Non-parametric synthesis of laminar volumetric texture

International audienceThe goal of this paper is to evaluate several extensions of Wei and Levoy's algorithm for the synthesis of laminar volumetric textures constrained only by a single 2D sample. Hence, we shall also review in a unified form the improved algorithm proposed by Kopf et al. and the particular histogram matching approach of Chen and Wang. Developing a genuine quantitative study we are able to compare the performances of these algorithms that we have applied to the synthesis of volumetric structures of dense carbons. The 2D samples are lattice fringe images obtained by high resolution transmission electronic microscopy (HRTEM)

Structural features of pyrocarbon atomistic models constructed from transmission electron microscopy images

International audienceWe report on atomistic models of laminar pyrocarbons constructed using a combination of 2D high resolution transmission electron microscopy (HRTEM) lattice fringe image analysis, 3D image synthesis and atomistic simulated annealing. In a first step, the effectiveness of the method and the convergence of the models with respect to the quench rate are checked on small systems. Then, the nanostructural features of large fully carbonaceous atomistic models obtained from the HRTEM images of a rough laminar pyrocarbon, asprepared and after partial graphitization, are discussed. Both models show a very pronounced sp2 character (≥ 97%), essentially made of hexagonal rings (≥ 88%) and pentagonal and heptagonal rings in similar amounts (≈ 6%). The latter mostly form pentagon-heptagon pairs or networks of line defects between misoriented hexagonal domains. Numerous pairs of screw dislocations, connecting different graphene domains, are also observed while edge dislocations with unsaturated carbon atoms are almost absent. The models are validated with respect to experimental pair distribution functions, showing excellent agreement

The self-referential method for linear rigid bodies : application to hard and Lennard-Jones dumbbells

The self-referential (SR) method incorporating thermodynamic integration (TI) [Sweatman et al., J. Chem. Phys. 128, 064102 (2008)] is extended to treat systems of rigid linear bodies. The method is then applied to obtain the canonical ensemble Helmholtz free energy of the alpha-N2 and plastic face centered cubic phases of systems of hard and Lennard-Jones dumbbells using Monte Carlo simulations. Generally good agreement with reference literature data is obtained, which indicates that the SR-TI method is potentially very general and robust

Algorithmes non paramétriques pour la synthèse de textures volumiques à partir d'un exemple 2D

National audienceCet article traite de synthèse non paramétrique de textures volumiques anisotropes à partir d'une observation 2D unique. Nous proposons différentes variantes originales d'un algorithme non paramétrique et multiéchelle de synthèse de textures. L'originalité réside d'une part dans le fait que le processus de synthèse 3D s'appuie sur l'échantillonnage d'une seule image 2D d'entrée en garantissant la cohérence selon deux des trois vues de la texture 3D de sortie et, d'autre part, dans les stratégies de parcours 3D et d'optimisation utilisées. Les différentes variantes algorithmiques proposées sont appliquées à la synthèse de textures volumiques de matériaux carbonés, à partir d'une observation 2D unique obtenue par Microscopie Electronique en Transmission (MET). Nous développons une étude qualitative et quantitative des résultats de synthèse qui permet d'une part d'identifier les stratégies les plus pertinentes pour la synthèse et d'autre part de les comparer de façon objective à certains algorithmes de la littérature

Atomistic models of pyrolytic carbons obtained with the IGAR method

International audienceThis study aims at obtaining atomistic representations of these RL and ReL pyCs and at describing their nano-texture/structure. We will first describe some improvements in the IGAR method. Then, the nature of defects in these materials, in terms of non-hexagonal rings and screw dislocations, as well as their hydrogen atoms contents and locations, will be thoroughly compared and discussed. These models will also be validated against experimental HRTEM images and pair distribution functions (PDF) (and structure factors) obtained from neutron diffraction

Prédiction de la structure et des propriétés de pyrocarbones à partir d'une reconstruction à l'échelle atomique

