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Characterization of ejecta in shock experiments with multiple light scattering
No full textInternational audienceUpon impact, the free surface of a solid metal may eject a cloud of fast and fine particles. Photon Doppler Velocimetry (PDV) is one of the optical diagnostics used to characterize these ejecta. Although the technique provides a direct way to estimate the particle velocities in the single scattering regime, it has been shown that multiple scattering cannot be neglected in real ejecta. Here we derive a model for PDV measurements starting from first principles of wave scattering. We establish rigorously the relationship between the specific intensity and the measured signal, as well as the radiative transport equation (RTE) that describes the evolution of the specific intensity upon scattering and absorption in a dynamic ejecta, including the effects of inelastic scattering and inhomogenities in the optical properties. We also establish rigorously the connection between the Monte Carlo scheme used for numerical simulations and the solution to the RTE. Using numerical simulations, we demonstrate the crucial contribution of multiple scattering to PDV spectrograms as well as the effect of statistical inhomogeneities in particle size distribution. These results could substantially impact the analysis of ejecta by PDV
Molecule Crowding Strategy in Polymer Electrolytes Inducing Stable Interfaces for All‐Solid‐State Lithium Batteries
No full textInternational audienceAbstract All‐solid‐state lithium batteries with polymer electrolytes suffer from electrolyte decomposition and lithium dendrites because of the unstable electrode/electrolyte interfaces. Herein, a molecule crowding strategy is proposed to modulate the Li + coordinated structure, thus in situ constructing the stable interfaces. Since 15‐crown‐5 possesses superior compatibility with polymer and electrostatic repulsion for anion of lithium salt, the anions are forced to crowd into a Li + coordinated structure to weaken the Li + coordination with polymer and boost the Li + transport. The coordinated anions prior decompose to form LiF‐rich, thin, and tough interfacial passivation layers for stabilizing the electrode/electrolyte interfaces. Thus, the symmetric Li–Li cell can stably operate over 4360 h, the LiFePO 4 ||Li full battery presents 97.18% capacity retention in 700 cycles at 2 C, and the NCM811||Li full battery possesses the capacity retention of 83.17% after 300 cycles. The assembled pouch cell shows excellent flexibility (stand for folding over 2000 times) and stability (89.42% capacity retention after 400 cycles). This work provides a promising strategy to regulate interfacial chemistry by modulating the ion environment to accommodate the interfacial issues and will inspire more effective approaches to general interface issues for polymer electrolytes
Simulations atomistiques de la dynamique de mouillage des nanogouttelettes d'eau sur du titane nanotexturé : implications pour les implants médicaux
No full textpublié le 29.10.2024International audienceThe development of high-performance biomedical implants requires a deep understanding of the molecular interactions between water molecules and titanium (Ti) surfaces. In this study, fully atomistic molecular dynamics simulations were used to study the static and dynamic wetting behavior of water nanodroplets on both flat and femtosecond laserinduced nanotextured Ti surfaces. Our findings reveal a clear transition from Wenzel to Cassie-Baxter wetting states as surface roughness increases, significantly affecting droplet spreading. We also observe the damping of nanodroplet vibrations and a roughness-dependent shift toward hydrophobicity, driven by stronger atomic interactions between water molecules and surface atoms. Furthermore, the interaction energy between water droplets and nano-textured Ti surfaces decreases with increasing roughness, reinforcing the observed changes in wettability. The discrepancies observed between classical wetting models and nanoscale behavior emphasize the limitations of current theoretical approaches and the importance of developing more advanced models. This study provides valuable insights into optimizing Ti surface properties for improved implant performance through controlled wettability.Le développement d’implants biomédicaux hautes performances nécessite une compréhension approfondie des interactions moléculaires entre les molécules d’eau et les surfaces en titane (Ti). Dans cette étude, des simulations de dynamique moléculaire entièrement atomistiques ont été utilisées pour étudier le comportement de mouillage statique et dynamique des nanogouttelettes d'eau sur des surfaces de Ti nanotexturées plates et induites par laser femtoseconde. Nos résultats révèlent une transition nette entre les états de mouillage de Wenzel et de Cassie-Baxter à mesure que la rugosité de la surface augmente, affectant de manière significative la propagation des gouttelettes. Nous observons également l’amortissement des vibrations des nanogouttelettes et un déplacement vers l’hydrophobicité en fonction de la rugosité, entraîné par des interactions atomiques plus fortes entre les molécules d’eau et les atomes de surface. De plus, l’énergie d’interaction entre les gouttelettes d’eau et les surfaces de Ti nano-texturées diminue avec l’augmentation de la rugosité, renforçant les changements de mouillabilité observés. Les écarts observés entre les modèles de mouillage classiques et le comportement à l'échelle nanométrique soulignent les limites des approches théoriques actuelles et l'importance de développer des modèles plus avancés. Cette étude fournit des informations précieuses sur l’optimisation des propriétés de surface du Ti pour améliorer les performances des implants grâce à une mouillabilité contrôlée
