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The kinetic and crystalline evolutions of UO2 during its oxidation into U3O8 at 250 degrees C in air were studied by isothermal thermogravimetry and calorimetry, coupled with an in situ synchrotron X-ray diffraction on the D2AM-CRG beamline at ESRF. This study was aimed at determining experimentally the validity of the kinetic assumptions made in existing literature to account for the oxidation Of UO2 into U3O8 and also to determine precisely the structural evolution, in relation to the kinetic behaviour. Our results provide evidence of four distinct kinetic time domains, and the assumption of a single rate-limiting step is verified only for two of them. The crystalline phases associated with these domains are also identified. In fact, the first kinetic domain corresponds to the reaction Of UO2 into U4O9; the second one is linked to the two simultaneous reactions, UO2 into U4O9 and U4O9 into U3O7. Finally, the transition from U3O7 into U3O8 corresponds to the third and fourth kinetic domains. These results show that the oxidation Of UO2 into U3O8 cannot satisfactorily be described with modelling approaches used in the literature. A new general outline is proposed to study the oxidation of uranium oxides. This outline will improve both the understanding and predictions of oxidation processes at the relatively low temperatures that are expected during interim storage of spent nuclear fuel. (c) 2006 Elsevier B.V. All rights reserved.

International audience; This paper evaluates and compares the impact of cellulose ethers (CE) on water transport and porous structure of cement-based materials in both fresh and hardened state. Investigations of the porous network (mercury intrusion porosimetry, apparent density, 2D and 3D observations) emphasize an air-entrained stabilisation depending on CE chemistry. We also highlight that CE chemistry leads to a gradual effect on characteristics of the water transport. The global tendencies brought by CE are: higher water retention, lower capillarity water absorption, lower liquid water permeability as well as higher water vapour permeability. All things considered, we demonstrate that CE chemistry is an important controlling factor on water transport and porous structure evolution.

Publié suite au congrès High Temperature Corrosion and Protection of Materials 7; International audience; Due to the specific in service VHTR conditions, the corrosion behaviour of Inconel 617, candidate alloy for the IHX design, has been investigated at elevated temperatures in representative helium containing impurities (CO, H2O, H-2 and CH4) in the range of mu bar. The role of Al in the corrosion behaviour of IN617 is investigated using Ni-22Cr-9Mo base model alloys containing different Al levels (from 0 to 2wt.%).

International audience; Chromium-rich nickel base alloys 617 and 230 are promising candidate materials for very high temperature gas-cooled reactors (VHTR) but they must resist corrosion in the impure primary cooling helium over very long times. The impurities of the hot helium can promote the development of chromium-rich surface oxides that appear to protect the alloys against intensive corrosion processes. However above a critical temperature (typically in the range 1173-1273 K), chromium oxide is reduced by carbon from the alloy and the surface layer is not stable anymore. Depending oil the gas composition, the unprotected material rapidly either gains or loses carbon with a dramatic impact on its mechanical properties. The deleterious reaction of chromia and carbon thus fixes all ultimate reactor operating temperature. Critical temperature measurements are presented for alloys 617 and 230 and the influence of carbon monoxide partial pressure in helium is discussed.

International audience; Despite recent insights into thermal stability of ettringite and structural changes during decomposition, a lack of knowledge on nucleation and growth mechanisms of metaettringite remained. Therefore, a better understanding of the kinetic modelling of this heterogeneous reaction was proposed. Thanks to an experimental approach allowing to check the validity of kinetic assumptions (rate-determining step, expression of the rate as d alpha/dt=k f(alpha)...), a good agreement was found between the calculated and experimental alpha vs. t and d alpha/dt vs. t kinetic curves. The reaction area of the region in which the rate-limiting step occurs was also elucidated. Finally, we demonstrated that the rate-limiting step of the growth process follows an Arrhenius law in non-isothermal conditions.

International audience; The 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 to 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.

