Jacques Werckmann

Structural studies of active carbon used in the growth of silicon carbide catalyst support

The microstructure of active carbon used in the growth of a silicon carbide catalyst support has been studied by BET area measurements, electron diffraction and high-resolution electron microscopy. The microstructure of the active carbon is consistent with different distributions of basic structural units (BSUs). These BSUs increase in size under heat treatment by forming adjacent distorted columns. Ordering processes are discussed from geometrical considerations based on reciprocal patterns. The reactivity of the interface between the active carbon and SiO vapour is controlled by the proportion of the highly reactive sites in the active carbon surface structure.

Si adatom surface migration biasing by elastic strain gradients during capping of Ge or Si1−xGex hut islands

Hut cluster formation during Ge or Si1−xGex solid source molecular beam epitaxial growth on Si(001) is a well-known kinetic pathway for partial strain relief. It results in undulated morphologies with {105} facets allowing a∥ lattice parameter relaxation on the island apexes. Here, we show how subsequent Si coverages, grown at 500 °C, avoid being tensile strained and impede further increase of stored elastic strain energy. Dominant inhomogeneous Si surface diffusions take place as proven by a Ge marker technique able to provide transmission electron microscopy or high-resolution transmission electron microscopy images of the initial Si morphology stages and by reflection high-energy electron diffraction examinations. This mechanism prevails for high enough Si growth rates, able to quench lateral Ge diffusion and limit chemical strain relief. Mediated by stress variations on the noncapped island curvatures, Si is depleted from the top of the islands and accumulates in the troughs of the ripples where it ac...

A sub-nanometer structural study of Pt-Rh catalysts supported on Ce doped SiC

In this paper, the HRTEM technique has been applied to the structural study of Pt-Rh catalysts supported on Ce doped SiC. The results on cerium doped active carbon and on cerium doped silicon carbide synthezised before its use as catalyst support are also reported. It was found that the Ce doped SiC catalyst support contains a new compound which is a Ce amorphous phase. The presence of such a compound within the support is thought to have the advantage of contributing to the improvement of the activity of the catalysts. At present, the HRTEM study on reduced Pt-Rh/SiC-Ce catalyst gives direct evidence of a selective association of Pt and Rh metals with the cerium oxide additive. This association, on the one hand, shows that cerium oxide is not inert towards noble metals, and on the other hand, it gives rise to a noble metal—cerium oxide composite to which the reduction is linked. The formation of an active interface, or a buffer oxygen layer which can be defined as a surface that serves as a common boundary between noble metals and cerium oxide, must be then considered in order to interpret the improvement in catalytic activities. After the ageing treatment, some structural changes were observed, particularly the formation a Rh suboxide phase formed by three to four monoatomic layers. Finally, it seems that the main oxidation and reduction processes occur through the active interface where the cerium oxide is easily able to change from cerium plus four (Ce4+) to cerium plus three (Ce3+) oxidation state. The metal will act only as a donor or acceptor of electrons. It must be emphasised that the SiC catalyst support showed a high chemical inertia since no structural modifications were observed on the noble metals caused by SiC.

Optical studies of carbon nitride thin films deposited by reactive pulsed laser ablation of a graphite target in low pressure ammonia

We report the characteristics revealed through optical investigations (microscopic studies, optical and IR transmission) of the thin films deposited by multipulse UV reactive laser ablation of a graphite target in 102 Pa ammonia. We observe that the films deposited at room temperature contain a mixture of carbon bonded to nitrogen in various configurations including the triple one and have a large optical band gap. On the other hand, the films deposited at 320°C have a lower content in nitrogen. We present evidence of C–N bonds with lower charge transfer. The layers are non-uniform and have a significantly narrower optical band gap. We consider that this difference is due to the desorption by heating of the highly reactive CN and CNH radicals.

Study of the structural evolutions of mesoporous MCM-48 silica infiltrated with carbon by different techniques

Abstract The physico-chemical properties of ordered mesoporous carbons obtained by a negative replication process are strongly influenced by their preparations. In order to get a better understanding of the carbon formation, the structural evolution of the mesoporous MCM-48 silica templates before and after C infiltration by two different processes (and a subsequent oxidation), was studied using various techniques such as XRD, N 2 adsorption/desorption and TEM. It was shown that the use of a liquid carbon precursor such as a sucrose solution led to a strong alteration of the silica template (loss of long-range ordering, disappearance of the narrow mesopore size distribution). This was attributed to (i) the high temperature of the process (1173 K) and (ii) to the water vapor released during the carbonization that hydrolysed the poorly hydrothermally stable silica network. On the contrary, the pyrolytic decomposition of a gaseous carbon precursor such as propylene, performed at lower temperature (1023 K) and without water release, only led to minor modifications. These behaviors may influence the physico-chemical properties of the resulting carbon. For both impregnation processes, carbon acts as a highly stable porogene agent. Heat-treatment at a temperature as high as 1473 K in an inert atmosphere and a subsequent oxidation for the removal of the carbon succeeded in preserving the long-range ordering of the starting silica template and its surface area.

