Christian Clinard

Carbon nanoparticles from laser pyrolysis

Carbon nanoparticles synthesised by laser pyrolysis of hydrocarbons in a flow reactor have been investigated as a function of laser power. Samples are cross-characterised by high resolution transmission electron microscopy (HRTEM) and infrared (IR) spectroscopy. Nanoparticles appear highly aromatic in character in all the experimental conditions explored here. As the flame temperature in the interaction zone increases, the nanoparticles evolve drastically from poorly organised, highly hydrogenated samples toward turbostratic concentric particles of carbon. The multiscale organisation of the samples and its evolution with the synthesis parameters are quantitatively determined and correlated to IR properties through an original development of HRTEM image analysis. The multiscale organisation does not reduce to the classical view of assemblies of basic structural units (BSU) of aromatic bricks. More refined models are proposed where non-stacked aromatic layers play a noticeable role and lead to a better understanding of the samples optical properties. Possible contribution to a better understanding of carbon cosmic dust is discussed from an astrophysical point of view.

Transmission electron microscopy studies on carbon materials prepared by mechanical milling

The effects of mechanical milling on the multiscale organization (structure and microtexture) of various carbon materials were investigated by means of Transmission Electron Microscopy. We show that mechanical grinding generates an increasing amount of disordered carbon at a rate depending on the type of grinding mode used (shear- or shock-type grinding). When the shock-type grinding is used, the triperiodic structure and the lamellar microtexture of the graphite completely break down to give microporous and turbostratic carbons made of misoriented nanometric Basic Structural Units (BSUs). Graphite grinding permits the elaboration of disordered carbons. The involved mechanism is different from a simple reverse graphitization, since not only structure but also microtexture are strongly modified by the grinding. After heat treatment at 2800°C, the graphite organization is not recovered, and a mesoporous turbostratic carbon is mainly obtained. All the carbon precursors studied, submitted to strong grinding, leads to similar microporous carbons. Shear grinding is less effective since remnants of graphitic carbon are still present within the disordered carbon.

Quantitative high-resolution transmission electron microscopy: a promising tool for carbon materials characterization

Carbon materials usually exhibit a multiscale organization from the subnanometric to the millimetric scales. Such an organization is the fingerprint of the conditions of formation and is responsible for numerous properties. High-resolution transmission electron microscopy (HRTEM) is a relevant tool to image directly the profile of the polyaromatic layers forming the skeleton of these carbons. Quantitative data are required to accurately describe such multiscale organization, to decipher the formation conditions and to foresee industrial properties. In this respect, an in-house image analysis procedure was developed based on the skeletonization of the HRTEM images, followed by the extraction of structural and microtextural data. Thanks to these quantitative data, the organization of disordered carbons can be relevantly revisited. Some applications of this approach are presently tested in the field of carbon characterization, in relation with environment problems, earth or universe sciences, or industrial carbon materials.

The first in situ 7Li nuclear magnetic resonance study of lithium insertion in hard-carbon anode materials for Li-ion batteries

We show the first continuous in situ static 7Li nuclear magnetic resonance (NMR) experiment on a plastic lithium/carbon cell. The electrochemical cycling was successfully performed inside the magnet. We particularly studied the insertion/extraction of lithium in a material designed to be used as negative electrode in a secondary battery; hard carbon fibers treated to reduce the irreversible capacity (x=0.5). The reversible capacity is x=1.5 (in LixC6), higher than with graphite for which the saturating composition corresponds to the first stage intercalation compound LiC6 (x=1). Ex situ quantitative transmission electron microscopy gives a statistical description of the fibers; a core made of disorganized carbon coated by better organized pyrocarbon. Crossed with the values of capacity given by the current generator, in situ NMR indicates the chronology of the insertion. From the observed data, we propose that a Li–C metallic alloy is formed. By comparing the Knight shift with that of metal (Li0), we esti...

TEM study of kaolinite thermal decomposition by controlled-rate thermal analysis

The dehydroxylation of kaolinite leading to metakaolinite was studied by transmission electron microscopy. TEM techniques allow the study of morphology, structure (by selected-area electron diffraction) and lattice imaging (by high resolution) in a non-destructive way. The samples were prepared using a controlled-rate thermal analysis method which provides a well-defined material (near thermodynamic equilibrium). The dehydroxylation begins near the defects of the structure (stacking defects, dislocation and edges) and progresses from them slowly, producing metakaolinite without any intermediate state. The formed metakaolinite is still organized: the two-dimensional hexagonal structure of the ab plane remains, but with a shorter extension (≈50 nm). A lamellar structure of a few layers with an interlayer period of about 1.2 nm appears in place of the kaolinite layers (0.71 nm). This structure is explained by the tetrahedral SiO4 sheet which remains but is highly deformed by a distortion wave due to the new aluminium and oxygen (remaining from the OH sheet) distribution after the loss of water.

