F. Hanic

Fast ionic conductivity of PbF2:MF2 (M = Mg, Ba, Cd) and PbF2:ScF3 single crystals and composites

Abstract The fluorite-structured heterovalent PbF2:ScF3 and isovalent PbF2:MF2 (M=Mg, Ba, Cd) concentrated solid solutions were prepared as single crystals in a broad concentration range. In the eutectic systems, PbF2–MgF2 and PbF2–ScF3, the eutectic and near-eutectic composites were prepared from the melt. Saturated solid solutions of each system form two phases of these composites. The fast ionic conductivity, static permittivity and microstructure of both the single crystals and the composites are examined. Fastest ionic conductivity, with a conduction enthalpy of 0.389 eV, is found in PbF2:7 m/o ScF3 solid electrolyte. In this material, the fluoride ion conductivity at 500 K is equal to 0.13 S/cm. Plausible models of defect structures and conduction mechanisms are proposed for all examined systems.

Influence of copper- and iron-doping on cubic yttria-stabilized zirconia

Cubic yttria-stabilized zirconias (YSZs) (15 mol% Y2O3) doped with 2 wt% CuO and 0.3 wt% Fe2O3, respectively, have been compared with undoped YSZ. The lattice constants were found to increase in the sequence:aYSZ <aYSZ/Fe <aYSZ/cu. Vickers microhardness of polycrystalline YSZ/Fe exceeds that of polycrystalline YSZ/Cu, whereas the hardness of the single crystalline materials YSZ/Fe and YSZ/Cu are nearly identical and consistently lower than the polycrystalline values. Raman and infrared spectra reveal a breakdown of the selection rules, i.e. these techniques probe the local, non-cubic arrangement of oxygen vacancies. The findings are discussed in terms of a substitutional versus an interstitial-doping model.

Thin YBCO films prepared by low-temperature spray pyrolysis

Thin YBCO films were prepared from aerosol by a low-temperature deposition process consisting of two steps-the deposition at atmospheric pressure and firing in vacuum degraded by partial pressure of oxygen-both at temperatures not exceeding 600 degrees C. A stoichiometric 1-2-3 aqueous nitrate solution of Y, Ba and Cu constituents was used as a source of aerosol. MgO and Al2O3 substrates were heated to 140 to 160 degrees C during deposition. The obtained films were 1 to 10 mu m thick with Tc>80 K and Jc=103 to 104 A cm-2. However, detailed TG, DTG and DTA studies performed by the authors and others showed that thermal decomposition of the nitrates used only starts at temperatures higher than 200 degrees C. The use of low substrate temperatures during deposition (140 to 160 degrees C in this case) is apparently the reason for rather low Jc values. Optimization of the preparation process is suggested, which could lead to higher Jc values at processing temperatures not exceeding 600 to 700 degrees C. Such temperatures are still interesting for substrates with elevated diffusivity into prepared films.

Hysteresis of transport critical currents in high-temperature Y1Ba2Cu3O7-x superconductors: bulk samples and thin films

The current-voltage characteristics and transport critical currents at T=4.2 K and 77.3 K in Y1Ba2Cu3O7-x bulk and thin film samples in magnetic fields up to 9 T have been measured. Large hysteresis of resistively measured critical currents was observed. This is caused by the magnetic field history as well as by the transport current history. An additional explanation of the observed hysteresis based on the different surface and volume pinning mechanisms is proposed.

High Tc Y-Ba-Cu-O thin films prepared by in situ low-temperature codeposition of Y, BaF2, and Cu on α-Al2O3 substrates

Thin Y‐Ba‐CU‐O films were prepared in‐situ by low‐temperature codeposition on Al2O3 substrates. The temperatures during preparation did not exceed 600 °C. As a source of Ba the BaF2 was used. The zero resistance Tc values were higher than 84 K, Jc =4×104 A/cm2 at 4.2 K/OT. The Auger electron spectroscopy analysis has shown almost a homogeneous distribution of the film components throughout the film thickness. X‐ray diffraction revealed the presence of unoriented 1‐2‐3 phase besides BaF2 and CuO and, one undefined phase, which might be oxyfluoride compound.

