Thermal Reconversion of Oxidised Lead White in Mural Paintings via a Massicot Intermediate

Lead white is the most ancient and common white pigment used in mural paintings. However, it tends to blacken with time due to its oxidation to plattnerite (\b{eta}-PbO2). Chemical treatments were used but they can put the pictorial layers supports at risks. Hereby we address the possibility of thermally reconverting black plattnerite to white lead carbonates via a massicot (\b{eta}-PbO) intermediate, with a view to developing a restoration procedure using continuous wave laser heating. We first investigated the conditions (temperature, time, and environment) in which pure powders react, before studying mural painting samples. Experiments were made in ovens and TGA and XRD and SEM characterization were achieved. Litharge ({\alpha}-PbO) and massicot were obtained from plattnerite respectively between 564 and 567 {\deg}C and at 650 {\deg}C. Lead carbonates (cerussite, hydrocerussite and plumbonacrite) formed from massicot in wet CO2 below 100 {\deg}C in a few hours. Lastly, when heating plattnerite based mural painting samples, lead species reacted with binders and mortar, yielding massicot, plumbonacrite but also lead silicate and calcium lead oxides. This demonstrates the viability of thermal reconversion of darkened lead in mural, while raising concerns about the formation of several lead species by reaction with mural painting constituents

Décontamination et nettoyage laser appliqués aux surfaces métalliques (études théorétiques et expérimentales)

Le nettoyage des surfaces métalliques est nécessaire dans différents domaines de l'industrie moderne. L'industrie nucléaire cherche de nouvelles méthodes de décontamination des surfaces oxydées, et les installations thermonucléaires nécessitent le nettoyage des composants face au plasma pour enlever la couche déposée contenant tritium. L'ablation laser est proposée comme une méthode efficace et sûre pour le nettoyage des surfaces métalliques et leur décontamination. Le facteur important influençant le chauffage et l'ablation laser est la distribution en profondeur de l intensité laser. Le modèle de propagation de la lumière dans une couche diffusant sur un substrat métallique est développé et appliqué pour analyser les caractéristiques de distribution de lumière. Pour simuler les surfaces contaminées, l'inox AISI 304L a été oxydé par laser et chauffé dans un four. La contamination radioactive de la couche d'oxyde a été simulée par l'introduction d europium et / ou de sodium. Un facteur de décontamination de plus de 300 a été démontré avec le régime de nettoyage optimal trouvé. Une diminution de la résistance à la corrosion a aussi été montrée après un nettoyage laser. Les seuils d'ablation des surfaces ITER-like (représentatives d ITER) ont été mesurés. Une vitesse de nettoyage de 0,07 m2/W h a été trouvée. Pour les surfaces miroir, les seuils de dommages étaient déterminés pour éviter les dommages lors du nettoyage au laser. La possibilité de restaurer la réflectivité après le dépôt d une couche mince de carbone a été démontrée. Les perspectives de développement ultérieur de nettoyage laser sont discutées.Metal surface cleaning is highly required in different fields of modern industry. Nuclear industry seeks for new methods for oxidized surface decontamination, and thermonuclear installations require the cleaning of plasma facing components from tritium-containing deposited layer. The laser ablation is proposed as an effective and safe method for metal surface cleaning and decontamination. The important factor influencing the laser heating and ablation is the in-depth distribution of laser radiation. The model of light propagation in a scattering layer on a metal substrate is developed and applied to analyse the features of light distribution. To simulate the contaminated surfaces, the stainless steel AISI 304L was oxidized by laser and in a furnace. Radioactive contamination of the oxide layer was simulated by introducing europium and/or sodium. The decontamination factor of more than 300 was demonstrated with found optimal cleaning regime. The decreasing of the corrosion resistance was found after laser cleaning. The ablation thresholds of ITER-like surfaces were measured. The cleaning productivity of 0.07 m2/hour W was found. For mirror surfaces, the damage thresholds were determined to avoid damage during laser cleaning. The possibility to restore reflectivity after thin carbon layer deposition was demonstrated. The perspectives of further development of laser cleaning are discussed.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Evguenii Konstantinovitch Zavoïskii (1907-1976) et la découverte de la résonance paramagnétique électronique

Evguenii Konstantinovitch Zavoïskii fut un éminent physicien expérimentateur russe. En 1944, à Kazan en URSS, il réalisa l’expérience décisive de la découverte de la résonance paramagnétique électronique (RPE). La RPE permet d’étudier la structure électronique de molécules paramagnétiques. Elle a un impact significatif en physique, chimie, minéralogie, biologie, médecine et physiologie. Avec son talent original d’expérimentateur et sa profonde intuition physique, Zavoïskii a aussi mené des recherches brillantes sur la physique nucléaire, la fusion thermonucléaire contrôlée, l’électronique physique et les phénomènes rapides

Développement d'une nouvelle génération de détecteurs micro-structurés à base de semi-conducteurs pour l'imagerie médicale de rayons X

