Christian Chatillon

Wettability of monocrystalline alumina by aluminium between its melting point and 1273 K

The wettability of Al2O3 single crystals by aluminium is investigated in the 933–1273 K temperature range by the sessile drop method under a low total pressure (4 × 10−5Pa) and an oxygen partial pressure of about 10−15 Pa. The variation of contact angle with temperature is shown to describe a linear decrease from 103 ± 6° (933 K) to 86 ± 6° (1273 K). The temperature coefficients of contact angle (dθdT = − 0.05 k−1) and work of adhesion (dWadT = 0.6 mJ m−2K−1) are found to be similar to but somewhat higher than those obtained with non reactive metals (such as Cu and Au) on alumina. Results of several wetting tensiometer experiments performed with sapphire cylinders are in close agreement with the above-described wetting behaviour. It is shown that, with the experimental conditions described, the influence of the oxide layer surrounding the liquid metal on wettability can be eliminated from the melting point of Al. By evaluating the gaseous exchanges between the experimental chamber and the oxide layer, using the kinetic theory of gases for molecular flow, the process whereby aluminium is released from its surrounding oxide layer can be discussed. This process depends on the initial thickness of the oxide layer, the oxygen partial pressure and the temperature.

High-temperature mass spectrometry: Instrumental techniques, ionization cross-sections, pressure measurements, and thermodynamic data (IUPAC Technical Report)

An assessment of high-temperature mass spectrometry and of sources of inaccuracy is made. Experimental, calculated, and estimated cross-sections for ionization of atoms and inorganic molecules typically present in high-temperature vapors are summarized. Experimental cross-sections determined for some 56 atoms are generally close to theoretically calculated values, especially when excitation–autoionization is taken into account. Absolute or relative cross-sections for formation of parent ions were measured for ca. 100 molecules. These include homonuclear diatomic and polyatomic molecules, oxides, chalcogenides, halides, and hydroxides. Additivity of atomic cross-sections supplemented by empirical corrections provides fair estimates of molecular cross-sections. Causes of uncertainty are differences in interatomic distances and in shapes of potential energy curves (surfaces) of neutral molecules and of molecular ions and tendency toward dissociative ionization in certain types of molecules. Various mass spectrometric procedures are described that render the accuracy of measured thermodynamic properties of materials largely independent of ionization cross-sections. This accuracy is comparable with that of other techniques applicable under the conditions of interest, but often only the mass spectrometric procedure is appropriate at high temperatures.

Thermodynamic assessment of the uranium-oxygen system

Abstract A thermodynamic assessment of the uranium–oxygen system is presented. A consistent set of experimental data is selected among the numerous data in the literature on the phase diagram and oxygen chemical potential. The thermodynamic properties of the phases are described using the compound energy model with ionic constituents for the solids and an ionic two-sublattice model for the liquid. For the uranium dioxide, the structure is described using three sublattices, one for the cations U3+, U4+ and U6+, one for the normal site of oxygen ions, and one for the interstitial oxygen ions. Vacancies are included in both oxygen sublattices. In this first approach, the homogeneity ranges of the U4O9−y and U3O8−y compounds are not represented. A set of consistent model parameters that describes both the phase diagram and the oxygen chemical potential data in the whole composition range is thus obtained. The description of this basic binary system will be used to calculate higher order systems such as O–U–Zr and Fe–O–U which are important for simulating severe nuclear accidents.

Critical analysis and optimization of the thermodynamic properties and phase diagrams of the III–V compounds II. The Ga-As and In-As systems

Abstract A critical assessment of thermodynamic and phase diagram data for the Ga-As and In-As systems has been performed by first carrying out a complex chemical equilibrium analysis of the conditions of measurements, then performing a critical analysis of all the experimental methods employed, and finally, after eliminating those results which seem erroneous or which show systematic deviation, and reevaluating the experimental accuracies, by optimizing the retained data together. The best self-consistent values obtained are the following: ΔH°f(GaAs, s, 298 K) =−19.54±0.30 kcal mol−1, S°298(GaAs, s) = 16.05±0.80 cal K−1 mol−1, Lf s→1 (GaAs, ) = 27.0± 0.80 kcal mol−1 at Tf = 1513.5±3 K, ΔH°f(InAs, s, 298 K) = −14.29±0.50 kcal mol−1, S°298 (InAs, s) = 17.84±0.80 cal K−1 mol−1, Lf(InAs, s→1) = 19.04±1.00 kcal mol-1 at Tf = 1212±3 K. The Gibbs energies of formation for the compounds are represented by the equations: ΔG°f(GaAs, s) = -19537+1.849T cal mol−1, ΔG°f(InAs, s) = -14291+2.257 T cal mol−1. Optimized partial pressures of In, Ga, As2, As4, InAs and GaAs molecules and phase diagrams have been obtained.

