Experimental study and Monte-Carlo simulation of the nucleation and growth processes during the dehydration of Li2SO4,H2O single crystals

9 pagesInternational audienceA kinetic model for the dehydration of lithium sulfate monohydrate is proposed in order to account for experimental data obtained on single crystals by thermogravimetry at 80°C under fixed water vapour pressure, and by optical microscopy. This model is based on the assumptions of Mampel's model, the nucleation takes place randomly at the surface of the solid and is followed by isotropic growth toward the centre of the crystal. Calculated rates d/dt are obtained by means of Monte-Carlo simulations and compared to the experimental ones, which leads to the determination of two kinetic constants: the areic frequency of nucleation (in number of nuclei.m-2.s-1) and the areic reactivity of growth (in mol.m-2.s-1)

Oxidation kinetics of ZrNbO in steam: differences between the pre and post transition stages

International audienceThe oxidation by water vapour of a zirconium based alloy, a ZrNbO alloy containing 1% of Nb, 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 (jumps) in temperature or water vapour pressure have also been performed. It comes out that the kinetic behaviour is different before and after the transition, even though the approximations of steady-state and rate-limiting step are justified in both stages : the influence of temperature jumps is greater in pre-transition, whereas the effect of water vapour partial pressure is more pronounced in post-transition (nevertheless, an accelerating effect is also observed before the transition). No influence of hydrogen partial pressure has been observed. Besides, the higher the Nb content in the alloy, the higher the oxidation rate (in pre-transition). A mechanism has been proposed to account for the results obtained in pre-transition, involving the diffusion of adsorbed species in the porous part of the oxide layer as rate-determining step. The transition is accompanied by a change in the oxidation mechanism: in the post-transition stage, the kinetic curves being linear, the oxidation may be controlled by an interface step, which is probably different from the steps proposed for the pre-transition mechanism

Kinetic study of a HGdP2O7.3H2O dehydration step

International audienceA kinetic study of the transformation of HGdP2O7.1.5H2O into HGdP2O7.0.5H2O has been achieved by means of isothermal thermogravimetry experiments under controlled water vapour pressure. This pressure was found to have an accelerating effect upon the rate of reaction. The kinetic curves giving the extent of reaction versus time can be described using a Mampel's model. Via this modeling, values of surfacic growth rate and frequency of nucleation have been determined

High temperature oxidation of a Cu-Ni – based cermet: kinetic and microstructural study

International audienceThe oxidation of a cermet composed of nickel ferrite (NixFe3-xO4), nickel oxide (NiyFe1-yO) and Ni-Cu alloy (CuzNi1-z) (with x=0.8, y=0.84 and z=0.44) has been studied by thermogravimetry, at 960°C, under controlled oxygen partial pressure. The rate of oxidation increases with the oxygen partial pressure up to 5.1 103 Pa, then it becomes independent of the oxygen pressure. This is due to the presence of the two copper oxides, CuO and Cu2O, above this oxygen pressure. SEM observation of oxidized samples has shown that the oxidation mechanism involves both external and internal oxidation

Application d'un modèle de réaction en phase gazeuse à l'oxydation d'un alliage liquide Al-Mg 5% sous faible pression d'oxygène

National audienceApplication of a model of reaction in the gaseous phase to the oxidation of a liquid Al-5%Mg alloy at low oxygen pressure. The oxidation of a liquid Al-5%Mg alloy was studied by isothermal (700°C) and isobaric thermogravimetry, under controlled oxygen partial pressure (lower than 50hPa). At low oxygen pressure, the reaction proceeds in the gaseous phase, due to the evaporation of magnesium from the alloy. A model exists in the literature to describe such reactions ; we verified experimentally that the assumptions of this model could be made for our reacting system; consequently, the variations of the oxidation rate with oxygen partial pressure were accounted for.L'oxydation d'un alliage Al-Mg5% à l'état liquide a été étudiée par thermogravimétrie isotherme (700°C) et isobare, sous pression d'oxygène contrôlée, inférieure à 50 hPa. En effet, à faible pression d'oxygène, la réaction se déroule en phase gazeuse, du fait de l'évaporation du magnésium de l'alliage. L'utilisation d'un modèle proposé dans la littérature, dont nous avons vérifié expérimentalement la validité des hypothèses, a permis d'interpréter les variations de la vitesse d'oxydation avec la pression d'oxygèn

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

International audienceThe 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

Oxidation behaviour of unirradiated sintered UO2 pellets and powder at different oxygen partial pressures, above 350°C

