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

Investigating carbon materials nanostructure using image orientation statistics

International audienceA new characterization method of the lattice fringe images of turbostratic carbons is proposed. This method is based on the computation of their orientation field without explicit detection of fringes. It allows meaningful insights into the material nanostructure and nanotexture at several scales, either qualitatively or quantitatively. The calculation of pairwise spatial statistics of the orientation field at short distance provides measurements of the coherence lengths along any direction, in particular along and orthogonally to the layers. These statistics also allow representing orientation coherence patterns typical of the observed nanostructure. At larger distances, the mean disorientation of the fringes is computed and information about the homogeneity of the sample is obtained. An experimental validation is carried out on various artificial images and an application to the characterization of four bulk turbostratic carbons is provided

A time-dependent atomistic reconstruction of severe irradiation damage and associated property changes in nuclear graphite

Detailed knowledge regarding the nature of and mechanisms causing neutron irradiation damage in graphite remains a scientific and technological challenge, particularly at high irradiation doses. Using electrons as a surrogate for neutron irradiation, we develop a time-dependent atomistic reconstruction strategy fed by a time series of high-resolution transmission electron microscopy (HRTEM) images, to monitor damage propagation in a graphite grain up to a dose of about one displacement per atom (i.e. well beyond the conventional irradiation simulations based on molecular dynamics). The reduction in crystalline order and the development of interlayer bonding observed in the models with increasing irradiation time induce significant modifications of the elastic constants and thermal conductivity. Homogenizing these properties to the case of isotropic polycrystalline graphite we are able to reproduce the increase in Young’s modulus and decrease in thermal conductivity observed experimentally for reactor graphites with increasing dose. Further validation of the models is provided via a comparison of simulated and experimental data from irradiated material such as: HRTEM images, carbon K-edge electron energy loss spectra, dose rate and stored energies

Nanoscale elasticity of highly anisotropic pyrocarbons

We report on the elastic properties of high-textured laminar pyrocarbons (PyCs) as obtained at the nanoscale using atomistic simulations on realistic models of the rough laminar (RL) PyC, the regenerative laminar (ReL) PyC, and the ReL PyC heat treated at temperatures up to 1700 °C. The purely longitudinal ( and ) and transverse ( and ) elastic properties of such materials have values of around 50–75% of those found for graphite. Conversely, cross longitudinal-transverse properties ( and ) are much larger in PyCs than in graphite (up to around one order of magnitude for ). Our results also show that stiffness decreases with the hydrogen content, that longitudinal properties increase with the extent of graphene domains and decrease with the misorientation of the layers, and that transverse and cross properties significantly increase with the amount of interlayer cross-links present in the materials. Comparison to known experimental data indicates that texture effect at superior scales play an important role in the effective macroscopic elasticity constants. Finally, from a materials perspective and in agreement with the structural evolution, the elastic properties of the ReL PyC evolve with heat treatment towards those of the RL PyC

Methane Diffusion in a Flexible Kerogen Matrix

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Methyldichloroborane Evidenced as an Intermediate in the Chemical Vapour Deposition Synthesis of Boron Carbide

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