Rayonnement du corps noir

Résumé

vignette|303px|Au fur et à mesure que la température diminue, le sommet de la courbe de rayonnement du corps noir se déplace à des intensités plus faibles et des longueurs d'onde plus grandes. Le diagramme de rayonnement du corps noir est comparé avec le modèle classique de Rayleigh et Jeans. vignette|303px|La couleur (chromaticité) du rayonnement du corps noir dépend de la température du corps noir. Le lieu géométrique de telles couleurs, représenté ici en espace x,y CIE XYZ, est connu sous le nom de lieu géométrique de Planck. Le rayonnement du corps noir est le type de rayonnement électromagnétique (rayonnement thermique) émis par un corps noir (corps qui absorbe parfaitement la chaleur) en équilibre thermodynamique avec son environnement. Le rayonnement possède un spectre spécifique et une intensité qui dépend seulement de la température du corps. Le rayonnement thermique spontanément émis par les objets ordinaires peut être approximé par un rayonnement de corps noir. Une encein

Source officielle

https://fr.wikipedia.org/wiki/Rayonnement_du_corps_noir

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Chargement

Embrittlement induced by the synergistic effects of radiation damage and helium in structural steels

Kun Wang

Ferritic/martensitic steels are candidate materials for fusion reactor structural components, liquid metal containers of spallation neutron sources, and accelerator driven systems, with good radiation resistance and thermo-mechanical properties. However, embrittlement resulting from the combined effects of radiation induced displacement damage (measured in dpa = displacement per atom) and transmutation products, especially helium gas, is one of the key issues. Four different steels were selected for mechanical and microstructural studies to understand the mechanisms of embrittlement induced by the combined effects of displacement damage and helium after irradiation in SINQ, the Swiss spallation source. The irradiations conditions were in the range: 10.7 ¿ 20.4 dpa with 850-1750 appm He at 160-300 °C. The evolution of the mechanical properties after irradiation was investigated by tensile and hardness tests. Radiation-induced defect clusters and helium bubbles were quantified by transmission electron microscopy (TEM). Emphasis was put on the deformation mechanisms under the different observed fracture mode, (ductile, quasi-cleavage and intergranular), whose occurrence depends on the irradiation conditions (dpa, He content and irradiation temperature). The tensile stress-strain curves and the scanning electron microscopy images of fracture surfaces showed distinct fracture mechanisms under different irradiation and test conditions. The tensile tests showed a yielding stress increase and loss of ductility of irradiated specimens. Hardness was measured on the specimens before tensile testing. The hardness results demonstrated an increasing trend with irradiation dose and helium content. TEM observations were done for all irradiated fractured specimens. Small defect clusters were observed in the 12.3 dpa specimen, but large defect clusters with loop-shape were very few. In the specimens of 17.2, 17.7 and 20.4 dpa, many large dislocation loops were detected besides small clusters. In addition, helium bubbles were observed in all specimens. The average size of defect clusters increased from 4.2 nm to 11.8 nm with dose increasing from 12.3 dpa to 20.4 dpa, whereas the number density did not change significantly. Meanwhile, the average size of visible helium bubbles increased from 1.03 nm to 1.93 nm. The microstructures in deformed area of irradiated specimens were observed and defect free channels with {110} and {112} slip planes were found in some specimens, indicating plastic flow localization. The average width of the channels is about 100 nm. Regarding the brittle samples, the TEM-lamella were extracted directly below intergranular fracture surfaces or cleavage surfaces by focused ion beam. Strikingly, deformation twinning was observed as the main feature in three irradiated specimens at high dose. Only twins with {112} planes were observed in all of these samples. The average thickness of twins is about 34 nm. Twins started from a fracture surface became gradually thinner with distance away from the fracture surface and stopped in the matrix finally. Features such as twin-precipitates interaction, twin-grain boundary and/or lath boundary interaction were observed. Twinning bands were seen to be arrested by grain boundaries or large precipitates, but could penetrate martensitic lath boundaries. Unlike the case of defect free channels, inside twins small defect-clusters, dislocation loops and dense small helium bubbles were observed.

EPFL2015

Chargement

The thesis presents the development of a custom made atomic force microscope (AFM) conceived to image biological molecules at low temperatures in ultrahigh vacuum conditions, and its application to DNA single molecule studies. Starting from a first prototype working in contact mode, our aim was to implement non-contact AFM in order to reduce possible sample damaging due to strong tip interactions. The first chapter of the thesis discusses the technical solutions developed to improve the previous system. A complete setup for in situ exchange of samples and tips was designed and implemented. This permits a continuous operation of the microscope under the experimental conditions of UHV. Thermal instabilities and temperature differences between the tip and the sample were solved with the design of a movable shield to avoid heating by thermal radiation. The introduction of a new high voltage signal generator with variable frequency and the development of a user-friendly LabVIEW interface to communicate with this device notably improved the piezomotors driving. These piezomotors perform the micro-positioning of all the microscope parts and are driven through this interface. Finally, the previous tip-holder design was modified to incorporate a piezo actuator behind the cantilever, which provides the necessary excitation for the implementation of dynamic AFM modes. The installation of a Phase Lock Loop (PLL) circuit to lock the cantilever frequency and the replacement of old feedback electronics for a digital SPM Controller, with advanced options of frequency-shift feedback, was another requirement for the correct implementation of non-contact AFM. The second chapter deals with DNA sample preparation for high resolution imaging by AFM. Based on our studies, the advantages and drawbacks of the most commonly used techniques for DNA adsorption are discussed in detail. Our measurements in contact mode demonstrate the convenience of using hydrophobic substrates for high resolution DNA imaging. At hydrophilic surfaces, like mica, the presence of a thin water layer could be responsible of strong adhesion forces between the cantilever and the sample. In order to overcome this limitation we tested HOPG as hydrophobic substrate. HOPG, chemically modified with amphiphilic compounds, allowed us to deposit DNA without molecular damage or stress. The HOPG surface treatment was significantly improved. New protocols were developed to control the formation of dodecylamine disordered monolayers or lamella structures on the substrate. The lamella formation induces a subsequent self assembly of DNA single molecules. In the last chapter we discuss our results on DNA imaging obtained with the different operation modes implemented in our microscope: contact mode, tapping mode and frequency modulation (non-contact) AFM. We conclude that the last one provides higher signal stability and the necessary sensitivity to observe DNA details at a single-molecule level. We also present a study of frequency shift versus distance dependence (force spectroscopy experiments) performed in DNA samples. We discuss for the first time a plausible explanation to the origin of the atomic force contrast between the substrate and the DNA molecules. Force spectroscopy measurements performed along DNA molecules reveal two distinguishable types of interaction. This result suggests that the chemical composition of the minor-major groove and/or of the sugar-phosphate backbone along the molecule can be detected, opening numerous possibilities for further studies and applications of this technique.

