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(α+Be) neutron production cross sections trends to improve neutron spectrum calculations using MCMC techniques
No full textInternational audienceThis work presents different methodologies to estimate trends on α+9Be neutron production cross sections to enhance neutron spectrum calculation of alpha beryllium sources. The simulation of an Am-Be source and its comparison with several experimental references are presented. This study highlights the contribution of neutron angular distributions to reduce the gap between experimental and calculated spectra. The use of EXFOR angular distributions shows interesting results for outgoing neutrons between 5 MeV and 10 MeV. Cross sections are re-estimated using a Markov Chain Monte Carlo algorithm. The global methodology is based on a coupling between a Geant4 tool, SaG4n, and a Metropolis-Hastings algorithm. The cross sections adjustments suggest the mis-estimation of the (α, α + n) reaction and partial (α, ni) reactions. In this context, two ways of modeling cross sections are compared: one using Bayesian inference, and one using Gaussian processes
Phenothiazylpropylsulfonate: A High‐Potential Posolyte for Redox‐Flow Batteries: Study of the Instability in the Charged State
No full textInternational audienceThe development of aqueous organic redox‐flow batteries (AORFB) requires finding new posolyte alternatives to the ferrocyanide salts used presently. Among the potential families, phenothiazine has been reported to be of interest if its solubility in aqueous medium is successfully increased. In this article, the phenothiazine propyl sulfonate (PTZPS) is evaluated as a promising high‐potential posolyte for neutral‐medium aqueous redox‐flow battery. It is demonstrated that electrochemical reversibility is highly dependent on pH, with namely a fast capacity loss in neutral medium. Through the preparation of the oxidized form of this molecule, a kinetic study is performed, confirming the crucial role of the electron transfer step between two molecules at this redox state. Even if density functional theory (DFT) calculations of the electron transfer are not successful due to the significant multireference character of the oxidized form's dyad, using MD simulations, the behavior of the oxidized form in various media is qualitatively predicted, including the effect of addition of chaotropic additives
A dive into formal explainable attributions for image classification
No full textInternational audienceIn the field of explainable Artificial Intelligence (xAI), attribution methods provide off-the-shelf tools to explain the decision of a machine-learning model. However, existing methods usually do not correctly reflect the actual model behaviour, and they are difficult to compare, due to a lack of ground truth and/or consensus on what a correct explanation should be. Meanwhile, existing work leveraging formal verification to attribution methods is promising but suffer from scalability issues.In this study, we propose a Formally Grounded Correctness (FGC) metric to measure the correctness of existing attribution methods using formal verification. FGC can be used to rank popular explanation methods according to their correctness, ensuring that explanations are correct with regards to the model decision. To allow the application of FGC on more challenging datasets, we propose to use sound, incomplete algorithms, that trade accuracy for scalability, with existing work on input segmentation. We extensively benchmark FGC on MNIST, GTSRB and a scaled down version of ImageNet with several state-of-the-art provers and show its practical usefulness. FGC surprisingly shows that the baseline saliency heuristic is a strong baseline with regards to correctness, sheding light on some caveats of other popular explanation techniques.</div
Enhanced Electromagnetic Wave Absorption in Mapbi<sub>3</sub> Hybrid Perovskite Through a Defect‐Tunable Green Synthesis
No full textInternational audienceDefect engineering and structure‐property relationship understanding in methylammonium lead iodide (MAPI) hybrid perovskites (HPs) attract significant scientific interest, as synthesis‐related defects may strongly influence intrinsic properties. We have explored a green solvent‐free synthesis—mechanosynthesis—leading to large quantities of MAPI powder with tuneable defect density suitable to study MAPI as an electromagnetic wave absorbing (EMWA) material. A dielectric loss enhancement was revealed at 11.4 GHz (X‐band) for 4 h‐ground MAPI powders (MAPI4h), compared to 30 min‐ground powders (MAPI30) when the particle size was <20 μm. MAPI powders display a fractal microstructure with agglomerates of clusters of (nano)grains (≈80 nm for MAPI4h), consisting further of oriented smaller nanograin (5–10 nm) clusters. A strong reabsorption in smaller particles was evidenced due to a surface‐defective layer. MAPI4h was shown to display a more surface‐defective layer with a higher defect density gradient from surface to (nano)grains core and unique open defects different from those in solution‐processed MAPI. These vacancy‐type surface defects would enhance dipole polarization by stabilizing methylammonium dipoles, thereby increasing permittivity. The improved dispersion of MAPI particles (<20 μm) in polymeric matrixes enhanced the surface effects and effective interactions with electromagnetic waves. This study demonstrated the potential of this green synthesis for producing large amount of HPs and tuning defects, opening new avenues for HPs EMWA application
Exploring Li pathways in Si-C/Gr electrodes and their SEI in Li batteries using Li isotope tracing by MAS NMR and FIB-SIMS
