M. Lomello-Tafin

Pathway for the formation of the Tl-2223 phase : an in situ neutron powder diffraction study

The reaction pathway for the formation of Tl2Ba2Ca2Cu3Oz (Tl-2223) has been investigated by in situ neutron powder diffraction. The experiments were carried out in an initially evacuated closed system on two samples with nominal compositions Tl1.7Ba2Ca2Cu3Oz and Tl2.3Ba2Ca2Cu3Oz. We find that, under these conditions, the formation path implies Tl2BaO4, Tl6Ba4O13 and Tl2Ba2CaCu2O8 (Tl-2212) according to the sequence: precursors/oxides→{Tl2BaO4}→{Tl6Ba4O13}→{Tl-2212}→{Tl-2223}. The difference with respect to the sequence observed in an open system (precursors/oxides→Tl-2201→Tl-2212→Tl-2223) is explained by the low temperature reaction (780 K) between the TlOx vapour and the free BaO resulting from the decomposition of BaCuO2 used as a precursor. The conversion from Tl-2212 to Tl-2223 occurs at 1075 K and does not involve the formation of a liquid phase but the solid-state diffusion of the Ca and Cu species and the restructuring of the Tl-2212 lattice. However, this conversion does not occur when the Tl content is 2.3, in agreement with previous works on phase equilibria. The reacted samples were characterized by high-resolution neutron powder diffraction, ac susceptibility measurements and SEM/EDX. Some original results on crystal structures and, to a lesser extent, on phase equilibria are also given.

Unsubstituted Tl-1223: a possible candidate for high current applications of superconductivity

Abstract The Tl–Ba–Ca–Cu–O system has been investigated. We report on the superconducting properties and the pathway for the formation of the Tl-1223 phase and on equilibrium phase relations. The formation of Tl-1223 implies the double TlO layer phases and occurs at high temperature (890–910°C) by a progressive de-intercalation of TlO layers from Tl-2223. As a consequence of this non-equilibrium process, transient intergrowth phases may be formed and have been unambiguously identified. This path of formation results in a platelet-like morphology for Tl-1223 grains which, in the view of the fabrication of tapes, is much more interesting than the globular shape observed for (Tl,Pb)(Sr,Ba) 2 Ca 2 Cu 3 O z . Moreover, the superconducting properties ( T c and irreversibility line) of the unsubstituted Tl-1223 phase compare well with those of (Tl,Pb)(Sr,Ba) 2 Ca 2 Cu 3 O z . With regard to phase equilibria, we show the existence of a three phase field Tl 2 Ca 3 O 6 –(Tl-2212)–(Ca,Tl) 1− x CuO y which prevents the formation of Tl-2223 for Tl contents higher than two.

Preparation and characterization of Au/ZrO2 nanoparticles obtained by oxidation of ZrXAuY alloy

Recently, we have shown that powders containing both gold and ZrO 2 nanosized particles could be prepared by low temperature (T< 100 °C) oxidation of cast crystalline Zr-Au alloys. In this report, we present the preliminary results of a study aiming at preparing the mixed nanosized particles from thin films. In a first step, pure Zr and Au layers are alternatively deposited by RF sputtering on Si and W substrates under vacuum. Then, the intermetallic compounds are formed at high temperature under an inert atmosphere or in vacuum. The resulting samples are characterised by X-ray diffraction and by scanning electron microscopy (SEM).

Intergrowth of structures in the Tl–Ba–Ca–Cu–O system

Abstract Three intergrowth phases between the high- T c superconducting cuprates TlBa 2 Ca 2 Cu 3 O y (Tl-1223) and Tl 2 Ba 2 Ca 2 Cu 3 O y (Tl-2223) have been detected and identified in a non-equilibrium sample with composition Tl 1.05 Ba 2 Ca 2 Cu 3 O y . Complete structure models including space group, cell parameters and atom coordinates are derived for quantitative analysis. X-Ray diffraction, AC susceptibility and scanning electron microscopy experiments show that these intermediate phases have a long range order. The results are discussed in the light of the formation path for Tl-1223 whose final step is the transformation from Tl-2223 to Tl-1223.

The reaction pathway for the formation of Tl-2223

Abstract The reaction pathway for the formation of Tl 2 Ba 2 Ca 2 Cu 3 O z (Tl-2223), a promising compound for applications in passive microwave devices, has been investigated by in situ neutron powder diffraction. The experiments were carried out in an initially evacuated closed system on two samples with compositions Tl 1.7 Ba 2 Ca 2 Cu 3 O z and Tl 2.3 Ba 2 Ca 2 Cu 3 O z . We find that, under the conditions specified above, the formation path implies Tl 2 BaO 4 , Tl 6 Ba 4 O 13 and Tl-2212 according to the sequence: precursors/oxides → Tl 2 BaO 4 → Tl 6 Ba 4 O 13 → Tl-2212 → Tl-2223. The difference with respect to the sequence observed when the experiments are carried out in an open atmosphere (precursors/oxides → Tl-2201→ Tl-2212 → Tl-2223) is explained by the low temperature reaction between the TlO x vapour which becomes appreciable at 700 K in closed system and the free BaO resulting from the decomposition of BaCuO 2 . The conversion from Tl-2212 to Tl-2223 occurs at 800 °C and does not involve the formation of a liquid phase but the solid state diffusion of the Ca and Cu species and the restructuring of the Tl-2212 lattice. This conversion however does not occur when the Tl content is equal to 2.3 in agreement with previous reports on phase equilibria.