National audienceNous présentons ici une approche originale permettant de construire des modèles à l'échelle atomique de matériaux nanotexturés à partir de leurs images de Microscopie Electronique en Transmission à Haute Résolution (HRTEM). Partant d'une image HRTEM filtrée à hautes et basses fréquences (a), un ensemble de descripteurs statistiques est collecté puis imposé à une synthèse d'image 3D aléatoire (b). Cette dernière est ensuite utilisée comme champ de potentiel attirant les atomes sur les franges lors d'une simulation de trempe d'un liquide de carbone par dynamique moléculaire (c). Une simulation d'image HRTEM à partir du matériau virtuel obtenu permet de valider la qualité de la reconstruction (d). Nous présentons en détail cette méthode ainsi que la structure et les propriétés mécaniques calculées pour des pyrocarbones laminaires rugueux, tels que préparés et traités thermiquement

Carbon

We present a combined experimental and computational study of the elastic behavior of a series of highly anisotropic pyrocarbons, with crystallite sizes La in the 2–10 nm range, under a-axis compressive load. The materials include a rough laminar and a regenerative laminar pyrocarbon, as-prepared by chemical vapor deposition and after various heat treatments up to 2600 °C, for which a-axis nanoindentation experiments have been performed, showing a significant decrease in the indentation modulus and hardness with increasing La (or heat treatment temperature). To rationalize this behavior, molecular dynamics simulations of the uniaxial compression of accurate atomistic models of the materials as well as pristine graphite were performed, unraveling significant out-of-plane deformations in the models with increasing compressive strain, leading to elastic softening. More precisely, significant kinks were observed around extended screw dislocation-like defects in the most disordered pyrocarbon at rather large strain levels (∼ 3%). Conversely, graphite rather shows the formation of extended buckles, starting at very low strain values. Finite element modelling shows that such kinking/buckling transitions should take place in a large area under the indenter tip within usual nanoindentation conditions. Both finite element calculation and analytical approximation of the indentation modulus predict the correct trend of decreasing modulus with increasing La when applied with the elastic tensors computed after the buckling/kinking transitions, certainly proving the importance of the latter in the observed experimental indentation moduli

The self-referential method combined with thermodynamic integration

The self-referential method [M. B. Sweatman, Phys. Rev. E 72, 016711 (2005)] for calculating the free energy of crystalline solids via molecular simulation is combined with thermodynamic integration to produce a technique that is convenient and efficient. Results are presented for the chemical potential of hard sphere and Lennard-Jones face centered cubic crystals that agree well with this previous work. For the small system sizes studied, this technique is about 100 times more efficient than the parameter hopping technique used previously

Experimental and theoretical investigation of BCl_3 decomposition in H_2

International audienceA combined experimental and theoretical study of the homogeneous decomposition of BCl3 in a H2 carrier gas is presented. A detailed description of the B/Cl/H thermodynamic equilibrium is first obtained from ab-initio calculations from which a restricted low energy chemical mechanism is identified to model the decomposition of BCl3. Transition state theory is then invoked to obtain reaction rates and the resulting kinetic mechanism is incorporated in a 1D model of a CVD reactor. Comparison of calculated steady state concentrations with in-situ FT-IR measurements shows a good agreement at low temperatures, thus validating the kinetic model. The divergence observed at higher temperatures is attributed to boron deposition

Chem Phys Lett

The annealing of a small nanodiamond cluster at 1500 K is studied by molecular dynamics. The transformation of the particle in an almost fully graphitized carbon onion is observed. The remaining 17% of sp3 atoms are delocalized on the whole particle, both under the form of isolated point defects and of small diamond-like clusters separating large graphite-like domains. It is also shown, that the Berendsen thermostat, previously used to fix temperature in such simulations, transfers kinetic energy from internal to global motions of the cluster. This can lead to severe artifacts like the freezing of the graphitization process