International audience; The 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.

International audience; The oxidation of a zirconium based alloy (recrystallised Zircaloy-4) has been studied at 500°C, by thermogravimetry under controlled partial pressures of water vapour and hydrogen. The study is restricted to the pre-transition state of oxidation, the aim being to identify the rate-limiting step of the oxide growth. It has been verified experimentally that the system is in a steady state during the pre-transition period. Moreover, using a method based on the isolation method, it has been shown that the oxidation could be described by a single rate-limiting step only when a weight gain of 7.5 mg.dm-2 has been reached. Since the kinetic curves are not parabolic, a diffusion model has been proposed, which takes into account the effect of barriers on the diffusion (such as pores, blisters, cracks... in the oxide layer) and which is in good agreement with the experimental curves. Then, numerical fitting has shown that the beginning of the oxidation could be interpreted by a mixed reaction-diffusion steady solution, which tends towards a single diffusion rate-limiting step when the oxide layer is thick enough (about 0.5 μm).

International audience; 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.

International audience; The carboreduction of UO3 into UO2 has been studied with an excess of carbon (C/U=3 mole ratio). The results of thermogravimetric (TG) studies, X-ray diffraction (XRD) and SEM reveal that the carboreduction depends upon the texture and the physical mixture of the precursors UO3 and carbon. Indeed, optimal carboreduction was obtained with an improvement of UO3 and carbon grains contacts during the reduction. The preparation of the mixture (UO3 + 3C) has been optimised by a slight manual grinding.

International audience; The reduction of U3O8 by hydrogen at 510 °C has been studied by thermogravimetry. After a first reduction to UO2, the sample is oxidised; then the analysis of the shape of the kinetic curves of a second reduction leads to the determination of the specific growth rate, according to a contracting-sphere model provided that the rate-limiting step occurs on the external surface of the particles. A mechanism of reduction is then proposed which accounts for the observed variations of the rate with the partial pressure in hydrogen.

International audience; A theorem on the separation of models is reported which shows how it may be possible to describe the kinetics of a reaction involving a solid: a physico-chemical model in which 'reactivity' has been defined, and a geometrical model to which a 'space function' corresponds. Then a series of rigorous experimental methods based on this theorem are presented. Their use improves and validates the kinetic modelling of a heterogeneous reaction.

International audience; To enable the ceramization of a barium sulphate-rich radioactive waste the synthesis of barium titanium oxide is studied by using anatase and barium sulphate. As a function of the calcination atmosphere, helium (or air) and Ar/H2, two reactions are studied. A mechanism of barium titanium oxide synthesis in helium (or in air) is proposed.

International audience; The influence of the gaseous atmosphere and the Ba/Ti ratio on the decrease in specific surface area in a BaTiO3 powder was studied at 850 °C. A model involving elementary steps and the BaTiO3 point defects was proposed to quantitatively account for the accelerating effect of the water vapour pressure and the Ba/Ti ratio.

The oxide film formed on nickel-based alloys in pressurized water reactors (PWR) primary coolant conditions (325 °C, aqueous media) is very thin, in the range of 1–100 nm thick, depending on the surface state and on the corrosion test duration. The nature and the structure of this scale have been investigated by Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). TEM observations revealed an oxide layer divided in two parts. The internal layer was mainly composed of a continuous spinel layer, identified as a mixed iron and nickel chromite (Ni (1− x ) Fe x Cr 2 O 4 ). Moreover, nodules of Cr 2 O 3 , with a size about 5 nm, were present at the interface between this spinel and the alloy. No chromium depletion was observed in the alloy, at the alloy/oxide interface. The external layer is composed of large crystallites corresponding to a spinel structure rich in iron (Ni (1− z ) Fe (2+ z ) O 4 ) resulting from precipitation phenomena. SEM and TEM observations showed a link between the nucleation and/or the growth of crystallites of nickel ferrite and the crystallographic orientation of the substrate. A link between the presence of surface defects and the nucleation of the crystallites was also underlined by SEM observations. Partially hydrated nickel hydroxide, was also observed by TEM in the external scale. Based on these results, some considerations about the mechanism of formation of this oxide layer are discussed.

