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Comparaison d’approches temporelle et fréquentielle dans la modélisation des vibrations non linéaires à l’origine du crissement roue/rail en courbe
No full textBruit des transports : bruit ferroviaire; GABE - Acoustique du Bâtiment et de l'Environnement: GVB - Vibro acoustique et Contrôle du BruitNational audienceLe crissement en courbe est un bruit tonal gênant produit par les véhicules ferroviaires dans les courbes de faible rayon, principalement attribué au glissement latéral à l'interface roue/rail. Il est souvent étudié en première approche par le biais d'une analyse de stabilité, au cours de laquelle les forces de contact roue/rail sont linéarisées. Cependant, les forces de frottement étant fortement non linéaires, ce type de méthodes ne permet pas de calculer les vibrations auto-entretenues (ou cycles limites) à l’origine du bruit de crissement émis. Pour cela, l'intégration des équations dynamiques dans le domaine temporel (calcul transitoire) est généralement utilisée, mais la question de la dépendance des cycles limites obtenus aux conditions initiales est rarement abordée. La présente étude met en évidence cette influence sur la base d’un modèle vibratoire multimodal de roue déstabilisée par une force de frottement latérale. En complément, une méthode d’équilibrage harmonique avec condensation au contact est proposée pour calculer directement des cycles limites périodiques dans le domaine fréquentiel à partir des mobilités roue/rail. La méthode conduit à un gain considérable en temps de calcul mais en revanche ne permet plus de relier les cycles aux conditions initiales. Les résultats sont comparés à ceux obtenus avec le calcul transitoire et les avantages et inconvénients des deux méthodes sont discutés
R-equivalence on group schemes
No full textInternational audienceWe define R-equivalence for group schemes over a semilocal ring and relate this with rational properties. Two main cases are investigated: tori and isotropic semisimple simply connected group schemes where we show in certain cases that R-equivalence coincide with Karoubi-Villamayor equivalence and is also related to the Kneser-Tits problem in this setting. Finally we construct specialization maps for R-equivalence in the case of regular algebras containing a field
Activation Volumes in Tribochemistry; What Do They Mean and How to Calculate Them?
No full textInternational audienceAnalyzing the effect of pressures and normal and shear stresses on chemical reaction rates, especially those occurring ina contact, remains the subject of controversy in the scientific community. This review article aims to clarify the principlesthat underpin the calculation of reaction rates based on transition-state theory (TST) and of activation volumes using aperturbation method of TST proposed by Evans and Polanyi. The goal is to aid researchers to calculate such tribochemicaland mechanochemical parameters. In this paper, the fundamental ideas behind the calculation of reaction rates in chemistryare outlined. This article describes how TST is used to account for the large numbers of molecules involved in chemicalreactions. The effects of individual stresses, and combination of normal and shear stresses on tribochemical reaction rates canbe understood using a thermodynamics analysis. These concepts are illustrated by two examples of normal-stress-modifiedprocesses from results in the literature: homogeneous-phase Diels–Alder reactions and the surface decomposition of adsorbedmethyl thiolate species on copper. The paper then reviews how to analyze tribochemical processes, which depend on couplednormal and shear stresses, showing that the effective activation volume is composed of multiple elementary-process activationvolumes. Compensation effects, in which the pre-exponential factors and the activation volumes are correlated have beenfound for tribochemical reactions and this arises naturally from the Evans–Polanyi analysis. Finally, the activation volumesthemselves depend on the applied stresses due to molecular distortions and a method for gauging the magnitude of theseeffects is described
Toward Real‐Time Assessment of Infrasound Event Detection Capability Using Deep Learning‐Based Transmission Loss Estimation
No full textInternational audienceAccurate modeling of infrasound transmission loss is crucial for assessing the performance of the International Monitoring System, which monitors compliance with the Comprehensive Nuclear-Test-Ban Treaty by detecting atmospheric explosions. This modeling supports the design and maintenance of the operating monitoring network. State-of-the-art propagation modeling tools enable transmission loss to be finely simulated using atmospheric models. However, the computational cost prohibits the exploration of a large parameter space in operational monitoring applications. To address this, recent studies made use of a deep learning algorithm capable of making transmission loss predictions almost instantaneously. However, the use of nudged atmospheric models leads to an incomplete representation of the medium, and the absence of temperature as an input makes the algorithm incompatible with long-range propagation. In this study, we address these limitations by using both wind and temperature fields as inputs to a neural network, simulated up to 130 km altitude and 4,000 km distance. We exploit convolutional and recurrent layers to capture spatially and range-dependent features embedded in realistic atmospheric models, improving the overall performance. The neural network reaches an average error of 4 dB compared to full parabolic equation simulations and provides epistemic and data-related uncertainty estimates. Its evaluation on the 2022 Hunga Tonga-Hunga Ha'apai volcanic eruption demonstrates its prediction capability using atmospheric conditions and frequencies not included in the training. This represents a significant step toward near real-time assessment of International Monitoring System detection thresholds of explosive sources
Endogenies and linearization in the non-virtually connected case