The crystal structure of ammonium metavanadate

AbstractThe paper deals with the structure analysis of ammonium metavanadate NH4VO3from photographs obtained with a Weissenberg goniometer using CnKαand MoKαradiation. The dimensions of the orthorhombic unit cell determined from these photographs of the rotating crystal are a=4.92, b=11.82, c==5.85 Å. A satisfactory space group is Pbcm (D2h11),with four molecules in the unit cell. The positions of the atoms in the unit cell were determined by means of the Patterson functions P(v, w), P(u, v) and from the projection of the electron density on the (001)plane. An analogy was found with the structure of the metasilicates from the pyroxen group. The oxygen atoms are coordinated tetrahedrally around the vanadium atoms. The tetrahedra VO4form infinite chains in the direction of the [001]axis. The VO4chains are mutually bonded by means of electrostatic bonds of the NH4ions. The splitting of the chains in an aqueous solution is connected with the dissociating and catalytic properties of the ammonium metavanadate.

Preparation and properties of transparent thoria-yttria ceramics

Abstract Precipitation and sintering conditions of thoria-yttria have been optimized to obtain transparent ceramics at low sintering temperatures. Density, micro- and X-ray structure, electrical conductivity and light transmission of the sintered bodies have been determined.

Study of copper-chromium oxide catalyst: I. Thermal decomposition of copper(III) chromate, CuCrO4

Abstract The kinetics, mechanism, and activation energy of the isothermal decomposition of CuCrO4 was studied using an isothermal TG method and an X-ray high-temperature diffraction technique in either air or a flowing atmosphere of N2. The enthalpy change ΔH of the decomposition reaction 2 CuCrO 4 → CuO + CuO + CuCr 2 O 4 + 3 2 O 2 was determined by DSC analysis. The mechanism of the thermal decomposition of CuCrO4 is well represented by the standard Avrami-Erofeev kinetic equation [− ln (1 − α)] 1 2 = kt . According to this mechanism, the reaction rate is controlled by the formation and growth of nuclei on the surface of the reactant. The activation energy EA of the process in air is EA = (248 ± 8) kJ mole−1, in flowing atmosphere of nitrogen EA = (229 ± 8) kJ mole−1. ΔH in air is 110 kJ mole−1, in flowing nitrogen 67 kJ mole−1. The lower values of ΔH and EA in the flowing atmosphere of nitrogen are due to the fast elimination of O2 from the reaction interface. However, the decay of the crystalline portion of CuCrO4 during its thermal decomposition, studied by the X-ray diffraction, is controlled by a different reaction mechanism (first-order kinetics). The reaction mechanism is discussed in the relation to the crystal structure of the reactants.

Epitaxial YBa2Cu3O7 superconducting films, without twin planes on Y2O3/YSZ/Si

Abstract Epitaxial c -axis oriented YBa 2 Cu 3 O 7 superconducting thin films have been grown by off-axis sputtering on Si substrates covered with an Y 2 O 3 /YSZ buffer layer. The films show good superconducting properties and are crack-free for thickness as high as 120 nm. TEM observations reveal an epitaxial growth of YBCO on Y 2 O 3 with an angle of 45° between the [100] directions. Both layers display a microstructure which consists of slightly misoriented rectangular twin-free monocrystalline mosaic blocks.

Study of copper-chromium oxide catalyst: II. Crystal data and thermal decomposition of basic copper(II) ammonium chromate

Abstract Basic copper(II) ammonium chromate (BCAC) of the composition CuCrO4(Cu(OH)2)x-(NH4OH)y, (NH3)z has been prepared. Depending on the pH of the precipitation reaction, quotients x, y and z vary in the intervals 0 C 2 m , Z is 4 and Dm = (2.98±0.04) Mg m−3). The thermal decomposition of BCAC proceeds in four steps. The activation parameters, Ea, A, ΔH≠ and ΔS≠, of the first step of the thermal decomposition were derived.