Absolute magnitude | EPFL Graph Search

Absolute magnitude (M) is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it were viewed from a distance of exactly , without extinction (or dimming) of its light due to absorption by interstellar matter and cosmic dust. By hypothetically placing all objects at a standard reference distance from the observer, their luminosities can be directly compared among each other on a magnitude scale. For Solar System bodies that shine in reflected light, a different definition of absolute magnitude (H) is used, based on a standard reference distance of one astronomical unit. As with all astronomical magnitudes, the absolute magnitude can be specified for different wavelength ranges corresponding to specified filter bands or passbands; for stars a commonly quoted absolute magnitude is the absolute visual magnitude, wh

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

Terrestrial ecotoxicity and effect factors of metals in life cycle assessment (LCA)

Sébastien Haye

Life cycle impact assessment aims to translate the amounts of substance emitted during the life cycle of a product into a potential impact on the environment, which includes terrestrial ecosystems. This work suggests some possible improvements in assessing the toxicity of metals on soil ecosystems in life cycle assessment (LCA). The current available data on soil ecotoxicity allow one to calculate the chronic terrestrial HC50EC50 (hazardous concentration affecting 50% of the species at their EC50 level, i.e. the level where 50% of the individuals of the species are affected) of nine metals and metalloids (As(III) or (V), Be(II), Cr(III) or (VI), Sb(III) or (V), Pb(II), Cu(II), Zn(II) and Ni(II)). Contrarily to what is generally advised in LCIA, the terrestrial HC50 of metals shall not be extrapolated from the aquatic HC50, using the Equilibrium Partitioning method since the partition coefficient (Kd) of metals is highly variable. The experimental ecotoxicology generally uses metallic salts to contaminate artificial soils but the comparison of the EC50 or NOEC obtained for the same metal with different salts reveals that the kind of salt used insignificantly influences these values. In contrast, depending on the metallic fraction of concern, the EC50 may vary, as for cadmium: the EC50 of Folsomia candida, expressed as free Cd in pore water is almost 2.5 orders of magnitude lower than that expressed as total metal. A similar result is obtained with Eisenia fetida, confirming the importance of metals speciation in assessing their impact on soils. By ranking the metals according to the difference between their terrestrial and aquatic HC50 values, two groups are distinguished, which match the hard soft acids and bases (HSAB) concept. This allows to estimate their affinity for soil components and potential toxicity according to their chemical characteristics.

2007

Local excitations in amorphous solids

Wencheng Ji

Amorphous solids are structurally disordered. They are very common and include glasses, colloids, and granular materials, but are far less understood than crystalline solids. Key aspects of these materials are controlled by the presence of excitations in which a group of particles rearranges. This motion can be triggered by (a) quantum fluctuations associated with two-level systems (TLS), which dominate the low temperature properties of conventional glasses and have practical importance on superconducting qubits; by (b) thermal fluctuations associated with activations, which are related to the famous and challenging

glass transition'' problem; or by (c) exerting an external stress or strain associated with shear transformations, which control the plasticity. Hence, it is important to understand how temperature and system preparation determines the density and geometry of these excitations. The possible unification of these excitations into a common description is also a fundamental problem.  These local excitations are thought to have a close relationship with

Quasi-localised modes (QLMs)'' which are present in the low-frequency vibrational spectrum in amorphous solids. Understanding the properties of QLMs and clarifying the relation between QLMs and these local excitations are important to the study of the latter. In this thesis: (1) we provide a theory for the QLMs, D_L(omega) ~ omega^alpha, that establishes the link between QLMs and shear transformations for systems under quasi-static loading. It predicts two regimes depending on the density of shear transformations P(x)~ x^theta (with x the additional stress needed to trigger a shear transformation). If theta>1/4, alpha=4 and a finite fraction of quasi-localised modes form shear transformations, whose amplitudes vanish at low frequencies. If theta

EPFL2021

Defect formation and mitigation during laser powder bed fusion of copper

Carl Tore Viktor Lindström

EPFL2021