No full textWe prove a linearization theorem for pre-rings of endogenies acting on a definable abelian group of finite dimension. Observe that no assumptions on the connectivity of A are made. We also prove a similar result when one of the two pre-rings is of quasi-endomorphisms. A corollary of these results is a generalization of Zilber's Field Theorem for finite-dimensional theories
Effects of the physico-chemical properties of amino acids and chemically functionalized surfaces on DIOS-MS analysis
No full textInternational audienceHighlights (85 characters) For initial zwitterionic forms, higher peak intensity at [M+H] + on porous silica. For cations higher peak intensity at [M+H] + with propyldimethylethoxy silane layer. The quantity of available protons controls peak intensity at [M+H] + of zwitterions. The surface interaction energy controls peak intensity at [M+H] + of cations. Water molecules and amino acid inside pores are required to get a peak at [M+H] + .</div
Simulation of the intermetallic compound layer formation in aluminum-steel arc welding
No full textWe investigate in this work the formation of the brittle intermetallic compound layer (IMC) developing at the faying surface during arc welding of aluminum to steel. First, a thermal finite element (FE) model is established to evaluate the full field, transient evolution of temperature in the weld for various welding conditions. Model parameters are calibrated with surface temperature measurements on the real setup in a couple of reference welding configurations. The FE model is then chained to a volume diffusion model and the simulated IMC thicknesses are compared to the experimental ones obtained from various cross-sections of the welded samples. The model turns out to be robust over a wide range of arc currents and welding speeds and properly captures the effect of evolving boundary conditions such as a change in backing plate material from steel to pure copper. The computed transient formation regime of the IMC highlights three stages of growth, delimited by the extrema of temperature variations. The calibrated model opens the way for efficient numerical optimization of welding parameters in order to control IMC formation at the interface in aluminum-steel joints
Mesoporous Silicon with Capacity‐Limited Lithiation: A Strategy for Stable High‐Capacity Lithium‐Ion Anodes
No full textInternational audienceWhile silicon anodes offer a high theoretical capacity for lithium‐ion batteries, they face challenges related to severe volume expansion and mechanical degradation. Herein, mesoporous structuring is systematically coupled with capacity‐limited (partial) lithiation to improve the durability of high‐Si composite anodes (70% Si). Using galvanostatic cycling and ex situ scanning electron microscopy/thickness measurements on bulk‐Si and mesoporous‐Si (PSi) electrodes at four lithiation depths (100%, 66%, 50%, 33%), it is shown that PSi consistently delivers higher capacity retention over 100 cycles and exhibits markedly lower swelling. At 33% lithiation, PSi retains ≈94% of its initial capacity, outperforming bulk Si, and expands ≈141% compared to 315% for bulk Si under the same cycling conditions. These benefits result from the porous framework's ability to accommodate the volume changes and stabilize the solid electrolyte interface (SEI), reducing the risk of fracture and electrical connectivity loss. These findings contribute to the ongoing research on mesoporous silicon materials and support a higher Si fraction in graphite‐containing anodes, delivering greater energy density with maintained SEI stability and low swelling
Enhancing Polarization and Reliability in HZO-Based FTJs Using WOx Electrodes and Interfacial Layers
No full textInternational audienceHfO₂-based ferroelectric devices have gained significant attention due to their excellent scalability andcompatibility with CMOS processes, making them promising for future memory and logic technologies. WhileAtomic Layer Deposition is currently the most widely used method for fabricating these devices, this studyexplores Radio-Frequency sputtering as an alternative, owing to its low carbon contamination, industrialrelevance, and the advantage of room-temperature deposition [1].TiN is commonly used as an electrode in HfO₂-based devices because it facilitates the stabilization of theorthorhombic phase, which is essential for ferroelectricity [2]. However, part of this stabilizationarises from oxygen vacancies induced by TiN’s oxygen scavenging behaviour [3]. While beneficial to someextent, excessive oxygen deficiency can introduce point defects and negatively impact device reliability—especially in ferroelectric tunnel junctions (FTJs), where direct tunnelling is the desired conduction mechanism.To mitigate this issue, we investigate the use of a WOₓ-based electrode or interfacial layer that acts as an oxygenreservoir [4], compensating for the oxygen vacancies caused by TiN. Various device structures were fabricatedusing a ferroelectric Hf0.5Zr0.5O2(HZO) layer, incorporating different sequences of rapid thermalannealing, to study its impact on ferroelectric properties, structure, chemistry, and interfacial states of HZOdevices. Initial electrical measurements showed promising ferroelectric characteristics, with a 2Pr value of 40 μC/cm² in films thinner than 6 nm, indicating suitability for tunnelling studies. Structural characterization using X-ray diffraction was performed to identify phase composition, while X-ray photoelectron spectroscopy was used to analyse chemical states and interfacial behaviour. Electrical characterizations were carried out to further evaluate polarization performance and overall device behaviour.These findings offer valuable insights into optimizing sputtered HZO devices for reliable FTJ applications andpresent a foundation for developing artificial synapses in neuromorphic computing systems.References :[1] Bouaziz, J., et al. (2019) APL Materials, 7(8).[2] Schroeder, U., et al. (2022). Nature Reviews Materials, 7(8), 653-669.[3] Hamouda, W., et al. (2020). Journal of Applied Physics, 127(6).[4] Wang, X., et al. (2024). Advanced Electronic Materials, 10(6), 230079