EPFL2008

Chargement

Aerosol Transport and Distribution

Quentin Bourgeois

The fourth report of the Intergovernmental Panel of Climate Change (IPCC) unambiguously indicates that the climate is changing. Climate change is induced by changes in atmospheric abundances of greenhouse gases and aerosols, solar radiation and land surface properties, which all alter the radiative balance of the Earth. Among them, greenhouse gases and aerosols are likely the compounds affecting the climate the most. While greenhouse gases warm the Earth, aerosols influence its radiative balance by scattering and absorbing solar radiation (the direct effect), and by modifying cloud amount and properties (the indirect effect). The radiative forcing of aerosols on the climate is however the less understood among the various forcings currently considered in the IPCC assessment. This is likely related to the lack of comprehensive and accurate observations of aerosols (and in particular of their vertical distribution) which thus makes global aerosol models difficult to constrain. Recent model intercomparisons have indicated that different assumptions regarding aerosol emissions, formation and growing properties, and removal processes generate large diversities within models. These large diversities are found in particular in the Arctic and the upper troposphere region where extreme surrounding conditions take place, limiting instrument sensitivity and accuracy. The objectives of this thesis are to better quantify the processes driving the aerosol distribution in regions where the uncertainties are the largest, including the Arctic region and the upper troposphere, and to assess the quality of CALIOP observations. For this purpose, we used the fully coupled global climate-aerosol-chemistry ECHAM5-HAMMOZ model conjointly with a suite of remote sensing observations including the CALIOP satellite instrument, which provides the vertical distribution of aerosols. The model predicts the size distribution and composition of aerosols as well as the number concentration of cloud droplets and ice crystals. We first investigated the processes that drive the transport of soluble and insoluble compounds toward the Arctic in the ECHAM5-HAMMOZ model. Recognizing that scavenging processes may be an issue in global models, we "re-visited" their properties in ECHAM5-HAMMOZ. We find that the model better reproduces the aerosol vertical distribution in the northern mid- and high-latitudes, especially in the free troposphere, by decreasing aerosol scavenging coefficients in the model. Using smaller aerosol scavenging coefficients results in an increase of aerosol burden and lifetime, especially in the Arctic. In addition, the Arctic haze phenomenon is better represented in the simulation using decreased aerosol scavenging coefficients. The relative contributions of different processes governing the transport of aerosols from the midlatitudes toward the Arctic together with the relative contributions of different geopolitical source regions were then quantified. We find that Europe and Siberia contribute the most to the Arctic pollution even though Asian anthropogenic emissions are much larger. The model suggests that BC emitted in Siberia and Europe is ten times more efficiently deposited onto the Arctic than BC emitted in Asia. We next investigated an aerosol layer that forms in the vicinity of the tropical tropopause layer (TTL) over the southern Asian and Indian Ocean region. Aerosols in this layer are observed with the CALIOP instrument even though ice clouds partly mask their signal. The aerosol layer follows the Inter Tropical Convergence Zone (ITCZ), which indicates that these aerosols are likely transported during convective processes. The ECHAM5.5-HAM2 model reproduces such an aerosol layer in the TTL over the same region but clearly overestimates CALIOP data. Model results suggest that aerosols in this layer are mostly sulfate particles transported during convective processes and originating from natural sources. The shortwave forcing of this aerosol layer is estimated. Finally, we assessed the accuracy of aerosol extinction retrievals from the CALIOP satellite instrument by comparing them with remote sensing observations such as AERONET, MODIS and MISR over the last four years. Overall, standard CALIOP products underestimate by 50% observed aerosol extinctions. However, this comparison is substantially improved by screening CALIOP data. The comparison of the CALIOP screened product with the ECHAM5.5-HAM2 model shows large disparities in model results. The model reproduces quite well the observed interannual variability and vertical distribution of aerosols in Europe and North Africa while it underestimates them in America and Asia. This is likely due to an underestimate of dust and anthropogenic emissions in North America and Asia.

EPFL2012

Chargement

Aerosol Transport and Distribution

Quentin Bourgeois

EPFL2012

Embrittlement induced by the synergistic effects of radiation damage and helium in structural steels

Kun Wang

EPFL2015

EPFL2008