Alain Derré

Shape and Temperature Memory of Nanocomposites with Broadened Glass Transition

Shape-memory polymers can revert to their original shape when they are reheated. The stress generated by shape recovery is a growing function of the energy absorbed during deformation at a high temperature; thus, high energy to failure is a necessary condition for strong shape-memory materials. We report on the properties of composite nanotube fibers that exhibit this particular feature. We observed that these composites can generate a stress upon shape recovery up to two orders of magnitude greater than that generated by conventional polymers. In addition, the nanoparticles induce a broadening of the glass transition and a temperature memory with a peak of recovery stress at the temperature of their initial deformation.

Synthesis and characterization of boron-substituted carbons

Abstract Thin films of boron-substituted carbons have been synthesized by chemical vapor deposition. X-ray diffraction measurements showed that the d(002) value depends both on B at.% in the deposits and on the reaction temperature. Addition of boron increases the crystallinity up to a certain boron concentration, but beyond this critical value, on the contrary, the crystallinity deteriorates as the boron content increases. 11B-NMR indicated that isolated boron atoms are dominant at low boron concentration in the deposits, and that the number of boron-to-boron connected atoms increases rapidly when the boron content is higher.

Film boiling chemical vapor infiltration. An experimental study on carbon/carbon composite materials

Several chemical vapor infiltration (CVI) processes have been developed to increase both the carbon yield and the densification rate for a controlled type of pyrocarbon deposit. Recently, the ‘film boiling technique’ (so-called Kalamazoo) has been successfully developed for making in particular C/C composite materials. To get a better insight on this process, we have built up two small laboratory reactors with an internal resistive heating. In-situ and ex-situ experiments have been carried out with various liquid carbon precursors. The kinetics analysis has shown that the nature of the precursor plays a role but that the fundamental point is the chemical–hydrodynamical coupling. This process is based on a moving reactive hot frontier with a steep densification profile. From our analysis, it results that the precursor feed comes mainly from the molecular diffusion through the porous preform. The balance between chemical and hydrodynamical constraints leads to different types of pyrocarbon microstructures.

Evidence for the solubility of boron in graphite by electron energy loss spectroscopy

We present the results of transmission electron microscopy (TEM) and electron energy loss spectrometry (EELS) studies on two carbon–boron alloys both prepared by chemical vapour deposition at ca. 1000°C and differing in their [Boron]/[Carbon] atomic ratio as well as in their morphology. In both samples, impurity concentrations, principally oxygen and nitrogen, were found to be low relative to boron dopant levels. For low boron contents, typically around 5–10 at.%, the sample consisted of onion-like spherical particles approximately 10 nm in diameter which exhibited a non-homogeneous distribution of boron, concentrated at a level of 5–6 at.% in the centre. For this sample, studies of the B–K- and C–K-ELNES (electron energy loss near-edge structure) together with associated modelling of the unoccupied density of electronic states, indicate a substitution of boron atoms on threefold coordinated sp2-sites within the graphite network. For higher boron doping levels, typically 25 at.%, the sample consisted of homogeneous thin films. In this case, the change in shape of the B–K-ELNES indicates that boron has higher coordinations than planar trigonal together with possibly some residual sp2 sites. This study unambiguously demonstrates the presence of boron substitution solely within an sp2-bonded graphite network in the case of low boron contents and, when combined with other studies, gives an indication of the solubility limit for boron in graphite for the chemical vapour deposition (CVD) process.

Boronated carbons: structural characterization and low temperature physical properties of disordered solids

BxC1–xcompounds (x≤0.25) have been prepared by the thermal CVD technique. The aim of this paper is to better characterize some specific boron rich compositions that are not yet satisfactorily described. Both structural and physical properties have been investigated; including low temperature direct current resistivity, dynamic and static magnetic susceptibilities, specific heat and thermal conductivity. The final purpose is to correlate the structural disorder, both positional and substitutional, with particular electronic and vibrational properties deduced from low temperature experiments.

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.

Boron-substituted carbons and their intercalation compounds

Abstract Boron-substituted carbons have been synthesized by chemical vapor deposition from acetylene and boron chloride. The highest boron concentration corresponded to BC 3 formulation as shown by electron probe microanalysis. The intercalation behavior of these materials has been investigated with electron donors (alkali metals) and electron acceptors (fluorine, bromine). Of particular interest is the intercalation of lithium from the vapor phase: whereas lithium carbide is formed with pristine graphite at temperatures above 450 °C, such a reaction does not occur with BC 3 , even at 700 °C. Very high Li contents corresponding to compositions such as (B 0.25 C 0.75 ) 3 Li can be obtained at 600 °C. On the other hand, halogens do not react with BC 3 , but bromine is intercalated in products with lower boron contents. Preliminary results are also presented on the intercalation in nitrogen- and boron-substituted carbons.

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 New Bio‐Inspired Route to Metal‐Nanoparticle‐Based Heterogeneous Catalysts

Onion-type multilamellar vesicles are made of concentric bilayers of organic surfactant and are mainly known for their potential applications in biotechnology. They can be used as microreactors for the spontaneous and controlled production of metal nanoparticles. This process does not require any thermal treatment and, hence, it is also attractive for material sciences such as heterogeneous catalysis. In this paper, silver-nanoparticle-based catalysts are prepared by transferring onion-grown silver nanoparticles onto inorganic supports. The resulting materials are active in the total oxidation of benzene, attesting that this novel bio-inspired concept is promising in inorganic catalysis.

Discrimination of dopamine and ascorbic acid using carbon nanotube fiber microelectrodes

The detection of dopamine is a scientific challenge of great importance for the understanding of neurobiological dysfunctions. However the presence of ascorbic acid at concentrations several times higher than that of dopamine and its oxidation at a very similar potential make a selective electrochemical detection difficult. Here we report the original and intrinsic selectivity of carbon nanotube (CNT) fiber microelectrodes (CNTFM) towards dopamine oxidation without significant interaction from ascorbic acid.