissociation distance, varies from one alloy to another. Improvements in ductility achieved by alloying are directly related to the ease and extent of this dissociation.

Stoichiometric iron nitride layers have been synthesized by high dose, high energy nitrogen implantation into Fe using a two-step implantation process. First, a nitrogen preimplantation at ~100 °C is used to form nitride precipitates. A low temperature is necessary to restrict the nitrogen mobility. Second, 1 MeV implantation at ~300 °C leads to the formation of a stoichiometric γ′–Fe4N layer at the position of the preimplanted N atoms. Growth of this nitride layer proceeds by diffusion of the implanted N through either the α–Fe matrix (for 200 or 500 keV preimplantations) or the nitride layer itself (for 1 MeV preimplantation). During annealing above 350 °C the γ′ layers dissolve in a planar fashion, characterized by an activation energy of 2.3 eV. Phase formation during preimplantation and phase transformations during subsequent annealing or hot implantation can be understood from the thermodynamics for the Fe–N system, while the kinetics of layer growth is influenced by the beam-induced defects. The experiment and model suggest that γ′ is not a thermodynamically stable phase below 310 ± 10 °C and should decompose into α (ferrite) and ∊ nitride.

Due to the large positive heat of mixing associated with the Cu–Cr binary system, solid solutions exist only as nonequilibrium states. In this study, a series of metastable Cu–Cr alloys ranging in composition from 14.1 to 75.4% copper was fabricated by sputter deposition. Symmetric, asymmetric, and grazing incidence x-ray diffraction geometries were used to trace the phase transition from bcc to fcc crystal structures with increasing Cu fraction. It is shown that the transition takes place not by a two-phase region suggested by equilibrium thermodynamics, but rather through gradual disordering of the bcc lattice as copper atoms are substitutionally accommodated. At a critical saturation near 71% Cu, the bcc structure becomes unstable relative to the fcc and a phase transition occurs. The free energies of the kinetically constrained Cu–Cr system are modeled and the results are found to agree well with observed behavior.