Topic 11 Gerard L. Vignoles # 582
Figure 6. Presentation of the global modelling strategy as applied to carbon foam reinforcement by pyrocarbon CVI.
This paper has summarized some efforts made in the characterization of C/C composites as a tool for modelling their preparation by chemicalvapourinfiltration. High-resolution X-ray tomographic imaging has proved extremely valuable, because of its excellent space resolution. A two-scale approach has been developed to account for the structure of the composite material. The studies have involved image processing and pattern recognition, morphological analysis, and direct numerical simulation of reaction/diffusion systems with moving boundaries. Results show that the assessment of geometrical and transport properties throughout the material evolution is possible on the basis of CMT images; the incorporation of the structure-property relationships in large-scale models of processes and/or degradation tests is indeed of large value and is still currently undergoing. An application to the reinforcement of carbon foams by pyrocarbon CVI, including the complex homogeneous chemistry of propane decomposition, is presented as a validation.
The matrix of Ceramic Matrix Composites (CMCs) and Carbon-Fibre Reinforced Carbons (CFRCs) is often deposited by ChemicalVapourInfiltration (CVI). This process gives the best matrix quality, but it is expen- sive and difficult to control. Experimental determination of the optimal condi- tions is time-consuming and expensive. That is why modelling of CVI is of great interest to optimize the final density and homogeneity of the compo- sites [3-5]. The keys of a CVI model are: (i) the determination of geometrical characteristics and transport properties of the preform at various stages of infiltration , namely: the effective gas diffusivity, either in continuum or in rarefied regime, the gas permeability to viscous flow in the case of pressure- driven CVI , and the heat conductivity, in the case of thermal-gradient modifications of CVI , (ii) the knowledge of chemical reaction rates, and (iii) the simulation of the infiltration itself.
BN interphases were synthesized by chemicalvapourinfiltration (CVI) from BCl 3 and NH 3 at 900 °C and
1200 °C within preforms made of a single 2D woven SiC fibre ply. Then CVI of a SiC matrix followed by molten silicon infiltration resulted in laboratory-made single-ply SiC/SiC-Si composites representative of similar industrial ceramic matrix composites. The interphases and interfaces were characterized by transmission electron microscopy. It has been found that, in the two cases, the interphases are mostly turbostratic BN. Those synthesized at 900 °C are very poorly crystallized and exhibit a quasi-isotropic microstructure. An equiaxed interlayer is obtained prior to the columnar CVI growth of SiC matrix. Interphases synthesized at 1200 °C exhibit high degrees of crystallization and structural anisotropy. The SiC matrix grows directly in columns on BN. For interphase thicknesses of 300 nm and more, triangular-shaped crystallites of rhombohedral BN appear in turbostratic BN and the columnar growth of SiC is disoriented.
LCTS, CNRS, University of Bordeaux, Herakles-Safran, CEA, 3 allee de la Boetie, F-33600, Pessac, France
Ceramic samples were prepared using a hybrid process in which the ceramic powder route was combined with Reactive ChemicalVapourInfiltration, a new gas phase route. In this technique, a carbide growth occurs from the conversion of a carbon- bearing powder and slows down with increase in carbide thickness due to solid-state diffusion limitation. This self-limitation is expected to allow a self-regulation of the growth between the interior and the surface of the sample and thus a better homogeneity. The chosen carbide was TiC obtained by reacting H 2 and TiCl 4 with a carbon powder introduced by slurry impregnation in felts. At 950°C, the carbide infiltration is homogeneous with depth but the powder is not consolidated, while at 1050°C the better conversion rate of carbon into carbide allows the consolidation but the infiltration is limited to only about 250 µm. The consolidation depth can be improved with time-temperature steps between both temperatures.
We present a computational tool for the modeling of Chemical Vapor Infiltration of carbon/carbon composites, which is based on 3D images acquired by X-ray Computerized Micro-Tomography with a very fine resolution, such that the fibers are clearly distinguishable from each other. Preliminary image processing is necessary in order to perform segmentation between solid and void phases. Then, morphological and transport properties are computed in the images. Random walkers are used for the simulation of gas transport in continuum and rarefied regimes. The image modification under chemical deposition is handled by a specific surface discretization technique and a pseudo-VOF method. Results are presented and discussed: the notion of infiltrability is introduced as a design tool for the CVI engineer.
preparation of unidirectional CMCs. Furthermore, the sintering additives can affect the final properties of the material.
