Atomic relaxation and dynamical generation of ordered and disordered chemical vapour infiltration (CVI) SiC polytypes

8 pagesInternational audienceThe coexistence of ordered and disordered polytypes in SiC deposits is discussed from the point of view of their formation under CVD/CVI conditions, i.e., far from equilibrium. Local deformations at layer n are considered to determine the orientation and the deformations of the (n + l)th layer. An analysis of this dependence is made by constructing a first return map of an atomic relaxation variable, based on the ab initio calculation data for some regular polytypes made by Cheng et al., J. Phys. (Condens. Matter) 2 (1990) 5115. Orientation sequences are linked to the local deformation parameter, and to 29Si and 13C NMR shifts. iterations of this map are considered as a one-dimensional dynamical system simulating layer-by-layer growth. Depending on control parameters, the dynamical system exhibits stationary, periodic or chaotic orientation sequences, reproducing many experimentally obtained polytypes. The occurrence of CVD- or CVI-grown one-dimensionally disordered polytypes can thus possibly arise from a deterministic process able to yield also regular structures for nearby values of control parameters

Image analysis, synthesis and image-based modeling of ceramic-matrix composites

This presentation will summarize some current research performed at LCTS on CMCs imaging and image-based modeling. Several aspects of SiC fiber-reinforced non-oxide CMCs production and use are envisaged. First, processing by chemical vapor infiltration is addressed through a series of modeling tools allowing the prediction of infiltration gradients as a function of the processing parameters and of the fibrous architecture. Second, the self-healing capability of multiphase, boron- and silicon-containing matrices is modeled through the use of a specific image-based method. Third, the high-temperature mechanical behavior of these materials is investigated by a thorough analysis of time series of X-ray µ-CT scans, followed by a virtual material reconstruction and a posterior FEM analysis. Please click Additional Files below to see the full abstract

Assessment of structural and transport properties in fibrous C/C composite preforms as digitized by X-ray CMT. Part I : Image acquisition and geometrical properties

International audienceRaw and partially infiltrated carbon-carbon composite preforms have been scanned by high-resolution synchrotron radiation X-ray CMT. 3D high-quality images of the pore space have been produced at two distinct resolutions and have been used for the computation of geometrical quantities : porosity, internal surface area, pore sizes, and their distributions, as well as local and average fiber directions. Determination of the latter property makes use of an originalalgorithm. All quantities have been compared to experimental data, with good results. Structural models appropriate for ideal families of cylinders are shown to represent adequately the actual pore space

Iterations of the sawtooth map as a dynamical model for CVD/CVI SiC polytype growth

14 pages, color figuresInternational audienceIn order to describe silicon carbide polytype growth under CVD/CVI conditions, a dynamical model based on iterations of the sawtooth map has been studied. A two-parameter bifurcation diagram has been computed for a simple logistic-like family, and its structure has been studied in more detail by means of kneading theory and study of the skeleton. All the routes to chaos are shown to derive from the application of a central theorem involved in the construction of the skeleton. A comparison, under a combinatorial point of view, between this mathematical model and the reported data on known SiC polytypes is carried out and discussed. Indeed, the model is able to generate any periodic or chaotic polytypic sequence

Direct 3D microscale imaging of C/C composites with computed holotomography

International audienceAs part of the modelling of CVI (Chemical Vapor Infiltration), one of the processing techniques of Carbon - Carbon composites, a better understanding of the relation between fiber architecture and transport properties is required. An excellent starting point for the pore-scale modeling of heat and gas transport is the acquisition of 3D images of the porous media through Computed Micro Tomography at various densification stages. Due to the low X-ray absorption rate of light materials such as carbon, a poor image contrast is obtained with absorption tomography. On the other hand, phase contrast imaging is readily feasible using the coherence properties of modern synchrotron beams. Holotomography has been performed on these materials and it provides quantitative density images where fibers and pyrocarbon matrix deposit are easily distinguishable. Such images are appropriate for the pore-scale computation of many effective transport properties

Monitoring density and temperature in C/C composites elaborated by CVI with induction heating