L amélioration des performances des détecteurs (au niveau de la résolution en énergie et de la résolution spatiale) pour la radiographie X médicale par l utilisation d un semi-conducteur montre l intérêt de remplacer les détecteurs à base de cristaux scintillateurs (majorité des systèmes commerciaux) par un détecteur à base de semi-conducteur. Avec une perspective de respect environnemental, ces travaux portent sur le développement d une nouvelle génération de détecteurs à base de semi-conducteur, différent du CdTe, pour l imagerie médicale de rayons X fonctionnant en mode comptage, ce qui permet la réduction de la dose de rayonnement envoyée sur le patient. Deux axes de recherches en découlent, avec le choix d un nouveau matériau semi-conducteur, le GaAs semi-isolant et d une nouvelle géométrie de détection, la géométrie 3D.Ces travaux ont consisté à évaluer expérimentalement le semi-conducteur, afin de choisir un matériau (fournisseur, croissance) et une électrode métallique qui ont la capacité de compter des photons. Puis, la structure de détection, au travers de caractérisations des procédés technologiques nécessaires pour la réalisation de la géométrie 3D (usinage des électrodes, dépôt des électrodes, connexion à un circuit électronique) et de la validation du concept avec un dispositif de test, a été également étudiée. Enfin, les premiers résultats d un détecteur 3D à base de GaAs semi-isolant, montrant la concrétisation de ces objectifs, sont proposés.In X-ray medical imaging, semi-conductors tend to replace scintillator crystals (most of the commercial devices), thank to their higher spatial resolution and energy resolution. Counting mode is another tendency, because it allows reducing the radiation dose delivered to the patient. This work aims at developing a new generation of semi-conductor detectors, different from CdTe / CdZnTe for environmental concerns, and associated with a new detection structure (3D geometry). Semi-insulating GaAs from specific growth and supplier, with adapted metallic electrodes, shows the ability to count photons. Then, a proof of concept of the 3D geometry is provided, through the characterization of electrodes machining in bulk material, metal deposition and connection to a read-out electronic circuit. Finally, the achievement of these objectives is reflected in the preliminary characterization of 3D detector based on semi-insulating GaAs.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

Diagnostics of nonuniform plasmas for elemental analysis via laser-induced breakdown spectroscopy: demonstration on carbon-based materials

International audienceWe investigate the plasma produced by the interaction of ultraviolet nanosecond laser pulses with a carbon fiber composite tile from the inner wall of a fusion reactor. The experiments are carried out in argon at a pressure of 5 x 10(4) Pa. Fast imaging is used to characterize the plume expansion dynamics and excited plasma species are followed by time-and space-resolved emission spectroscopy. The measurements show that ionized and highly excited plasma species are located in the front of the expanding plume, whereas neutral atoms and lower excited species dominate in the plasma core. By measuring the excitation temperature of metallic ions and neutral atoms, we evidence the existence of a temperature gradient that appears during the early expansion stage and remains up to delays typically used in material analysis via laser-induced breakdown spectroscopy. The knowledge of the spatial distribution of the plasma properties is then used to model the plasma emission spectrum. Assuming two plasma zones in local thermodynamic equilibrium of different temperatures and electron densities, we calculate the spectral radiance and compare it to the spatially integrated spectrum recorded with an Echelle spectrometer. From the best agreement between measured and computed spectra we deduce the elemental concentrations of carbon, hydrogen and metal impurities. The validity of the model is critically discussed and the measurement uncertainties are evaluated. The present approach is foreseen to improve the accuracy of analysis via laser-induced breakdown spectroscopy in many applications, in particular when materials of elements with significantly different ionization potentials are investigated

Extraction du carbone dans des échantillons d’acier par ablation laser

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Growth of micrometric oxide layers for the study of metallic surfaces decontamination by laser

The nuclear industry produces a wide range of radioactive waste in term of level of hazard, contaminants and material. For metallic equipment like steam generators, the radioactivity is mainly located in the oxide surface. In order to study and develop techniques for dismantling and for decontamination in a safe way, it is important to have access to oxide layers with a representative distribution of non-radioactive contaminants. We propose a method of formation of oxide layer on stainless steel 304L with europium (Eu) as contaminant marker. In this method, an Eu-solution is sprayed on the stainless steel samples. The specimen are firstly treated with a pulsed nanosecond laser and secondly the steel samples are exposed to a 600°C furnace for various durations in order to grow an oxide layer. The oxide structure and in-depth distribution of Eu in the oxide layer are analysed by scanning electron microscopy coupled with energy dispersive X-ray microanalyzer, and by glow discharge optical emission or mass spectrometry. The oxide layers were grown to thicknesses in the range of 200 nm to 4.5 μm regarding to the laser treatment parameters and the heating duration. These contaminated oxides have a ‘duplex structure’ with a mean weight percentage of 0.5% of europium in the volume of the oxide layer. It appears that europium implementation prevents the oxide growth by furnace but has no impact on laser heating. These oxide layers are used to study the decontamination of metallic surfaces such as stainless steel 304L using a nanosecond pulsed laser

Extraction du carbone dans des échantillons d’acier par ablation laser

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Development of a tip enhanced near-field laser ablation system for the sub-micrometric analysis of solid samples

International audienceA near-field laser ablation system was developed for the analysis of inorganic solid samples in the nanometer resolution range. The instrument is based on the coupling of a nanosecond Nd:YAG laser with an atomic force microscope. The technique uses a tip enhancement effect obtained by the interaction of laser radiation with the conductive tip of the AFM maintained at a few nanometers above the sample surface. By applying this technique to conducting gold and semiconducting silicon samples, a lateral resolution of 100 nm was demonstrated. With a single laser pulse, craters of about 100 nm in diameter and a few nanometers in depth were obtained. A multi-parametric study was carried out in order to understand the effect of different experimental parameters (laser fluence, tip-to-sample distance, sample and tip nature) on the near-field laser ablation efficiency, crater dimensions and amount of ablated material. Numerical simulations of the localized heating with a home-made 3-D code presented a good explanation for the nanometer-sized crater diameters obtained in our experiments

Extraction du carbone dans des échantillons d’acier par ablation laser

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