Jacques Rogez

Thermodynamics of open networks: ordering and entropy in NaAlSiO4 glass, liquid, and polymorphs

The thermodynamic properties of carnegieite and NaAlSiO4 glass and liquid have been investigated through Cp determinations from 10 to 1800 K and solution-calorimetry measurements. The relative entropies S298-S0 of carnegieite and NaAlSiO4 glass are 118.7 and 124.8 J/mol K, respectively. The low-high carnegieite transition has been observed at 966 K with an enthalpy of transition of 8.1±0.3 kJ/mol, and the enthalpy of fusion of carnegieite at the congruent melting point of 1799 K is 21.7±3 kJ/mol. These results are consistent with the reported temperature of the nepheline-carnegieite transition and available thermodynamic data for nepheline. The entropy of quenched NaAlSiO4 glass at 0 K is 9.7±2 J/mol K and indicates considerable ordering among AlO4 and SiO4 tetrahedra. In the liquid state, progressive, temperature-induced Si, Al disordering could account for the high configurational heat capacity. Finally, the differences between the entropies and heat capacities of nepheline and carnegieite do not seem to conform to current polyhedral modeling of these properties

Re-investigation of the La–Mg phase diagram

Literature results on the La–Mg binary system are critically reviewed. It is shown that most recent experimental results are not in agreement with the previously published assessments of the system. Thus, an experimental re-investigation of this phase diagram is required. Several alloys are synthesized from pure constitutive elements and characterized using X-ray diffraction, scanning electron microscopy, and differential thermal analysis. This study clarifies the phase equilibria in the La–Mg system. These results combined with the recent study of the enthalpies of formation of the intermediate compounds of this system provide a sound basis for the Calphad optimization of the system.

High Temperature X-ray Diffraction Study of the Oxidation Products and Kinetics of Uranium–Plutonium Mixed Oxides

The oxidation products and kinetics of two sets of mixed uranium-plutonium dioxides containing 14%, 24%, 35%, 46%, 54%, and 62% plutonium treated in air were studied by means of in situ X-ray diffraction (XRD) from 300 to 1773 K every 100 K. The first set consisted of samples annealed 2 weeks before performing the experiments. The second one consisted of powdered samples that sustained self-irradiation damage. Results were compared with chosen literature data and kinetic models established for UO2. The obtained diffraction patterns were used to determine the temperature of the hexagonal M3O8 (M for metal) phase formation, which was found to increase with Pu content. The maximum observed amount of the hexagonal phase in wt % was found to decrease with Pu addition. We conclude that plutonium stabilizes the cubic phases during oxidation, but the hexagonal phase was observed even for the compositions with 62 mol % Pu. The results indicate that self-irradiation defects have a slight impact on the kinetics of oxidation and the lattice parameter even after the phase transformation. It was concluded that the lattice constant of the high oxygen phase was unaffected by the changes in the overall O/M when it was in equilibrium with small quantities of M3O8. We propose that the observed changes in the high oxygen cubic phase lattice parameter are a result of either cation migration or an increase in the miscibility of oxygen in this phase. The solubility of Pu in the hexagonal phase was estimated to be below 14 mol % even at elevated temperatures.

Thermodynamic properties of saponite, nontronite, and vermiculite derived from calorimetric measurements

Abstract The stability of clay minerals is an important factor in assessing the durability of containment barriers for deep waste storage. In that context, the complete thermodynamic data set of three 2:1 ferro-magnesian clay minerals have been determined at 1 bar and from 2 to 520 K, using calorimetric methods. The studied clay samples were, respectively, the Na-saturated saponite Sap-Ca-1, Na0.394 K0.021Ca0.038(Si3.569Al0.397Fe3+0.034)(Mg2.948Fe2+0.021Mn0.001)O10(OH)2, the Ca-saturated nontronite NAu-1, Ca0.247K0.020(Si3.458Al0.542)(Mg0.066Fe3+1.688Al0.268Ti0.007)O10(OH)2, and the Ca-saturated Santa Olalla vermiculite, Ca0.445(Si2.778Al1.222)(Al0.192Mg2.468Fe3+0.226Fe2+0.028Ti0.018Mn0.007)O10(OH)2. The standard enthalpies of formation were obtained by solution-reaction calorimetry at 298.15 K. The heat capacities were measured between 2 and 520 K, using low-temperature adiabatic calorimetry, heat-pulse calorimetry, and differential scanning calorimetry. The standard entropies and the Gibbs free energies of formation at 298.15 K have been calculated from these values. Finally, the equilibrium constants at 298.15 K have been determined. A comparison between these experimental data and estimated values obtained from prediction models available in the literature enabled the most usual calculation methods available to date to be assessed for each thermodynamic property.

Biphasic MO2+x–M3O8–z Domain of the U–Pu–O Phase Diagram

The reduction of six mixed-oxide samples containing 14, 24, 35, 46, 54, and 62 mol % Pu was studied in situ by X-ray diffraction. The samples were first oxidized in air and subsequently reduced in a controlled atmosphere corresponding to a stoichiometric composition with an O/M = 2.00. After oxidation, we observed two structures, one cubic and one orthorhombic, MO2+x and M3O8-z. The two phases were subsequently reduced back to their stoichiometric O/M = 2.00 in a controlled atmosphere. The plutonium contents of the two resulting cubic structures differed from the initial one. We conclude that strong cation transport took place during oxidation, according to the shape of the tie lines in the biphasic MO2+x/M4O9-M3O8-z domain. The resulting overall O/M after oxidation was estimated. We propose the shape of the tie lines in the aforementioned biphasic domain and suggest a maximal plutonium solubility in the M3O8 structure at 8 ± 2 mol % (Pu/U + Pu) at 1573 K.

Dissolution de l'hydroxyapatite et du phosphate tricalcique β dans les solutions d'acide nitrique

Using an isoperibol calorimeter for rapid reactions and a Calsol type microcalorimeter for slow processes, are applied to determine the enthalpies of solution of two synthetic phosphate products in nitric acid. Namely, β tricalcium phosphate Ca3(PO4)2 and the calcium hydroxyapatite Ca10 (PO4)6 (OH)2 are measured by varying pH value of the solvent. Some dissolution mechanisms are proposed for various pH values. They are ensured by complementary reactions of solution of Ca(NO3)2, Ca(H2 PO4)2 and H3 PO4 in the same solvents. An extrapolation of solution enthalpies to pH=7 leads to the enthalpy of solution of these products in the pure water. These values are ΔsolH °=–138.3 kJ mol–1 for Ca3 (PO4)2 and –393.6 kJ mol–1 for Ca10 (PO4)6 (OH)2 .

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.