Diagramme d'existence et proprietes de composites carbone-carbone

Abstract

The pyrolytic carbons are prepared by chemical vapor deposition of carbon from hydrocarbons (CVD carbons); different kinds are known [1–5]. —bulk pyrolytic carbons deposited on a fixed substrate. —pyrolytic carbons deposited on solids in a fluid bed. —carbons obtained by densification processing of porous or fibrous substrates. Our work has to deal with the last series of CVD carbons which have been studied during recent years because of the aerospace applications [6]. We have prepared and investigated a family of carbon felt-carbon matrix composites. In relation with the different microstructures we have studied the electronic and thermal properties. The isothermal process with a resistor-graphite furnace working between 1000 and 1300°C was used. The hydrocarbon was methane diluted in nitrogen and sometimes with hydrogen additions. The substrate was a carbon felt from “Le Carbone Lorraine” Company (see Table 1). Pyrolysis of methane occurs under conditions far from thermodynamical equilibrium. In order to describe such a process it is useful to distinguish [9]: The constraints: These are variables which might be controlled by the experimenter; some are fixed by the method (shape and size of the furnace, no thermal gradient because of the isothermal process, constant pressure of gases) and others adjustable (deposition temperature T D , composition and flow ( D) of gases, time of deposition t D ). The responses: These are the variables measured by the experimenter, all the observations and the physical properties which are involved in the study (see Table 2). The system is the whole set of responses, depending on the processing constraints for which an existence diagram can be established in the constraints space. In order to do that the pertinent response chosen was the nature of the microstructure as determined by optical microscopy under polarized light illumination [7]. Different microstructures were characterized: rough laminar (L.R.), smooth laminar (L.L.), granular (L.G.) isotropic (I) and a mixture of them in agreement with previous investigations [6] except for a difference quoted for the granular microstructure (Figs. 3–6). For a quasi-constant flow a section of the existence diagram characteristic of the carbon felt is drawn (Fig. 7) using textures as a descriptive variable. Physical properties: They are presented in Table 2 for the CVD as deposited: The apparent and powder densities, the X-ray data which are in agreement with results on similar composites [5,6]. The magnetic properties: The EPR line widths are characteristic for each microstructure. The study of diamagnetism furnished very interesting results. There are significant differences between the various characterized microstructures (see Fig. 7). The relative diamagnetic anisotropy (Δχ%) allows us 10 confirm quantitatively the optical anisotropy of the different samples. The electrical and thermal conductivities support the picture: the thermal variation of the thermal conductivity gives further evidence of two classes of materials (Fig. 8). Graphitization process: after heat-treatment at 2500°C during 1 h 30 min the studies of structural and physical properties show that only the rough laminar microstructure is graphitized, the other ones behaving as hard carbons (Table 3). We show in this study that a graphitable microstructure (L.R.) exists between the non-graphitable ones (see Fig. 7). This behavior could be correlated with the formation of a mesophase from the gas phase reaction processes which might occur only for the given constraint conditions. The existence diagram shows how to obtain the desired microstructures under definite conditions. They change with the fixed and adjustable constraints. Finally we propose a method to analyze the parameters which define an evolution and make possible a selection of a specific material for a given application.