Structure interne de la Lune | EPFL Graph Search

thumb| right|thumb|320px|Schéma de la structure interne de la Lune La structure interne de la Lune est celle d'un corps différencié composé d'une croûte géochimique distincte, d'un manteau et d'un noyau liquide. Cette structure est le résultat supposé de la cristallisation fractionnée d'un océan lunaire de magma peu de temps après sa formation il y a 4,5 milliards d'années. L'énergie nécessaire à la fusion de la partie extérieure de la Lune est généralement attribuée à un impact géant (un événement qui est supposé être à l'origine du système Terre-Lune) et à l'accrétion postérieure de matière dans l'orbite terrestre. La cristallisation de cet océan de magma aurait donné naissance à un manteau mafique et à une croûte riche en plagioclases. La cartographie géochimique depuis l'espace laisse supposer que la croûte de la Lune est largement composée d'anorthosite, ce qui est cohérent vis-à-vis de l'hypothèse de l'océan de magma. En termes d'éléments, la croûte lunaire est composée princip

Unmoderated molten salt reactors design optimisation for power stability

Andrea Bellè, Axel Guy Marie Laureau, Andreas Pautz

The critical region of unmoderated molten salt reactors consists in a cavity filled with a liquid fuel. The lack of internal structure implies a complex flow structure of the circulating fuel salt. A preliminary core shape optimization has been performed during the EVOL European project to limit recirculation and hotspots. This optimization was based on a Reynolds Averaged Navier Stokes (RANS) approach, but the latter only provides time-averaged values for velocity and temperature. However, the power stability is sensitive to thermal fluctuations induced by the flow turbulence itself, even at steady state without pump flow rate or heat extraction variation. This phenomenon is studied using a Detached Eddies Simulation approach to solve the turbulence in the reactor and get a time dependent temperature distribution and then the reactivity fluctuations. A new geometry is proposed to limit the total power fluctuations from 7.5% for the preconceptual EVOL geometry down to 1.2%. (C) 2022 Published by Elsevier Ltd.

PERGAMON-ELSEVIER SCIENCE LTD2022

Construction of Functional Superhydrophobic Biochars as Hydrogen Transfer Catalysts for Dehydrogenation of N-Heterocycles

Zhaowen Dong, Feng Zhou

To endow metal-free materials with the high catalytic activity that is typically featured by a metal-based catalyst is yet a constant pursuit in the field of catalytic chemistry. In this work, novel functional biochars (DCNs) were prepared from wheat straw for the first time via a simple strategy of reconstructing the catalytic active sites in carbon precursors and subsequent controlled carbonization, which may be further applied in the oxidative dehydrogenation of N-heterocycles with ambient air as the oxidant. Whereas the superhydrophobicity of DCN-850 can effectively remove the only byproduct water to effectively reduce the possible effects of water on the catalyst, it also can decrease mass transfer resistance on active sites, thereby ensuring the reaction with high efficiencies and good generality. Especially, after several reuses, the activity and structures of DCN-850 remained unchanged in the catalytic system with water as a solvent. Furthermore, various characterization technologies and the model reaction were used to investigate the architectural attributes of DCNs, and the results show that there is a positive correlation between the catalytic performance and hydrophobicity of DCNs, as well as reveal that the catalytic active sites may be made up of a five-membered-ring ketone or its enol form and possibly a phenolic unit, which could be encapsulated in internal structures of catalysts and promote the reaction via the recycling of -C-C-OH and -C-C=O groups by the pathway of catalytic hydrogen transfer.

AMER CHEMICAL SOC2021

Thin-film-specific elastic effects in ferroelectric domain structures

Konstantin Shapovalov

An essential feature of ferroelectric thin films is the presence in them of domain structures. In order to efficiently implement ferroelectric films into potential new ferroelectrics-based devices, it is of high interest to understand the behaviour of domain structures. Some properties of ferroelectric domain structures, such as internal structure of domain walls, orientation of domain walls, which were investigated and formulated for bulk systems, are typically believed to be applicable to thin films as well. In this work, importance of thin-film-specific effects on domain structure formation is discussed. It is shown that in thin films nonferroelastic domain walls may interact by elastic means on distances far exceeding the domain wall width, which is sometimes considered as the maximal range of their interaction. Theoretical study of films of practical interest shows that the force of elastic repulsion between the walls can be strong enough to be important during the polarization switching and for control of nonferroelastic domain structures. Further, this work discusses mechanisms of ferroelastic domain wall orientation specific to thin films. Typically, when a ferroelastic wall orients violating the condition of mechanical compatibility of the domains, macroscopic elastic fields appear leading to increase of the energy of the system restraining the deviation of the wall from the compatible state. It is shown that in thin films this increase of the energy can be small enough to allow competition with other contributions to the energy, such as domain wall self-energy, leading to large deviations of the wall orientation from its compatible state. Another investigated mechanism of ferroelastic domain wall orientation is interference of elastic fields generated by an incompatible wall with the film-substrate misfit stresses. A system is investigated where such interference, which leads to reduction of elastic energy of the structure, results in deviation of the wall from its compatible state, violating the domain wall electric neutrality as well. Using this result, the concept of elasticity-driven generation of charged conductive domain walls is developed; the concept is supported by reports of experimental observation of metallic-like domain wall conduction in Pb(Zr,Ti)O3 films. A theoretical description is provided for ferroelastic structures with narrow domain inclusions into dominant domain state. It is obtained that in sparse structures the domain periodicity follows linear dependence on the film thickness, not the classical Kittel-like square root dependence. Equilibrium parameters of the domain structure, such as periodicity of the structure, width of the narrow inclusions, are obtained; the results for Pb(Zr,Ti)O3 films are compared with experimental observations of the films. Finally, antiphase boundary with local ferroelectricity in antiferroelectric is investigated. The mechanical response of such boundary on external electric field is modelled. It is shown that the electric field gradient can be used as the driving force for displacement of the boundary. Conditions for displacement of the boundary sufficient for Scanning Probe Microscopy observations are formulated.

EPFL2016

Thin-film-specific elastic effects in ferroelectric domain structures

Konstantin Shapovalov

EPFL2016