Peter E. D. Morgan

Structural and electrical properties of La0.5Sr0.5CoO3 epitaxial films

Epitaxial La0.5Sr0.5CoO3 films with very smooth surface morphology were grown on (100) SrTiO3 and (100) MgO substrates by pulsed laser deposition. Scanning tunneling microscopy reveals that the thin film is formed by the coalescence of many aligned square mesas. The growth proceeds from the edges of terraces which are stacked on the mesa. Spiral growth is never observed. Films display an crystallographically isotropic metallic‐like electrical conductivity but become semiconductor‐like after vacuum annealing. The energy for carrier activation is 0.3 eV. The change of the electrical resistivity of La0.5Sr0.5CoO3 with oxygen pressure at high temperature is much less sensitive than that of YBa2Cu3O7−x.

Monazite-containing oxide/oxide composites

Abstract An extremely simple processing route has been used to produce LaPO 4 (La-monazite)/alumina continuous fiber-reinforced ceramic composites. In this paper, the processing, microstructure and tensile properties are reviewed. In particular, the damage tolerance and notch insensitivity of this system, which contains monazite-coated fibers, will be compared to the properties reported for other oxide composites.

Functional interfaces for oxide/oxide composites

Abstract Potentially useful interfacial coatings for composites of sapphire fibers in polycrystalline aluminum oxide matrices are discussed, subject to the constraints of phase compatibility in high temperature oxidizing environments and low mechanical interface strength ( i.e. to allow pull-out). Results of preliminary tests to investigate feasibility of some candidate coatings ( β -aluminas, LaPO 4 and CaF 2 ) are presented. These systems exhibited interfacial debonding under certain conditions, as required for high toughness composites, although several issues remain to be resolved before they can be used successfully.

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.

Oxide Composites of Al2O3 and LaPO4

Abstract Some properties of oxide composites based on Al 2 O 3 and LaPO 4 (La-monazite) are examined. A composite consisting of woven Al 2 O 3 fibers with a porous matrix of Al 2 O 3 and LaPO 4 is shown to be damage tolerant and notch insensitive. The feasibility of achieving fiber sliding and pullout in a composite with a fully dense matrix is investigated using a small hot-pressed composite of sapphire fibers and LaPO 4 matrix.

Pauling's second crystal rule for nitrogen-substituted crystal structures

The use of Paulings second crystal rule possibly for predicting anion aliovalent substitutions, particularly those of nitrogen replacing oxygen, in crystallographic isotypes is examined. Bond length/bond strength sums are also reviewed for some structures where nitrogen-substitution is known.

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.

Reactive ion beam deposition of thin films in the bismuth‐calcium‐strontium‐copper oxide ceramic superconductor system

Reactive ion beam deposition has been used to grow c‐axis‐oriented superconducting thin films in the Bi‐Ca‐Sr‐Cu‐O (BCSCO) system around the cation ratio 1:1:1:1 on single‐crystal (001) MgO. The films show a single superconducting transition with an initial onset near 85 K and a critical current of 5×104 A/cm2 at 10 K. Two different BCSCO‐containing phases have been identified in the thin films: one with a tetragonal pseudo‐body‐centered subcell, c=24.4 A, which is not superconducting above 28 K, and a second with c=30.6 A, which is responsible for the superconductivity. Electron diffraction measurements on the 30.6 A phase are consistent with those previously reported for the bulk ceramic.