T.L. Aselage

Liquidus relations in Y--Ba--Cu oxides

The liquidus relations in the system YO/sub 1.5/--BaO--CuO/sub x/ in air in the compositional region near the superconducting oxide YBa/sub 2/Cu/sub 3/O/sub x/ were studied by differential thermal analysis, x-ray diffraction, electron microprobe analysis, and visual observation. The temperatures of 11 invariant points and the corresponding reactions were determined. YBa/sub 2/Cu/sub 3/O/sub x/ was found to melt incongruently at 1015 /sup 0/C to Y/sub 2/BaCuO/sub 5/, which in turn melts incongruently to Y/sub 2/O/sub 3/ at 1270 /sup 0/C. These reactions mean that preparing the superconducting phase by melting and rapid cooling will result in the presence of these two phases as well. The peritectic reaction YBa/sub 2/Cu/sub 3/O/sub x/+CuO..-->..Y/sub 2/BaCuO/sub 5/+liquid at 940 /sup 0/C accounts for the observation of partial melting, improved synthesis purity, and grain growth at temperatures of 950 /sup 0/C. The determination of these invariant temperatures and reactions provide insight into optimal processing conditions.

A proposed boron-carbide-based solid-state neutron detector

Its large cross section for absorption of thermal neutrons has made B10 a frequent candidate for use in neutron detectors. Here a boron-carbide-based thermoelectric device for the detection of a thermal-neutron flux is proposed. The very high melting temperatures and the radiation tolerance of boron carbides make them suitable for use within hostile environments (e.g., within nuclear reactors). The large anomalous Seebeck coefficients of boron carbides are exploited in proposing a relatively sensitive detector of the local heating that follows the absorption of a neutron by a B10 nucleus in a boron carbide.

Synthesis and properties of Tl–Ba–Ca–Cu–O superconductors

We review the synthesis methods and properties of single crystal, powder and thin film TlBaCaCuO high-temperature superconducting (Tl-HTS) materials. With transition temperatures {ge}100K for several compounds, Tl-HTS materials present real opportunities for applications above 77 K. Experiments using (1) single crystals: determined precise structural parameters and identified the complex Tl{sup 1+}{minus}Tl{sup 3+} equilibrium model; (2) powders: studied the complex thermodynamic phase diagram; and (3) epitaxial films: have studied fundamental properties such as electron pair symmetry and the effect of controlled extrinsic defects on flux pinning strength, as well as providing the large-area surfaces required for device applications. {copyright} {ital 1997 Materials Research Society.}

Neutron powder diffraction refinement of boron carbides nature of intericosahedral chains

Abstract Our Rietveld structure refinements of neutron powder diffraction data of boron carbides with 10, 13, 16, and 20 at.% carbon indicate 15–25% vacancies at the central position of the intericosahedral chains. In addition, we find a comparable number of interstitials residing off the intericosahedral chains. However, X-ray diffraction studies on unirradiated boron carbide single crystals do not generally find these defects. This discrepancy may indicate that the observed defects are the remnants of defect cascades created by the energetic fission products of 10 B during the neutron diffraction measurements. The tendency to form vacancies in the center of the intericosahedral chain is consistent with the weak and soft binding of chain-center atoms that we previously inferred from other measurements.

Preparation and Properties of Icosahedral Borides

The electronic and thermal transport properties of refractory icosahedral boron-rich solids suggest the utilization of these unique solids as high-temperature semiconductors. Such applications will require high-quality, well-characterized materials. A variety of techniques have been used to prepare powders, ceramics, and single crystals of boron carbides and the icosahedral boron pnictides. X-ray diffraction, Raman spectroscopy, and ultrasound measurements have been used to probe boron carbide sample quality. Each of these measurements supports the structural model of boron carbides developed from analysis of transport data. Initial electrical conductivity measurements on these samples show no thermal hysteresis. The thermally-activated conductivity (Ea ≈ 0.17 eV) is consistent with a bipolaronic hopping mechanism.