International audience; Corrosion of a chromia-forming nickel base alloy, Haynes 230_, has been investigated under impure helium containing a few Pa of CO and H2O at 900 °C. It has been found that this alloy reacts simultaneously with CO and H2O. Oxidation by CO has been revealed to occur mainly in the first hours. CO diffuses through the scale via short-circuit pathways and oxidizes Al, Cr and Si at the oxide/metal interface. Kinetics of CO oxidation has been investigated and several rate limiting steps are proposed. In the long term, H2O is the major oxidant of chromia-forming nickel base alloys in impure helium.

International audience; The oxide film formed on nickel-based alloys in Pressurized Water Reactors (PWR) primary coolant conditions (325°C, aqueous media) has been investigated by Transmission Electron Microscopy (TEM). TEM observations revealed an oxide layer divided in two parts. The internal layer was mainly composed of a continuous spinel layer, identified as a mixed iron and nickel chromite (Ni(1-x)FexCr2O4). Moreover, nodules of Cr2O3 were present at the interface between this spinel and the alloy. The external layer is composed of large crystallites corresponding to a spinel structure rich in iron (Ni(1-z)Fe(2+z)O4) resulting from precipitation phenomena. The influence of alloy surface defects was also studied underlining two main consequences on the formation of the passive film e.g. the internal layer. On one hand, the growth kinetics of the internal spinel rich in chromium increased with the surface defect density. Besides that, when the defect density increased, the oxide scale became more finely crystallized. This result agrees with a growth mechanism due to a rate limiting process of diffusion through the grain boundaries of the oxide. On the other hand, the quantity of Cr2O3 nodules increased with the number of surface defects, revealing that the nodules nucleated preferentially at defect location.

International audience; Nickel base alloys Haynes 230 and Inconel 617 are of interest for gas cooled reactors. At high temperature in impure helium, they generally form surface chromium-rich oxides. However above a critical temperature called T-A, the scales are not stable anymore and the chromia destruction comes with a production of carbon monoxide. Reactivity tests on model alloys, with and without carbon, prove that chromia is reduced by the carbon from the alloy. TA VS P(CO) curves were also plotted for the two commercial alloys based on the experimental determination of T-A in various atmospheres with increasing partial pressures of carbon monoxide. Unexpectedly, both materials exhibit an almost ideatical behavior although a basic equilibrium approach suggests that the chromia scale would be reduced in different conditions due to the thermodynamic particularity of the interfacial alloy/scale system.

International audience; A microscopic model based on the appearance, diffusion, and aggregation of point defects allows to calculate the time of appearance of the first nucleus on a surface during a reaction between a solid and a gas. Calculated distributions of these times of appearance of the first nucleus are qualitatively compared to experimental ones, previously determined. The appearance of the point defects seems to be the most influential step on the time of appearance of the first nucleus. Moreover the comparison between experimental and calculated distributions allows to conclude that the frequency of appearance of the defects is higher on the edges than on the faces of the single crystal.

International audience; Oxidation mechanism of Alloy 690 has been investigated in Pressurised Water Reactor (PWR) primary coolant conditions (325°C, aqueous hydrogenated media). Experiments performed with gold marker and RBS technique reveal that the passive film formation is the consequence of an anionic mechanism. This result is confirmed by experiments achieved with two sequences of corrosion in a H2 16O media and in a mixed H2 16O/ H2 18O media. The localisation of 18O by SIMS analysis in the thin passive layer underlines an oxidation mechanism due to oxygen diffusion by short circuits (like grain boundaries) in the oxide scale. Moreover grain boundary diffusion coefficient in chromite like oxide was estimated to be in the range 2 10-18 - 1 10-17 cm2.s-1 and compared to values extrapolated from higher temperature.

