Thermodynamique des melanges binaires LiPO3-Pb(PO3)2: Diagramme de phases et étude thermodynamique du liquide

Study of solid-liquid phase diagram of LiPO₃-Pb(PO₃)₂ binary system, in certain calcination conditions, shows the existence of several metastable phasis. When heated at a temperature of 723 K the binary mixtures lead uncompletely to a defined compound Pb₂Li(PO₃). On heating these ternary solid mixtures, three eutectic reactions have been observed: LiPO₃+Pb(PO₃)₂→Liquid at a temperature of 793 K(1) LiPO₃+Pb₂Li(PO₃)₅→Liquid at a temperature of 843 K (2) Pb₂Li(PO₃)₅+Pb(PO₃)₂→Liquid at a temperature of 891 K (3) The metastable liquid phase appears in the system at temperature of 793 K. DTA experiments performed on the binary LiPO₃-Pb(PO₃)₂ mixtures, show a superposition of two diagrams. The first one is metastable and the second represents the stable equilibrium phase diagram. Measurements of liquid enthalpy of binary LiPO₃-Pb(PO₃)₂ system at temperature of 979.65 K were reported. The corresponding values were very small and so the binary system can be considered as athermal. Assuming an ideal behaviour, the liquidus curves in the metastable diagram were calculated and the eutectic reaction (LiPO₃-Pb(PO₃)₂→Liquid) was confirmed at 793 K. This revised version was published online in August 2006 with corrections to the Cover Date.     

Diagramme de phases du systeme binaire AgNO3—RbNO3

Phase diagram of the binary system AgNO₃—RbNO₃ was studied using thermal analysis technique, differential scanning calorimetry and X-ray diffraction. This binary exhibits a congruently melting compound Ag_0.5Rb_0.5NO₃ (m. p.=138°C), an incongruently melting one Ag_0.33Rb_0.66NO3 with two polymorphic varieties, two eutectics at (36 mol% RbNO₃, 128°C) and at (60 mol% RbNO₃, near 134°C) respectively and a peritectic at (60.5 mol% RbNO₃, 141°C). This system contains also three invariant reactions at 164, 222 and 282°C due to the phase transitions of RbNO₃ and another one at 164°C due to the phase transition of AgNO₃.

Ce travail a été présenté aux 32èmes JCAT, tenues à Hammamet, TN, du 12 au 14 Mai 2001.     

Oxygen exchange study in phase transformations of superconducting bismuth cuprates (Pb2212 and Pb2223)

The oxygen exchange between the condensed phase(s) and the gas phase was studied as a function of temperature for superconducting phases of the system (Bi,Pb)-Sr-Ca-Cu-O.An oxygen probe apparatus allowed confirmation that the variation in oxygen composition in the Pb2212 and Pb2223 superconducting phases is a reversible phenomenon on heating and cooling. It was demonstrated that the mass loss of both phases, for the 905 and 980°C isotherms, was due to the oxygen composition variation and to PbO loss (this latter phenomenon begin irreversible).     

Structural, microstructural and surface properties of a specific CeO2-Bi2O3 multiphase system obtained at 600 °C

Polycrystalline samples of (1−x) CeO₂−x/2 Bi₂O₃ phases, where x is the atom fraction of bismuth have been synthesized by the precipitation process and after the thermal treatment at 600 °C, under air. Samples are first characterized by the X-ray diffraction and scanning electron microscopy. To determine the samples specific surface areas, Brunauer-Emmett-Teller (BET) analyses have been performed. In the composition range 0≤x≤0.20, a cubic solid solution with fluorite structure is obtained. For compositions x comprised between 0.30 and 0.90, two types of T′ (or β′) and T (or β) tetragonal phases, similar to the well-known β′ or β Bi₂O₃ metastable structural varieties, are observed. However, the crystal cell volumes of these β′ or β Bi₂O₃ phases increase with the composition x in bismuth: this might be due to the presence of defects or substitution by cerium atoms, in the tetragonal lattices. Using X-ray diffraction profile analyses, correlations between bismuth composition x and crystal sizes or lattice distortions have been established. The solid-gas interactions between these polycrystalline materials and air-CH₄ and air-CO flows have been studied as a function of temperature and composition x, using Fourier transform infrared (FTIR) analyses of the conversions of CH₄ and CO gases into the CO₂ gas. The transformations of CH₄ and CO molecules as a function of time and temperature are determined through the intensities of FTIR CO₂ absorption bands. Using the specific surface areas determined from BET analyses, these FTIR intensities have been normalized and compared. For all bismuth compositions, a low catalytic reactivity is observed with air-CH₄ gas flows, while, for the highest bismuth compositions, a high catalytic reactivity is observed with air-CO gas flows.     

Elaboration de capteurs de gaz a partir de ceramiques conductrices ioniques de type NaSICON

The elaboration of Na⁺ super-ionic conductor (NaSICON) ceramics is studied in this work. These solid electrolytes can be used as sensor for detection of polluted gases in air. Two sorts of ceramics with different chemical compositions are synthesised by soft chemistry route: a zirconium-based NaSICON and a hafnium-based NaSICON. DTA-TG and temperature depending X-ray diffractometry were used to follow the thermal decomposition of the precursor phases. The electrical properties of these ceramic sodium ionic conductors are investigated by complex impedance spectroscopy (CIS). The substitution of the zirconium by the hafnium increases the ceramic conductivity and decreases the activation energy E ^a (from 0.29 to 0.12 eV). This revised version was published online in August 2006 with corrections to the Cover Date.