P. Delhaes

Chemical vapor deposition and infiltration processes of carbon materials

Abstract The chemical vapor deposition (CVD) and the chemical vapor infiltration (CVI) processes of carbon materials are reviewed starting from the historical aspects and including the latest developments in the preparation of C/C composites. Our presentation is based on an analysis of the different types of reactors, of the composite materials with different types of pyrocarbon as matrices and a comparison between the different processes. In particular, the classical isothermal–isobaric technique and temperature or pressure gradient reactors, which lead to a higher deposition efficiency, are compared. A complementary aspect is the structural and physical analysis of the deposited pyrocarbons: they are considered as reproducible metastable phases which are obtained under non-equilibrium thermodynamic conditions. The final relevant point concerns the relationship between the process parameters and the type of pyrocarbon. In particular, the so-called rough laminar microstructure, useful for most composite applications, is described.

Graphite and Precursors

1. Polymorphism of Carbon 2. Electronic Band Structure of Graphites 3. Electronic Conduction 4. Magnetic Properties of Graphite Carbons 5. Thermal Properties and Nuclear Energy Applications 6. Mechanical Properties 7. Carbon Surface Chemistry 8. Applications of Polycrystalline Graphite 9. Carbonization and Graphitization 10. Preparation and Properties of Mesophase Pitches 11. Amorphous and Non-Crystalline Carbons 12. Physical Properties of Preg

Physical Properties of One Dimensional Conductors

Abstract In the course of investigations of organic metals we have investigated the cation-radical series TMTTF2-X′ with X′ = BF4, ClO4, PF6, SCN and Br. On this isomorphous series a large range of physical properties have been studied: electrical conductivity, magnetic susceptibility, EPR, optical reflectivity and specific heat. In correlation with crystallographic results, which indicate the existence of TMTTF diads, the dominant interactions are analyzed. It results that the metallic behavior at high temperatures and the occurrence of a Peierls distorsion below 100 K arise mainly from a competition between electron-electron and electron-phonon coupling inside the cation-radical chains

Phase transitions in new BEDT-TTF κ-phase salts with hexacyanometalate anions [M(CN)63− M=Co(III) and Fe(III)]☆

The preparation, crystal structure determination and physical properties of the compounds formulated as κ-(BEDT-TTF)4((C2H5)4N)M(CN)6.3H2O (M= CoIII and FeIII) are presented. Organic ET layers with packing of orthogonalized dimers containing charge carriers and inorganic octahedral hexacyanometalate anions with diamagnetic or paramagnetic transition metals coexist in the title compounds. Two phase transitions occuring respectively at 150 K and in the 230–260 K range have been evidenced by magnetic (SQUID and ESR), DSC measurements. However, preliminary X-Ray studies revealed a structural change around 240 K only.

Diagramme d'existence et proprietes de composites carbone-carbone

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 TD, composition and flow (D) of gases, time of deposition tD). 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.

Layer-by-layer self-assembly of Prussian blue colloids.

The adsorption of Prussian blue (PB) colloids within layers of polyelectrolytes has been achieved by a reiterative immersion-rinse approach. Multilayer assemblies consisting of alternate layers of these components have been prepared by the layer-by-layer (LbL) self-assembly technique. Both processes have been carefully monitored by cyclic voltammetry and infrared and UV-visible spectroscopy. Linear increase in the IR and UV-visible light absorbance with the number of deposited layers indicates that well-organized lamellar systems have been elaborated. Size and distribution of Prussian blue nanoparticles in these systems have been investigated by AFM. The effect of the molar concentration of the PB dipping solution on the adsorption process and the distribution of the PB colloids has also been described. Finally, magnetic properties of these assemblies have been studied by low-temperature ESR measurements. Indeed, this new approach of hybrid LbL films opens the way to a new class of nanostructured lamellar compounds.

High temperature thermal and mechanical properties of high tensile carbon single filaments

The final quality of carbon/carbon composite materials depends strongly on the tight fit of the components. This could be predicted with a better knowledge of the intrinsic thermal and mechanical behavior of the carbon fibers. With this object, we have built-up an experimental set-up for in situ studying of dimensional variations and the bending strain at failure of single filaments from high tensile carbon fibers at temperature up to 2500 °C. We have shown off and separated the purely physical and the chemical parts of the dimensional evolution of these PAN-based filaments, in comparison to the fundamental behavior of pure graphite.

A PHOTOMAGNETIC EFFECT FOR CONTROLLING SPIN STATES OF IRON(II) COMPLEXES IN MOLECULAR MATERIALS

Abstract The electronic spin-state crossover of an iron(II) complex may be triggered through the cis / trans photoisomerization of the ligand. Such a photoinduced spin change has been observed, first at 140 K, then at room temperature, using cis / trans photoisomerizable ligands. A photosensitive iron(II) complex exhibiting a thermal spin crossover was stabilized in a Langmuir monolayer. The thermal spin-crossover process has also been demonstrated on the corresponding Langmuir–Blodgett film.

A metal-insulator phase transition close to room temperature: (BEDTTTF)2SbF6 and (BEDTTTF)2AsF6

Abstract We report the synthesis and characterization of two new salts of BEDTTTF: (BEDTTTF)2SbF6 and AsF6. Near to room temperature these two salts show a strong anomaly of conductivity, also visible on the EPR results. The crystal structure of the high temperature phase is described and a short discussion of the nature of the metal-insulator phase transition is given.

Triplet excitons in (TEA) (TCNQ)2

Abstract We report the results of EPR studies on the ionic-radical salt (TEA)+ (TCNQ)2- composed of an oganic free radical anion and a diamagnetic cation. Between about 40 and 80 K this crystal exhibits the triplet exciton EPR spectrum characteristic of an alternating chain of spins. The triplet spin Hamiltonian parameters are |D| = 44 ± 2 G and |E| = 5.5 ± 1 G. The directions of the zero field splitting principal axes are determined through single crystal rotation studies at 55 K and related to the crystal structure.