International audience; High temperature corrosion properties of Haynes 230 were investigated in a purposely-designed facility under a typical very high temperature reactor (VHTR) impure helium medium. The study was focused on the surface oxide scale formation and its stability at about 1223 K. The alloy developed a Mn/Cr rich oxide layer on its surface under impure helium at 1173 K. Nevertheless, a deleterious reaction destructing the chromium oxide was evidenced above a critical temperature, TA. Reagents and products of this last reaction were investigated.

The kinetics of the dolomite partial decomposition into MgO and CaCO 3 have been investigated using isothermal and isobaric thermogravimetry ( T = 953 K, CO 2 pressure in the range 0.025–0.4 atm). The characterization of the reaction products using XRD, SEM and XPS indicated that fine particles of MgO are formed at the surface of the initial dolomite needles. The elucidation of the mechanism of growth and of the rate-limiting step was based on a series of experiments devoted first to verify the steady-state assumption (by coupling thermogravimetry to calorimetry), second to verify the existence of a rate-limiting step, and third to determine the variations of the areic growth reactivity, ϕ , in mol m − 2 s − 1 , as a function of CO 2 pressure. A growth mechanism in eight elementary steps, involving interface and diffusion steps has been proposed. By comparing the experimental results to the possible rate laws calculated from the mechanism on the assumption of a rate-limiting step, it was found that the growth of MgO and CaCO 3 can be controlled by the diffusion of magnesium through the MgO phase surface.

In the context of nuclear waste management, the release of hydrogen can generate safety problems. It has been shown that amorphous cobalt hydroxo-sulphide is able to decrease the apparent production of radiolytic hydrogen yield by hydrocarbon radiolysis. This paper presents a study of the effect of cobalt hydroxo-sulfide on the radiolysis of propane induced by a proton beam. Hydrogen and hydrocarbon partial pressures obtained after propane irradiation were compared in different conditions: (i) without cobalt hydroxo-sulphide, (ii) in the presence of solid, irradiated or not, by adjusting the gas pressure in the irradiation cell. The evolution of those partial pressures as a function of the time of contact between the amorphous cobalt hydroxo-sulphide and the formed gaseous species is discussed. The solid is characterised by X-ray diffraction, infra-red and Raman spectroscopies before and after hydrogen trapping. It is shown that the solid does not change the composition and amount of organic radiolytic products but acts as a trap of hydrogen. The hydrogen consumption by the solid seems to be proportional to the initial hydrogen partial pressure. This study shows that radiolytic hydrogen is rapidly produced by propane irradiation whereas the trapping of hydrogen by cobalt hydroxo-sulphide is a slower phenomenon.

International audience; The 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 h Pa, 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.

International audience; The 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.

International audience; A pyrochlore-related Ce2Zr2O8−x phase has been prepared in a reduction reoxidation process from Ce0.5Zr0.5O2 powders. Ce2Zr2O8−x, based on a cubic symmetry with a=1.053 nm, decomposes in nitrogen at 800 °C, but remains stable up to 900 °C in air. It shows mixed oxygen ionic and electronic conductivity. The bulk conductivity at 700 °C is 4×10−4 S cm−1 in air and 1×10−2 S cm−1 in nitrogen, and the activation energy is 1.27 eV in air. In nitrogen, the Arrhenius law is not obeyed, and a curved plot was obtained from 400 to 700 °C; then, the conductivity decreased rapidly due to the thermal decomposition of Ce2Zr2O8−x.

International audience; 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.

International audience; 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.