The use of a hybrid process in which the slurry impregnation is followed by the gas phase route would overcome the limitations of each route used separately. However, a submicrometer powder compact is difficult to infiltrate by conventional CVI in large enough thickness due to a clogging effect. The gaseous precursor infiltrates in the fine porosity of the powder with difficulty. The carbide growth rate is lower at the interior of the powder than at the surface. This case is illustrated in Fig. 1. Because of the gaseous reactive species depletion within the powder, a pronounced deposition gradient is established around the particles and finally the seal-coating of the powder surface prevents further in depth infiltration.
Chemicalvapour deposition (CVD) is a versatile process of prime interest to achieve this goal. CVD coatings are generally smooth, dense and crack free, leading to particularly high fracture strength [7-10]. In proper experimental conditions, this process allows an accurate control of the composition, the structure of the coatings, and more particularly the thickness uniformity through porous substrates. It is usually referred to as CVI –I standing for infiltration– in this case. It has led to the development of high performance carbon and ceramic matrix composites (CMC) . The CVD or CVI of refractory carbides has already been applied to reinforce reticulated vitreous carbon or mesophase-pitch-based carbon foams. [12-15]. But these few studies have not reported in details on the beneficial effect of the densification degree on the mechanical properties of the foams.
smooth a curve, however, the presence of a discontinuity is partly justified by the study of the relation between deformations and orientation sequences.
6. SYMBOLIC DYNAMICS AND ORIENTATION SEQUENCES
It has already been mentioned that the n values are very neatly split into two groups, one of h orientation and one of k orientation. More precisely the first return map suggests to divide them into four groups, on the criterion of both U n and U n+1 . These groups may be referred to as A, B, C and D in exact agreement with the groups of chemical shifts in silicon carbide NMR [19,20]. Going further these groups are split again on the criterion of U n +2 , as shown in fig. 5. This is consistent with the observed differences between the group A chemical shifts of 3C (kkk ) and of 6H or 15R (kkh) and a suggestion that a more refined NMR study of SiC would lead to recognize further splittings of the chemical shift groups . The ordering of the U n U n +1 U n +2 sequences following n is particularly regular: if one replaces k with 0 and h with 1 then it is exactly the ordering of binary numbers: 000 < 001 < 010 < … .
Here, we perform film-boiling chemicalvapourinfiltration of a mixture of liquid alkoxides which contains all necessary species to synthesize BAS in a woven preform made of Nextel TM 312 alumina-boria-silica fibers. The aim of our study is to prepare a fully oxide CMC of which the matrix consists of the hexacelsian phase. In this paper, we first describe the experimental apparatus, then we report several characterization results confirming the composition and phase of the matrix.
This paper deals with the modelling of vapour-liquid phase transition and the relaxation towards the thermo- dynamical equilibrium in liquid-vapour mixture. Focusing on isothermal problems, vapour bubbles may appear in a liquid through a decompression process, which leads to a pressure decrease below the saturation pressure and cavitation. If the process of depressurization is slow or weak enough, then the system may remain entirely liquid, although the pressure is below the saturation pressure. There is a delay in the phase transition, which may be shortened by a (strong enough) dynamic perturbation. This liquid state is commonly called metastable. A similar behavior holds in a vapour system submitted to a compression. All these phenomena result from the coupling between hydrodynamics and thermodynamics, the former inducing dynamical perturbations of thermodynamical equilibrium states.
0.286 0.286 0.264 0.054 0.030 0.030
Example. Consider a wall of 4-inch reinforced concrete with an inside finish of winch plaster over
1 inch of foamed plastic insulation that separates an internal environment of 73°F and 35 per cent RH from an outside environment of 0°F and 80 per cent RH. The actual vapour pressures are, respectively, 0.818 x 0.35 = 0.286 in. Hg and 0.038 x 0.80 = 0.030 in. Hg, and the total pressure difference is 0.256 in. Hg. This pressure difference must be apportioned among the various components of the envelope in proportion to their resistance to vapour flow. These calculations are tabulated in Table I and the resulting vapour pressure gradient for continuity of flow is plotted in Figure 1 as curve p c . Up to this point the method is the same as that for the arithmetical determination of temperature gradient. No value for the permeability of foamed plastic insulation is given in the 1963 edition of the ASHRAE Guide, but depending on the type it varies between 0.75 and 5.0 perm-in.
