Origin of the excitonic recombinations in hexagonal boron nitride by spatially resolved cathodoluminescence spectroscopy

The excitonic recombinations in hexagonal boron nitride (hBN) are investigated with spatially resolved cathodoluminescence spectroscopy in the UV range. Cathodoluminescence images of an individual hBN crystallite reveals that the 215 nm free excitonic line is quite homogeneously emitted along the crystallite whereas the 220 nm and 227 nm excitonic emissions are located in specific regions of the crystallite. Transmission electron microscopy images show that these regions contain a high density of crystalline defects. This suggests that both the 220 nm and 227 nm emissions are produced by the recombination of excitons bound to structural defects

Numerical investigation of generalized Graetz problem in circular tube with a mass transfer coupling between the solid and the liquid

International audienceThe unsteady diffusion equation in a tube and the advection/diffusion equation in laminar flow in a liquid of a solute are theoretically established. The most important parameters are the migration strength α and the so-called Graetz number Gr. The former dimen-sionless number is the product of the partition coefficient of the solute at the solid/liquid interface, the ratio of the tube thickness to the interior radius and the ratio of diffusion coefficients of the solute in the liquid and in the solid. The Graetz number Gr is the ratio of the diffusion time scale in the liquid based on the interior radius of the pipe to the advection time over the tube length. The problem is applied to the plasticizer migration from a polyvinyl chloride material to a liquid with the safety food applications in mind. The migration of the plasticizer is solved numerically for various tube sizes, flow conditions and partition coefficients of the plasticizer in the liquid. For comparison, the numerical results obtained in static condition are also provided. The average plasticizer concentration in the liquid behaves following two main regimes as a function of the migration strength. When α is much smaller than one, the average plasticizer concentration in the liquid is an algebraic function of the axial coordinate, z, at the power two third and increases linearly with time. Conversely, when α is much larger than one, the average plasticizer concentration is linear as a function of axial distance and increases as a function of the square root of time. Moreover, the concentration is much smaller of few orders of magnitude in dynamic condition than in static working showing that in the context of food contamination the dynamic approach is more appropriate and relevant for demonstration of compliance with the safety authorities requirements

Growth mechanism of catalyst-free 0001 GaN and AlN nanowires on Si by molecular beam epitaxy

International audienceReal-time in-situ X-rays scattering experiments were performed to study the nucleation process of GaN nanowires grown by plasma-assisted molecular beam epitaxy on AlN(0001)/Si(111). From the comparison between the case of nanowires and the case of GaN quantum dots, it is concluded that nanowire precursor islands are completely relaxed from the beginning. Next, based on high resolution electron microscopy analysis, it is demonstrated that relaxation of nanowire precursors is associated to formation of dislocations at the AlN/GaN interface, establishing that three-dimensional islanding and plastic strain relaxation of GaN are two necessary conditions for nanowire growth. Along the same lines, we demonstrate that three-dimensional islanding of AlN on SiO2 leads to formation of AlN nanowires with structural and optical properties characteristic of high quality relaxed material. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Thermodynamic analysis of the reaction-induced phase separation of solutions of random copolymers of methyl methacrylate andN,N-dimethylacrylamide in the precursors of a polythiourethane network

International audiencePoly(methyl methacrylate) and random copolymers of methyl methacrylate (MMA) and N,N-dimethylacrylamide (DMA) containing 7.5, 15, or 20 wt % DMA were dissolved in a stoichiometric mixture of m-xylylene diisocyanate and 4-mercaptomethyl-3,6-dithia-1,8-octanedithiol, precursors of a polythiourethane network. Phase separation, which took place during polymerizations at 60, 90, and 120 °C,exhibited a lower critical solution temperature behavior. The cloud-point conversions, which were determined by the iodometric titration of free thiol groups of samples chilled in ice at the cloud point, increased with the weight fraction of DMA in the random copolymer. This could be used to control the cloud-point conversion and determine the characteristic size of the dispersed domains. A thermodynamic analysis was performed with the Flory–Huggins equation, taking into account the polydispersities of both the thermoplastic and thermoset polymers and using an interaction parameter depending on the temperature and on the three binary interaction energies. A reasonable fitting of the experimental curves was obtained with negative values for the interaction energies of the MMA–thermoset and DMA–thermoset pairs and with a positive value for the MMA–DMA pair

Influence du poids à 10 jours et de l'âge à la castration des porcelets mâles sur la croissance et la qualité des carcasses à l'abattage

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