International audience; In the non-isothermal approach to the determination of kinetic constants, the mathematical analysis of the α-time curve is based on the assumption of a rate equation, which in some cases, is not consistent with the isothermal curves. For example, during the reduction of U3O8 by hydrogen, we have shown from isothermal and isobaric experiments that a model of nucleation and anisotropic growth was able to fit the experimental rate curves. This makes the analysis of non-isothermal experiments very complex. A first attempt to solve this problem is presented here, in which the effect of a sudden change in the reaction temperature during an experiment is analysed. Taking into account the differences due to the sudden change in the calculation of the rate, a good agreement was found between the calculated and experimental dα/dt vs. α curves.

International audience; The kinetics of the reaction, a-SiO2+t-ZrO2→ZrSiO4, were monitored by isothermal HTXRD experiments. A series of diffractograms were recorded at constant temperature (1250°C, 1300°C, 1350°C, 1400°C), after heating the precursor powder. Integrated intensities of the peaks relating respectively to the ZrSiO4 (200) and tetragonal t-ZrO2 (101) planes, were used to obtain the conversion rate. According to the shape of the curves, λ=f(t), the limiting step of the zircon formation is not the nucleation process, but, in fact, diffusion of silicon species through a zircon layer, from an external continuous or granular silica phase to the surface of zirconia grains. To account for the results obtained, a model of a contracting sphere is proposed: the system is described as either a series of spherical t-zirconia grains surrounded by a shell of zircon, which is covered by amorphous silica, or as spherical t-zirconia grains surrounded by a shell of zircon, which is in contact with spherical amorphous a-SiO2 particles. In the latter case, fast superficial diffusion of Si4+ and electrons originating from silica grains on the external surface of the zircon layer produce a homogenisation of the silicon concentration. The validity of this model is confirmed by the fact that the experimental data, λ=f(t), fit the kinetic law of Valensi and Carter, considering chemical diffusion through the reaction product (oxide layer) as the rate limiting step. Consequently, the apparent diffusion coefficient of silicon can be estimated.

International audience; 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.

International audience; The reduction of uranium oxide U3O8 into uranium dioxide UO2 has been studied by temperature-programmed thermogravimetry, up to 700°C. Experiments have been carried out either in ammonia (PNH3 ranging from 35 to 125 hPa) or hydrogen (PH2 ranging from 42 to 243 hPa). The gases evolved and consumed during the reduction were followed simultaneously by mass spectrometry. The reduction of U3O8 by ammonia into UO2 begins at 550°C, and is completed at about 650°C. It has been noticed that ammonia decomposition occurs at 700°C; moreover, it is catalysed by UO2 produced by the reduction of U3O8, since no decomposition is observed in the absence of UO2. Besides, some isothermal experiments carried out at 510°C have confirmed that ammonia reacts directly with U3O8 since the shape of the curves obtained either in ammonia or in hydrogen are different, particularly, the reaction is faster with ammonia compared to hydrogen, for the same partial pressure of the reducing gas.

