Évolution stellaire | EPFL Graph Search

L'évolution d'une étoile, ou évolution stellaire, désigne l'ensemble des phénomènes allant de la formation à la d'une étoile. Elle peut être décomposée en plusieurs phases principales dont la formation de l'étoile, son séjour sur la séquence principale et sa phase finale. Durant sa vie, une étoile émet des particules et des rayonnements électromagnétiques (dont une partie sous forme de rayonnements visibles) grâce à l'énergie dégagée par les réactions de fusion nucléaire produites dans les zones internes de l'étoile. L'analyse spectrale de ces émissions révèle certaines caractéristiques de l'étoile, et par conséquent permet de déterminer le stade d'évolution où elle est parvenue. La plus grande partie de l'existence de l'étoile se passe sur la séquence principale, où elle fusionne de l'hydrogène pour former de l'hélium. Une fois le cœur de l'étoile épuisé en hydrogène, elle quitte la séquence principale pour évoluer vers d'autres stades d'évolution ; le stade ultime étant un objet

Operando X-ray diffraction during laser 3D printing: from machine design to experiments and computer simulations

Samy Hocine

The importance of additive manufacturing, also known as 3D printing, has grown exponentially in the recent years thanks to its high flexibility to manufacture intricate parts. This process can be divided into many different sub-categories, depending on the feedstock form and the source used. However, the principle stays the same: building the part one layer after another. Selective Laser Melting (SLM) is one them, combining between a fine metallic powder bed and a laser beam scanning the surface to selectively fuse the particles together. SLM is already used in the medical field where custom shapes implants are needed, or in the automotive and aerospace industries where it eases the production time of certain parts. SLM exhibits very high heating and cooling rates, up to 105-106 K/s as demonstrated in this work. Therefore, it poses several challenges such as the formation of pores, the accumulation of residual stresses and the creation of metastable phases in the microstructure. Post-processing techniques such as long heat treatments are used to counter the effect of these problems. A better way to address these different issues would be to adapt the processing parameters to reduce or remove the generation of such unwanted features. To do so, it is necessary to understand the underlying causes creating these defects. Ex situ characterisation of SLM-printed parts can be done using microscopy techniques. It consists in exploring the microstructure changes for a certain range of parameter values (laser power, scanning speed, etc). However, this is very time consuming. Simulations could be used as a prediction tool, but the degree of complexity needed here is quite high, which makes them computationally expensive. This PhD thesis is part of the CCMX-Challenge AM3 (Additive Manufacturing and Metallic Microstructures) and is dedicated to the development of a new device reproducing the SLM manufacturing process to characterise the phase evolution of metallic microstructures in real time, or operando, by X-ray diffraction (XRD) techniques. A Ti-6Al-4V (wt%) alloy is used to demonstrate the capabilities of this new device, thanks to its compatibility with the X-ray energies available at the Swiss Light Source (SLS) synchrotron beamlines of the Paul Scherrer Institut (PSI). This dissertation introduces the design of the machine and its software, as well as a characterisation of the printed Ti-6Al-4V samples to determine the optimum printing parameters. To demonstrate the measurement capabilities of the machine, different operando XRD experiments will then be presented, giving new insights into the phase evolution during SLM processing. These observations will be linked to the sample microstructure after printing. Additionally, the unique properties of this setup allow to capture the rapid temperature evolution within the probed materials during the XRD measurement. The obtained temperature profile will be compared to some results derived by Finite Elements Method (FEM) analysis on already existing models established for Ti-6Al-4V.

EPFL2020

Long-range magnetic ordering in Li2MnGeO4 and precursor short-range spin correlations

Artem Korshunov, Irina Safiulina

Long-range magnetic ordering and short-range spin correlations in layered noncentrosymmetric orthogermanate Li2MnGeO4 were studied by means of polarized and unpolarized neutron scattering. The combined Rietveld refinement of synchrotron and neutron powder diffraction data at room temperature within the Pmn2(1) space group allowed us to specify the details of the crystal structure. According to the additional Bragg peaks in low-temperature neutron diffraction patterns a long-range antiferromagnetic ordering with the propagation vector k = (1/2 1/2 1/2) has been found below T-N approximate to 8 K. Symmetry analysis revealed the model of the ground state spin structure within the C(a)c (no. 9.41) magnetic space group. It is represented by the noncollinear ordering of manganese atoms with a refined magnetic moment of 4.9 mu(B)/Mn2+ at 1.7 K, which corresponds to the saturated value for the high-spin configuration S = 5/2. Diffuse magnetic scattering was detected on the neutron diffraction patterns at temperatures just above T-N. Its temperature evolution was investigated in detail by polarized neutron scattering with the following XYZ-polarization analysis. Reverse Monte Carlo simulation of diffuse scattering data showed the development of short-range ordering in Li2MnGeO4, which is symmetry consistent on a small scale with the long-range magnetic state below T-N. The reconstructed radial spin-pair correlation function S(0)S(r) displayed the predominant role of antiferromagnetic correlations. It was found that spin correlations are significant only for the nearest magnetic neighbors and almost disappear at r approximate to 12 angstrom at 10 K. Temperature dependence of the diffuse scattering implies short-range ordering long before the magnetic phase transition. Besides, the spin arrangement was found to be similar in both cases above and below T-N. As a result, an exhaustive picture of the gradual formation of magnetic ordering in Li2MnGeO4 is presented.

2020

The influence of sodium and potassium hydroxide on alite hydration: Experiments and simulations

Caterina Gianocca, Aditya Kumar, Karen Scrivener

The basic nature of alkali hydroxides (NaOH, KOH) when added to mixing water, increases the pH in proportion to the level of salt addition. For alite (impure tricalcium silicate; MIII-Ca3SiO5) hydration, this pH increase accelerates the rate of hydration and reduces the duration of the induction, acceleration and deceleration regimes. This study evaluates alite hydration in solutions of varying compositions and alkalinities (0.1 M. 0.2 M and 0.5 M NaOH and KOH) in context of their heat release behavior and analysis of the solid/liquid phases. The modeling platform, mu ic, is used to simulate, describe and discriminate the impact of the pore solution chemistry and reaction product formation parameters on alite hydration (Bishnoi and Scrivener, 2009 [1]). Numerical predictions of the solid and liquid phase compositions and the heat release response show good agreement with experimental determinations. The simulations indicate that the effects up to the end of the induction period follow directly from a change in the pore solution composition under a solution controlled dissolution mechanism. which leads to the faster precipitation of portlandite. The changes in the main heat evolution peak appear to be related to an increase in the nucleation density of C-S-H in alkali hydroxide solutions. Examination under the SEM did not indicate significant difference in C-S-H morphology and composition in the presence of NaOH/KOH. (C) 2012 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2012