M. Couzi

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).

Phase transitions in elpasolites (ordered perovskites)

Abstract Many compounds with general chemical formula A 2 BB′X 6 and with tolerance factor value t

Brillouin and Raman scattering study of borate glasses

Abstract We report light scattering results obtained from Brillouin and Raman scattering experiments in a series of B 2 O 3 − x M 2 O glasses (M  Li, Na, K, Rb, Cs and Tl). Raman lines related to the internal vibrations of structural units give information on the composition and concentration dependence of the short range arrangement in the glasses. The “Boson peak” is analysed on the basis of the Martin-Brenig model. The short correlation range is lower than 10 A and increases with the cation radius. From these experiments the very low frequency light scattering (Light Scattering Excess) is extracted. The temperature dependence of the LSE intensity in pure B 2 O 3 glass as well as its variation with composition at room temperature are studied. The Theodorakopoulos-Jackle theory relates this intensity to the acoustic attenuation deduced here from Brillouin linewidth measurements. While a theoretical description of the relation between the temperature variation of both quantities agrees well with experimental results, disagreement is apparent in their composition dependence. Finally, the sound velocity plotted versus concentration gives evidence of the contradictory influence of the hardening due to the change of the boron atom coordination number and the softening related to the distortion of the network by large cations.

An infrared and Raman study of new lonic-conductor lithium glasses

New vitreous fast ionic conductors in the system B2O3Li2OLiCl are described. The conductivity of these glasses increases with the Li2O and particularly with the LiCl contents. A Raman and infrared study undertaken to elucidate the “structure” of the glasses and the conduction mechanism indicates that the structure consists of a “covalent” boron-oxygen network, in which LiCl is “diluted” without producing detectable interactions with the latter.

SiC filament/titanium matrix composites regarded as model composites

Two types of large diameter SiC CVD filaments have been investigated on both chemical and mechanical standpoints: a 100μm filament with a tungsten core (from SNPE) and three 140μm filaments with carbon cores and surface coatings (from AVCO). On the basis of microprobe (X-ray, Auger and Raman), X-ray diffraction and SEM analyses, it appears that the former is made of a single homogeneous stoichiometric SiC deposit while the latter are mainly made of two concentric shells (the inner being a SiC+C mixture and the outer almost pure SiC). All the C-core filaments had received a surface coating (either pure pyrocarbon or SiC+C mixture) presumably to protect the brittle SiC deposit against abrasion due to handling in opposition to the W-core filament which seems to have no surface coating at all. The W-core filament, although smaller in diameter, is weaker than the C-core filaments (average UTS of 3 and 4 GPa respectively for a 40 mm gauge length). However, its strength distribution is much narrower (Weibull moduli of 7–8 and 2–5 respectively). Failures of most filaments appear to have a multimodal character.

Nonlinear optical properties of some tellurium (IV) oxide glasses

Abstract Thermal and optical properties of glasses of the TeO 2 | ZnO, TeO 2 | Nb 2 O 5 and TeO 2 | M 2 O | Nb 2 O 5 (M = Li, K) systems have been investigated. Linear and nonlinear indices of the materials increase with the TeO 2 rate. The highest values are obtained for the niobium tellurite glasses emphasizing the particularly significant role of NbO 6 octahedra in the optical response.

Characterization of nearly stoichiometric SiC ceramic fibres

A comparative study of the chemical composition and microstructure of Hi-Nicalon, Hi-Nicalon type S, Tyranno SA, Sylramic and Carborundum fibres has been conducted. This analysis has confirmed results already published but has also evidenced some original features. The Hi-Nicalon type S fibre has a near stoichiometric composition but it still contains some oxygen (≈1 at. %) and free carbon (≈2 at. %). The expected near stoichiometric composition of both the Tyranno SA and the Sylramic fibres is only effective near the edge region, while the core of the fibres contains some amount of free carbon (e.g., up to ≈14 at. % and ≈6 at. % respectively in large diameter fibres) as well as some residual oxygen (≈0.5 at. %). The composition of the Carborundum fibre is very close to stoichiometric SiC except rare and localised free carbon or B4C inclusions. The properties of the different fibres, some of them still beeing at a development stage, are discussed from a chemical and a phase composition point of view, on the basis of what is known about their respective preparation process.

Low frequency Raman spectra of ammonium halides in the disordered phases I and II

The low frequency Raman spectrum of single crystals of NH4 Cl, NH4 Br, and NH4 I has been measured in disordered phases I and II. Particular attention has been paid to the polarization selection rules. The temperature dependence of the intensity of the disorder induced spectrum on entering ordered phase III or IV has also been measured. The data for disordered phase II in NH4 Cl were interpreted by performing a calculation of the Raman spectrum, based on the theory of Loveluck and Sokoloff of Raman scattering from phonons in a disordered system, using the lattice dynamical calculations performed by Teh. Excellent agreement is obtained with the major features of the observed polarization selection rules. The two polarization configurations are shown to pick out either NH4+ vibrations or Cl− vibrations alone. The implications for phase II in NH4Br are also discussed in the light of these calculations. A brief discussion of NH4I in phase I is also given.

Structural approach of the (xPbCl2-(1−x)Sb2O3) glass system

A structural approach of xPbCl2, (1 − x)Sb2O3 glasses, quenched from the melt, is proposed. The crystallization process of the glasses is analyzed and a Raman scattering investigation is reported. The two investigations give valuable information on the structural arrangement of the glass network. Observations within the xPbCl2, (1 − x)Sb2O3 system show variations of local structure and a comparison is made with previously reported data for some Sb or As based glasses.

Synthesis and characterization of new solid electrolyte conductors of lithium ions

Abstract New vitreous electrolytes with fast lithium ion carriers have been obtained in the B2O3- Li2O-LinX(n = 1, X = F, Cl, Br, I;n = 2, X = SO4) systems. Some of these glasses have a conductivity of the order of 10-2 ω-1 cm-1 at 300°C. The conductivity is all the more important as the content of oxide and lithium salt is higher. Studies by techniques sensitive to local ordering have shown that these materials seem to be constituted by a B2O3-xLi2O “framework” in which the coordination of boron atoms shifts from III to IV with rising x. The lithium salt is diluted in the vitreous matrix without detectable interaction with the surrounding, although lithium ions participate in the conduction.