Classical vs generalized Kirchhoff's law in anisothermal structures
No full textInternational audienceWe quantitatively analyze the thermal emission by anisothermal structures. By comparing the heat flux radiated by simple multilayered systems calculated using the classical Kirchhoff's law to the exact flux derived from a fluctuational-electrodynamics theory, we highlight the limits of validity of the radiometric theory to predict the thermal emission with respect to the geometric configuration, the temperature gradient, and the type of materials and also highlight discrepancies as high as 50%
CTH:YAG : from laser medium to luminescent concentrator
No full textInternational audienceThis work presents what we believe is a new way to use a CTH:YAG crystal for spontaneous emission instead of laser emission. The spontaneous emission is collected in one main direction thanks to a luminescent concentrator configuration. The CTH:YAG is indirectly LED-pumped by a Ce:YAG delivering 3.5 ms pulses at 10 Hz with an energy of 2 J in the visible (550-650 nm). In a configuration optimized for light extraction, the CTH:YAG luminescent concentrator provides a broadband emission between 1.8 µm and 2.1 µm with a unique combination of power (1 W) and brightness (21.2 W/cm 2 /sr) that could be useful for short-wave infrared (SWIR) lighting applications
Real-time acousto-optic imaging using a high peak power long-pulsed illumination
No full textInternational audienceAcousto-optic imaging (AOI) of absorbing objects embedded in highly scattering media remains challenging since the detectable signal suitable for image reconstruction is weak. To increase the detected signals to a level required by live biological applications, we designed a high peak power quasi-continuous laser source based on the coherent combination of two pulsed amplifiers, delivering 100 µs-long pulses with a 9 W peak power at a 100 Hz repetition rate while maintaining an average power below 100 mW; jointly used with a digital holographic detection which maximises the amount of collected signal, we demonstrate for the first time the optical imaging of 2-cm-thick highly scattering media (µ ′ s ∼ 10 cm -1 ) at near video frame rate (0.2 Hz) using Fourier Transform-AOI
VANDOR: Mitigating SEUs into Quantized Neural Networks
No full textInternational audienceEmbedded neural networks are increasingly de- ployed in critical applications, such as avionics and autonomous vehicle control. However, their reliability is challenged by various sources of soft errors, including radiation-induced faults from cosmic ray strikes, leading to Single Event Upsets (SEUs). To en- sure the reliability of such systems, we present a novel hardware- based fault protection strategy tailored for embedded neural networks. The idea is based on mitigating faults by adapting at run-time any erroneous values (parameters, intermediate data) due to SEU towards zero upon fault detection. As neural networks exhibit heterogeneous sensitivity to fault direction, our hardware-based approach triplicates the sign bit (TMR) and uses a Voter block based on logical AND/OR gates to handle fault directionality. Through a comprehensive and exhaustive fault injection study, conducted on a Convolutional Neural Network (CNN) model, implemented on FPGA using fixed-point quantization, we show that our method is applicable to various hardware architectures while optimizing hardware cost, a crucial aspect in the context of embedded systems. Obtained results show that VANDOR protection efficiency is near 90.97% for the LeNet-5 CNN inference model, suitable for an embedded system. Additionally, it significantly reduces area overhead compared to existing approaches
Toward Free Space Local Characterization Method in Microwave
No full textInternational audienceAn electromagnetic jet operating at 30 GHz is examined in this article to detect, image, and characterize a material with high precision. First, we show the jet's characteristics, such as the high intensity of the focused beam, the FWHM below the diffraction limit, and the plane form of the wavefront at the focal point. Secondly, we use these properties to study the electromagnetic jet's ability to detect small objects relative to wavelength. Furthermore, the investigation was also carried out to analyze the spatial resolution of the jet in order to identify two adjacent tiny objects. Finally, we introduced the possibility of the electromagnetic jet to characterize materials locally in the plane wave approximation
Films Sol-Gel de TiO2 micro-nanostructurés par nano impression pour la génération photocatalytique d’hydrogène.
