Robert M. Housley

Low level mobile liquid droplet mechanism allowing development of large platelets of high-Tc “Bi-2223” phase within a ceramic

Abstract A VLS-like mechanism is illustrated of how 3-Cu-layer Bi-HTSC (high temperature superconductor) is slowly formed from precursors by small, Bi/Pb rich, liquid droplets migrating over growing platelets, depositing ledges (steps) of product in their wakes. The difficulty of obtaining a pure 3-layer, i.e. without syntactic intergrowths, is explained.

High-temperature stability of monazite-alumina composites

Abstract The chemical and morphological stability and debonding characteristics of monazite/alumina interfaces were examined at temperatures of 1400 and 1600 °C. In the absence of impurities, the interface was found to be stable for long periods (24 h) at 1600 °C and to retain its ability to prevent crack growth by debonding. However, in the presence of alkali metal or divalent elements (K, Mg, Ca and others) at a free surface, La-containing β-alumina-magnetoplumbite platelets form near the interface. It is proposed that these elements stabilize the β-alumina-magnetoplumbite structure by tending to form the more stable ‘stuffed’ structure closer to the magnetoplumbit type.

Ceramic composites for thermal protection systems

Abstract Advanced coating systems based on monazite, a weak interphase for oxide composites, are being investigated as a means to increase the service temperatures of thermal protection blankets for re-entry space craft. Preliminary evaluations, including chemical compatibility, tensile strengths of coated, heat-treated fibers and fabrics, and durability in a modulated wind tunnel facility have been conducted.

Use of a thermal gradient to study the role of liquid phase during the formation of Bismuth high temperature superconductors

Abstract Firing a long specimen in a thermal gradient furnace is an efficient way of looking for the fine details of synthesis/morphology behavior in Ag-“doped” (Bi, Pb)-high temperature superconductors as a function of temperature and time. The 110 K, three-Cu-layer phase, begins to form apparently only upon the appearance of an eutectic-liquid at ∼822°C. The accompanying morpological changes are readily seen and interpreted by SEM/EDS of fractured surfaces.

Growth and characterization of large YBa2Cu3O7−x single crystals

Abstract Centimeter sized crystal plates of YBa 2 Cu 3 O 7−x have been obtained using a bulk composition and method described by Schneemeyer et al. modified for growth in a pure oxygen atmosphere. A porous MgO crucible and long soak time promote large crystals. Mg was not detectable in the crystals. Electron microbeam analysis using a 60 A monochromator crystal for the oxygen determination gave the composition: Y 1.05 Ba 2.02 Cu 2.94 O 6.52 ± 0.06. As-grown crystals from the melt surface exhibit a fairly sharp T c with zero resistance at 80 K, and show complex twinning and crack patterns. Optical micrographs of a lightly polished surface normal to the c -axis of a ∼ 3 mm crystal delineate twinning and fine scale microcracking; this crystal surface now has a zero resistance at 68 K. Microhardness measurements and micro-fracture observations indicate intrinsic properties that may inhibit or impede large scale manufacture of electronic ceramics.

Study of chemically-etched Bi2Sr2−xCa1+xCu2O8+δ surfaces

Abstract X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) are used to characterize the surfaces of polycrystalline Bi 2 Sr 2− x Ca 1+ x Cu 2 O 8+δ thin films which have been chemically etched in a solution of Br 2 in absolute ethanol. XPS measurements from individual grains (∼1 mm 2 ) are obtained, and the spectra presented here are comparable to spectra in the literature obtained from single crystals cleaved in vacuum. XPS valence band spectra show a clear Fermi edge, indicating that the room temperature conductivity near the surface is metallic. This is the first report of a detectable Fermi edge from a high temperature superconductor surface which has been treated in vacuum.

Thallous-ion-rich-liquid-phase synthesis of TlSr2Ca2Cu3Ox

Abstract The 1223 TlSr 2 Ca 2 Cu 3 O x compound has now been synthesized relatively pure for the first time we believe; preliminary electrical and magnetic properties are reported. The evidence for the presence of a thallous-ion-rich liquid phase which promotes synthesis is discussed. We give some preliminary resistance/temperature and magnetic susceptibility/temperature data.

High temperature creep of La-monazite

Abstract Compressive creep of La-monazite is investigated in the temperature range 1100 °C to 1500 °C. The study includes both high-purity single-phase material and material with excess phosphorus located in amorphous grain boundary phases. The results indicate that the presence of small amounts of excess P in polycrystalline LaPO4 has a large effect on microstructural stability and creep at high temperature. Materials with La/P ratio close to unity (within ∼500 ppm) show little grain growth at temperatures up to 1400 °C and deform by creep at rates similar to those of alumina and zirconia, with stress exponent ∼1. Materials containing excess P (as little as ∼1 %) show more rapid grain growth, higher creep rates, and cavitation during creep. The results are compared with creep rates of other refractory oxides and oxide fibers. Implications for the behavior of oxide composites containing La-monazite are considered.

Use of a Thermal-Gradient Method and Eds, with Image Processing, To Elucidate the Operative Mechanism(s) During the Formation of Tl-1223

The microstructural requirements of a sheathed high temperature superconductor ceramic wire for high Jc, are long and platy grains, with good percolative connectivity from grain to grain to allow a high critical current. This microstructure is encouraged by grain-growth through solution-reprecipitation (S-P) from a small amount of liquid phase. In earlier work on both Tl- and Bi-HTSC [1–4], we found that liquid was necessary to crystallize the three-Cu-layer 2223-types by S-P; without liquid none formed. This viewpoint is gaining support [5]. It was of interest to inquire if the same type of situation applies to the Tl-1223 case, which invites attention due to its superior flux-pinning properties [6]. We strongly believe higher Jc will be achieved through paying attention to the microstructural/percolative development with liquid just above the eutectic (or peritectic) temperature.

Further Implications and Utility of Low-Level Liquid During the Fabrication of 3-Cu-Layer HTSC

Much attention has been paid to the details of the solid phase in HTSC. However, the solid crystals are best formed from a liquid for high Jc; thus we alternatively are concentrating on the details of how the liquid effects the solid phase synthesis, which will lead to more understanding of orientation effects from thin films of liquid and consequent opportunities to improve weak-link properties.