The oxidation into MgO of an industrial Al-Mg alloy in the liquid state has been studied at 700°C by thermogravimetry. Since the kinetic curves were not reproducible, it was not possible to use them directly. Thus, it has been necessary to use a method based on the isolation method to obtain the influence of oxygen partial pressure on the areic reactivity (mole of MgO.m-2.s-1) of growth of MgO. Introduction During the elaboration of aluminium-magnesium alloys, the surface of the liquid metal may be oxidised, mainly leading to the formation of MgO. The industrial system is quite complex, since several gases of the atmosphere can react with the alloys (oxygen, water vapour, nitrogen, carbon dioxide...). So, we have studied a « simplified » system : the oxidation of an Al-Mg 5% alloy by oxygen. The aim of this work is to obtain the influence of the oxygen pressure on the areic reactivity (mole of MgO.m-2.s-1) of growth of MgO, and to propose a reaction mechanism able to account for the experimental results, in order to predict the behaviour of the alloy in industrial conditions of elaboration and to limit its oxidation. After giving the oxidation kinetic curves, the methodology used for the kinetic study of the MgO growth will be detailed. Finally, the influence of the oxygen pressure on the areic reactivity of growth of MgO will be obtained, and thanks to the results of an IR study, some indications for a reaction mechanism will be given. Experimental The alloy is an industrial Al-Mg 5% alloy, supplied by Pechiney. The samples are cylinders of 1mm height and 9mm diameter (this height was chosen in order to ensure an constant oxygen pressure around the sample). For higher samples (15mm to 3.5mm), more oxidised parts were observed (a ring of white powder at the surface of the samples), the localisation of this ring along the cylinders varying with the oxygen pressure fixed for the experiment. Thus, we have assumed that a gradient of oxygen pressure exist along the high samples. This was not observed for the 1mm-high cylinders. The oxidation of the liquid alloy was followed by isothermal thermogravimetry at 700°C (thermobalance SETARAM TAG 24). The experiments were carried out at atmospheric pressure, under a flowing mixture of helium and oxygen (1l.h-1), the partial pressures being controlled by mass-flowmeters (Brooks 5850E). The heating and the melting of the samples were carried out under oxygen at atmospheric pressure since it has been observed that a high oxygen pressure allows to limit the oxidation of the liquid alloy; the oxygen pressure chosen for the experiment is established after 30 minutes of isotherm at 700°C. The experiments of simultaneous calorimetry and

International audience; Carboreduction of UO3 has been studied by in situ infrared spectroscopy in order to detect the presence of carbon monoxide during the reduction. From thermogravimetry, the reaction was observed to occur in two steps: (i) reduction of UO3 to U3O8 by carbon at 440°C and (ii) reduction at 590°C of U3O8 to UO2 by carbon. No carbon monoxide was observed during the reaction. Nevertheless, formation of carbonate species is suggested at temperature above 550°C with absorption bands around 1005 and 1300 cm−1.

International audience; The aim of this work is to study the reaction synthesis of colbat chromite in the solid state between tricolbat tetroxide and bichomium trioxide. the diffusing species ave been first identified. The experimental curves have been obtained using thermogravimetry under isothermal conditions and various partial pressures of oxygen. A qualitative interpretation of the transformation is proposed, as well as a geometrical model that leads to the variations of the degree of conversion with time.

International audience; A polysilazane precursor was studied from 500 to 1600°C by means of thermogravimetry and associated mass spectrometry. At 550°C, opposite influences of methane and hydrogen partial pressures on the weight loss were evidenced. At 1600°C, the amorphous solid crystallizes into silicon, silicon carbide and silicon nitride phases by producing carbon and silicon monoxides and nitrogen.

High surface area ceria was doped with lanthanum ions and calcined at 900 K under various partial pressures of oxygen and water vapour. The kinetic rates of crystallite size increase were observed to vary as P − 1 6 O 2 . By addition of 0.5 and 1% of lanthanum ions, the rates were lowered, and then significantly decreased for 10% (cat.). A model of this process involved association between lanthanum ions and oxygen vacancies in order to explain the better stability of doped ceria.

International audience; The influence of water vapour and additives on the decrease in surface area of gamma Al2O3 at 1378 K, before its transformation into alpha Al2O3, was investigated. The experimental rate of this decrease was found to vary as P H2O at constant value of surface area and for a value of water vapour pressure ranging from 0.1 to 1 kPa. This reduction in surface area was inhibited by a slight addition of Mg2+, Ca2+, La3+, Th4+ or Zr4+ ions but its rate was drastically increased by increasing both additive content up to 4% (cationic fraction) and oxygen pressure. The influence of water vapour can be explained quantitatively using a mechanistic model for initial sintering of oxide powders. This model is based on a description of material transport by means of structural elements of alumina, point defects, and their chemical interaction with the calcination atmosphere.

