In situ XRD analysis of the oxide layers formed by oxidation at 743 K on Zircaloy 4 and Zr–1NbO

Abstract Two alloys, having different oxidation behaviour (Zy4 and Zr–1NbO), have been investigated during oxidation at high temperature (743 K) and low oxygen pressure (10 kPa) by in situ X-ray diffraction (XRD). Tetragonal phase content and ‘pseudo-stresses’ on the monoclinic phase have been measured as a function of the oxide layer thickness. The tetragonal phase contents are similar for both alloys and decreased with the oxide layer thickness. Pseudo-stresses were much more compressive on Zr–1NbO alloy, with limited changes at the corrosion kinetics transition. On cooling, the tetragonal fractions do not change, while ‘pseudo-stresses’ decreased in different ways for the two alloys. Wit…

Studies on Instabilities and Patterns in Evaporating Liquids at Reduced Pressure and/or Microwave Irradiation

This paper summarizes our recent experimental and theoretical work on the instabilities in liquids and at interfaces which form during evaporation at reduced pressure and/or microwave irradiation. We have observed a variety of patterns (Benard rolls, Marangoni waves, Hickman interface deformations) which depend on the value of the reduced pressure and the power of the incident beam.

Oxidation of niobium sheets at high temperature

Abstract This work shows the role of oxygen pressure on growth kinetics, morphology, phase composition and texture of oxide scale in the oxidation of niobium sheets at 900 °C. The particular morphology observed at low pressure shows the existence of competition between edge and surface growth mechanisms. Pressure and conversion rate influence the nature of preferential orientations of the Nb2O5 oxide layer: a [110] fibre axis is stabilized at high pressure but, at low pressure, the higher is the conversion rate, the more evident is this fibre axis. This behaviour is observed not only after cooling but also in situ. At high pressure, the evolution is not only temporal but also spatial: with …

Periodic behaviour in heterogeneous chemical reactions

Abstract The authors present an analytical and numerical analysis for a solid-gas oxidation process represented by a set of coupled reaction rates equations. The equations describe the time evolution of four elementary process that govern the overall heterogeneous kinetics. The description formation of a new oxide unit considers: (1) an internal interface (oxide-metal) reaction by which an activated complex is formed; (2) the dissolution of the complex produce a chemical element σ; (3) the diffusion of σ through the oxide layer; and (4) an external interface (oxide-gas) reaction. The results reported here delinate the parameter region where chemical oscillations are present.

Spatiotemporal dynamics in the oxidation of groups IV?V metals: study of zirconium

Abstract This paper focus on the mechanism of zirconium oxidation at 850 °C by X-ray diffraction (XRD) and the interactions between chemical (oxide growth, phase proportion) and physical processes (oxide scale texture, mechanical stresses). The structural gradients in the oxide scale composed of monoclinic ZrO 2 (higher proportion) and tetragonal ZrO 2 were revealed by XRD at variable incidence. The spatial evolution of the monoclinic zirconia texture through the oxide scale was shown and quantified. Relaxation of the oxide compressive stresses was observed for different oxidation times and correlated to kinetic breakaway evidenced by other methods. A structural mechanism of the oxide forma…

Absorption and desorption of liquid water by a superabsorbent polymer: Effect of polymer in the drying of the soil and the quality of certain plants

In this work, we were interested in the study of the kinetic behavior of the reaction of drying soil representing the Marrakesh region (south of Morocco) in the presence of an superabsorbent polymer. The presence of a polymer in the soil diminished the kinetic drying of the soil, which will have as an application a reduction in water loss in the process of irrigation. The presence of a polymer in the soil enables a complete modification of the kinetic regimes controlling the soil-drying kinetic. We also realized the first study of the behavior of certain selected plants in the melange of a soil and a polymer. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 234–243, 2002

Improved Approach to Liesegang Phenomena

Determination of Zircaloy -4 Radiocristallographic Elastic Constants from Room Temperature to 300°C

A new experimental apparatus, allowing the determination of radiocristallographic elastic constants (REC) as a function of the temperature is presented. The material studied the zirconium alloy Zircaloy-4, showing low elastic anisotropy, an isotropic treatment of the problem is possible. As a consequence, the classical sin 2 ψ method is used to determine the REC from room temperature to 300°C.