Soot formation from heavy hydrocarbons behind reflected shock waves

Soot formation from heavy hydrocarbons (n-hexadecane, tolnene, n-heptylbenzene, and 1-methylna-phatalene) was studied behind reflected shock waves, using a light extinction technique. The highly diluted mixtures (99 to 99.8% of argon) were heated between 1300 and 2700 K. The pressure ranged from 200 to 1800 kPa. Soot induction delay times, growth rates, and yields were determined under pyrolysis and for two equivalence ratios (5 and 18). The effect of aromaticity, oxygen content, temperature, and pressure on these parameters were investigated. Small quantities of oxygen have little influence on soot induction delay times and soot growth rates but decrease the soot yield. The maximum soot yield was found to depend strongly on the initial number of aromatic rings of the fuel. The optimum temperature for which the soot yield is maximum is a function of the hydrocarbon nature (aromatic or linear). Samples of soot particles formed behind shock waves and collected after experiments were analyzed by transmission electron microscopy at a magnification of 5×104 in order to determine the size of elementary spheres. This parameter was studied in relation with the experimental conditions. The elementary sphere size decreases with increasing temperature, especially in the presence of oxygen. For relatively low equivalence ratios (φ=5), the shape of the primary spheres deviates strongly from the spherical form.

HRTEM IMAGE FILTRATION: NANOSTRUCTURAL ANALYSIS OF A PILLARED CLAY

Nanostructural analysis of pillared clay samples using high-resolution transmission electron microscopy has been developed. Montmorillonite samples were pillared using partially hydrolyzed Al and Fe solutions. Two samples, M01 and M05, corresponding to Fe/(Fe+Al) ratios of 0.1 and 0.5, respectively, were analyzed. The different steps of image filtration, resulting from filtration by ring-shaped masks, are illustrated and discussed from lattice imaging of sample M01. This procedure is used to show the heterogeneous distribution of the basal spacings in the different ordered domains. Domains of mesoporosity and distribution of the different Fe species are studied specifically in the sample M05. The quantitative HRTEM results are discussed and compared with X-ray diffraction patterns obtained from the same sample.

A transmission electron microscopy study of interfaces and matrix homogeneity in ultra-high-performance cement-based materials

Ultra-high-performance cement-based materials produced under different conditions have been characterized by transmission electron microscopy (TEM), scanning transmission electron Microscopy (STEM), high resolution transmission microscopy (HRTM) and chemical analysis. In addition to cement, these materials contain large amounts of crushed quartz and amorphous submicrometre silica. A post-set heat treatment was also applied in some cases. An abrasive thinning method combined with grazing angle ion etching allowed the preparation of 100 nm thick specimens with wide observation surface areas while avoiding any water or CO2 contact which may cause changes. Clinker, silica fume and crushed quartz reactivity as a function of the curing processes have been studied, as well as the interfacial zones with the hydrated matrices. The Ca/Si ratio spatial distribution in hydrated products has been analyzed and shown to undergo strong local fluctuations. Nevertheless, the composition fluctuations were less pronounced and the average Ca/Si ratio was lower than in silica-free cement paste. HRTM lattice imaging shows the coexistence of nanocrystalline phases and mesoscale ordered regions within an amorphous matrix. A d-spacings analysis of the nanocrystalline phase suggests a tobermorite-like structure for the calcium silicate hydrates, whereas the mesoscale order might reflect modulations in the water content.

Optical properties of synthetic carbon nanoparticles as model of cosmic dust

Carbon nanoparticles synthesised by laser pyrolysis of small hydrocarbons are deposited at low energy on a silicon substrate. Infrared spectroscopy of the as-formed films are studied as a function of the synthesis parameters and post-treatments, such as annealing and heavy ion irradiation. Correlation between infrared spectroscopy and multiscale organisation of the samples is made through transmission electron microscopy, including image analysis. Changes in infrared spectra are analysed in terms of the carbon network building. The relevance of the results to model the structure and spectroscopy of carbon dust in the carbon-rich circumstellar media is discussed.

Microporosity and optical properties of some activated chars

Abstract The aim of this work was to understand the structural and microtextural changes responsible for microporosity formation in saccharose-based chars activated with CO 2 at 850 °C. High-resolution transmission electron microscopy (HRTEM) and optical microscopy (reflectance measurements) allow to follow the changes in the chars organisation from nanometric to micrometric scales. Quantitative structural and microtextural data can now be extracted from the HRTEM images with an in-house image analysis procedure. Our results allow to suggest that the development of the microporosity with an increasing burn-off could be due to the development of slit-shaped pores. Such changes in pore shape and dimensions affect optical properties; a relationship was established between BET surface area and mean reflectance. By coupling these experimental data and a theoretical approach, a model of activated carbons is proposed to explain the reflectance changes during activation.