International audience; In numerous industrial fields, materials are manufactured from powders. The most classical process consists in pouring the powder into a die and pressing in between two punches. The resulting component can be used in this state, as in the pharmaceutical industry. But, most often, for example in powder metallurgy or ceramic processing, it is submitted to a thermal treatment, called sintering, during which the particles are welded together. These two stages, compaction and sintering, have been extensively investigated in the last decades. The main physical mechanisms arising during both stages have been identified and more or less sophisticated models have been developed. However most of these studies have concerned single-component powders whereas industrial processes more frequently use mixtures of powders with different physical and mechanical. The interest of using mixtures instead of single-component powders may be to facilitate the compaction or the sintering or to create alloys or composite materials with outstanding features. Understanding and modelling the behavior of powder mixtures require taking into account the mechanical and chemical interactions between both phases. Concerning mechanical problems, in most cases, a simple law of mixture is not appropriate. Classical models of multiphase materials hardly give better results. Accounting for the granular nature of the material is thus absolutely necessary. Relevant issues are interparticle contacts, particle rearrangement, agglomeration, phase percolation. The compressibility of bimodal mixtures, for example, is mainly related with the ratio of the size of particles of one phase to the size of particles of the other phase. Chemical phenomena involve more or less complex phase transformations and chemical reactions. Such transformations or reactions usually occur during sintering but they can also be generated when the material is still in the powder state, for example during milling. Most research studies in the field of powder processing at the Federation concern powder mixtures. Three examples are presented in the following, illustrating recent research studies on the behavior of powder mixtures during compression or sintering.

National audience; The thermodynamical study of the CaO-Al2O3-CO2-H2O system, applied to the hydration of monocalcium aluminate at ambient temperature, allows to explain both the experimental precipitation of unstable hydrates (C2AH8 and C4ACH11) and the apparent complexity of conductimetric curves, by the manifestation of supersaturation phenomena. The role of some anhydrous phases (C12A7 and CA2) present in aluminous cements, and particularly their influence on the acceleration or delay in the precipitation process, are also clearly pointed out from the observation of phase diagrams.

International audience; To obtain high-quality produces from aluminium alloys powders, degassing is usually carried out before consolidation. But, in the case of Al-Zn-Mg alloys an evaporation of alloying elements may occur during outgassing. The aim of this work was to determine the influence of degassing conditions (temperature, pressure, nature of the gaseous atmosphere) on the stability of the alloy composition. These studies were performed on X7091 alloy by means of thermogravimetry and temperature-programmed desorption technique. The results show that liberation of alloying elements is closely connected with the evolution of the surface oxide layer which itself depends on various thermodynamic and kinetic parameters. The role of water vapour in the gaseous environment was particularly emphasized.

The more favourable experimental conditions of cooling and grinding was defined for calcium aluminates obtained by solid-solid reaction with C/A ratio in the range 0.96 – 1.04. The composition of the samples was related to the apparition of different phases : “substoichiometric” solid including CA and CA 2 , with a calcium deficiency “super-stoichiometric” samples including C 12 A 7 with an excess of calcium as well as mixtures of sub and super-stoichiometric solids could be obtained. “Super-stoichiometric” samples presented ionized anionic vacancies. During hydration, “super-stoichiometric” aluminate led to the formation of a first hydrate. The hydration kinetics depended on the ratio C 12 A 7 /CA 2 of the samples.

L'etude de l'equilibre entre CaSO4 (Y) et la vapeur d'eau a ete effectuee sur du sulfate de calcium pur obtenu par deshydratation de CaSO4, 2H2O issu de la reaction entre K2SO4 et CaCl2 purs. Lors de nos essais, le produit etait etale en couche mince (1mm) dans la nacelle d'une thermobalance de maniere a favoriser au maximum le contact entre le solide et la vapeur d'eau. Nos conclusions ne sont donc valables : - qu'en couche mince. La reversion, a l'echelle industrielle, de l'anhydrite soluble (CaSO4Y) en semihydrate (CaSO4, 1/2 H2O) est regie dans de tres nombreux cas par la diffusion de la vapeur d'eau dans la masse de matiere, d'ou l'existence d'une energie d'activation apparente qui ne peut pas etre reliee aux mecanismes elementaires. - Que pour certains sulfates de calcium, la presence d'impuretes modifie non seulement la thermodynamique de l'equilibre mais aussi les cinetiques. C'est le cas notamment des phosphogypses.

