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The two-phase flow approach has been the conventional method designed to study the sediment transport rate. Due to the complexity of sediment transport, the precisely numerical models computed from that approach require initial assumptions and, as a result, may not yield accurate output for all conditions. This research work proposes that Machine Learning algorithms can be an alternative way to predict the processes of sediment transport in two-dimensional directions under oscillating sheet flow conditions, by utilizing the available dataset of the SedFoam multidimensional two-phase model. The assessment utilized linear regression and gradient boosting algorithm to analyze the lowest average mean squared error in each case and search for the best partition method based on the domain height of the simulation setup.

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Chloride-induced corrosion of reinforcing steel in concrete is a worldwide problem. In order to predict how chlorides penetrate in concrete and how other ionic species in con-crete pore solution affect the penetration of chlorides, this thesis presents a numerical study on multi-phase modelling of ionic transport in concrete dominated by migration process. There are many advantages in rapid chloride migration test (RCM) method and numeri-cal approach. However, most of models in the literature predicting chloride diffusivity in concrete are diffusion models, which not consider the action of externally applied electric field. In view of this, the specific aim of this thesis is to develop a rational nu-merical migration model to simulate chloride migration tests. By using this model, the diffusion coefficient of chlorides in concrete will be efficiently predicted. Furthermore, other mechanisms of ionic transportation in composite materials can be scientifically in-vestigated in the meantime. In most existing work, researchers tend to use the assumption of electro-neutrality con-dition, which ensures that no external charge can be imported (Bockris and Reddy, 1998), to determine the electrostatic potential within concrete as well as considering a 1-D problem with only one phase structure and single species (i.e. the chlorides) for pre-dicting the ionic migration. In contrast, this thesis presents a number of sets of multi-phase migration models in more than one dimension and uses the Poisson’s equation for controlling the multi-species interactions. By solving both mass conservation and Pois-son’s equations, the distribution profiles of each ionic species and electrostatic potential at any required time are successfully obtained. Some significant factors, i.e. the influ-ence of dimensions, aggregates, interfacial transition zones (ITZs), cracks and binding effect have also been discussed in detail. The results reveal a series of important features which may not be seen from existing numerical models. For quantitative study, this thesis also provides the prediction method of chloride diffu-sivity not only by the traditional stationary diffusion models but also by the migration models presented in the thesis. The obtained results are compared with three proven analytical models, i.e., Maxwell’s model (Dormieux and Lemarchand, 2000), Brug-geman’s equation (Bruggeman’s, 1935) and the lower bound of the effective diffusion coefficient proposed by Li et al. (2012) as well as validated against experimental data sets of an accelerated chloride migration test (ACMT) brought by Yang and Su (2002).

Les technologies de filtres, résonateurs, oscillateurs et capteurs sont des éléments essentiels dans lesindustries des télécommunications, automobile, militaire, médical, etc. Les monocristaux de LiTaO3 (LT) etLiNbO3 (LN) sont les matériaux les plus utilisés pour la fabrication de filtres de radiofréquence à ondesélastiques des téléphones portables, car ils possèdent un facteur de couplage électromécanique (K2) élevé.Cependant, ils présentent une problématique liée à la variation de la fréquence de fonctionnement avec latempérature (CTF), dont la valeur est environ de -40 à -95 ppm/°C. D’autre part, il a été démontré dans lalittérature que les propriétés de LT et LN changent avec la non-stoechiométrie du Li.L’objectif de cette thèse a été l’étude de l’effet de la concentration en Li2O sur la performance desdispositifs acoustiques à ondes élastiques de surface, utilisant comme substrat piézoélectrique desmonocristaux de LT coupe YXl/42 (42 RY-LT) et LN coupe YXl/128 (128 RY-LT). Cette étude vise àl’amélioration du CTF sans la dégradation d’autres propriétés (K2 et pertes d’insertion) dans le cas du 42 RYLTet la stabilité de dispositifs utilisés à haute densité de puissance dans le cas du 128 RY-LN. Tout d’abord,nous avons préparé des monocristaux de LT et LN avec différente concentration en Li2O :48.5-50 mol%, enutilisant la méthode d’équilibration par transport en phase vapeur (VTE). Ensuite, nous avons fabriqué etcaractérisé des dispositifs à ondes élastiques de volume et de surface à base de LT et LN, traités par VTE, afind’étudier l’effet de la non-stoechiométrie de Li et l’effet des domaines ferroélectriques sur leur performance. / The filter technologies, resonators, oscillators and sensors are essential elements fortelecommunications, automotive, military, medical industries. The most of radio frequency surface acousticwave (RF-SAW) filters, present in mobile phones, are based in LiNbO3 (LN) and LiTaO3 (LT) single crystalsbecause they have high electromechanical coupling factor (K2). However, these materials have a problemrelated to the variation of the operating frequency with temperature (TCF), whose value is about -40 to -95ppm / ° C. On the other hand, it has been previously shown in the literature that the physical and structuralproperties of LT and LN change with Li non-stoichiometry, including elastic properties.The aim of this work was the investigation of the impact of Li2O concentration on the performance ofSAW devices based on YXl/42 (42 RY-LT) and YXl/128 (128 RY-LN) single crystals. In the case of 42 RY-LT,we focused in the reduction of TCF without the degradation of other properties (K2 and insertion losses) andin the case of 128 RY-LN crystals we focused in the stability of devices at high power densities. First, singlecrystals of LT and LN with different Li2O concentration: 48.5-50 ml% were prepared, by using the VaporTransport Equilibration (VTE) method. Afterwards, SAW and bulk acoustic wave (BAW) devices based on LTand LN VTE treated crystals, were fabricated and characterized, in order to study the effect of Li nonstoichiometryand the effect of ferroelectric domains on the performance of devices.