Françoise Valdivieso

Surface and structural characterization of CexZr1-xO2 CEZIRENCAT mixed oxides as potential three-way catalyst promoters

The textural and structural properties of high specific surface area (HS) CexZr1-xO2 mixed oxides (CeZrMO) and their modification upon thermal aging have been investigated. Results from BET area determination and complete porosity analysis, as well as high-resolution electron microscopy, XRD, Raman and FTIR spectroscopies are presented. Some relationships between the molar composition of the binary systems and their textural and structural properties are presented. In addition, it has been possible to propose an explanation for structural features of the mixed oxides (solid-solution structures and possible phase segregation) as a function of Ce content, a point of great interest for the knowledge of phase composition and stability of CeZrMO.

Differences in reactivity of oxide growth during the oxidation of Zircaloy-4 in water vapour before and after the kinetic transition

Abstract The oxidation of Zircaloy-4 by water vapour has been studied between 500 and 550 °C, the water vapour partial pressure ranging in 13–80 hPa, using isothermal and isobaric thermogravimetry, and calorimetry. During gravimetry experiments, sudden changes in temperature or water vapour pressure have also been performed. It results that the approximations of steady state and rate-limiting step are only valid before the kinetic transition. In the post-transition region, a significant influence of water vapour and hydrogen partial pressures has been found, contrarily to the kinetic behaviour before the transition (which is in this last case, in good agreement with a rate-limiting step of diffusion of oxygen vacancies). It comes out that the post-transition kinetic behaviour is definitely not the same as before the transition.

Kinetic study of the oxidation by oxygen of liquid Al–Mg 5% alloys

The oxidation into MgO of an industrial Al–Mg 5% alloy in the liquid state has been studied at 700°C by thermogravimetry, under controlled oxygen partial pressure. Since the kinetic curves were not reproducible, it was not possible to obtain directly from them the variations of the areic conversion rate (mol of MgO s−1 m−2) versus the oxygen partial pressure. Therefore, it has been necessary to use a method based on the isolation method, which allowed us to overcome the problems of non-reproducibility of the curves. It was found that the areic rate of growth of MgO decreases when the oxygen pressure increases. A reaction mechanism in elementary steps has been proposed to account for these results, involving two competitive oxygen adsorptions on the MgO surface.

Reduction by hydrogen of U3O8 into UO2: Nucleation and growth, influence of hydration

The reduction of U3O8 by hydrogen has been studied by thermogravimetry in the range 510–600°C and 1–40 kPa hydrogen partial pressure. The kinetic curves have been interpreted by a new model involving steps of nucleation and growth in which the rate limiting step of growth is located at the external interface. This allowed us to determine the variations in the areic nucleation frequency and the areic reactivity of growth as a function of hydrogen pressure and temperature. Mechanisms have been proposed which account for the variations in the areic nucleation frequency and reactivity of growth as a function of the partial pressure of hydrogen and the temperature of reduction. The influence of the hydration of UO3 prior to its reduction by hydrogen on the kinetics could also be explained by the same model. It was found to lower the areic nucleation frequency without changing the areic growth reactivity.

Thermodynamic description of the nonstoichiometric defect structure in Ce1−xZrxO2 solid solution powders

Equilibria between gaseous oxygen (from 870 to 20 000 Pa) and several high surface area Ce1−xZrxO2 powders (0≤x≤0.36) were studied by means of thermogravimetry in order to get a better insight into the nature and behaviour of the point defects in these solid solutions. By measuring the weight changes between two different oxygen pressures, we could obtain the variations of the oxygen vacancy concentration at the surface of the solutions as a function of the oxygen pressure and zirconium concentration. The anionic sublattice was shown to follow an ideal thermodynamic behaviour, the oxygen vacancy concentration being proportional to PO2−1/6. By comparing the experimental and theoretical variations of the oxygen vacancy concentration versus the zirconium concentration, we found that the cationic sublattice was well described by a nonideal (regular) model in which the activity coefficients of the structural elements CeCex, ZrCex and of the associated defect (ZrCe, CeCe′)′ are different from 1.

Thermal stability of Ce1 − xZrxO2 solid solution powders

High surface area Ce1 − xZrxO2 powders (0 < x < 0.36) were annealed at 950 °C. The surface area variation versus time of calcination was followed by BET method for all the samples. When the experimental rates of the surface area decrease are plotted versus x, two extremes are observed: a minimum around 0.09 and a maximum around 0.19. Our previous model involving oxygen and cerium diffusion proposed to interpret the stabilizing effect of zirconium at low content in ceria cannot account for the present experimental rate variations. We propose that the assumption of the ideal behaviour of the solutions is no longer valid when x reaches values as high as 0.32 and 0.36. Thus, provided that the Ce1 − xZrxO2 phases are considered as strictly regular solutions and on the basis of the kinetic model for ceria, it can be shown that the present experimental rate can be interpreted

Kinetic study of the dehydration of lithium sulphate monohydrate

Abstract The dehydration of lithium sulphate monohydrate has been studied by isothermal thermogravimetry at 80 °C, under various partial pressures in water vapour. The kinetic curves are sigmoid, they are interpreted with a model involving nucleation and growth steps: the nucleation is supposed to occur at the surface of the grains and is followed by a rapid two-dimensional growth, the new phase developing towards the centre of the grains. The studied powder being composed of ellipsoidal particles, this geometry is taken into account in the model.

Mechanisms of Corosion and Oxidation of Metals and Alloys

Selected topics in field of the study of the mechanisms of corrosion and of oxidation of metals or alloys are presented. The first part reports a new model for the mechanism of the breakaway oxidation of ferritic stainless steels in water vapour. The second part is devoted to the physico-chemical aspects of oxidation and presents experimental methods useful in the kinetic modelling applied to two alloys, the zircalloy-4 and an AlMg5 % in the liquid state. In the third part the physical and numerical modelling of the stress corrosion cracking behaviour in face-centered cubic (fcc) alloys is detailed, which enables the study of the influence of macroscopic parameters (such as the temperature or hydrogen activity) on the fracture process.

Etude cinetique et modelisation de l'oxydation du magnesium par l'oxygene

Kinetic study and modelling of the oxidation of magnesium by oxygen. The oxidation of magnesium to MgO was studied by isothermal thermogravimetry (484 to 518°C), under controlled oxygen partial pressure. The variations of the growth reactivity (mole of MgO.s−1.m−2) with the oxygen pressure were obtained : they are quite complex, since the growth reactivity increases first with the pressure, then decreases and finally increases again. Reaction mechanisms were proposed to account for these variations, which were attributed to a change in the reaction mechanism, depending on the oxygen pressure range.

Kinetic study and modeling of coarsening in a BaTiO3 powder at 850 °C

Abstract The influence of the gaseous atmosphere and the Ba Ti ratio on the decrease in specific surface area in a BaTiO 3 powder was studied at 850 °C. A model involving elementary steps and the BaTiO 3 point defects was proposed to quantitatively account for the accelerating effect of the water vapour pressure and the Ba Ti ratio.