M.C. Hash

Oxide-ion conductivity of bismuth aluminates

Abstract Novel compounds in the Bi-Al-O and La-Bi-Al-O systems were prepared and tested for conductivity at high temperatures (500–800°C) in an oxygen gradient cell. The purpose is to develop an electrolyte material that will permit operation of a solid oxide fuel cell at 500–800°C instead of the 1000°C now used. The Bi-Al-O and La-Bi-Al-O compounds were found to have conductivities of about 10 −2 ohms −1 cm −1 and 10 −1 ohms −1 cm −1 at 800°C, respectively, in an oxygen gradient and ma y have the needed conductivity and stability toward humidified hydrogen for use as fuel cell electrolytes.

Solid state synthesis of Bi2Sr2CaCu2Ox superconductor

Abstract The high-temperature superconducting material Bi 2 Sr 2 CaCu 2 O x (2212) was synthesized by reaction of oxides and carbonates. Several stoichiometries and a range of temperatures and oxygen partial pressures were investigated for the production of pure 2212. The reaction sequence to form Bi 2 Sr 2 CaCu 2 O x was found to consist of initial formation of Bi compounds, their subsequent reaction to form the Bi 2 Sr 2 CuO x phase and CaCu oxides, and a slow reaction to form the Bi 2 Sr 2 CaCu 2 O x phase. The best powder was made from a Bi:Sr:Ca:Cu composition equal to 2.0:1.7:1.0:2.0. The optimum heat treatment was found to consist of initial heating to about 725 °C in flowing oxygen at a reduced total pressure of 5 x 10 2 Pa, followed by brief heating in air to about 885 °C, and, finally heating in air at 842 °C for 2 to 3 days.

Thermomechanical processing of reactively sintered Ag-clad (Bi, Pb)2Sr2Ca2Cu3Ox tapes

Abstract Agclad tapes of Bi 1.8 Pb 0.4 Sr 2 Ca 2 Cu 3 O x annotated as 2223, were fabricated from polyphase powders which reactively sinter in situ to form 2223, and from completely reacted 2223. Annealing temperatures, as well as mechanical processing schemes involving intermittent cold uniaxial pressing, were investigated to arrive at a thermomechanical regimen that would promote increased critical current density (J c ). Reactive sintering of polyphase tapes is very sensitive to annealing temperature and must be optimized to allow maximum transformation to, and prevent decomposition of, the 2223 phase. Continued intermittent pressing, after initial grain-growth annealing, is necessary to advance the reaction and allow consumption of alkaline earth cuprate second phases, as well as to promote grain texturing. The highest J c values (9000 A/cm 2 at 77 K and 0 T field) were obtained in tapes containing a polyphase starting powder composed of nearly equal fractions of 2212 and 2201 annealed for 350 h at 845°C with three intermittent pressings. Similarly treated tapes containing completely reacted starting powders attained J c values of only 3000 A/cm 2 .

Calcination of YBa2Cu3O7−x powder☆

YBa2Cu3O7−x was synthesized by mixed-oxide reaction of Y2O3, BaCO3, and CuO. The starting constituents were mixed, followed by multiple steps of calcination with interposed grinding. Heating rapidly to 900°C was shown to minimize formation of liquid during calcination while still producing a phase-pure powder. Slow heating to 900°C or fast heating to 950°C resulted in formation of a liquid phase deleterious to YBa2Cu3O7−x synthesis.

Oxygen stoichiometry, phase stability, and thermodynamic behavior of the lead-doped Bi-2223 and Ag/Bi-2223 systems

Abstract Electromotive-force (EMF) measurements of oxygen fugacities as a function of stoichiometry have been made in the lead-doped Bi-2223 superconducting system in the temperature range 700–815°C by means of an oxygen titration technique that employs an yttria-stabilized zirconia electrolyte. The results of our studies indicate that processing or annealing lead-doped Bi-2223 at temperatures ranging from 750 to 815°C and at oxygen partial pressures ranging from ∼ 0.02 to 0.2 atm should preserve Bi-2223 as essentially single-phase material. Thermodynamic assessments of the partial molar quantities Δ S ( O 2 ) and Δ H ( O 2 ) indicate that the plateau regions in the plot of oxygen partial pressure versus oxygen stoichiometry (x) can be represented by the diphasic CuOCu2O system. In accord with the EMF measurements, it was found that lead-doped Bi-2223 in a silver sheath is stable at 815°C for oxygen partial pressures between 0.02 and 0.13 atm.

