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Mo-enhanced passivity of stainless steels studied at metal/oxide interfaces by DFT atomistic modeling
No full textInternational audienc
Compact segmented meta-liners for enhanced acoustic absorption with grazing flow
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A hybrid memetic metaheuristic for medical staff assignment in major public health emergencies
No full textInternational audienceDuring major public health emergencies, effective assignment of medical staff is crucial for saving lives andcontrolling the spread of epidemics. This work focuses on the assignment of doctors and nurses to hospitals toform treatment groups that carry out patient treatment tasks. We consider the practical constraints of skilltypes of medical staff and the severity of patients’ conditions and propose a mixed integer programmingmodel with the objective of maximizing demand satisfaction and personnel skill matching. To solve thisproblem, we introduce a hybrid memetic search algorithm that combines a specialized crossover operatorfor generating promising offspring solutions and a variable neighborhood search procedure to improvetheir quality. Computational results demonstrate that our algorithm outperforms the general mixed integerprogramming solver GUROBI. The key components of the proposed algorithm are experimentally analyzedand managerial insights are derived
Enhancing thermal management of GaN transistors with direct oil jet impingement: Experimental insights for high-power electronics
No full textInternational audienceThis study experimentally investigates the cooling performance of oil jet impingement on high-power GaN transistors, focusing on the effects of nozzle geometry (diameter: 0.5 mm, 1 mm; nozzle-transistor distance: 5 mm, 10 mm) and properties of two fluids (a NewOil-A and Siloil M40). At a flow rate of 80 mL/min, Siloil M40 achieved a heat transfer coefficient of 13 848 W/m2K, reducing junction temperatures to 120 °C under 38 W power dissipation (corresponding to a heat flux density of 135 W/cm2). In contrast, NewOil-A yielded a junction temperature of 90 °C under identical power and heat flux conditions, demonstrating NewOil-A's superior cooling performance. The optimized system enabled a 198 % improvement in power handling compared to uncooled operation. Smaller nozzles (diameter: 0.5 mm) and reduced nozzle-transistor distance (5 mm) enhanced cooling efficiency. Additionally, a novel correlation linking Nusselt, Reynolds, and Prandtl numbers is proposed, offering practical design guidelines for GaN-based power electronics in applications like electric vehicles
Development and validation of a 2D Mason model for ultrasonic transducer arrays: A simplified approach simulating crosstalk and dissipation mechanisms
No full textInternational audiencePiezoelectric elements arranged in 1D or 2D configurations are commonly used in ultrasonic transducer arrays for medical imaging and non-destructive testing (NDT) applications. These arrays are usually modeled using complex 2D or 3D numerical methods. Based on the equivalent circuit approach previously developed in our paper One-dimensional equivalent circuit for ultrasonic transducer arrays” (Bybi. et al. Applied Acoustics 2019) [1], this work proposes a novel and accurate two-dimensional equivalent circuit to model and simulate the electromechanical behavior of piezoelectric transducer arrays. The model consists of a lossy 2D Mason circuit taking into account the width and thickness modes and also the modes coupling, the crosstalk phenomenon, and various dissipation mechanisms, including dielectric, piezoelectric, and mechanical losses. The model is tested on a fabricated seven elements transducer array made of PZ27 and its results (electrical impedance, displacement, and crosstalk curves) are validated by comparison with experimental data. A good agreement is obtained between simulation results and measurements, confirming the validity of the model. Key advantages of the proposed model include its simplicity, ease of implementation, and a significant reduction in computation time compared to traditional numerical methods. Furthermore, this approach allows for the analysis of the crosstalk between array elements without the need for expensive experiments, such as laser vibrometer displacement measurements. Finally, the model will be extended to a 64-element piezoelectric array, with individual matching layers, backing, and its associated electronics, to deepen the study of the crosstalk phenomenon and propose solutions to reduce it
