Strong Suppression of Thermal Conductivity in the Presence of Long Terminal Alkyl Chains in Low-Disorder Molecular Semiconductors

While the charge transport properties of organic semiconductors have been extensively studied over the recent years, the field of organics-based thermoelectrics is still limited by a lack of experimental data on thermal transport and of understanding of the associated structure–property relationships. To fill this gap, a comprehensive experimental and theoretical investigation of the lattice thermal conductivity in polycrystalline thin films of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (Cn-DNTT-Cn with n = 0, 8) semiconductors is reported. Strikingly, thermal conductivity appears to be much more isotropic than charge transport, which is confined to the 2D molecular layers. A direct comparison between experimental measurements (3ω–Völklein method) and theoretical estimations (approach-to-equilibrium molecular dynamics (AEMD) method) indicates that the in-plane thermal conductivity is strongly reduced in the presence of the long terminal alkyl chains. This evolution can be rationalized by the strong localization of the intermolecular vibrational modes in C8-DNTT-C8 in comparison to unsubstituted DNTT cores, as evidenced by a vibrational mode analysis. Combined with the enhanced charge transport properties of alkylated DNTT systems, this opens the possibility to decouple electron and phonon transport in these materials, which provides great potential for enhancing the thermoelectric figure of merit ZT

Plutonium in the environment: key factors related to impact assessment in case of an accidental atmospheric release

International audienceThis paper deals with plutonium and key factors related to impact assessment. It is based on recent work performed by CEA which summarize the main features of plutonium behaviour from sources inside installations to the environment and man, and to report current knowledge on the different parameters used in models for environmental and radiological impact assessment. These key factors are illustrated through a case study based on an accidental atmospheric release of Pu in a nuclear facility

Collective Molecular Switching in Hybrid Superlattices for Light-Modulated Two-Dimensional Electronics

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Computing the Lattice Thermal Conductivity of Small-Molecule Organic Semiconductors: A Systematic Comparison of Molecular Dynamics Based Methods

While the Green-Kubo and non-equilibrium molecular dynamics methods have been compared quite extensively to calculate the thermal conductivity in inorganic compounds, there is currently a lack of comparison of these algorithms with the more recently developed approach-to-equilibrium molecular dynamics (AEMD) method for other types of materials such as organic semiconductors. To fill this gap, this article reports a theoretical description of thermal transport in single crystals made of terthiophene as prototypical system based on the three most popular molecular dynamics approaches. A systematic comparison of the computed values of thermal conductivity and its anisotropy is carried out and the strengths and weaknesses associated with each method are discussed. Although the three algorithms give essentially the same trends, this study points to the "AEMD" approach as the most suitable compromise between accuracy and computing cost. On the material aspects, the theoretical modeling yields an anisotropic character of the thermal transport in crystals whose out-of-plane thermal conductivity component is approximately twice larger than the in-plane components. The AEMD approach is further used to investigate the influence of temperature on thermal transport in terthiophene. The trends are utterly rationalized by relying on the concepts of phonon mean free paths and phonon group velocities

A Simple and Adaptive Methodology to use a Commercial Solvent Extraction Microsystem as Screening Tool Validation with the U-TBP Chemical System

International audienceThe solvent extraction of a U (VI) present at several g.L-1 from the aqueous phase (HNO3) = 3 M to the organic one (30 percent TBP) involves a change of the viscosity of the two phases during the extraction process in microsystem. This shifts the position of the interface between the two phases and then affects the quality of the separation at the outlet of the microsystem. The aim of this study is to present a simple method to stabilize the quality of the separation by controlling the interface position during solvent extraction in microsystem. First, a study at low concentration is operated in order to set flow rates and contact time that allow optimal parallel flows and mass transfer. Second, the influence of the high grade concentration (up to 140 g.L-1) of solute on the flows and extraction yield is studied

Microsystèmes d'extraction par solvant pour l'analyse radiochimique

National audienceL’analyse des radionucléides dans les solutions de combustibles irradiés ou dans les déchets de haute ou moyenne activité implique l’enchaînement de plusieurs opérations de séparation/purification avant la mesure par spectroscopies nucléaires ou ICP-MS. Ces étapes sont sources de déchets et d’exposition aux radiations, et implique l’utilisation de volumes non négligeables de réactifs chimiques ou de solvants. Résoudre ce problème passe par la miniaturisation des outils d’analyse et le développement de laboratoires sur puce dédiés aux analyses liées au domaine nucléaire. La micro extraction liquide-liquide ($\mu$-ELL) a été développée pour des applications d’analyse de radionucléides dans des effluents radioactifs

