CHARACTERIZATION OF THE 105 K SUPERCONDUCTOR Tl2Ba2CaCu208 ("2212")

International audienceHomogeneous Tl1.8Ba2CaCu208 ceramics were synthesized by a novel route, starting from Tl2Ba205 precursors and using high gas pressures. This method allows tight control of thallium losses, resulting in dense, large-grained samples with sharp superconducting transitions above 105 K. Results of a characterization by X-ray diffraction, optical micrographs and micro-probe analysis are presented, together with selected physical properties

Preparation of pure Tl2Ba2CuO6+x: the contribution of phase equilibrium studies

International audienceThe formation of the pure thallium-based cuprate Tl2Ba2CuO6+x (2201) using a convenient process in the ternary TIO1.5-BaO-CuO phase diagram is described. This process is based on the congruent formation of the oxide Tl2Ba2O5 to which CuO is mixed, constituting a quasi-binary system. In the resulting 2201 compound no trace of the main poisoning magnetic impurity BaCuO2 could be detected by AC susceptibility measurements. The orthorhombic to tetragonal transformation is asserted to be due to thallium deficiency which occurs during the heat treatments at a temperature depending on the oxygen partial pressure

The Ca0.80CuO1.93 phase : New way of synthesis by the sol-gel method

We report on the reaction kinetics for the Ca0.80CuO1.93 phase prepared by the sol-gel method using acrylamide for gelification. The phase was prepared 95% pure after a 2 hour treatment at 800°C. Comparative stability under vacuum and oxygen has been determined. The kinetics parameters for the decomposition have been established : the activation energy is Ea= 626 kJ/mol and the pseudo reaction order n = 0.5

The oxidation mechanisms of Cu54Ni45Mn1 (Constantan) tapes: Kinetic analysis

International audienceThe kinetic parameters, namely the triplet activation energy EA, model function f(alpha) or g(alpha) and pre-exponential factor A of the oxidation of Constantan tapes in 1 atm of oxygen have been determined from both isothermal and non-isothermal thermogravimetry. For isothermal experiments, with temperatures ranging from 650 degrees C to 900 degrees C, the results from direct conversion of the weight increase as a function of the time and curve fitting, are compared with the isoconversion method. For the non-isothermal experiments, with heating rates from 1 degrees C/min to 20 degrees C/min, comparison is made between the Friedman differential method and the integral methods of Kissinger, Ozawa and Li and Tang. All methods give apparent activation energies with relative standard deviations as low as 3%. The results converge to the identification of three stages in the oxidation behaviour. A parabolic law for reaction extents alpha below 15% with E-A = 246 +/- 7 kJ mol(-1), In A = 14.3, is followed by two linear stages with E-A = 244 +/- 4 kJ mol(-1) and In A= 15.3 for 0.18 45%, respectively

Tl2Ba2CuO6+x : from normal state to superconductivity

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METALLURGY, HRTEM, MAGNETIC-PROPERTIES AND SPECIFIC-HEAT OF TL2BA2CU1O6+DELTA 90 K-SUPERCONDUCTORS OBTAINED BY A NEW PROCESS

International audienceHigh-quality Tl2Ba2CuO6+delta (2201) compounds were synthesized using a high-pressure route. Starting from Tl2Ba2O5 and CuO, the reactions yielding either the orthorhombic or the tetragonal (superconducting) phases were carried out under 100 bar of oxygen or argon and helium, respectively. Preferential thallium losses are prevented and high sintering temperatures can be used to obtain pure samples with large 2201 grains. The experiments performed include metallographic examinations, X-ray diffraction, microprobe analysis, plasma emission spectroscopy, HRTEM, AC susceptibility, Meissner field cooling, hysteresis loops, normal-state susceptibility and specific heat measurements