Absorption et désorption de l’eau liquide par un polymère superabsorbant

Resume L’etude cinetique du sechage d’un gel (polymere mouille) a montre que les reactions de sechage effectuees sous pression reduite de vapeur d’eau sont generalement rapides meme a basse temperature, ce qui est un avantage par rapport aux systemes zeolithiques. En effet, les systemes zeolithiques presentent une cinetique de regeneration lente ce qui peut limiter leurs utilisation dans le domaine des energies renouvelables et en particulier celui des pompes a chaleur chimique. L’utilisation des masses elevees du gel permet de modifier completement le regime cinetique limitant la reaction de sechage; il passe en chauffage classique d’un phenomene controle par l’evaporation suivie d’une dif…

Absorption and desorption of liquid water by a superabsorbent polyelectrolyte: Role of polymer on the capacity for absorption of a ground

To study the potentialities of a polyelectrolyte superabsorbent for the storage of water, we have studied a ground representative of the area of Haouz (area of Marrakesh, South of Morocco) in the presence of polymer. We sought the optimal proportions allowing the best retention of water by the mixture made up of the polymer and the ground and to prolong the storage period of water in the ground. We have also studied the effect of certain ions and the pH on the capacity for absorption of polymer. These ions can be naturally present in the ground or introduced by the means of manure and pesticides. The presence of polymer in the ground makes it possible to increase the capacity for absorption…

Adsorption et desorption de l'eau vapeur sur un polymère polyacrylique absorbant

Resume Cet article porte sur la cinetique d'adsorption de l'eau vapeur sur un polymere reticule de l'acide acrylique. L'allure des courbes isocinetiques reportees dans le diagrame d'equilibre ( p , T ) est la manifestation directe de l'effet exothermique de la reaction d'adsorption et de sa consequence sur l'ecart a l'equilibre de vaporisation de l'eau; L'existence de cet effet thermique n'entraine pas de modification du mecanisme reactionnel comme ceci est indique lors d'etudes cinetiques de rehydration, ou dans les differentes approches de l'effet Smith-Topley (evaporation, deshydration, decomposition de carbonates, ou d'ammoniacales).

Oxidation of Titanium, Zirconium and their Alloys, Texture of their Oxide Scales

Microwave heating as a tool for coupling Marangoni and Hickman instabilities

The aim of this paper is the linear stability analysis of an evaporating interface in order to justify hydrodynamic behaviors observed during evaporation of water and ethanol simultaneously under microwave irradiation and reduced pressure. In relation to the local character of the thermal conversion of electromagnetic energy, and of the thermal dependency of the dielectric loss, it seems possible under microwave heating to induce a specific hydrodynamic compared to those induced by conventional heating.

Caractéristiques des réactions endothermiques d'interface

The knowledge of the different thermodynamic states of an evaporative interface leads us to propose a formulation of the evolution laws (evaporation rate vs pressure) adapted to each regime (interfacial equilibrium or non equilibrium). Starting from fluid dynamics and irreversible thermodynamics, the model is able to determine the interface flux-forces relations which express the transport coefficients and the thermodynamic coupling effects. The theoretical results are compared to experimental data relative to water evaporation al T =313 K. Taking into account the different regimes arising for the deviation from equilibrium, the model reproduces quantitatively the experimental kinetic curve…

Mechanical, Kinetic and Morphological Correlations during the Scale Growth of Niobium and Zirconium Oxides

Oxidation of niobium at 700°C and zirconium at 850°C, in air or oxygen pressures from 20 to I 000 hPa, lead to the formation in one case of a stratified Nb 2 O 5 oxide scale and in the other case a more compact ZrO 2 oxide scale. Analyses of the evolutions of phases, crystalline textures and mechanical stresses during oxidation are determined by X-ray diffraction techniques. Interactions between chemical processes, crystalline building and mechanical processes constitute together the driving force of the reaction and can explain the differences in morphologies of oxide scales.

INTERACTION BETWEEN MECHANICAL STRESSES AND OXIDATION OF THE NIOBIUM

Valorisation de machines frigorifiques a adsorption sur zeolithes

Abstract The regeneration stage using intermittent thermodynamical cycles based on zeolite adsorption (zeolite 13X-H2O) has been studied. This point is of particular interest for understanding of systems of energy storage and cooling. This paper examines the use of microwave energy sources for analysing the regeneration problem. The method proposed allows a considerable decrease in the regeneration time without an increase of the temperature of the material. The energy yields are high and the adsorption-desorption time cycles are short if the microwave field is well tuned. This last is a function of time and of the filling coefficient. The results obtained using microwave irradiation are co…