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

Modelling Binary, Knudsen and Transition Regime Diffusion Inside Complex Porous Media

The problem of gaseous diffusion inside complex porous media arises in the modeling of many chemical processes (e.g. Chemical Vapor Infiltration (CVI), heterogeneous catalysis in porous catalysts, filtration, etc...). A program computing effective diffusivities in the bulk, Knudsen and transition regimes has been designed and tested, which uses a Monte-Carlo mean-square-displacement algorithm. The porous medium has been represented from a special interpretation of a computer 3D discretized image. Simulations were carried out in a typical case of complex structured porous medium : a stacking of tissues (e. g. 2D woven fiber preform for CVI-densified composite materials). The results are presented as tortuosity factors, i.e. deviations from an equivalent medium made of straight cylindrical pores. The evolution of the diffusivities with the geometrical parameters of the tissues, and with the stacking mode has also been studied. It appears that the perpendicular diffusivity is closely related to the proportion of matching holes between different layers of tissue. The intermediate regime appears for Knudsen numbers lying between 100 and 10-l. In this domain, the Bosanquet formula only gives a good description if the Knudsen number is multiplied by a factor [MATH] = 1/4. This phenomenon had been reported, to a lesser extent, for unidirectional random fiber packings

Rarefied Pure Gas Transport in Non-isothermal Porous Media: Validation and Tests of the Model

Viscous flow, effusion, and thermal transpiration are the main gas transport modalities for a rarefied gas in a macro-porous medium. They have been well quantified only in the case of simple geometries. This paper presents a numerical method based on the homogenization of kinetic equations producing effective transport properties (permeability, Knudsen diffusivity, thermal transpiration ratio) in any porous medium sample, as described by a digitized 3D image. The homogenization procedure -- neglecting the effect of gas density gradients on heat transfer through the solid -- leads to closure problems in R^6 for the obtention of effective properties ; they are then simplified using a Galerkin method based on a 21-element basis set. The kinetic equations are then discretized in R^3 space with a finite-volume scheme. The method is validated against experimental data in the case of a closed test tube. It shows to be coherent with past approaches of thermal transpiration. Then, it is applied to several 3D images of increasing complexity. Another validation is brought by comparison with other distinct numerical approaches for the evaluation of the Darcian permeability tensor and of the Knudsen diffusion tensor. Results show that thermal transpiration has to be described by an effective transport tensor which is distinct from the other tensors

Corrigendum to “Analytical modeling of the steady state ablation of a 3D C/C composite”, [. Int. J. Heat Mass Transfer 51 (2008) 2614–2627.]

The authors regret that after a careful re-reading of the paper by Z. H., some errors have been identified in three formulae. The original authors agree with the modifications.(...) The conclusions of the article are not changed by the discovery of these errors. The authors would like to apologise for any inconvenience caused

A 2D image-based multiphysics model for lifetime evaluation and failure scenario analysis of self-healing ceramic-matrix mini-composites under a tensile load

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Thermal modelling of a carbon/carbon composite material fabrication process

The rapid densification process is a variant of Chemical Vapour Infiltration. It allows to fabricate composite materials starting from a fibrous preform lying in a boiling hydrocarbon precursor, the cracking of which results in a solid deposit constituting the matrix of the carbon/carbon composite. A phenomenological analysis emphasises the significant role played by temperature in the couplings between various physical and chemical phenomena. It is shown here that the control of the thermal couplings makes it possible to decrease the power consumed while guaranteeing the quality of the final material

Thermographic and tomographic methods for tridimensional characterization of thermal transfer in silica/phenolic composites

International audienceSilica/phenolic composite materials are often used in thermal protection systems (TPS) for atmospheric re-entry. The present work aims to compare two different approaches to assess heat transfer properties of these materials: i) using standard and specific experimental methods, and ii) with the development of 3D thermal transfer multiscale model using 2D (microscopy) and 3D (tomography) images. The latter procedure, based on computations on images, is a two-step change of scale from microscopic scale to mesoscopic scale and then to the macroscopic one. Two silica/phenolic composites with different spatial organizations are studied and their 2 thermal properties are compared. Several experimental methods have been used, including space-resolved diffusivity determinations. Numerical results are compared to experimental ones in terms of transverse and longitudinal thermal conductivities of the composites, and were found to be in good agreement. A discussion is made on the different possible sources of uncertainty for both methods

Complex geometry macroporous SiC ceramics obtained by 3D-printing, polymer impregnation and pyrolysis (PIP) and chemical vapor deposition (CVD)

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Nanoscale structure and texture of highly anisotropic pyrocarbons revisited with transmission electron microscopy, image processing, neutron diffraction and atomistic modeling

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