Carlo U. Segre

Methanol Oxidation on Single‐Phase Pt‐Ru‐Os Ternary Alloys

Methanol oxidation was studied on arc-melted Pt-Ru-Os alloys and on fuel cell catalysts prepared by the NaBH{sub 4} reduction of metal chloride salts. Both the arc-melted alloys and the high surface area catalysts have x-ray diffraction patterns indicative of single-phase face-centered cubic lattices. Hydrogen adsorption/desorption measurements on the polished alloy electrodes, in the presence of adsorbed CO (25 C), show that selected ternary alloys have significant hydrogen adsorption/desorption integrals at adsorption potentials where Pt:Ru (1:1) was fully blocked and higher integrals at all adsorption potentials studied up to 400 mV vs. the reference hydrogen electrode. In situ diffuse reflection Fourier transform infrared spectroscopy of the fuel cell anodes showed that the alloy catalysts had reduced CO coverage relative to Pt, with the ternary catalyst showing the least coverage. Steady-state voltammetry of the arc-melted alloys at 25 C confirmed that Pt-Ru-Os (65:25:10) is more active than Pt-Ru (1:1), particularly above 0.6 V. Pt-Ru-Os (65:25:10) methanol fuel cell performance curves were consistently superior to those of Pt-Ru (1:1) (e.g., typically at 90 C, 0.4 V; 340 mA/cm{sup 2} with Pt-Ru-Os vs. 260 mA/cm{sup 2} with Pt-Ru).

Humidity sensing properties of Nation and sol-gel derived SiO2/Nafion composite thin films

Abstract Nafion, sol-gel derived SiO2 and SiO2/Nafion composite films were prepared through casting and dip-coating methods. Impedance response of the films to relative humidity and the stability of the response against different aging environments were examined. The response to relative humidity of each film was analyzed with their complex impedance spectra at different humidities.

Phase diagram and superconductivity in the Y‐Ba‐Cu‐O system

We have determined the phase diagram of the Y‐Ba‐Cu‐O system through structural, superconducting critical temperature and critical current density characterization. Our results show that a single‐phase compound with a stoichiometry YBa2Cu3Oy is responsible for the high‐temperature superconductivity (92.5 K) in this system.

Structural behavior and chemical order of Fe in YBa2(Cu1 − xFex)3O7 + δ

Abstract Neutron and Mossbauer effect measurements have been made as a function of Fe concentration in orthorhombic ( x = 0.01, 0.02) and tetragonal ( x ⩾ 0.05), YBa 2 (Cu 1 − x Fe x ) 3 O 7 + δ . A systematic Rietveld analysis of the neutron data for all concentrations shows that Fe principally occupies a site slightly displaced ( y , y , 0) from the Cul (0, 0, 0) site in order to approach tetrahedral coordination. Neutron data for the x = 0.05 compound at 10 K show no significant difference from those obtained at 297 K. Local ordering of oxygen about an Fe substituent is conducive to placement of a second Fe atom in an adjacent site leading to local aggregation into chains of various lengths. This clustering is reflected in the character of low temperature magnetic Mossbauer spectra and in specific heat measurements. The Mossbauer spectra yield the relative changes in oxygen configurations about Fe atoms as the Fe concentration changes.

Electronic and magnetic properties of rare-earth ions in REBa2Cu3O7-x (RE=Dy, Ho, Er)☆

Abstract Heat capacity, resistivity, and magnetic susceptability data have been obtained for the compounds REBa2Cu3O7-x, where RE = Dy, Ho or Er. Neutron diffraction data on the Ho compound show a structure identical to that of YBa2Cu3O7-x. Magnetic transitions are observed at Tm=0.95, 0.17 and 0.59 K for Dy, Ho and Er compounds, respectively. It is argued that these are due predominantly to dipolar interactions. Resistivity data show that the magnetic state is coexistent with superconductivity in all cases. From the heat capacity data, the degeneracies of the crystal field ground states are determined, and estimates are given for the magnetic moment in the ground state and the energy separation of the first excited crystal field state.

Determination of chemical diffusion coefficient of SrFeCo0.5Ox by the conductivity relaxation method

A conductivity relaxation experiment has been conducted on an SrFeCo0.5Ox sample by abruptly changing the oxygen partial pressure in the atmosphere and monitoring the change of conductivity as a function of time. The re-equilibrium process obeys Ficks second law. By fitting the relaxation data to the solution of the diffusion equation with appropriate boundary conditions, we could determine the oxygen chemical diffusion coefficient and the activation energy. The oxygen diffusion coefficient is 8.9 × 10−7 cm2/s at 900 °C and it increases with increase in temperature. Measured activation energy is 0.92 eV, which is slightly lower than that of other oxides in the system SrFe1 − xCoxOy.

Solubility and superconductivity in RE(Ba2-xREx)Cu3O7+δ systems (RE=Nd, Sm, Eu, Gd, Dy)

Solid solutions of RE(Ba2-xREx)Cu3O7+ delta (RE=Nd, Sm, Eu, Gd, Dy) for x=0 to x=0.5 have been investigated. X-ray and resistivity measurements show that there exists a solid solution region, through which the structure changes from orthorhombic to tetragonal and the superconducting properties are depressed. The solubility limits depend strongly on the size of the rare-earth ion, with the smallest (Dy) showing no appreciable solubility. The superconducting transition temperature versus x for all of the rare-earth-ion substitutions falls on a universal curve, indicating that the Ba sites are extremely ionic and magnetically isolated.

The MRCAT insertion device beamline at the Advanced Photon Source

The Materials Research Collaborative Access Team (MRCAT) beamline 10-ID at the Advanced Photon Source has been in Commissioning since January 1998. This Beamline has been designed as a flexible spectroscopy, scattering, and diffraction facility for the study of materials in situ. Over 70 users have performed experiments at the beamline during this period. Details about the beamline layout, components, performance and configuration for typical experiments will be presented

Origin of enhanced growth of the 110 K superconducting phase by Pb doping in the Bi‐Sr‐Ca‐Cu‐O system

Kinetics of the 110 K superconducting phase formation has been studied in the Bi‐Sr‐Ca‐Cu‐O system with and without the addition of small amounts of lead. The kinetics of the phase transformation has been greatly affected by lead doping. The time required for the maximum formation of the 110 K phase is substantially reduced by the addition of lead. This phenomenon has been shown to be associated with the lead enhancing the nucleation and growth process of the 110 K phase and the diffusivity of calcium and copper.

The superconductivity of Sc5T4Si10 (T = Co, Rh, Ir) and isomorphous compounds☆

Abstract Superconductivity is reported for a new class of ternary compounds of general formula R 5 T 4 X 10 where R is Sc, Y or a rare earth, T a transition metal of the platinum or iron groups and X either Si or Ge. Critical temperatures up to 8.6 K are observed. The relationship with non-superconducting compounds of similar composition and structure is discussed.