Correlation between hardness and structure of carbon-nitride thin films obtained by reactive pulsed laser deposition

Abstract Carbon-nitride thin films were synthesized by reactive pulsed laser deposition from graphite targets in low-pressure nitrogen. X-Ray photoelectron spectroscopy and nanoindentation measurements were performed in order to establish a connection between the composition, structure and hardness of the obtained thin films. We studied the variation of the sp 3 /sp 2 C bonded to N ratio with the increase of the N content of the thin layers. We found that the value of this ratio mainly determines the hardness of the carbon-nitride layers. The stability in time and/or under thermal heating of the CN bonds formed was also tested.

Ultrastructure of regenerated bone mineral surrounding hydroxyapatite–alginate composite and sintered hydroxyapatite

We report the ultrastructure of regenerated bone surrounding two types of biomaterials: hydroxyapatite-alginate composite and sintered hydroxyapatite. Critical defects in the calvaria of Wistar rats were filled with micrometer-sized spherical biomaterials and analyzed after 90 and 120 days of implantation by high-resolution transmission electron microscopy and Fourier transform infrared attenuated total reflectance microscopy, respectively. Infrared spectroscopy showed that hydroxyapatite of both biomaterials became more disordered after implantation in the rat calvaria, indicating that the biological environment induced modifications in biomaterials structure. We observed that the regenerated bone surrounding both biomaterials had a lamellar structure with type I collagen fibers alternating in adjacent lamella with angles of approximately 90°. In each lamella, plate-like apatite crystals were aligned in the c-axis direction, although a rotation around the c-axis could be present. Bone plate-like crystal dimensions were similar in regenerated bone around biomaterials and pre-existing bone in the rat calvaria. No epitaxial growth was observed around any of the biomaterials. A distinct mineralized layer was observed between new bone and hydroxyapatite-alginate biomaterial. This region presented a particular ultrastructure with crystallites smaller than those of the bulk of the biomaterial, and was possibly formed during the synthesis of alginate-containing composite or in the biological environment after implantation. Round nanoparticles were observed in regions of newly formed bone. The findings of this work contribute to a better understanding of the role of hydroxyapatite based biomaterials in bone regeneration processes at the nanoscale.

High-resolution transmission electron microscopy study on SiC grown from SiO and C*: crystal growth and structural characterization

Abstract The results of a high resolution transmission electron microscopy study on the growth of the catalytic support silicon carbide, obtained by reacting active carbon with silicon monoxide, are presented. On the basis of this study, it is concluded that the reaction results first in the formation of microareas of SiC randomly distributed where the carbon has been converted into carbide. The growth was found to he preferentially favourable along the 〈111〉 direction of a fcc crystal structure resulting in the formation of plates parallel to a {110}-type plane. The stacking of the Si-C elementary layers along 〈111〉 is random, which enhances the probability of occurrence of stacking faults and twins. These defects were the most striking feature in the carbide obtained. Heavily faulted 3C-SiC or one-dimensionally disordered SiC polytype was obtained.

Effect of strontium ranelate on bone mineral: Analysis of nanoscale compositional changes.

Strontium ranelate has been used to prevent bone loss and stimulate bone regeneration. Although strontium may integrate into the bone crystal lattice, the chemical and structural modifications of the bone when strontium interacts with the mineral phase are not completely understood. The objective of this study was to evaluate apatite from the mandibles of rats treated with strontium ranelate in the drinking water and compare its characteristics with those from untreated rats and synthetic apatites with and without strontium. Electron energy loss near edge structures from phosphorus, carbon, calcium and strontium were obtained by electron energy loss spectroscopy in a transmission electron microscope. The strontium signal was detected in the biological and synthetic samples containing strontium. The relative quantification of carbon by analyzing the CK edge at an energy loss of ΔE = 284 eV showed an increase in the number of carbonate groups in the bone mineral of treated rats. A synthetic strontium-containing sample used as control did not exhibit a carbon signal. This study showed physicochemical modifications in the bone mineral at the nanoscale caused by the systemic administration of strontium ranelate.

Three-dimensional chemistry of multiphase nanomaterials by energy-filtered transmission electron microscopy tomography.

A three-dimensional (3D) study of multiphase nanostructures by chemically selective electron tomography combining tomographic approach and energy-filtered imaging is reported. The implementation of this technique at the nanometer scale requires careful procedures for data acquisition, computing, and analysis. Based on the performances of modern transmission electron microscopy equipment and on developments in data processing, electron tomography in the energy-filtered imaging mode is shown to be a very appropriate analysis tool to provide 3D chemical maps at the nanoscale. Two examples highlight the usefulness of analytical electron tomography to investigate inhomogeneous 3D nanostructures, such as multiphase specimens or core-shell nanoparticles. The capability of discerning in a silica-alumina porous particle the two different components is illustrated. A quantitative analysis in the whole specimen and toward the pore surface is reported. This tool is shown to open new perspectives in catalysis by providing a way to characterize precisely 3D nanostructures from a chemical point of view.