Phénomène | EPFL Graph Search

Un phénomène est la manière dont une chose, un fait du monde physique (objet, action…), psychique (émotion, pensée…) ou social (produit d'interactions sociales) se manifeste à la sensibilité d'un être vivant. Certains phénomènes de la physique, tels le magnétisme, la lumière ultra-violette, les rayons X, etc. n'affectent pas de la même manière la sensibilité des différents êtres vivants. Le mot phénomène vient du latin , emprunté au grec , « phénomène naturel », dérivé du verbe , « faire paraître, faire voir, rendre visible ». On contraste parfois, notamment Emmanuel Kant qui a fait de la notion un problème philosophique, le phénomène tel qu'il se présente à notre esprit à ce qu'est en soi une chose réellement existante supposée (das Ding an sich). Par exemple, je vois là un arbre (phénomène), mais qu'est-ce qui est réellement ? Nous n'en savons rien, que ce soit intuitivement, scientifiquement ou métaphysiquement. Au phénomène peut aussi s'opposer le noumène au sens également de Ka

Tribological Approach to the Watch Movements

Valentine Agnès Madeleine Magnin

Mechanical watches are complex and sensitive systems that have to maintain a faultless precision over their lifetime. However, this precision can be altered by disturbances occurring during the functioning time, such as friction, lubricant drying, or wear, leading to surface modification. Due to the significant number of contacts, and diversity of materials, geometries, motions, and lubricants, these disturbances and their consequences over time are difficult to control. To deeply understand these phenomena and limit their effect on watches precision over time, a tribological study of watch movements is necessary. The goal of this Ph.D. research is to contribute to the development of a scientific methodology to appraise friction and wear evolution over time in watch movements. To achieve this, a system approach combined with a modelling of the phenomena was used to anticipate the evolution of two critical watch components: the barrel and the escapement. Based on existing theories of lubrication, the performance of the Swiss lever escapement was appraised as a function of well defined geometrical, kinematic, and dynamic parameters. The developed formalism allowed to highlight the relevance of the radii of curvature of the escapement tooth and the anchor pallet on the effectiveness of the fluid lubrication of the Swiss lever escapement. The tribology of the barrel, a brass piece coated with a nickel sublayer and a flash of gold, was then investigated. Wear characterization of real pieces allowed to identify the relevant wear mechanisms at the barrel drum/spring contact. Friction tests were performed to reproduce the observed phenomena in controlled conditions. A predictive law was proposed to estimate the loss of power at the escapement wheel and showed that the wear of the barrel drum can significantly affect the energy efficiency of the watch. Model surfaces composed of a nickel substrate of a specific waviness profile coated with different gold thicknesses were used for further investigation on the wear mechanisms of the gold coating. A simple mechanistic model considering contact pressure, geometrical, and frictional properties was developed to predict the minimum gold thickness necessary to lubricate the system. Two different relations were proposed according to the sliding direction. These relations were used to evaluate possible improvement strategies in terms of materials properties and surface finishing. With the intention to perform further tribological studies of any contacts of a mechanical watch, a methodology was proposed. This methodology stands on three phases: analysis, modelling and application to the watch. This thesis emphasizes the need of a tribological study to better understand the tribological response of a mechanical watch. By combining reviews of literature, experimental observations, and theoretical laws, analytical models were developed to identify critical parameters, materials and design strategies for improving the tribological response of watch components.

EPFL2020

Kinks in the angle resolved photoemission and inelastic x-ray spectra of high temperature superconductors

Jeff Graf

Though the high superconducting transition temperature (Tc) is the most interesting technological aspect of high temperature superconductors, the complex way in which the spin, lattice and electronic degrees of freedom interplay, makes them of the highest scientific interest. This is illustrated by their rich phase diagram characterized by a variety of exotic groundstate including; Mott insulating, pseudogap and high temperature superconductivity. One of the most serious barriers that has prevented our understanding of the mechanism of high temperature superconductors thus far is the lack of information on how low energy Landau quasiparticles result from an organization of real particles. This organization, often colloquially referred to as "dressing", is a fundamental general concept in physics that explains a variety of physical phenomena, such as exotic particle formations and phase transitions. The role of the lattice in this dressing is particularly controversial. Phonons are quanta of lattice vibration energy, and play a crucial role in conventional superconductivity. They provide an attractive interaction allowing the electrons to condensate in superconducting Cooper pairs. However, high temperature superconductivity in the cuprates in achieved through hole doping in an antiferromagnetic Mott insulator. In this case, the antiferromagnetic background, the strong coulomb repulsion and the anisotropic superconducting gap all suggest a marginal role of the phonons. In order to assess experimentally the exact role of the phonon, angle resolved photoemission spectroscopy (ARPES) is the weapon of choice given its unique ability to probe the electronic structure of solids in an energy- and momentum-resolved manner. In this thesis, I will use ARPES to characterize the various form of dressing of the quasiparticles in the high temperature superconductor from the Fermi level all the way to the valence band complex. I'll show that when combined with isotope substitution and inelastic x-rays scattering, the strong electron-phonon interaction can be probed in vivid details. This thesis presents three fundamental results. 1) Using the isotope effect, the important and unusual role of the phonon is demonstrated, as well as the strong interplay between the magnetic and phonic degrees of freedom. 2) Using inelastic scattering, the intriguing interplay between the Fermi surface and the bond stretching phonon is exposed. And 3) By exploring the ARPES data up to high energy, two new energy scales at high binding energy as well as the spectral waterfalls phenomenon are revealed. When all these new elements are considered together, they clearly show the need to consider simultaneously the spin, lattice, Fermi surface topology and electronic degrees of freedom. The spectacular progress in ARPES techniques over the past two decades was critical for these results, and I will present in this thesis our current effort in pushing the techniques even further. In particular I'll present my efforts in building a laser based ARPES setup with a hemispherical analyzer and a spin resolved time of flight analyzer.

EPFL2008