Recuit simulé

Résumé

En algorithmique, le recuit simulé est une méthode empirique (métaheuristique) d'optimisation, inspirée d'un processus, le recuit, utilisé en métallurgie. On alterne dans cette dernière des cycles de refroidissement lent et de réchauffage (recuit) qui ont pour effet de minimiser l'énergie du matériau. Cette méthode est transposée en optimisation pour trouver les extrema d'une fonction. Elle a été mise au point par trois chercheurs de la société IBM, S. Kirkpatrick, C.D. Gelatt et M.P. Vecchi en 1983, et indépendamment par V. Černy en 1985. La méthode vient du constat que le refroidissement naturel de certains métaux ne permet pas aux atomes de se placer dans la configuration la plus solide. La configuration la plus stable est atteinte en maîtrisant le refroidissement et en le ralentissant par un apport de chaleur externe, ou bien par une isolation. Recuit réel recuit La description des phénomènes physiques et quantiques liés au processus de recuit s'appuie sur la stat

Source officielle

https://fr.wikipedia.org/wiki/Recuit_simul%C3%A9

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This thesis aims to find, for the first time, a direct relation between the size and morphology of small metallic nanostructures (gold in this case) supported on a metal-oxide surface to their catalytic activity. In this perspective, three main topics have been treated during this thesis. The first part concerns the design and realization of a Scanning Tunneling Microscope (STM) supposed to work over a wide temperature range (4K < T ≤ 300K). This new device replaces an existing solution. The coarse approach of the scanning tip towards the sample is realized by an axial motor based on a sapphire prism gliding on shear piezos in the stick&slip mode. The new STM is very rigid moving the resonance frequencies to higher values with respect to the existing solution. Operation of the motor down to T = 8K has been proven, however topographic imaging has been performed only in the temperature range 77K < T ≤ 300K. The second part of this work focuses on a study of the evolution of the morphology of gold nanoparticles on a TiO2(110) surface. Size-selected clusters Aun+ (n = 5, 7) are deposited at a well defined kinetic energy on the surface held at room temperature. Subsequent annealing of the sample has been performed stepwise. After each temperature increase, the morphology has been determined by STM. The evolution as a function of surface temperature has been studied for two different surface reconstructions, TiO2(110)-(1×1) and TiO2(110)-(2×1). The deposition process leads only to small fragmentation and the morphology is stable up to T = 400K. Further increase of the surface temperature leads to sintering of the particles by Ostwald ripening as shown by an exponential decrease of the island density with temperature. The main topic of this thesis, the correlation between morphology and catalytic activity, is described in the last part of this manuscript. For the first time we are able to relate the onset of CO2 production from CO and O2 to a clear change in the morphology. The catalytic activity of the particles strongly depends on their size and dimensionality. The relative activity per particle has been determined and we find a clear maximum for clusters containing 60 atoms and are 3 to 4 monolayers high. These results are discussed in contrast of literature data on the same but also on different metal-oxide surfaces.

EPFL2009

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Simulated annealing study of the disordered quantum magnet LiHoxEryY1-x-yF4

Thermal and quantum phase transitions of some rare earth compounds (LiErF4, LiYbF4, LiGdF4 and LiTmF4) are established using the mean field theory. These preliminary calculations allowed evidencing the existence of a novel high-field antiferromagnetic phase in LiErF4, and a still unexplained symmetry breaking in LiGdF4. But the discrepancies with experimental results impel a more sophisticated method. We then present analytical and numerical evidence for the validity of an effective approach to the description of the dipolar coupled antiferromagnet LiErF4. We show that the approach, when implemented in mean field calculations, is able to capture both the qualitative and quantitative aspects of the physics of LiErF4 at small external field and low temperature, yielding results that agree with those obtained in the full Hilbert space using mean field theory. This model nevertheless still fails to describe the LiHoF4 system and needs to be improved. We finally use this toy model as a basis for classical Monte Carlo simulations of LiErF4, which allows the calculation of thermodynamical quantities of the system, as well as the evolution of the order parameters as a function of field H and temperature T. These calculations yield results that are much closer to the experiments than those based on the mean field approximation. Although the theoretical critical temperature is still overestimated by 34%, the critical exponents computed from this effective model correspond to those found experimentally.

2012

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Optimal Process Design for Power Production from Wet Waste Biomass by Hydrothermal Gasification coupled to a Solid Oxide Fuel Cell

This report presents a system analysis of the production of electricity from wet waste biomass. A system of Hydrothermal Gasification is coupled with a Hybrid Cycle composed by a SOFC and two Gas Turbines, driven in an inverted Brayton--‐Joule cycle. A thermodynamic optimization approach, based on the system energy integration, is used to analyze several configuration options. The influence of the process design on performances is discussed for the most representative scenario that highlights the key aspects of the system. Optimization results prove that a systematic system design may reaches efficiencies higher than 60%.

2011

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EPFL2009