Kinetics of phase separation in polyurethane/polystyrene semi-1 interpenetrating polymer networks. 2. Microscopy observations and theoretical approach

The phase separation process of in-situ polyurethane/polystyrene semi-1 interpenetrating polymer networks has been followed by optical microscopy completed by image analysis. The final morphology has been examined by scanning electron microscopy, and its relation to the phase separation process has been established. When phase separation occurs before gelation of the medium, polystyrene is dispersed both as large and small noduli in the polyurethane network; a polyurethane-rich interphase separates the noduli from the matrix. These observations explain the previous results on the turbidity of such systems measured by light transmission. In the opposite cases, no visible phase separation is observed on the micrographs. The thermodynamics of the phase separation process has also been examined.     

Evolution structurale par substitution cationique dans les perovskites hexagonales fluorees: II - Les systèmes Cs1−xRbxMF3 (M = Mg,Co, Ni,Zn)

The substitution of rubidium for cesium in CsMF₃ phases has the same influence as pressure. While no phase transformation is observed if M is magnesium or zinc, the sequences 2H → 9R → 6H and 9R → 6H → 3C are respectively observed in the case of nickel and cobalt with increasing rubidium concentration. The 9R-Cs_0.70 Rb_0.30 NiF₃ phase is antiferromagnetic and the 6H-Cs_0.50 Rb_0.50 CoF₃ compound is a ferrimagnet (T_C = 62 K).     

An A Contrario Decision Method for Shape Element Recognition

Shape recognition is the field of computer vision which addresses the problem of finding out whether a query shape lies or not in a shape database, up to a certain invariance. Most shape recognition methods simply sort shapes from the database along some (dis-)similarity measure to the query shape. Their main weakness is the decision stage, which should aim at giving a clear-cut answer to the question: “do these two shapes look alike?” In this article, the proposed solution consists in bounding the number of false correspondences of the query shape among the database shapes, ensuring that the obtained matches are not likely to occur “by chance”. As an application, one can decide with a parameterless method whether any two digital images share some shapes or not.     

Facteur de couplage phase-amplitude des diodes lasers avec absorbant saturable

The phase-amplitude coupling factor (α-factor) is one of the fundamental parameters of semiconductor lasers. We show that this factor can be evaluated by time-resolved spectroscopic measurements in pulsed regime. Experiments are carried out with injectionseeded single-mode GaAs lasers whose active stripe includes regions of saturable absorption made by ion implantation. We show that the measured pulse downchirp is proportional to pulse energy. The downchirp amplitude is found to approach four laser cavity intermode spacings at the highest energies. Correspondingly, the ±-values can be five times higher than those reported to date in litterature. High a-values are confirmed in pulse compression experiments. Several mechanisms are proposed to interprete these new results.     

Opérations de cylindre et applications séquentielles gauches inverses

Résumé Dans cet article, on étudie les rapports entre la principauté d'une famille de langages en tant que cylindre et en tant que famille fermée par application séquentielle gauche (gsm) inverse. Pour cela on considère les familles de langages closes pour les opérations de cylindre et les gsm inverses. On montre qu'une telle famille est principale en tant que cylindre seulement si elle est principale vis à vis des gsm inverses, mais que l'inverse n'est pas vrai. Cependant, si l'on se restreint aux familles de langages vérifiant une propriété de clôture que l'on explicite, on obtient cette réciproque.

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In this paper, the relations between the principality of a family of languages as a cylinder and its principality as family closed under inverse gsm mappings are studied. For that purpose, the families of languages closed under the operations of cylinder and under inverse gsm mappings are considered. It is shown that such a family is a principal cylinder only if it is principal through the operation of inverse gsm mapping, but that the converse is not true. However, the converse is true for the families satisfying a closure property given here.

     

Least-squares support vector machines modelization for time-resolved spectroscopy

By use of time-resolved spectroscopy it is possible to separate light scattering effects from chemical absorption effects in samples. In the study of propagation of short light pulses in turbid samples the reduced scattering coefficient and the absorption coefficient are usually obtained by fitting diffusion or Monte Carlo models to the measured data by use of numerical optimization techniques. In this study we propose a prediction model obtained with a semiparametric modeling technique: the least-squares support vector machines. The main advantage of this technique is that it uses theoretical time dispersion curves during the calibration step. Predictions can then be performed by use of data measured on different kinds of sample, such as apples.     