International audienceThe oxidation of sintered UO2 pellets and powder into U3O8 has been studied by thermogravimetry at 370°C, under controlled oxygen partial pressures (PO2 ranging from 2-40 kPa). Sigmoidal curves of oxidation weight gain were measured for both pellet and powder test samples. The rate of oxidation increased as the oxygen partial pressure increased. It has been shown, by simultaneous TG-DSC, that the reaction proceeds in a pseudo steady state. An experimental methodology based on temperature or PO2 jumps has shown that the assumption of a rate-limiting step is validated, and a mean value of activation energy for the formation of U3O8 of 103 kJ.mol-1 was estimated

Modelling of the kinetic transition in zirconium based alloys: Application to the oxidation of Zircaloy-4 by water vapour

International audienceThe kinetic curves of oxidation of Zircaloy-4 exhibit a transition, which is a sharp increase in the oxidation rate when the oxide thickness reaches a critical value. The pre-transition stage is controlled by the diffusion of oxygen vacancies in the oxide layer. In the post-transition stage, oxygen or water vapour have an accelerating effect on the oxidation (whereas they have no influence during the pre-transition) and the oxide layer is damaged, with large cracks parallel to the metal/oxide interface and connected to the gaseous atmosphere by pores. Consequently, it is clear that the post-transition stage cannot be accounted for by the same mechanism as in pre-transition. In this paper, we propose a geometrical modelling allowing to describe the progressive transformation of the oxide layer during the transition. This model is based on a random appearance of pores (connected to the external surface) which leads to the transformation, from a pre-transition stage to the post-transition stage, of small sections s0 of the oxide layer (analogy with the models of thermal transformations of powders, involving the processes of nucleation and growth of a new phase). The model allows to describe the kinetic curves obtained for the oxidation by water vapour of Zircaloy-4

Transformation de mélanges de poudres de trioxyde d'uranium et de carbone

National audienceLe dioxyde d'uranium UO2 intermédiaire de base dans la fabrication du combustible nucléaire, est généralement obtenu par réduction du trioxyde d'uranium UO3. La réduction, d'après la littérature, (Tomlinson) s'effectue en deux étapes successives (1) la transformation du trioxyde d'uranium UO3 en oxyde U3O8 vers 600-700°C et (2) la réduction de l'oxyde U3O8 en dioxyde d'uranium par l'hydrogène vers 520°C. Il est également possible de réduire le trioxyde d'uranium en dioxyde d'uranium par le carbone, mais dans ce cas la réaction se produit grâce à la mise en contact des grains des deux phases. Le déroulement de la réaction doit alors dépendre fortement de la qualité du mélange. Afin de mieux comprendre et contrôler les phénomènes mis en jeu au cours de la carboréduction, nous avons étudié par thermogravimétrie l'effet d'un broyage manuel du mélange de deux poudres. Une partie de ce travail a été consacrée à la caractérisation des mélanges d'un point de vue morphologique, avant et après broyage. La seconde partie consiste en l'étude de leur réactivité sous atmosphère d'hélium par thermogravimétrie

Kinetic study of the oxidation by oxygen of a zirconium based alloy: ZrNbO. Differences between the pre- and post-transition stages

International audienceThe oxidation by oxygen of a zirconium based alloy, M5TM (which is a ZrNbO alloy, containing 1% of Nb) has been studied. The M5TM alloy, like many zirconium alloys, undergoes a kinetic transition. The aim of the present work is to achieve a better understanding of the oxidation in the pre-transition stage, and to clearly identify the differences between the pre- and post transition stages from the kinetic point of view. The oxidation of M5TM was followed by isothermal gravimetry at 490°C, under a controlled partial pressure of oxygen (in the range 7 to 200hPa). First, we have verified the steady state assumption, by coupling thermogravimetry and differential scanning calorimetry (DSC) : it is shown that the system is in a steady state from the beginning of the oxidation, in the pre- and post-transition stages. Then, the existence of a rate-limiting step was verified in the pre-transition stage using an experimental method based on temperature or pressure jumps; this assumption is no more verified in the post-transition stage, which means that the oxidation does not proceed in the same way as in the pre-transition stage. Finally, we have obtained the variations of zirconia growth reactivity with the oxygen pressure, in the pre-transition stage (using the pressure jump method). The oxygen pressure has a slightly accelerating effect, which cannot be interpreted by the diffusion of oxygen vacancies through a dense oxide layer (in that case no effect of the oxygen pressure would be observed). In the post-transition stage, the oxygen effect is more important