Absorption (physique) | EPFL Graph Search

L’absorption est un phénomène ou processus physique et chimique dans lequel des atomes, molécules ou ions pénètrent dans une phase gazeuse, liquide ou solide. Les espèces absorbées, quant à elles, entrent à l'intérieur de la phase (i. e. en profondeur dans le volume). L’absorption est donc la rétention d’une espèce par une substance. Absorption Vs. adsorption Ce phénomène est différent de l’adsorption où les espèces adsorbées restent à la surface. L’absorption et l'adsorption peuvent être regroupées sous le terme sorption. Partage entre deux phases On considère deux phases « 1 » et « 2 » auxquelles on ajoute un troisième constituant i qui va être absorbé par les deux phases ; i va se partager entre les deux phases suivant l'équilibre : : i_\text{phase1} \rightleftharpoons i_\text{phase2} L’absorption lors de ce partage obéit à la loi de cloison de Nernst : La grandeur K est nommée « coefficient de partage » et sa valeur dépend de la températ

Defect formation and mitigation during laser powder bed fusion of copper

Carl Tore Viktor Lindström

Additive manufacturing (AM) is a group of processing technologies which has the potential to revolutionize manufacturing by allowing easy manufacturing of complex shapes and small series. One AM-method which is of high interest for processing of metals is laser powder bed fusion (LPBF), where a powerful laser is used to melt powder in a layer-by-layer fashion into a consolidated part.Many defects can form during LPBF processing, but copper, copper alloys, and similar metals are particularly prone to the formation of pores.The reason why these metals tend to have these defects is the high reflectivity decreasing the effective energy input, and the high thermal conductivity transporting the absorbed energy away. Several ways of decreasing the pore formation tendency are known: increasing the power, increasing the absorptivity, and alloying to decrease thermal conductivity.The relative effectiveness of these strategies are however not known, making it difficult to choose the correct strategy for the different industrial applications of these metals.In this thesis a simple analytical model for predicting the occurrence of these defects is proposed, allowing a lowest possible processing power given the material properties to be estimated. The model agrees well with experiments, and can be used to predict the transition from so called balling mode processing to conduction mode processing, and for qualitative analysis of different processing strategies. Two strategies of decreasing the porosity are proposed and tested: coating of the powder with an absorbing layer and processing with a green laser, which is mor e strongly absorbed by copper than the conventional IR-lasers.The copper powder was coated with thin layers of tin and nickel using a simple and cheap immersion deposition method, and it is shown that the amount of porosity is decreased more than can be expected of the alloying alone, showing both that the method is working and that laser interaction with the solid is important for the heat balance of the system.The interaction with the solid is shown through computational fluid dynamics simulations to arise from periodic fluctuations of the molten metal which cause laser light to be reflected forward, pre-heating the powder bed.The preheating is larger the higher the solid absorptivity is, explaining the porosity decrease when using coated powders, and shows that processing using a green laser increase the laser coupling more than the absorptivity of the liquid would.Single line tracks made from fine 5 µm pure copper with a green laser source show that the desired conduction mode melting can be achieved at a power below 70 W, an order of magnitude lower than what is needed for processing of the conventionally used 45 µm powder with an IR-laser.

EPFL2021

Toward smart carbon capture with machine learning

Seyedmohamad Moosavi, Mohammad Rahimi, Berend Smit

Machine learning (ML) is emerging as a powerful approach that has recently shown potential to affect various frontiers of carbon capture, a key interim technology to assist in the mitigation of climate change. In this perspective, we reveal how ML implementations have improved this process in many aspects, for both absorption-and adsorption-based approaches, ranging from the molecular to process level. We discuss the role of ML in predicting the thermody namic properties of absorbents and in improving the absorption process. For adsorption processes, we discuss the promises of ML techniques for exploring many options to find the most cost-effective process scheme, which involves choosing a solid adsorbent and designing a process configuration We also highlight the advantages of ML and the associated risks, elaborate on the importance of the features needed to train ML models, and identify promising future opportunities for ML in carbon capture processes.

ELSEVIER2021

Spectral cola or how to turn green laser light red

Csaba Forro, Thibaud Humair, Xavier François Mettan

A green laser pointer shone through a glass of cola will appear red when looking from above the glass. Lowering the laser so that it is deeper in the cola, the colour will vary from orange to deep red. Fluorescence and absorption spectra were recorded consistent with the reddening of the laser. The depth dependence of the colour of the immersed laser is attributed to a more significant absorption of the short wavelengths by the cola, making the laser look redder as it is lowered deeper into the cola. To confirm this hypothesis, we simulated the fluorescence spectrum we would get at various depths by applying the Lambert law on our measured fluorescence spectrum. These spectra were then converted into red, green and blue (RGB) values. These values were compared to those measured on images of the immersed laser beam at corresponding depths.

Iop Publishing Ltd2014

Defect formation and mitigation during laser powder bed fusion of copper

Carl Tore Viktor Lindström

EPFL2021