Détection de claquages produits par laser (LIBD) pour la caractérisation de nanoparticules en suspension

National audienceLa technique LIBD (laser-induced breakdown detection) a été mise au point dans les années 90 dans le but de détecter et caractériser des nanoparticules en suspension dans un liquide. Elle consiste à focaliser un faisceau laser impulsionnel dans le milieu et détecter les claquages sur les particules soit par voie acoustique, soit par voie optique. Dans le premier cas, on détermine la probabilité de claquage en fonction de l'énergie de l'impulsion. Le seuil et la pente de la courbe en S ainsi obtenue permettent d'obtenir la taille et la concentration moyenne des particules. Dans le deuxième cas, on fixe l'énergie de l'impulsion et on image le volume focal en accumulant un certain nombre de tirs laser. La distribution des claquages dans l'axe de propagation du laser permet également de déterminer la taille et la concentration moyenne des particules.La LIBD a plusieurs avantages essentiels par rapport aux techniques conventionnelles de caractérisation de nanoparticules en suspension, notamment celles basées sur la diffusion de lumière ou sur la microscopie électronique elle est rapide, elle permet de détecter des nanoparticules individuelles jusqu'à des tailles inférieures à 10 nm et en très faible teneur, et on peut la mettre en oeuvre in situ. Elle permet donc de caractériser des suspensions de colloïdes environnementaux, de particules manufacturées, ou encore de déterminer précisément l'initiation de mécanismes de précipitation. Ces différents domaines ouvrent la voie a des applications innovantes notamment dans le secteur de la chimie en milieu nucléaire. Un montage de LIBD a été mis en place au laboratoire. Son pilotage a été réalisé sous LabView, en mode de détection acoustique et optique. Les premiers résultats expérimentaux seront présentés, obtenus sur des suspensions de nanoparticules de polystyrène

direct detection and identification of uranium(vi)-bearing solids by trlfs and chemometrics analysis

International audienceUranium can be naturally found in the environment in the form of minerals, precipitates, and/or associated to other compounds like iron (hydr)oxides or clay rocks. Identification of the nature of the uranium compounds is of major importance for geological survey, mining exploration, and management of the environmental impact of industrial sites. Many studies have been conducted to characterize uranium-bearing solids with low uranium content from ppm to hundreds of ppm. This requires a careful search of uranium-rich zones in the sample, and the use of characterization methods such as SEM, XRD, and total elemental analysis. Such a methodology is rather long and difficult, and limits the survey to a small number of samples.Time-resolved laser induced fluorescence spectroscopy (TRLFS) is a classical technique that enables the detection of U(VI) at low level, and has been largely used for quantitative and speciation analyses, in solution [1] and in minerals [2]. The rapidity of the analysis is particularly interesting to investigate a large number of samples with no or limited preparation steps, to detect traces of U(VI) and to obtain spectroscopic information that can be related to the nature of the uranium(VI)-bearing phases such as phosphates. In this study, about twenty U(VI) fluorescence spectra have been recorded on a large set of samples from several environmental sites. The fluorescence intensity is however highly variable regarding the phases due to enhancing and quenching effects on the fluorescence. Interpretation of these measurements is not straightforward. Measurements on cooled or cryogenized samples can considerably increase the fluorescence intensity and the resolution of the spectra, and help identifying the chemical family of the phases [3]. However the resulting spectral analysis is often ambiguous for phase identification and improved data treatment methods need to be developed.Chemometrics provide useful multivariate methods for spectral and classification analysis, but is seldom used in TRLFS data analysis. We have applied multivariate methods to a set of U(VI) minerals and synthetic phases (phosphates, vanadates, arsenates, silicates) as reference materials. Independent Components Analysis (ICA) has been used to determine pure independent signals contributing to the fluorescence spectra. The resulting spectral signatures have been interpreted regarding the nature of the phases, and may evidence mixtures of compounds or U(VI) chemical sites in natural minerals. Using ICA, the spectral data sets have been categorized into classes that were successfully described using Principal Components Analysis (PCA). Iterative PCA has proved a good efficiency to discriminate most of the samples in our reference data set (Figure 1). Several classes cannot be easily distinguished, and exhibit a significant variability, which may suggest less pure compounds. The use of predictive methods with a dedicated spectral data base is under development in order to identify U(VI) phases in unknown samples. This combined TRLFS and chemometrics approaches has already demonstrated its great performance for U(VI) phases identification, and perspectives will be presented