Distributed bragg reflector resonators with cylindrical symmetry and extremely high Q-factors

A simple non-Maxwellian method is presented that allows the approximate solution of all the dimensions of a multilayered dielectric TE0qp mode cylindrical resonant cavity that constitutes a distributed Bragg reflection (DBR) resonator. The analysis considers an arbitrary number of alternating dielectric and free-space layers of cylindrical geometry enclosed by a metal cylinder. The layers may be arranged along the axial direction, the radial direction, or both. Given only the aspect ratio of the cavity, the desired frequency and the dielectric constants of the material layers, the relevant dimensions are determined from only a set of simultaneous equations, and iterative techniques are not required. The formulas were verified using rigorous method of lines (MoL) calculations and previously published experimental work. We show that the simple approximation gives dimensions close to the values of the optimum Bragg reflection condition determined by the rigorous analysis. The resulting solution is more compact with a higher Q-factor when compared to other reported cylindrical DBR structures. This is because it properly takes into account the effect of the aspect ratio on the Bragg antiresonance condition along the z-axis of the resonator. Previous analyses assumed the propagation in the z-direction was independent of the aspect ratio, and the layers of the Bragg reflector were a quarter of a wavelength thick along the z-direction. When the aspect ratio is properly taken into account, we show that the thickness of the Bragg reflectors are equivalent to the thickness of plane wave Bragg reflectors (or quarter wavelength plates). Thus it turns out that the sizes of the reflectors are related to the free-space propagation constant rather than the propagation constant in the z-direction.     

A non linear integral constitutive equation for viscoelastic fluids

An intégral constitutive equation is written using a particular reference frame, built with unit vectors along the principal axes of the rate-of-deformation tensor, and using the associated intrinsic rate-of-rotation. This equation is easier to handle in calculations than corotational or codeformational models. The material functions for a rheological model including the first six terms of the constitutive equation have been studied in steady and unsteady shear flows, as well as in elongational flows. Material functions are readily written from six memory functions and no inconsistency comes out.     

Etude des copolymères acryliques utilisés dans l'encollage de fils de polyester. I. Caractéristiques physicochimiques des copolymères et leurs interactions avec des fils de polyester

Sizing is generally employed to protect yarns when submitted to mechanical stress during weaving. Two commercial acrylic copolymers used for sizing of polyester yarns are studied: an acrylonitril-acrylic acid copolymer (A) as a bad sizing product and an acrylonitril-acrylic acid-ethyl acrylate copolymer (B) as a good one. A relationship has been established between physico-chemical parameters and sizing efficiency. Interactions between polyester yarn and size (and samples used as models) are studied both in solution and in dry state. It must be pointed out that the action of water is basically due to its plasticizing properties. The presence of ethyl acrylate favorizes the polyester yarn impregnation with size B in aqueous solution. Adhesion in the dry state is mainly caused by acrylic acid. Finally the action of neutralizing agents and imbeding oils is discussed.     

On the Characterisation of Silver Species for SCR of NO x with Ethanol

Reducing of nitrogen oxides (NO _x ) in a lean exhaust gases has become one of the most important environmental concerns. Among the different active phases studied for NO _x reduction reaction, silver-based catalysts supported over alumina show good performances using, as reducing agents, either hydrocarbons or oxygenated compounds. Nevertheless, a good understanding of the mechanism reaction has not been reached yet. This comprehension requires a better characterisation of the silver-based catalysts system. In our study, Ag/Al₂O₃ catalysts showed high efficiency in NO _x reduction using ethanol as reducing agent. The conversion plots, in steady state conditions for the different samples Ag/Al₂O₃ (0.8–3.5% Ag wt), show a great dependance of the activity with the metal loading. The optimal silver loading has been established around 2 wt.% Increasing the silver loading, the temperature of maximal NO _x conversion shifted toward the lower temperatures. According to the literature, a reduced and an oxide phase of silver have been observed by UV–Vis spectroscopy. The ratio between the two phases is changing with the silver loading. However, temperature programmed reduction (TPR) measurements reveal the presence of two types of oxide phases. TPR reveal the coexistence of a silver oxide phase (Ag₂O), according to a production of water in the course of the reaction, and a non-oxygenated phase attributed to isolated Ag⁺ cation. Thus, an original way using TPR measurements has been developed to differentiate the various oxidized phases. The aim of this characterisation is to correlate the catalyst’s activity with the observed silver phases, in order to understand the nature of phase active for NO _x reduction at low temperatures.