International audienceCarbon/Carbon composites are processed by Chemical Vapor Infiltration (CVI) with radio-frequency inductive heating, which leads to inside-out temperature gradients, suitable for the production of homogeneously densified pieces if properly controlled throughout the whole processing. We present here a 2D axisymmetrical case where a comprehensive numerical model is tested against experimental runs. The numerical thermal model takes into account induction heating, radiative, conductive, and convective effects, intermediate regime diffusion and densification reactions in the pores, and the evolution of the porous medium. The results are the time evolution of the temperature, concentration and composite material density field, as well as the input power necessary to ensure a given maximal temperature in the preform. Experimental data are measurements of the temperature and density fields at various infiltration stages. Comparison between experience and simulation, yielding an useful agreement, allows to show that porosity gets trapped inside the preform as densification proceeds, because of the progressive lowering of the temperature gradient steepness. The discrepancies between computations and experimental data rely on the only approximate knowledge of some quantities, principally the reaction kinetics, which are currently under investigation

A Cluster Approach for the modelling of the layer-by-layer growth of SiC polytypes

10 pagesInternational audienceA cluster approach has been designed in order to confirm the physical bases of a previously presented dynamical model for chemical vapor deposition-chemical vapor infiltration SiC growth (Vignoles, G. L. J. Cryst. Growth 1992, 118, 430). The clusters consist of two or three Si-C bilayers; the relaxation of the bond lengths in the upper bilayer of the clusters simulates the impingement of a new bilayer on the crystal surface. The quantities relevant to the model (energies and optimized geometries) have been calculated at the semiempirical level. The use of regular series of clusters allowed us to obtain extrapolated values for infinite surfaces. Qualitative agreement has been obtained between the cluster calculations and the assumptions made for the dynamical model

Ablation of carbon-based materials : investigation of roughness set-up from heterogeneous reactions

International audienceAblation of carbon-based materials is a key issue in atmospheric reentry ; it displays a strong coupling between mass, momentum and heat transfers, the importance of which relies on the surface roughness. A new possible physical cause for roughness set-up is investigated, based on the coupling between diffusive transfer in the surrounding fluid on one hand, and heterogeneous reaction or sublimation on the other. Considering mass transfer in a 2D, isothermal, vertical-flux approximation, the surface is proved to be able to acquire, among others, a stable stationary morphology made of circle arcs connected by symmetrical singular points. Such a morphology has indeed been observed in the case of graphite ablation, and the computed roughness length scale, arising from the diffusion-to-reaction ratio, is compatible with observed data. A similar model based on the presence of a thermal gradient yields similar results, but with a larger length scale, also compatible with other observations

Periodic and disordered polytypes of SiC: A dynamic model based on local deformations

International audienceSilicon carbide exists under a great variety of polytypes that can be described by a sequence of orientations of diatomic Si-C layers in a stacking direction. A dynamical model is proposed for the formation of polytypes during the layer-by-layer growth of the material that occurs in CVD/CVI. A recurrence law is determined for the sequence of layer deformations ; the orientation sequence is derived from it. The evolution of this law allows one to reproduce qualitatively the whole polytypism of SiC: short- or long- period periodic polytypes, as well as one-dimensionally disordered polytypes. A two-parameter bifurcation diagram has been studied : its fractal structure allows one to visualize various types of ''roads to chaos"

Modelling of pyrocarbon chemical vapor infiltration

International audienceThe chemical vapor infiltration (CVI) of pyrocarbon is used to produce carbon matrix of C/C composites. This process involves complex physico-chemical phenomena such as the transport of gas mixtures (hydrocarbons and hydrogen) in the reactor and inside the fibrous preform, chemical reactions (pyrolysis and deposition), and the structural evolution of the preform. A global modelling approach has been developed for isobaric CVI. The most difficult point is to find a realistic chemical model for pyrocarbon deposition chemistry, simple enough to be implemented in a 2D or 3D fluid dynamics code. Such a model is proposed in this study, featuring a group of light species leading to smooth laminar pyrocarbon, a group of heavier species (polycyclic hydrocarbons) leading to rough laminar pyrocarbon, and associated homogeneous and heterogeneous reaction kinetics. This model has been developed and validated according to results of pyrocarbon CVD experiments from propane, and isothermal, isobaric CVI in a 1D model porous medium made of compact stacks of 100 μm diameter filaments