No full textInternational audienceCe travail vise à développer de nouveaux concepts permettant le confinement de la lumière incohérente grâce à des structures micro et nanostructurées à des fins photocalytiques. Deux objectifs ont été définis (1) l’élaboration d’une couche par voie sol-gel de TiO2 nanostructurée sur une grande surface et (2) l’étude des propriétés photocatalytiques en utilisant un outil de caractérisation optique de haute performance en relation avec la production photocatalytique d’H2. Pour y parvenir et, basé sur de précédents résultats1, une méthode a été développée comprenant une simulation électromagnétique pour concevoir les paramètres optimums de la structure photonique ciblée, la préparation de couches sol-gel à base de TiO2, le micro-nanopatterning sur de larges zones2 à l’aide de la technologie « Step-and-Repeat NanoImprint Lithography (NIL) », et la caractérisation optique et photocatalytique du système. Ce procédé permet d’obtenir des motifs complexes et bien contrôlés, adaptable à différents substrats (grands et/ou non conventionnels), sur une large zone permettant des expériences de photocatalyse et avec des coûts de production compatible avec le développement industriel futur. Nous présenterons ici les différentes étapes conduisant à une surface utile pour la génération d’hydrogène par reformage d’alcool
Deterministic Graphene Folds Through Ultra-Fast Laser Nano-texturing : A new approach to Graphene Origami
No full textInternational audienceGraphene exhibits physical and chemical properties that depend on its curvature [1,2], in this study we explore those limitations. Previously the research of creating deterministic or stochastic folds in graphene or n-layer graphene stacks has been approached through several methods. Different techniques have been tested to induce certain curvatures on graphene, such as subjecting the sample to specific environmental conditions. Nevertheless the control of these has yet to be obtained. The goal of the presented work is to create an original method for the realization of such folded graphene surfaces with tunable multiscale texturing through the use of ultra fast lasers. Inspired by the Japanese art of origami, we aimed to transform a flat 2D surface into a 3D structure and have this process open to repeatability. The idea being that with deterministic controlled pre-textured patterns we would gain customizable physical/chemical properties that would help engineer novel nano devices later on[3]. In order to transform a smooth surface into a desired nanostructured surface with a determined and controllable pattern we have used femtosecond lasers. This approach has offered us several advantages such as using tight focusing and spatial shaping for example using Bessel beams to obtain well defined holes in the elastomer at the sub-micrometric scale without relying on periodic surface generation. The advantages of using ultrashort pulses for nanopatterning over other methods are numerous: there is no chemistry involved, we eliminate the need for using in-situ experiments, we gain a lot of processing time, a lot of different patterns at the nanoscale can be attained, which makes it one of the most versatile methods for what we intend.This study brings to light our pioneering approach to create and control folds on a graphene sample, illustrating a deterministic, reversible, and repeatable process of folding and unfolding. In order to comprehensively characterize these results, we employed the use of Atomic Force Microscopy to gain insights into the topological features of the graphene folds that appear under the applied stress to the elastomer. Additionally, Raman Spectroscopy using a distinct focus on the D-band, was utilized to probe the defects (or lack thereof) generated throughout this transformative process. Our findings represent a significant advancement in the precise manipulation of graphene structures, showing promise for how these could be implemented in applications for nanotechnology and material science