Effect of Tin Precursor on the Catalytic Properties of Pt-Sn/Al2O3 Sol-Gel Prepared Catalysts

Tin-alumina supports were prepared by cogelation of aluminum tri-sec-butoxide and tetrabutyltin or tin tetrachloride. Aqueous or acetone platinum solutions were used for impregnation. The results show that the specific surface area and the acidity of the catalysts were only slightly modified by the metal precursors used. On cyclohexane dehydrogenation andn-hexane conversion, high selectivity to olefins and low cracking products were detected.     

Structural effects on the photodissociation of alkoxyamines

The search for photosensitive alkoxyamines represents a huge challenge. The key parameters governing the cleavage process remain unknown. The dissociation process of light sensitive alkoxyamines is studied as a function of their chemical structures. The photochemical properties of 6 selected compounds are investigated by ESR and laser flash photolysis. It is found that (i) the selectivity of the cleavable N-O vs. C-O bond and (ii) the efficiency of the nitroxide formation are strongly related to the alkoxyamine structure. The distance between the chromophore and the aminoxy group is a key parameter for an efficient pathway of the radical generation as displayed by the photopolymerization ability of these alkoxyamines.     

In vivo quantitative NMR imaging of fruit tissues during growth using Spoiled Gradient Echo sequence

Nondestructive studies of physiological processes in agronomic products require increasingly higher spatial and temporal resolutions. Nuclear Magnetic Resonance (NMR) imaging is a non-invasive technique providing physiological and morphological information on biological tissues. The aim of this study was to design a robust and accurate quantitative measurement method based on NMR imaging combined with contrast agent (CA) for mapping and quantifying water transport in growing cherry tomato fruits. A multiple flip-angle Spoiled Gradient Echo (SGE) imaging sequence was used to evaluate the intrinsic parameters maps M₀ and T₁ of the fruit tissues. Water transport and paths flow were monitored using Gd^3 +/[Fe(CN)₆]^3 −/D − mannitol nanoparticles as a tracer. This dynamic study was carried out using a compartmental modeling. The CA was preferentially accumulated in the surrounding tissues of columella and in the seed envelopes. The total quantities and the average volume flow of water estimated are: 198 mg, 1.76 mm³/h for the columella and 326 mg, 2.91 mm³/h for the seed envelopes. We demonstrate in this paper that the NMR imaging technique coupled with efficient and biocompatible CA in physiological medium has the potential to become a major tool in plant physiology research.     

General Limit Distributions for Sums of Random Variables with a Matrix Product Representation

The general limit distributions of the sum of random variables described by a finite matrix product ansatz are characterized. Using a mapping to a Hidden Markov Chain formalism, non-standard limit distributions are obtained, and related to a form of ergodicity breaking in the underlying non-homogeneous Hidden Markov Chain. The link between ergodicity and limit distributions is detailed and used to provide a full algorithmic characterization of the general limit distributions.     

Two-dimensional quantitative mapping of arsenic in nanometer-scale silicon devices using STEM EELS–EDX spectroscopy

Field emission gun (FEG) nanoprobe scanning electron transmission microscopy (STEM) techniques coupled with energy dispersive X-ray (EDX) and electron energy loss spectroscopy (EELS) are evaluated for the detection of the n-type dopant arsenic, in silicon semiconductor devices with nanometer-scale. Optimization of the experimental procedure, data extraction and the signal-to-noise ratio versus electron dose, show that arsenic detection below 0.1% should be possible. STEM EDX and EELS spectrum profiles have been quantified and compared with secondary ion mass spectrometry (SIMS) analyses which show a good agreement. In addition, the arsenic doping level found inside large and small epitaxial devices have been compared using STEM EDX-EELS profiling. The average doping level is found to be similar but variable interface segregation has been observed. Finally, STEM EDX arsenic mapping acquired in a BiCMOS transistor cross-section shows strong heterogeneities and segregation in the epitaxially grown emitter part.     

Étude expérimentale de la dispersion active d'un polluant hydrocarboné en milieu poreux hétérogèneActive dispersion of a hydrocarbon pollutant in heterogeneous porous media: experimental study

This study deals with Non Aqueous Phase Liquid (NAPL) dissolution in subsurface water in order to predict the pollutant plume development and to optimize remediation processes. An experimental study of NAPL dissolution in porous media is presented. Local water saturation and effluent pollutant concentration measurements are presented for several kinds of porous media. Experimental results show clearly the influence of microscopic and/or macroscopic heterogeneities of the porous media and the distribution of the pollutant on the active dispersion of the NAPL. The NAPL dissolution occurs in several steps which highlights the existence of non-local equilibrium related to the heterogeneity of the porous media. To cite this article: A. Yra et al., C. R. Mecanique 334 (2006).     

Inverse thermal analysis of the drying zone of the evaporator of an axially grooved heat pipe

An inverse approach is performed to characterize the thermal behaviour of an axially grooved heat pipe, in steady state, for various operating conditions. For this purpose, an experimental set up, as well as a network conduction model, are developed to simulate the heat transfer in the wall at the evaporator section. The minimization of an objective function, taking into account the discrepancy between measured temperatures and computed ones, allows then the estimation of a heat transfer coefficient as well as the drying out front positions for all the axial grooves. Hence, at the burnout point, the significant temperature increase in the evaporator extremity is considered to be a direct consequence of the restriction of the evaporative zone. Therefore, the distribution of liquid phase in the capillary structure of the heat pipe can be obtained through the analysis of the measured temperature gradient in the evaporator section where the dry out front was expected to occur. Furthermore, the dry out front expansion can be observed when the input heat load is increased or when the adiabatic temperature is decreased. Introducing an adverse tilt angle also shows the effect of the puddle.     

Fast formation of magnetic islands in a plasma in the presence of counterstreaming electrons

With the help of 2D-3V (two dimensional in space and three dimensional in velocity) Vlasov simulations we show that the magnetic field generated by the electromagnetic current filamentation instability develops magnetic islands due to the onset of a fast reconnection process that occurs on the electron dynamical time scale. This process is relevant to magnetic channel coalescence in relativistic laser plasma interactions.