Caracterisation de materiaux carbones par microspectrometrie Raman

Abstract

A fast and non destructing characterization of the local state of a material may be achieved with the Raman microprobe, since spectra from areas as small as 1 square micrometer can be obtained and recorded. An obvious application of this new technique is in the field of composite materials. Characterization of the degree of gaphitization is one of the most important problems in the case of carbons: we have undertaken to investigate quantitatively the effects of gaphitization on the Raman spectrum. Various materials (both graphitizing and non graphitizing) were selected: pitch cokes, anthracene cokes, and saccharose cokes, fibers from PAN, and pyrocarbons deposited from methane at 2100°C. These carbons were subsequently heat-treated at temperatures up to 3000°C. Their degree of graphitization was first characterized by measurements of the mean diamagnetic susceptibility \\ ̄ gc and by X-ray determination of the average interlayer spacing d 002. Raman spectra from all the samples were then recorded with the microprobe and four “graphitization indices” were selected: the frequency ν E 2 g and the line-width Δν( E 2 g) of the E 2 g line (Fig. 1) the ratio R of the intensities of the 1350 cm −1 and E 2 g lines (Fig. 1), and the line-width Δν (2700) of the main line in the second order spectrum (Fig. 2). A good correlation was found for all types of carbons between these indices and both \\ ̄ gc and d 002 (Figs. 3–6). The values of ν E 2 g and Δν( E 2 g) are easily determined with a good precision and their range of variation is sufficient: they may be used to characterize the extent of two-dimensional graphitic domains. The line width Δν(2700) is also easily measured and its wide range of variation, with a minimum value, is convenient for an estimation of both the two-dimensional growth of graphitic layers and the three dimensional ordering of the graphite lattice. The final stage of the graphitization process may be monitored by the splitting of 2700 cm −1 line (Fig. 7). The complete “graphitization path” of a carbon is shown with the Δν(2700) and Δν( E 2 g) indices as an example in Fig. 8. An application of these findings is presented in the field of carbon-carbon composite materials, where the Raman spectra of the substrate and matrix can be obtained separately and compared (Fig. 9). Several carbon fiber substrates were selected: FV (carbon felt from a viscose precursor), TC (carbon cloth from PAN heat-treated at 2000°C), FC (cloth from T 300 high strength PAN fibers), G (polycrystalline graphite). They were densified by vapor phase deposition (CVD) from methane at 1000–1100°C of two different matrices: LR (rough laminar) is a graphitizing pyrocarbon, while LL (smooth laminar) is non-graphitizing. The Raman spectra of the substrates and matrices were separately recorded with the Microprobe, in the as-deposited state ( TD) and after heat-treatments ( HIT) at 1900, 2100 and 2700°C. A comparison was made of the spectra of the matrix at increasing distances (0–8μm) from the fiber edge. The characteristic differences in the evolutions of each type of substrate (Fig. 11) and pyrocarbon (Fig. 13) are clearly shown. No evidence is found of any influence of either the matrix on the evolution of the substrate (Fig. 10) or the substrate on the evolution of a non-graphitizing matrix. But there are indications that a non-graphizing substrate does inhibit slightly the evolution of a graphitizing pyrocarbon matrix at 2100 and 2700°C (Fig. 12).