Dynamic behaviour of dry and water-satured sand under planar shock conditions

International audiencePlane shock-wave experiments were performed on dry and partially water-saturated sand, using three water contents, in order to validate predictive models of material behaviour at stress levels between 1 and 10 GPa. Gas and powder guns were used to load the sample under uni-axial strain conditions at low and high stress levels, respectively. Wave motions were detected by piezoelectric pins in the samples and a VISAR (Velocity Interferometer System for Any Reflector) recorded the free-surface velocity on the back target. This study presents both experimental and simulated results. Experimental data are used to determine shock Hugoniot states. Significant differences are observed in the dynamic response of the materials under various water-saturated conditions, and are reproduced with good agreement by numerical simulations using the ARMORS (A Rheological MOdel of Rocks Saturated) model

Transmission electron microscopy characterization of GaN layers grown by MOCVD on sapphire

International audienceWe characterize by transmission electron microscopy (TEM), GaN layers deposited by metal organic chemical vapor deposition (MOCVD) on (0001) sapphire. Different GaN films with different surface morphologies have been observed and their crystallographic quality determined. Polarity and surface diffusion are the important factors that determine the surface morphology. The lack of an adapted buffer layer leads to a layer with a dominant N-polarity that contains many inversion domains (IDs) (of Ga-polarity) that grow faster than the surrounding material and form pyramids. All the flat unipolar GaN films we have observed have a Ga-polarity. Unipolarity (Ga-polarity) is realized with the recrystallization of the low temperature buffer layer or/and of the nitridation of the sapphire substrate. An intermediate cubic phase has been observed at the sapphire/buffer layer interface of optimized nitridated samples. In non optimized samples, IDs (of N-polarity) can remain near the buffer layer, but they tend to disappear during the growth of the Ga-polar GaN layer. A high growth temperature (about 1000 degrees C) was necessary to obtain flat GaN layers. (C) 1997 Elsevier Science S.A

Quantitative characterization of GaN quantum-dot structures in AlN by high-resolution transmission electron microscopy

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