Jean-Luc Bruneel

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.

Raman Spectroelectrochemistry of a Lithium/Polymer Electrolyte Symmetric Cell

A symmetric lithium cell Li/P(EO) 20 LiTFSI/Li has been studied at 80°C by confocal Raman microspectrometry while current densities of up to 0.5 mA cm -2 are passed through the cell. The Raman observation is performed on the edge of the cell along a line of points extending from one electrode to the other at a depth of about 20 μm within the electrolyte. Local salt concentration is measured in the electrolyte as a function of time, current density I, and electrolyte thickness L (90 and 160 μm). When the steady-state regime is reached, linear and symmetric salt concentration gradients are observed. They are proportional to I and L as expected from the theoretical predictions for a binary electrolyte containing a fully dissociated salt with negligible ionic associations. In addition, preliminary results have been obtained concerning the establishment and relaxation of the steady state. From these data, it is shown that salt diffusion coefficient and ionic transport numbers can be determined with a reasonable precision. Confocal Raman microspectrometry can therefore be considered as a new and powerful spectroelectrochemical method to study transport properties in polymer electrolytes.

In situ probing of interfacial processes in the electrodeposition of copper by confocal Raman microspectroscopy

Confocal Raman microspectroscopy is applied to the in situ probing of interfacial processes in pulsed-current copper electrodeposition. This technique provides time-resolved characterization of the vibrational spectra of sulphate ions whenever in solution or adsorbed on the growing electrode. It also confirms the formation of cuprous oxide in the reduction process as the solution pH is increased by proton reduction. Moreover, when the passage of current is terminated, this technique provides evidence for the recombination of copper ions with copper metal to produce cuprous oxide on the outermost branches of the deposit.

Mapping dynamic concentration profiles with micrometric resolution near an active microscopic surface by confocal resonance Raman microscopy. Application to diffusion near ultramicroelectrodes : first direct evidence for a conproportionation reaction

Abstract Confocal Raman microspectroscopy is a very efficient means for probing the molecular composition of micrometric-sized samples. Its coupling with Raman resonance spectroscopy allows the specific tracking of very dilute species by considerably enhancing its Raman bands. Thus, spatially resolved information on the chemical composition of diffusion layers, which build up spontaneously near an active surface placed in a solution, can be obtained with a micrometric resolution. In this work, the applicability of the method for imaging diffusional transport towards ultramicroelectrodes with a micrometric resolution is examined. The efficiency and versatility of confocal resonance Raman microspectroscopy have been tested by probing the composition of the two different diffusion layers which build up in the vicinity of an ultramicroelectrode during the reduction of tetracyanoquinodimethane (TCNQ) on its first or second electrochemical wave. Besides the establishment of the method, this work affords the first direct experimental evidence of the existence and role of conproportionation reactions, which take place on the second reduction wave of EE electrochemical systems. In both cases, the concentration profiles of the radical anion TCNQ − agree extremely well with the theoretical predictions.

The photodimerisation of trans-cinnamic acid and its derivatives: a study by vibrational microspectroscopy.

Infrared and Raman spectroscopies have been used to monitor the [2 + 2] photodimerisation reactions of alpha-trans-cinnamic acid and of a number of its derivatives. The principal changes observed in the spectra upon dimerisation are decay of a band around 1637 cm(-1), which is assigned to v(C=C) of the ethene bond of the monomer, and growth of bands just above 3000 cm(-1), which result from v(C-H) of saturated carbon atoms of the dimer. The use of microscope attachments has allowed us to follow the reactions of single crystals and we conclude that the reactions are topotactic in nature. We have carried out preliminary kinetic experiments in which spectra of one single crystal are recorded after sequential periods of photolysis. We find that the rates of dimerisation of differently substituted cinnamic acids are similar, with physical effects, such as the thickness of an individual crystal, outweighing any observed electronic effects (inductive or mesomeric).

Mapping concentration profiles within the diffusion layer of an electrode: Part I. Confocal resonance Raman microscopy

Abstract Confocal microspectroscopy is known to be a very efficient means for probing composition of spatially resolved micrometric volumes inside a macroscopic sample. In this paper, the applicability of confocal Raman microspectroscopy for imaging molecular diffusion at microelectrodes with a micrometric resolution is established. The efficiency and versatility of the method have been tested by probing the composition of the two different diffusion layers which build up in the vicinity of an ultramicroelectrode during reduction of tetracyanoquinodimethane (TCNQ) on its first or second electrochemical wave. This is performed by mapping the concentration profiles of the TCNQ − anion radical under each condition using its resonance Raman spectrum. As a correlation, this provides the first direct experimental proof of a conproportionation reaction taking place when the electrode potential is poised on the second wave of a two-wave EE electrochemical system. In both cases, the concentration profiles of the radical anion TCNQ − agree extremely well with theoretical predictions.

Investigations on natural rubber filled with multiwall carbon nanotubes

Composites based on natural rubber (NR) and multiwall carbon nanotubes (MWNTs) were prepared by using different mixing processes. The state of dispersion was evaluated by transmission electron microscopy and the extent of reinforcement imparted to the elastomeric matrix was assessed by equilibrium stress–strain measurements. Dynamic mechanical analysis was performed in order to get information on the polymer–filler interface. Molecular investigations including infrared analysis under strain or Raman spectroscopy were carried out in order to point out possible alterations in the crystalline phase or interfacial interactions between the filler and the matrix.

Raman spectroscopy of white wines.

The feasibility of exploiting Raman scattering to analyze white wines has been investigated using 3 different wavelengths of the incoming laser radiation in the near-UV (325 nm), visible (532 nm) and near infrared (785 nm). To help in the interpretation of the Raman spectra, the absorption properties in the UV-visible range of two wine samples as well as their laser induced fluorescence have also been investigated. Thanks to the strong intensity enhancement of the Raman scattered light due to electronic resonance with 325 nm laser excitation, hydroxycinnamic acids may be detected and analyzed selectively. Fructose and glucose may also be easily detected below ca. 1000 cm(-1). This feasibility study demonstrates the potential of the Raman spectroscopic technique for the analysis of white wines.

Raman study on carbonaceous materials prepared by mechanical milling

Abstract Through Raman spectroscopy studies, we show that mechanical grinding generates an increasing amount of unorganized carbon at a rate depending on the type of grinding mode used (Shear and Shock-type grinding). The first-order Raman spectrum for pure unground graphite has a well-known G sharp band at 1579 cm−1, which corresponds to the E2g vibration while the ground samples present a broadened G band accompanied by new components at about 1610 cm−1 (D′), 1510 cm−1 (D″) and 1348 cm−1 (D), usually explained as arising from disorder and defects1. Shock-type grinding produces a faster disorder increase than shear-type grinding. The latter preserves part of the graphitic character. The general effect of mechanical milling remains however opposite to that of Thermal Treatment (Graphitization).

Spectroscopic and theoretical investigations of phenolic acids in white wines

Model solutions of white wines containing phenolic acids have been investigated by means of UV-vis, laser induced fluorescence and Raman spectroscopic techniques. In order to interpret the spectra, density functional theory calculations of phenolic acids have been performed. This work demonstrates that only hydroxynamic acids are in resonance with a laser excitation line with 325nm wavelength and are therefore at the origin of the strong enhancement of the Raman light scattering. Real white wines also display such resonance Raman scattering so that their content in hydroxycinnamic acids may be quite precisely determined. The analysis of the Raman spectrum of a real dry white wine reveals qualitatively the preponderance in its composition of p-coumaric and caftaric acids.