From thermodynamic analysis, areas of investigation to generate a set of materials with the strongest propensity for amorphization as well as useful guidelines for the target phase material deposition are provided.
Prospective to develop MOCVD (metalorganic chemicalvapour deposition) and ALD of intermetallic ﬁlms, in view of fabrication of metallic glass thin ﬁlms is proposed. Examples from selected ALD and MOCVD single element metallic deposition processes will be described to illustrate the effect of deposition parameters on the physico-chemical properties of the ﬁlms. This processing approach is particularly promising for metallic glass thin ﬁlms.
4.2 Lake Chad chemical and isotopic data
Data from Carmouze (1976), Chantraine and Lemoalle (1976a), Chantraine and Lemoalle (1976b), Chantraine (1977), Chantraine (1978), Gac (1980), Gaultier (2004), Lemoalle (1979), Maglione (1976), Roche (1980), and Zairi (2008) were collected and completed by new samplings per- formed between 2008 and 2012. Due to its endorheism and climate characteristics, the lake is expected to be a concen- trated basin of dissolved salts. However, the water salinity is low and relatively time constant. Exhaustive studies car- ried out by Carmouze (1976) on the whole lake during its normal state, between 1968 and 1971, provide a description of its geochemical features. These data show a global trend of increasing concentrations from the Chari–Logone delta (σ = 50 µS cm − 1 ) to the northern pool (σ = 1000 µS cm − 1 ) and to a lower extent to the archipelagos (σ = 600 µS cm − 1 ). The Lake Chad waters are dominated by HCO − 3 while Cl − and SO 2− 4 accounts for only 2 % of the anionic balance with chlorine concentrations below 1 mg L − 1 . Calcium is the most abundant cation near the Chari–Logone mouth, while sodium becomes dominant in more concentrated waters because it does not react with the lake substratum or the vegetation (Carmouze, 1976). Therefore, sodium and chlorine can be considered conservative elements except in the northern mar- gin where limited deposits of natron (Na 2 CO 3 ) are described.
Previously, our group designed and developed a new, solvent-free and scalable surface functionalization technique called photo-initiated chemical vapor deposition (PICVD) (Dion et al. 2014). In this work, we carry out surface functionalization of SPIONs using PICVD, and assess its impact as a secondary functionalization tool on the physicochemical properties of these nanomaterials as a function of their initial surface charge.
The CVG of transition metals has proved to be a viable technique for efficient introduction of analyte for atomic spectrometric techniques. There is definitely potential for further improvement. Unfortunately, at least for silver, its analytical use is still plagued by pronounced memory effects, arising from the complex CVG processes involving many chemical and physical factors which are not yet completely understood. The current study has raised more questions than answers. Positive identification of the volatile species remains essential for further progress, which can possibly be obtained by either mass-spectrometric methods or those used to characterize particles.
of chemical and roughness changes) [ 44 ]. Aside from CA approaches, an inverse gas
chromatography-surface energy analyzer (IGC-SEA) may be used to assess expected wettability
[ 45 , 46 ]. For example, Li et al.(2016) measured surface energy of carbon fibres coated with carbon
nanotubes. They could successfully fabricate CNT coated Carbon fibres by electrospray method resulted in a multi-scale hierarchal structure without any tensile strength changes of fibres [ 47 ]. Both CA and IGC-SEA approaches applied to CNT rely on measurements of the nanomaterials in a bucky paper configuration (or deposited/grown on a surface) – in other words, in aggregate form. To measure the wettability of individual CNTs, indirect measurements must be applied, typically
Nowadays airlines are bidding on composite materials for their high specific mechanical performances even if those materials are known for being impact-sensitive. The existing repair solutions are usually expensive in time and well trained work force, and are immoderate regarding the most common damage found in aeronautics: low energy impact. In this study we present an original repair process based on liquid resin infiltration in the crack net generated by an impact. An analytical model has been developed in order to ensure that the repair resin can propagate in such small crack nets. Mechanical testing has been realized on repaired panels in order to demonstrate the capabilities of the repair process. CAI was chosen upon several structural tests because it is widely used at industrials scale and because this test is particularly discriminating for composites. Complementary fracture mechanical tests were also realized for understanding purpose. Unfortunately structural testing gives very good results for this kind of repair whereas for the elementary tests the quality of the repair seemed to be completely limited by the toughness of the repair resin. Further numerical investigation must be done in order to understand the solicitation of the repaired interface in structural tests.