Phenomena, Ideas and Mechanisms. Introducing the Essays in the History of Physics and Mathematics
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Carleman-Based Reconstruction Algorithm on a wave Network
No full textInternational audienceIn the context of a network of vibrating strings, modelled by interconnected linear partial differential equations, we are interested in the reconstruction of a zeroth order term of each one-dimensional wave equation involved, using some appropriate external boundary measurements. More precisely, we are interested in an inverse problem set on a tree shaped network where each edge behaves according to the wave equation with potential, external nodes have Dirichlet boundary conditions and internal nodes follow the Kirchoff law. The main goal is the reconstruction of the potential everywhere on the network, from the Neumann boundary measurements at all but one external vertices. Leveraging from the Lipschitz stability of this inverse problem, we aim at providing an efficient reconstruction algorithm based on the use of a specific global Carleman estimate. The proof of the main tool and of the convergence of the algorithm are provided; along with a detailed description of the numerical illustrations given at the end of the article
Procédé de réalisation de transistors MOSFET intégrant des cavités d'air pour la réduction du couplage capacitif en régime radiofréquence
No full textNuméro(s) de priorité: FR20230009552 20230911 - Numéro de demande: FR20230009552 2023091
Épitaxie de van der Waals d'hétérostructures à base de TaSe₂ et HfSe₂ sur substrats semi-conducteurs
No full textLayered transition metal dichalcogenides (TMDCs) have attracted a significant attention in last two decades owing to the large variety of properties they offer together with the unique ability of two-dimensional (2D) materials be isolated in atomically thin layers or integrated in complex heterostructures regardless of the lattice mismatch. However, the lack of scalable fabrication techniques has hindered their use in practical applications. Especially, if molecular beam epitaxy (MBE) is a technique capable of growing high-quality semiconductors, the weak coupling between 2D layers represents a serious challenge for epitaxial growth because it favors rotational misalignment and twin formation. This makes the choice of an appropriate substrate an important and yet open question.In this thesis, MBE is employed to grow TaSe₂ on GaP(111)B and GaAs (111)B substrates and HfSe₂ on GaP(111)B and Si(111). These semiconducting three-dimensional (3D) substrates are commercially available, but they require a passivation of their surface prior to 2D material growth. This has been achieved through an annealing under a Se flux for GaP and GaAs, whereas a self-limiting GaSe growth has allowed the deposition on a GaSe half-monolayer on Si. In each case, once the electronic properties and the morphology of the resulting surface has been fully characterized, the influence of the growth parameters during TMDC epitaxy is discussed. The focus on a metallic TMDC (TaSe₂) and a semiconducting one (HfSe₂) finally offers the opportunity to grow a 2D metal/semiconductor heterostructure.In TaSe₂ monolayers, Mott insulating 1T or metallic 1H phase can be selectively grown at high or low substrate temperature, respectively. On the other hand, the stable H polytypes are favored in few-layers, for which the rotational alignment and crystallinity improve with increasing growth temperature. However, a rougher morphology is observed around 400—500 °C, whereas at higher temperature, a Se-deficiency is revealed in the TaSe₂ few-layers accompanied by an interdiffusion with the GaP substrate. Still, this substrate appears more suited than GaAs, which suffers from a large Se incorporation during TaSe₂ growth. The 1T-TaSe₂ monolayers reveal a moiré induced by the different GaP lattice parameter, in addition to the expected √13 × √13 charge density wave and Mott insulating phase stable at room temperature, contrasting with the metallic conductivity of H phases.By contrast, HfSe₂ grown on GaP exhibits