The coexistence of silicon borides with boron-saturated silicon: Metastability of SiB 3

The silicon-rich end of the Si-B phase diagram, defining the silicon boride(s) that coexist in equilibrium with boron-saturated silicon, is poorly known. Understanding this equilibrium has implications for the processing of p{sup +} silicon wafers, whose boron concentrations are near the solubility limit. Additionally, silicon boride precipitates produced by boron-ion-implantation and annealing of crystalline silicon have recently been shown to be efficient internal getters of transition metal ions. The experiments described in this paper probe the stability of these silicon borides. A phase with a boron-carbide-like structure, SiB{sub 3}, grows from boron-saturated silicon in both the solid and the liquid state. However, SiB{sub 3} is not found to be stable in either circumstance. Rather, SiB{sub 3} is a metastable phase whose formation is driven by the relative ease of its nucleation and growth. The silicon boride that exists in stable equilibrium with boron-saturated silicon is SiB{sub 6}. A qualitative understanding of the metastability of SiB{sub 3} comes from recognizing the conflict between the bonding requirements of icosahedral borides such as SiB{sub 3} and the size mismatch between silicon and boron atoms. {copyright} {ital 1998 Materials Research Society.}

Two‐zone equilibria of Tl‐Ca‐Ba‐Cu‐O superconductors

Phase equilibria in the Tl‐Ca‐Ba‐Cu‐O system have been studied at sample temperatures that are near, but below, the onset of melting. Independent control of the partial pressures of oxygen P(O2) and thallous oxide P(Tl2O) was achieved through the use of a two‐zone furnace. Stable ranges of P(Tl2O), P(O2), and sample composition for individual Tl superconductors, including 2122, 2223, 1122, and 1223 (the latter intergrown with about 10% 2223) have been determined. A qualitative understanding of these stable ranges, based on the relative Tl content and formal oxidation state of the cations within a structure type, has been developed. From this information, equilibrium phase diagrams for Tl superconductors with Ca:Ba:Cu ratios of 1:2:2, 2:2:3, and 3:2:4 are proposed. These diagrams provide process information for the two‐zone synthesis of bulk materials and thin and thick films. The superconducting transition temperatures of the individual phases produced by near‐equilibrium processing are both reproducible ...

Occurence of free CuO in YBa2Cu3O6+δ and its effect on melting and solidification

Abstract An endothermic reaction is frequently observed in samples of YBa2Cu3O6+δ (123) at temperatures of about 940°C in air or 965°C in O2. The origin of this endotherm was examined by differential thermal analysis, microstructural analysis, and by a critical review of the pertinent literature. It is concluded that this endothermic reaction is caused by the presence of free CuO in 123 samples. Potential sources of free CuO in 123 are considered, and the effect of free CuO on the melting and solidification of 123 is discussed.

Phase stability and properties of near-equilibrium TlCaBaCuO superconductors

Abstract High-temperature phase stability limits of TlCaBaCuO superconductors were determined as a function of Tl 2 O partial pressure at constant O 2 partial pressure (≈630 Torr), sample temperature (880°C), and sample Ca:Ba:Cu ratio (1:2:2, 2:2:3, and 3:2:4). Both phases with double CuO 2 sheets in the structure, 2122 and 1122, were stable over wide ranges of P (Tl 2 O) for stoichiometric (1:2:2) samples. At relatively high P (Tl 2 O), 2122 was also the stable superconducting phase even for the Ca-and Cu-rich stoichiometries. Energy-dispersive X-ray spectra and c -axis differences suggested variable cation stoichiometry in 1122 and 2122 prepared under such a range of conditions. However, the superconducting transition temperatures and a -axes of both 1122 and 2122 were nearly constant for all conditions of preparation, suggesting a nearly constant carrier density. No range of P (Tl 2 O) was found where either 1223 or 2223 was stable as a single superconducting phase for the P (O 2 ) and temperature employed. Intergrown mixtures of 1223 and 2223 or Tl-free phases were found when P (Tl 2 O) was below the 2122 formation boundary.

Anomalous Seebeck Coefficient in Boron Carbides

Boron carbides exhibit an anomalously large Seebeck coefficient with a temperature coefficient that is characteristic of polaronic hopping between inequivalent sites. The inequivalence in the sites is associated with disorder in the solid. The temperature dependence of the Seebeck coefficient for materials prepared by different techniques provides insight into the nature of the disorder.