Oxidation of graphite by atomic oxygen: a first-principles approach

Abstract The main goal of this paper is to investigate the interaction of atomic oxygen with graphite surfaces, using first principles total energy calculations. Oxygen coverage, position and crystallographic orientation of the surfaces are the main parameters of this study. At the basal surface, the binding energy is given by the contribution of the in-network interaction between oxygen atoms and between oxygen and graphite. Relaxation effects are non-negligible at low and medium coverages. Zig-zag surfaces are the most reactive ones, followed by armchair and basal ones. For both surfaces, the oxidative etching process that forms the CO and CO2 gases is also discussed on the basis of our calculations.

Thermodynamics of the O–U system. I – Oxygen chemical potential critical assessment in the UO2–U3O8 composition range

A critical assessment of oxygen chemical potential of UO2+x, U4O9 and U3O8 oxide non-stoichiometric phases as well as of diphasic related domains has been performed in order to build up primary input data files used in a further optimization procedure of thermodynamic and phase diagram data for the uranium–oxygen system in the UO2–UO3 composition range. Owing to the fact that original data are very numerous, more than 500 publications, a twofold process is used for the assessment – (i) first a critical selection of data is performed for each method of measurement together with a careful estimate of their uncertainties, (ii) second a reduction of the total number of data on the basis of a chart with fixed intervals of temperature and composition that allows a comparison to be made of the results from the various experiments. Results are presented for chemical potentials of oxygen with their associated uncertainties.

Congruent vaporization of GaAs(s) and stability of Ga(l) droplets at the GaAs(s) surface

The existence of a liquid gallium film or droplets on the surface of a GaAs(s) single crystal at high temperature is analyzed using the thermodynamics of vaporization and interfaces in the AsGa system. The steady-state of vaporization is calculated by taking into account the congruent vaporization of the liquid phase, the vaporization of solid GaAs in equilibrium with the liquid phase and the congruent vaporization of the GaAs(s) crystal. The stability of a liquid film or droplets at the surface of a crystal is estimated by examining the surface and interfacial tensions of the liquid phase. The composition of the liquid depends on the vaporization steady-state and this is compared with independent free vaporization studies. Two temperature domains are determined: at high temperature (T > 905 K) gallium rich droplets are formed by arsenic excess vaporization and at low temperature (T < 905 K, which is the maximal temperature for congruent vaporization of the solid GaAs) the liquid droplets must disappear by gallium vaporization since the GaAs system tends to come back to the congruent vaporization of the pure crystal.

Thermodynamic calculations in CVD growth of GaAs compounds: I. Critical assessment of the thermodynamic properties for the gaseous molecules of the Ga-Cl system

Abstract Thermodynamic calculations of CVD growth are successful for prevision of the deposition rate, but also for selecting the main kinetic process which governs the growth rate. When changing the pressure and temperature conditions, the gaseous phase composition usually changes drastically. So, for a chemical system, the set of existing gaseous molecules must be well-known for the understanding of the CVD process. We analyse by complex equilibrium calculations the experimental works which have been performed on the Ga-Cl system and estimate or check with a dimensional model the minor species which have not been completely studied. Thermodynamic data are available for the GaCl 3 , Ga 2 Cl 6 , GaCl, Ga 2 Cl 2 , Ga 2 Cl 4 and GaCl 2 molecules, which are important in CVD-chloride deposition of GaAs .

Thermodynamic calculations of congruent vaporization in III–V systems; Applications to the In-As, Ga-As and Ga-In-As systems

Abstract Conditions for congruent vaporization of GaAs, InAs and Ga y In 1− y As compounds under vacuum are calculated using thermodynamic data of basic binary and ternary systems. Erosion rates are predicted for the GaAs and InAs compounds that explain their tendancy to form liquid metallic droplets at their surface at high temperature. For the ternary Ga y In 1− y As compounds, partial congruent vaporization is defined that is useful for molecular beam epitaxy to avoid the droplet appearance at the compound surface. The maximal temperature for partial congruent vaporization is calculated as a function of the composition varying from pure InAs to pure GaAs. The curve has a maximum at 992 K for the Ga 0.99 In 0.01 As composition which explains the higher stability of these substrates under vacuum.

Thermodynamic properties of the O–U system. II – Critical assessment of the stability and composition range of the oxides UO2+x, U4O9−y and U3O8−z

Abstract The published data concerned with the determination of the composition ranges of uranium oxides, UO 2+ x , U 4 O 9− y and U 3 O 8− z , which have been determined using thermogravimetric, X-ray diffraction and electrochemical techniques are critically assessed. U 4 O 9 and U 3 O 8 have quite small domains of composition and the assessment of such data has carefully considered the uncertainties in the experimental determinations. In addition, the thermodynamic properties of U 4 O 9 and U 3 O 8 , enthalpies of formation and transformation, entropies, and thermal capacities are analyzed and selected to build a primary data base for compounds.