L'hydratation du sulfate de calcium est suivie par conductimétrie électrique. Le palier de sursaturation correspond à un ETAT QUASI-STATIONNAIRE, et non à un état d'équilibre. Cette interprétation est en accord avec les différences de réactivité observées sur divers types d'hémihydrates. // The calcium sulfate hydratation has been mainly studied by electrical conductivity. The sursaturation level may be seen as a STATIONARY STATE rather than an equilibrium one. The interpretation agrees with several observations of hemihydrate reactivity variations.

International audience; The isothermal kinetics of the transformation of gamma calcium sulphate anhydrite to bêta anhydrite have been determined. All the curves obtained have a sigmoidal form. We have also observed an influence of the water vapour pressure on the rate of formation of bêta anhydrite. On each plot of rate vs. water vapour pressure one observes a maximum whose position shifts to lower values of water vapour pressure when the temperature decreases. The formation of bêta anhydrite at low temperatures may be explained by the geometry of the water vapour pressure-temperature diagram of the hydrated forms of calcium sulphate, and by the "mineralizing" action of water vapour at or close to saturation. Les courbes cinétiques isothermes de la transformation anhydrite γ → anhydrite β du sulfate de calcium ont été obtenues. Toutes ont une allure sigmoídale. On observe une influence de la pression de vapeur d'eau sur la vitesse de formation de l'anhydrite β. Sur chaque courbe (vitesse - pression de vapeur d'eau) on remarque la présence d'un maximum dont l'abscisse diminue lorsque la température décroît. L'explication de l'obtention de l'anhydrate β à basse température est à rechercher dans la géométrie du diagramme (pression de vapeur d'eau - température) des formes hydratées du sulfate de calcium et dans l'action "minéralisatrice" de la vapeur d'eau saturante ou voisine de la saturation.

International audience; The influence of a SO2 treatment on the electrical properties of SnO2 has been studied. It is shown that some gaseous compounds induce an important maximum on the curve of the electrical conductance as a function of the temperature of SnO2 treated with SO2. The temperature at the maximum conductance is connected with the nature of the electrical contacts (gold or platinum) and an important part is associated with the metal-oxide interface process. The selective aspect of the electrical response can be exploited to develop a benzene detector.

International audience; The dehydration reaction rate of calcium sulphate dihydrate decreases monotonically with the water vapour pressure. The kinetics curves, of sigmoidal form, may be described, beyond the point of inflexion by a process of interface advance. The influence of foreign gases provide evidence that it is necessary for crystallizing water to pass through an adsorbed state. ¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤ Les variations de la vitesse de la réaction de déshydratation du sulfate de calcium à deux molécules d'eau en fonction de la pression de vapeur d'eau sont monotones, décroissantes. La courbe cinétique, d'allure sigmoïdale, peut être décrite au delà du point d'inflexion par un processus d'avancement d'interface. L'influence de gaz étrangers confirme le passage préalable des molécules d'eau de cristallisation du gypse par l'état adsorbé.

National audience; The comminution of monocalcium aluminate depends both on the grinding process and the composition of the solid : an "Aurec" vibrating crushing mill is more acting than a planetary ball mill. The subsequent reactivity of the solid versus water is more enhanced for "super-stoichiometric" aluminate (C/A=1.04) than for "sub-stoichiometric" one. Moreover, a connection between both the "balling fitness" of the solid and its activation during grinding was pointed out, what let suppose that entities responsible of the balling phenomenon take a prominent part in the hydration kinetics and can be considered as aluminate vacancies precedently identified.