Modelling of synchrotron SAXS patterns of silicalite-1 zeolite during crystallization

Synchrotron small angle X-ray scattering (SAXS) was used to characterize silicalite-1 zeolite crystallization from TEOS/TPAOH/water clear sol. SAXS patterns were recorded over a broad range of length scales, enabling the simultaneous monitoring of nanoparticles and crystals occurring at various stages of the synthesis. A simple two-population model accurately described the patterns. Nanoparticles were modeled by polydisperse core-shell spheres and crystals by monodisperse oblate ellipsoids. These models were consistent with TEM images. The SAXS results, in conjunction with in situ light scattering, showed that nucleation of crystals occurred in a short period of time. Crystals were uniform in size and shape and became increasingly anisotropic during growth. In the presence of nanoparticles, crystal growth was fast. During crystal growth, the number of nanoparticles decreased gradually but their size was constant. These observations suggested that the nanoparticles were growth units in an aggregative crystal growth mechanism. Crystals grown in the presence of nanoparticles developed a faceted habit and intergrowths. In the final stages of growth, nanoparticles were depleted. Concurrently, the crystal growth rate decreased significantly.     

Large Eddy Simulation of a Motored Single-Cylinder Piston Engine: Numerical Strategies and Validation

This paper describes a compressible Large Eddy Simulation (LES) used to investigate cyclic variations for nonreacting flow in an optical single cylinder engine setup. The simulated operating point is part of a large experimental database designed to validate LES for cycle-to-cycle prediction, and constitutes a first step towards the realization of fired operating points. The computational domain covers almost the whole experimental setup (intake and exhaust plenums, intake and exhaust ducts, cylinder) to account for acoustic phenomena. The assessment of the computation is performed in two regions of the domain: the intake and exhaust duct predictions are compared to the results of a Helmholtz solver and the experiment (pressure transducers and Particle Image Velocimetry (PIV)) while the in-cylinder dynamics are compared to PIV measurements. The ability of the developed methodology to capture the correct level of cycle-to-cycle variations is demonstrated considering in-cylinder pressure and velocity fields predictions. Cycle-to-cycle variations in velocity are highlighted and localized using a proper orthogonal decomposition analysis.     

On the Global Regularity for a Wave-Klein—Gordon Coupled System

In this paper we consider a coupled Wave-Klein—Gordon system in 3D, and prove global regularity and modified scattering for small and smooth initial data with suitable decay at infinity. This...     

Structures de Poisson sur une intersection complète à singularités isoléesPoisson structures over a complete intersection with isolated singularities

We study Poisson structures over singular varieties. For this purpose, we consider the Koszul complex associated to the equations of a complete intersection. This complex forms a differential graded algebra which is equivalent to the algebra of the variety. We show that a Poisson structure is equivalent to a sequence of multiderivations over the Koszul complex. If the variety has isolated singularities, then we can construct a sequence of multiderivations of reduced form. To cite this article: B. Fresse, C. R. Acad. Sci. Paris, Ser. I 335 (2002) 5–10.     

Influence of columnar defects on the thermodynamic properties of BSCCO

Amorphous columnar defects strongly affect the reversible magnetization of Bi₂Sr₂CaCu₂O_8+δ single crystals both in the vortex solid, where the change reflects the change in vortex energy due to pinning, and in the vortex liquid, where the randomly positioned columns disrupt the interaction between superconducting fluctuations.     

A Consistent BGK-Type Model for Gas Mixtures

We introduce a relaxation collision operator for a mixture of gases which satisfies several fundamental properties. Different BGK type collision operators for gas mixtures have been introduced earlier but none of them could satisfy all the basic physical properties: positivity, correct exchange coefficients, entropy inequality, indifferentiability principle. We show that all those properties are verified for our model, and we derive its Navier–Stokes limit by a Chapman–Enskog expansion.     

The mass spectra of carboxylic acids—I: Fragmentation mechanisms in maleic and fumaric acids and related compounds

The mass spectra of maleic acid, maleic acid-2,3-d, fumaric acid and fumaric acid-2,3-d have been examined and fragmentation mechanisms are proposed for these compounds. The molecular ion of the cis-acid fragments via H atom transfer from one carboxyl group to the other followed by loss of CO₂. The trans acid does not fragment significantly by this route and the former effect may be characteristic of molecules containing two carboxyl groups cis-oriented to each other. This hypothesis was successfully tested by examining the mass spectra of citraconic, itaconic and phthalic acids. Itaconic and mesaconic acids show some of the fragmentation characteristics of fumaric acid.     

Abnormal viscosimetric behavior of homopolymers in mixed solvents and preferential solvation

It is shown that the applicability of the Stockmayer–Fixman–Burchard plot is much more limited in the case of solvent–precipitant mixtures than in the case of pure solvents. The observed deviations can be explained by taking into account the variation of preferential solvation as a function of the molecular weight of the polymer. More precisely the average density of the segments in the region occupied by the coiled molecule seems to be the parameter governing both changes in viscosity and in preferential solvation.