only the 1T phase, and its crystallinity is almost independent of the growth conditions, but rather depends on the annealing temperature. The thickness also plays an important role, the small lattice mismatch with GaP inducing a slight strain in thin layers which is relaxed with thickness. On Si substrates, however, HfSe₂ suffers from the instability of the GaSe half-monolayer and the formation of an intermediate SiSeₓ phase. Electronic characterization suggests a n-type doping in thin HfSe₂ layers which is reduced in thicker films, leading to a high resistivity. This n-type character is largely preserved in the TaSe₂/HfSe₂ heterostructures although the latter suffer from a significant intermixing at the interface.Les dichalcogénures de métaux de transition (TMDC) ont suscité un intérêt considérable au cours des deux dernières décennies en raison de la grande variété de propriétés qu'ils offrent, ainsi que de la capacité unique des matériaux bidimensionnels (2D) à pouvoir être isolés en couches atomiquement minces ou intégrés dans des hétérostructures complexes, quel que soit le désaccord de maille. Cependant, le manque de techniques de fabrication à grande échelle a freiné leur emploi pour des applications pratiques. Notamment, si l'épitaxie par jets moléculaires (MBE) est une technique permettant de produire des semi-conducteurs de haute qualité, le faible couplage entre les couches 2D représente un défi majeur pour la croissance épitaxiale, car il favorise le désalignement dans le plan et la formation de macles. Le choix d'un substrat approprié est donc une question importante, mais toujours ouverte.Dans cette thèse, la MBE est utilisée pour faire croître du TaSe₂ sur des substrats de GaP(111)B et GaAs(111)B, et du HfSe₂ sur des substrats de GaP(111)B et Si(111). Ces substrats semi-conducteurs tridimensionnels (3D) sont disponibles commercialement, mais leur surface doit être passivée avant la croissance de matériaux 2D. Ceci a été réalisé par un recuit sous flux de Se pour GaP et GaAs, tandis qu'une croissance autolimitée de GaSe a permis le dépôt d'une demi-monocouche de GaSe sur Si. Dans chaque cas, une fois les propriétés électroniques et la morphologie de la surface obtenue entièrement caractérisées, l'influence des paramètres de croissance lors de l'épitaxie de TMDC est discutée. L'étude d'un TMDC métallique (TaSe₂) et d'un semi-conducteur (HfSe₂) offre enfin la possibilité de développer une hétérostructure métal/semi-conducteur 2D.Dans les monocouches de TaSe₂, la phase isolante de Mott 1T ou la phase métallique 1H peuvent être sélectivement fabriquées à haute ou basse température du substrat, respectivement. En revanche, les polytypes H, plus stables, sont favorisés en multicouches, pour lesquelles l'alignement rotationnel et la cristallinité s'améliorent en augmentant la température de croissance. Cependant, une morphologie plus rugueuse est observée autour de 400-500 °C, tandis qu'à plus haute température, une déficience en Se est révélée dans les multicouches de TaSe₂ accompagnée d'une interdiffusion avec le substrat GaP. Cependant, ce substrat semble plus adapté que GaAs, qui souffre d'une importante incorporation de Se lors de la croissance de TaSe₂. Les monocouches 1T-TaSe₂ présentent un moiré induit par la différence de paramètres de maille par rapport à GaP, en plus de l'onde de densité de charge en √13 × √13 attendue pour ce matériau et de la phase isolante de Mott stable à température ambiante, contrastant avec la conductivité métallique des phases H.En revanche, HfSe₂ préparé sur GaP ne forme que la phase 1T, et sa cristallinité est presque indépendante des conditions de croissance, mais dépend plutôt de la température de recuit. L'épaisseur joue également un rôle important, le faible désaccord de maille avec GaP induisant une légère contrainte dans les couches minces qui se relâche avec l'épaisseur. Sur substrats de silicium, cependant, HfSe₂ souffre de l'instabilité de la demi-monocouche de GaSe et de la formation d'une phase intermédiaire de SiSeₓ. La caractérisation électronique suggère un dopage de type N dans les couches minces de HfSe₂, qui est réduit dans les couches plus épaisses, conduisant à une résistivité élevée. Ce dopage N est largement préservé dans les hétérostructures TaSe₂/HfSe₂, bien que ces dernières subissent une importante interdiffusion à l'interface