Effect of heat treatment time and temperature on the properties of YBa 2 Cu 3 O 7− x

Two YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} powders were investigated: one was stoichiometric, and the other, CuO-rich. After being sintered at 950 {degree}C, samples from both powders were heat-treated in oxygen at 845, 860, and 910 {degree}C for up to 10 days. The samples were characterized by differential thermal analysis, x-ray diffraction, optical and scanning electron microscopy, iodometric titrations, and measurement of their superconducting properties (electrical resistivity, critical temperature, and critical current density). The electrical properties of the nearly stoichiometric material improved with time at all temperatures. On the other hand, the electrical properties of the CuO-rich material deteriorated with temperature, possibly owing to problems associated with grain growth at 910 {degree}C.

Synthesis of 85 K BiSrCaCuO superconductor

Abstract Synthesis of the compound Bi 2 Sr 2-y CaCu 2 O x , 0≤y≤0.5, was attempted from 10 4 to 10 6 Pa oxygen pressure. Lower pressures and temperatures less than 800°C proved most effective in forming the 85-K superconducting material. A Sr-deficient stoichiometry exhibited highest phase purity. Results suggest, however, that presence of a small amount of a phase such as the low-temperature superconductor, Bi 2 Sr 2 CuO 6 , has little effect on the resulting superconducting properties.

Synthesis and sintering of Tl2Ca2Ba2Cu3Ox

The composition TlCa3BaCu3Oy has been reacted to form the high-temperature superconducting phase Tl2Ca2Ba2Cu3Ox. Compacts were sintered in sealed containers from 870 to 925 degrees C in various oxygen partial pressures. Zero-resistance temperatures (T0) reached 118 K; the highest values were obtained from sintering in pure oxygen at 915 to 920 degrees C for 2 to 4 h. Critical current densities (Jc) at 77 K in zero field reached about 700 A cm-2; no correlation was found between T0 and Jc.

Effect of heating rate on properties of YBa2Cu3O7-x

YBa2Cu3O7-x powders, made at Argonne National Laboratory by solid-state reaction or supplied by Rhone-Poulenc, were sintered in oxygen at 910 or 950 degrees C. Heating rates to the sintering temperature were varied from 0.5 to 5 degrees C min-1. Sintered specimens were characterised by differential thermal analysis, X-ray diffraction, optical and scanning electron microscopy, and measurement of electrical resistivity, superconducting transition temperature and critical current density. It was found that the heating rate affected only the properties of specimens which were heated to a temperature at which second phases present could react. Slower heating improved electrical properties as a consequence of inducing greater reaction to desired phases such as YBa2Cu3O7-x.

Oxygen stoichiometry, phase stability, and thermodynamic behavior of the lead-doped and lead-free Bi-2212 systems

Abstract Electromotive-force (EMF) measurements of oxygen fugacities as a function of stoichiometry have been made on lead-doped and lead-free Bi2−zPbzSr2Ca1Cu2Ox superconducting ceramics in the temperature range ≈ 700–815°C by means of an oxygen-titration techique that employs an yttria-stabilized zirconia electrolyte. Equations for the variation of oxygen partial pressure with composition and temperature have been derived from our EMF measurements. Thermodynamic assessments of the partial molar quantities Δ H (O2) and Δ S (O2) for lead-doped Bi-2212 and lead-free Bi-2212 indicate that the solid-state decomposition of these bismuth cuprates at low oxygen partial pressure can be represented by the diphasic CuOCu2O system.