Dispersion (mécanique ondulatoire)

vignette|Dispersion de la lumière blanche au passage d'un dioptre. En mécanique ondulatoire, la dispersion est le phénomène affectant une onde se propageant dans un milieu dit « dispersif », c'est-à-dire dans lequel les différentes longueurs d’onde constituant l'onde ne se propagent pas à la même vitesse. On rencontre ce phénomène pour tous types d'ondes, comme la lumière, le son et les ondes mécaniques (vagues, séismes, etc.). À l'exception du vide, tous les milieux sont dispersifs à des degrés divers. Le phénomène de dispersion possède de multiples origines qui peuvent varier selon le type d'onde. Ondes dans un milieu dispersif Relation de dispersion Une onde sinusoïdale est caractérisée par sa fréquence \nu ou sa pulsation \omega = 2 \pi \nu (en rad/s), et par son vecteur d'onde de norme k=2\pi/\lambda (en rad/m), où \lambda est la longueur d'onde. On observe alors qu'il existe deux vitesses caractéristiques différent

Solitary waves in longitudinally wrinkled and creased helicoids

Florian Paul Robert Maurin

Elastic ribbons subjected to twist and stretch handle multiple morphological instabilities, amongst others, the longitudinally wrinkled and creased helicoids are investigated in the present paper as promising periodic nonlinear waveguides. Modeling the ribbon by isogeometric Kirchhoff-Love shells, the first longitudinal buckling mode is recovered numerically and used into the Bloch-Floquet method to obtain dispersion curves. After analyzing the effects of the buckling pattern on the different wavemodes, it is shown that classical linear axial waves interact with bending ones and become dispersive. Additionally, as buckling involves geometrical nonlinearities, the structure is expected to host stable nonlinear waves. Indeed, clear supersonic rarefaction trains are observed experimentally and their characteristics are found in agreement with the weakly nonlinear Boussinesq model.

Elsevier2017

Precipitation, Characterization, Kinetic, and Thermodynamic Modelling of Synthetic C-S-H Systems (C-S-H, CASH, CSH+$)

Maya Nicole Harris

This thesis investigates the effects of aluminates, sulfates, and heterogeneous substrates on the nucleation and growth of synthetic calcium-silicate-hydrates (C-S-H) produced by dropwise precipitation. The use of synthetic C-S-H, separate from other cementitious phases, allows formation mechanisms and kinetics effects to be investigated in a simplified system. The addition of other ions and substrates then allows an approach to the more complicated, multi-phase system of cement and concrete. The first goal of this thesis was to reproducibly synthesize single-phase C-S-H with Ca:Si from 1 to 2. Guidelines were described for collection, drying, and handling of the precipitates to prevent the formation of Ca(OH)2 during synthesis and minimize carbonation. Interlayer spacing in precipitates was studied by transmission XRD to compare the effects of Ca:Si, relative humidity, and time, using the 14Å tobermorite structure as a baseline for synthetic C-S-H. The growth and nucleation kinetics of C-S-H were then investigated with the combined thermodynamic and population balance equation model (PBEM). The surface characteristics of C-S-H, C-A-S-H, and C-S-H +

were investigated by acoustophoresis to compare the effects of Ca:Si, washing, and dispersion media on zeta potential measurements. Results showed the necessity of full characterization of both solutions and precipitates in order to understand and compare acoustophoresis results. In the final section of the thesis C-S-H, C-A-S-H, and C-S-H +

were grown on the surface of heterogeneous substrates, quartz and calcite, to approach C-S-H growth in real systems. Overall, the results from this thesis provided helpful information on synthetic C-S-H, C-A-S-H, and C-S-H + $ at the full range of Ca:Si molar ratios, particularly ratios above 1.5, as observed in Portland cement. As we get closer to understanding and controlling C-S-H in real systems, this work will help provide insight as to how heterogenous substrates and aluminates introduced by SCMs, in addition to sulfates, commonly added to cement via gypsum, affect the early age strength and kinetics of C-S-H formation.

EPFL2022

The growth by laser ablation and electronic properties of thin cuprate films

Claudia Cancellieri

In my thesis work I have concentrated on the growth and the in-depth analysis of high temperature superconducting thin films with the central aim to elucidate their electronic properties, predominantly by in-situ angle resolved photoemission spectroscopy (ARPES). I have used two somewhat complementary approaches and two laser ablation set-ups. The first one, developed previously in Wisconsin, was used mainly for studies of strained La2-xSrxCuO4 (LSCO) with a transfer to the Scienta analyzer via an appropriate suitcase. The second one, at the EPFL, where I have built a new pulsed laser deposition (PLD) system, was used to optimize the growth of Bi2Sr2-xLaxCuO4 (Bi-2201) and study in-situ ARPES. In-situ ARPES is the most direct tool to probe the electronic structure. We performed it at the Synchrotron Radiation Center (SRC, University of Wisconsin), where we used the aforementioned experimental set-up consisting in a dedicated PLD system coupled with the SCIENTA beamline. The sample transfer procedure assures that the surface quality is preserved on the way to the SCIENTA analyzer. There we studied in detail the effect of strain in LSCO thin films. In a previous work the in-plane compressive strain was studied and the main result was that the Fermi surface (FS) topology changed from hole-like to electron-like. The tensile strained films showed completely different results. ARPES analysis show evidence for a 3-dimensional (3D) electronic dispersion relation in contrast to the strictly 2-dimensional (2D) dispersion observed in all other studied LSCO films. In this thesis this result has been confirmed mapping the FS at different photon energies. We found that the strain related to the thickness of the films, is playing an important role in inducing a 3D dispersion. Furthermore, the 3D parameters, evolve according to the level of strain. Moreover, we observe a staircase structure for different photon energies, revealing both the 3D nature of the electronic dispersion and the quantization of the electron wave vector along the direction normal to the film surface. Taking advantage of the wavevector quantization we were able to determine directly the band parameters and map the FS without using the nearly-free-electron approximation (NFEA). Moreover, introducing an effective anisotropic photoelectron effective mass, related to the local structure of the excited band, improves the use of the NFEA for single photon energy measurements. In parallel, I have built an improved PLD system at the EPFL which can be connected to the SCIENTA analyzer, and which enables us to perform in-situ ARPES measurements at any time rather than only during allocated beamtimes at the synchrotron. We also produced our own targets for the laser ablation and all the films were fully characterized at the EPFL performing X-ray diffraction (XRD), resistivity and magnetic measurements. I analyzed in detail the growth mechanism of Bi-2201 and I investigated the presence of random intergrowths. We developed a model to explain the presence of these polytypes and studied their presence as a function of the deposition parameters and the annealing treatment. The model predicts a very particular spatial distribution of defects: a Markovian-like sequence of displacements along the grow direction, as well as a two-component in-plane correlation function, characteristic of self-organized intercalates. We varied the growth conditions in order to study the presence of intergrowths and to produce single-phase samples. Subsequently, we performed in-situ photoemission experiments on thin films of Bi-2201 films free from intergrowths and we analyzed their FS. This method is successful and can be extended to other related oxide films.

EPFL2008