André Marchand

Caracterisation de matériaux carbonés par microspectrométrie Raman

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 d002. Raman spectra from all the samples were then recorded with the microprobe and four “graphitization indices” were selected: the frequency νE2g and the line-width Δν(E2g) of the E2g line (Fig. 1) the ratio R of the intensities of the 1350 cm−1 and E2g 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 d002 (Figs. 3–6). The values of νE2g and Δν(E2g) 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(E2g) 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).

Pyrolytic carbon deposition from methane: An analytical approach to the chemical process

Abstract The present work was conducted in order to investigate the effects of three important constraints, namely, temperature, flow-rate, and pressure, in the deposition process of pyrolytic carbon from methane, in the vicinity of 1000–1100°C, and at pressures between 150 and 400 mm Hg. The properties of the deposited carbon were studied, and the experimental setup enabled us to obtain a fairly complete qualitative and quantitative description of all the important intermediate reaction products. It is found that more intense pyrolysis conditions (higher temperature and pressure or lower flowrate) accelerate the later steps of the process more than the early ones, and that some particular intermediate polyaromatic hydrocarbons (e.g., anthracene) lead more efficiently to carbon than other ones (e.g., acenaphtylene). The pyrolysis mechanism remains essentially the same when methane is diluted with hydrogen.

Local order and electrical properties of boron carbonitride films

Thin films of carbon-boron-nitrogen alloys were prepared by chemical vapor deposition from gaseous mixtures of C2H2, BCl3, NH3, and H-2. Their turbostratic graphitic structure was investigated by electron probe micro analysis (EPMA), X-ray diffraction (XRD), Raman spectroscopy, and nuclear magnetic resonance. Those compounds were shown to be solid solutions with compositions varying in a continuous domain bounded by carbon, boron nitride, and BC3. Measurements of the electrical transport properties (conductivity and Hall coefficient) were performed from 295 down to 4.2 K and in the presence of magnetic fields up to 5 T. The behavior of these solid solutions can be interpreted in terms of a metal-non metal transition, when the carbon content decreases. Heat-treatment of these compounds between 1400 and 2000 degrees C involves strongly modified transport properties. The resistivity of all materials is lowered. But the main property change is that of the magnetoresistance.

Fluorine-intercalated carbon fibers-I. structural and transport properties

The intercalation of fluorine in various types of carbon fibers (PAN-based or pitch-based, asreceived or high-temperature treated) has been investigated at room temperature in the presence of gaseous HF. Stage-1 compounds with C2.5F to C4F compositions are obtained for 10 bar F2 pressures, whereas lower pressures (1 bar F2) lead to stage-2 compounds. Although in higher stages (≥2) the electrical conductivity is generally larger than in the pristine fiber, in stage-1 compounds a drastic increase of resistivity is observed, ρ being more than one order of magnitude larger than that of the starting material. Finally, fluorine-intercalated GICs have been found appropriate to investigate the effects of disorder and reduced dimensionality.

Galvanomagnetic properties of bromine-intercalated carbon fibers

Abstract Three different types of carbon fibers were submitted to bromine intercalation and subsequent desorptions. The changes of the electrical resistivity, Hall effect and transverse magnetoresistance were measured at 300 and at 77 K. The results are interpreted in terms of a classial two-carriers model and the carriers concentrations and mobilities are determined from the experimental values of the electronic properties. A general relationship between the concentrations and the mobilities is found, not only in the case of the fibers, but also for boron-doped pyrolytic carbons as well as for graphite single crystals and pyrographites. A charge transfer coefficient of approximately 0.3 electronic charge per Br atom can be determined

Structural and transport properties of bromine intercalated carbon fibers

Abstract Intercalation of P-120 pitch-based fibers with bromine, from the vapor phase or liquid bromine, has been investigated. The maximum weight uptake was 20–25%, nearly independent of intercalation temperature. Intercalated samples were characterized by X-ray diffraction, Raman scattering and differential scanning calorimetry. Thermogravimetry experiments showed that a large amount (up to 50%) of bromine remains trapped in the fibers up to temperatures in the order of 1000 °C. Measurements of the transport properties (electrical conductivity, magnetoresistance and Hall coefficient) gave estimates of the concentration and mobilities of the charge carriers and of the charge transfer from carbon to bromine (0.5–0.6 electronic charge per Br2).

Various Kinds Of Solid Carbon: Structure and Optical, Properties

Graphitic carbons are considered from two complementary points of view: as imperfect forms of graphite and also as very large aromatic molecules. The various existing materials are presented and a short overview of the carbonization and graphitization processes is given, with an emphasis on the defects which may arise from the finite size of the crystallites, from the increased distance between the carbon layers, and from their non-planarity. The known data concerning the optical spectra of carbon materials are then reviewed, and suggestions are presented for the spectra of very small graphitic or aromatic grains.

Unusual behavior of the magnetoresistance of boron carbonitride films at low temperature

We have performed resistivity and magnetoresistance measurements down to 0.3 K, and under fields up to 37 T of boron carbonitride and BC3 films prepared by chemical vapor deposition. The turbostratic structure of the as-deposited materials favors a 2D weak localization effect which is invoked to explain the negative magnetoresistance (MR) as well as the log T variation of the resistivity. However, at very low temperature a positive component is superimposed on the negative MR. At high fields, the total MR is positive and almost isotropic. Usual theories are unable to account for the observed phenomenon. Increasing heat treatments up to 1800 degrees C increase the 2D character of the deposits, which show an increasingly negative magnetoresistance. For still higher treatments, the change of the films to a 3D graphitic-like structure leads to a vanishing of the negative magnetoresistance.

Effet hall et diamagnetisme de composes residuels pyrocarbone-bore-halogene

Abstract In order to clarify the electronic structure of halogen residue compounds, particularly the amount of charge transfer that occurs between the carbon π band and the intercalated species, bromine and iodine chloride were intercalated into pyrolytic carbons with various levels of boron doping, and the Hall effect and diamagnetic anisotropy of the resulting residue compounds were studied. The positive Hall coefficient is always independent of temperature and yields the concentration of holes in the π band, while the amount of intercalated halogen can be obtained from weight variation measurements. Table 1 shows the Hall coefficients and hole concentrations of the boronated pyrocarbons before intercalation, and Table 2 gives the results of measurements performed on residue compounds between 77 and 300 K. Increasing heat treatments reduce the amounts of halogen (Table 3). From these various data it can be stated that the halogen content of the residue compounds does not depend on the boron doping and the resulting initial position of the Fermi level (see Tables 2 and 3). The charge transfer from carbon to halogen is always found to be quite small: 0.02–0.07 electronic charge per Br 2 or ICl molecule (Fig. 1). The charge transfer also varies if the compounds undergo thermal cycles without loss of halogen. This was shown by in situ measurements of the Hall coefficient and diamagnetic anisotropy during such cycles (Figs. 2–6). Successive intercalations and deintercalations, as suggested by the Marchand-Rouillon model[5,6], are responsible for the hysteresis cycles observed[1–7] with many properties of these compounds.

Deplacements du niveau de Fermi de composes d'insertion sodium-pyrocarbone-bore

Abstract The Fermi level of pyrolytic carbons is displaced through boron doping and intercalation and de-intercalation of sodium. Diamagnetic anisotropy and Hall effect measurements show that, whatever the initial doping level, the intercalation of sodium proceeds until the Fermi level is raised about 3 eV above the Brillouin zone corner. The average ionization of sodium atoms is estimated at 0.2−0.4 electronic charge. As theoretically predicted the diamagnetic anisotropy is maximum when the Fermi level is at the Brillouin zone corner.