Paul-Henri Duvigneaud

The sintering of activated CaO

The sintering of CaO has been investigated from oxides of different “activity” resulting from the decomposition of CaCO3 at 1000‡C or Ca(OH)2 between 600 and 1000‡C.Nitrogen adsorption, X-ray broadening and electrical conductivity were carried out in order to correlate the activity and the sintering ability.Cobles models of first and intermediate stages of sintering can be applied to these oxides, considering that the self-diffusion coefficient of rate-controlling species (i.e.DCa2+) is time dependent. The variations ofDCa2+ are connected with the presence of non-stoichiometric defects, related to the conditions of preparation, which are eliminated during sintering. The occurrence of such an excess of defects as well as the decrease of grain radius explain the improved sintering of the oxide activated by transformation of original carbonate to the hydroxide form.

Effect of zinc and iron on the (micro-)structure and copper charge excess of the YBaCuO superconductor

Samples of YBa2(Cu1−yZny)3O7−y and YBa2(Cu1−yFey)3O7+x, withy in the range 0.0–0.16 for zinc and 0.0–0.30 for iron were synthesized by solid-state reaction. The solubility limit has been found to be equal to 7% and 19 at % for zinc and iron, respectively. Zinc has little effect on the structure which remains orthorhombic throughout all the zinc concentration range, while iron induces an important structural modification (tweed structure formation). Both impurities induce grain-growth inhibition but densification appears to be independent of the impurity content with respect to the undoped material. However, iodometry shows that the oxygen stoichiometry decreases in zinc-doped samples while it increases in iron-doped samples. Charge balance resulting from the dopant charge and the evolution of the copper charge with doping have been invoked. In zinc-doped samples, the copper charge excess (copper charge fraction>2) decreases sharply for 0.0⩽y<0.04, then it shows a plateau-like behaviour for 0.04⩽y<0.06, while in iron-doped samples, it decreases almost monotonically. Confirming some of our previous results there is a correlatedTc decrease in the case of zinc-doped samples (occurrence of aTc plateau) and in the case of iron-doped samples (quasi monotonical decrease). This difference has been interpreted in terms of structural changes related to the different substitution behaviour of zinc and iron.

Properties of epitaxial YBa2(Cu1-xZnx)3O7 thin films prepared by inverted cylindrical magnetron sputtering

Epitaxial YBa2(Cu1-xZnx)3O7-(delta ) thin films were prepared by an inverted cylindrical magnetron sputtering technique. X-ray diffraction shows an epitaxial growth with the c axis perpendicular to the substrate surface. The surface morphology of the films was studied by scanning electron microscopy. Auger electron microscopy analysis reveals that there is a good homogeneity throughout the film thickness. The transport properties such as the resistivity and the Hall effect were studied. The critical current densities were obtained indirectly from the imaginary part of the complex AC- susceptibility, and directly from the transport measurement performed on film microbridges. Both indirect and direct measurements gave comparable results. The critical current densities are 1.6 X 107 A/cm2 and 8 X 106 A/cm2 at 77 K for the pure films deposited on SrTiO3 and MgO substrates, respectively. The epitaxy and the values of Tc and Jc of the Zn-doped films are decreased with respect to those of the undoped films. It is found that the thermally activated flux creep model gives the best agreement with the Jc values of the pure and Zn-doped films, with a decrease of the activation energy UO for the Zn-doped films.