A portable X-ray diffraction apparatus for in situ analyses of masters’ paintings

It is rare that the analyses of materials in paintings can be carried out by taking micro-samples. Valuable works of art are best studied in situ by non-invasive techniques. For that purpose, a portable X-ray diffraction and fluorescence apparatus has been designed and constructed at the C2RMF. This apparatus has been used for paintings of Rembrandt, Leonardo da Vinci, Van Gogh, Mantegna, etc. Results are given to illustrate the performance of X-ray diffraction, especially when X-ray fluorescence does not bring sufficient information to conclude.     

On instability of elliptical hetons

Using the method of contour surgery, we examine the evolution of an initially vertically aligned elliptical heton. A classification of quasi-stable and unstable regimes for the case of two-layered vortex structure is suggested.     

Fluorescent Brighteners as Visible LED‐Light Sensitive Photoinitiators for Free Radical Photopolymerizations

The photochemical and electrochemical investigations of commercially available, safe, and cheap fluorescent brighteners, namely, triazinylstilbene (commercial name: fluorescent brightener 28) and 2,5‐bis(5‐tert‐butyl‐benzoxazol‐2‐yl)thiophene, as well as their original use as photoinitiators of polymerization upon light emitting diode (LED) irradiation are reported. Remarkably, their excellent near‐UV–visible absorption properties combined with outstanding fluorescent properties allow them to act as high‐performance photoinitiators when used in combination with diaryliodonium salt. These two‐component photoinitiating systems can be employed for free radical polymerizations of acrylate. In addition, this brightener‐initiated photopolymerization is able to overcome oxygen inhibition even upon irradiation with low LED light intensity. The underlying photochemical mechanisms are investigated by electron‐spin resonance‐spin trapping, fluorescence, cyclic voltammetry, and steady‐state photolysis techniques.

     

Slow motion of a conducting fluid between two rotating walls

Summary An investigation is presented of the flow of a viscous, conducting fluid between two plane walls, which rotate around a common axis towards each other. The flow is considered to be under the influence of a magnetic field, set up by a line-current along the axis of rotation.Under the assumption of low hydrodynamic and magnetic Reynolds-number, expressions are given for the velocity and the pressure gradient.At present at Brussels: Ecole Royale Militaire, Physique des Plasmas.     

Réduction de modèle en convection naturelle par une méthode d'identificationModel Reduction in natural convection using an identification method.

In this numerical study, a natural convection Reduced Model (RM) is built by using results of a Detailed Model (DM, obtained by a pseudo-spectral method), associated with an optimisation problem. RM's structure is defined from a matrix formulation obtained after spatial discretisation of the problem local equations. RM's matrices are identified through the minimisation of a functional build on the discrepancy between DM's and RM's responses, for several values of Rayleigh number. The method is applied to a laminar steady flow in a 2D differentially heated cavity (Window problem). The RM allows one to strongly reduce computing time. To cite this article: M. Girault et al., C. R. Mecanique 332 (2004).     

H2 evolution and molecular electrocatalysts: determination of overpotentials and effect of homoconjugation

In an effort to standardize the determination of overpotential values for H(2)-evolving catalysts in non-aqueous solvents and allow a reliable comparison of catalysts prepared and assayed by different groups, we propose to adopt the half-wave potential as reference potential. We provide a simple method for measuring it from usual stationary cyclic voltammograms, and we derive the formulas to which the measured potential should be compared, taking into account the effect of homoconjugation. We also revisit tabulated values of the standard reduction potential of protons in nonaqueous solvents, E(H+/H(2))°.     

Gold doped by palladium: Building of Au–Pd electrodes showing exceptional capability for achieving electrocatalytic reductions

Smooth gold is easily palladized by means of very thin galvanostatic deposits of Pd⁰. Such chemically modified materials are mainly tested in acetonitrile in the presence of tetraalkylammonium salts (TBABF₄). All exhibit large usable electrochemical windows: between ? 1.8 V and + 1.5 V vs. saturated calomel electrode (SCE). When employed as cathodic material, they show exceptional catalytic capabilities especially for achieving the one-electron cleavage of organic halides (RX). Thus, these modified surfaces, found to be chemically and electrochemically stable, lead to amazing potential shifts compared to those obtained with glassy carbon, smooth platinum, or gold cathodes under the same experimental conditions. Specifically, these palladium-doped gold interfaces were preliminarily used for the reduction of alkyl iodides RIs. The presence of gold is expected to boost up the catalytic effect of palladium already reported with such compounds.     

Capillary electrophoretic and nuclear magnetic resonance studies on the opposite affinity pattern of propranolol enantiomers towards various cyclodextrins

In the present study the migration order of the propranolol enantiomers with various native CDs and neutral and charged CD derivatives was examined in capillary electrophoresis (CE). The reversal of the enantiomer migration order was observed due to sulfation of β‐CD on its primary hydroxy groups. The structures of intermolecular selector‐select and temporary diastereomeric associates in solution were elucidated based on 1D rotating frame nuclear Overhauser effect spectroscopy (1D ROESY) experiments. Major structural differences were observed between the propranolol complexes with native β‐CD and heptakis(6‐O‐sulfo)‐β‐CD.     

Optimization of the LC enantioseparation of chiral pharmaceuticals using cellulose tris(4‐chloro‐3‐methylphenylcarbamate) as chiral selector and polar non‐aqueous mobile phases

The resolving power of a new commercial polysaccharide‐based chiral stationary phase, Sepapak‐4, with cellulose tris(4‐chloro‐3‐methylphenylcarbamate) coated on silica microparticles as chiral selector, was evaluated toward the enantioseparation of ten basic drugs with widely different structures and hydrophobic properties, using ACN as the main component of the mobile phase. A multivariate approach (experimental design) was used to screen the factors (temperature, n‐hexane content, acidic and basic additives) likely to influence enantioresolution. Then, the optimization was performed using a face‐centered central composite design. Complete enantioseparation could be obtained for almost all tested chiral compounds, demonstrating the high chiral discrimination ability of this chiral stationary phase using polar organic mobile phases made up of ACN and containing an acidic additive (TFA or formic acid), 0.1% diethylamine and n‐hexane. These results clearly illustrate the key role of the nature of the acidic additive in the mobile phase.     

Scalar Tachyons in the de Sitter Universe

We provide a construction of a class of local and de Sitter covariant tachyonic quantum fields which exist for discrete negative values of the squared mass parameter and which have no Minkowskian counterpart. These quantum fields satisfy an anomalous non-homogeneous Klein–Gordon equation. The anomaly is a covariant field which can be used to select the physical subspace (of finite co-dimension) where the homogeneous tachyonic field equation holds in the usual form. We show that the model is local and de Sitter invariant on the physical space. Our construction also sheds new light on the massless minimally coupled field, which is a special instance of it.