No full textInternational audienceOne promising way for enhancing Li-ion battery performance is the use of silicon-graphite-composite anodes. Even though this complex system has been extensively studied, the lithium dynamics in silicon/graphitecomposite electrode material, as well as at the interfaces, are still not well understood. To investigate lithium mobility and trapping effects in the solid electrolyte interphase (SEI) during lithiation and at rest state in Si-C/Gr composites, we use here a methodology based on combined MAS NMR and FIB-SIMS, both sensitive to lithium isotopic labeling. Labeling electrolyte and Li metal counter electrode with 6Li and SEI with 7Li allows distinguishing the origin of lithium ions within graphite and silicon active materials. Following electrochemical lithiation, the SEI shows the lowest 7Li fraction while it is higher in both Si and graphite, providing a direct observation of the so-called knock-off mechanism for the movement of lithium ions through the SEI. However, the lower 7Li fraction of silicides points out that another mechanism of Li transport, possibly through vehicular mechanism (solvated or desolvated Li), is also at play, enabling a more direct transfer of lithium ions from the electrolyte to silicon particles. In addition, in Open Circuit Voltage (OCV) experiments, Li movement seems to be different in Li-rich and Li-poor silicides, as the Li-rich phase exchanges faster with 6Li enriched electrolyte. The innovative 6/7Li isotope tracing approach described here paves the way for further understanding of the transport properties between electrodes and electrolyte, controlling these phenomena and developing knowledge for the development of high-performance battery technology
Li-Mg alloys as anode for Li batteries in liquid electrolytes: Preparation, shaping, characterization and parametric electrochemical study
No full textInternational audienceLithium-metal batteries offer significant energy density advantages over classical Li-ion technology but their practical implementation is hampered by the poor reversibility of the lithium deposition/stripping processes. To solve this issue, Li-Mg alloys have already been considered as possible alternative and are examined in greater details herein. Homogeneous Li-rich alloys (β-phase) with controlled composition (5–13 at.%Mg) were fabricated and electrochemically reacted with Li to explore the kinetics of the Li-driven β−α phase equilibrium. We showed that the (de)lithiation of the β-phase that occurs at ∼0 Volt vs Li+/Li°, and therefore the most interesting, is strongly limited with the early appearance of Mg-rich α-phase at higher voltages. We demonstrated that this β-domain can be largely extended by using moderate cycling temperature (40 °C), low current density, high initial Li/Mg ratio and rest sequences. Post-cycling observations reveal that repeated (de)alloyings proceed without dendritic growth but rather involve the continuous growth of a porous surface film at the expense of the dense Li-Mg alloy resulting in a sudden drop in capacity when the electrode is becoming completely porous. By limiting the capacity, it is possible to control the progression of this porous layer, improve the efficiency of the Li plating-stripping process, and thus achieve a thousand cycles (0.8 mA.cm-2, 0.8 mAh.cm-2). In full cells, better reversibility is spotted for LiFePO4/Li17Mg compared to LiFePO4/Li cells provided that cycling rate is not pushed over ∼1.2 mAh.cm-2. Overall, we hope these results will provide useful insights for the design of lithium alloy electrodes to improve lithium-metal batteries
ÉVALUATION DE LA SENSIBILITÉ THÉORIQUE D'UN CAPTEUR DE CHAMP ÉLECTRIQUE BASÉ SUR DES CRISTAUX PHOTONIQUES SUR COUCHES MINCES DE NIOBATE DE LITHIUM
No full textInternational audienceLa détection des champs électriques nécessite des capteurs de haute performance en terme de sensibilité et de bande passante ainsi qu'une chaîne de mesure optimisée. Nous présentons un capteur basé sur des cristaux photoniques sur couche mince de niobate de lithium, exploitant une résonance de Fano, avec une sensibilité de 32 V • m -1 • Hz -1 . Dans ce travail, nous proposons une évaluation théorique de la sensibilité du capteur en prenant en compte les paramètres critiques de la chaîne de mesure, afin d'identifier les limites du système actuel et de proposer des pistes d'amélioration. L'analyse montre qu'une amélioration du RIN (Relative Intensity Noise) du laser, passant de -100 dB • Hz -1 à -160 dB • Hz -1 , permettrait de renforcer la sensibilité du capteur d'un facteur 20.MOTS-CLEFS : Capteur de champ électrique ; Sensibilité théorique ; Couche mince de niobate de lithium ;</div
Mechanisms of sodium hydroxide (NaOH)-catalyzed sealing of SiO2/SiO2 direct bonding interfaces
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Tuning Anisotropy Axis Orientation and Dipolar Coupling in Triple-Stranded Dinuclear Dy 3+ Helicates Via Ligand Protonation State
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Seasonal Transition in the Dominance of Photoautotrophic and Heterotrophic Protists in the Photic Layer of a Subtropical Marine Ecosystem
No full textInternational audienceProtists are major functional players in the oceans. Time‐resolved protist diversity and succession patterns remain poorly described in subtropical ecosystems, limiting current understanding of food web dynamics and responses to environmental changes in these major world‐ocean regions. We used amplicon sequencing data and trait‐based annotation to examine the seasonality of planktonic protists in the subtropical Gulf of Aqaba (Red Sea). Temperature and nutrients were the major drivers of succession. We detected marked seasonal shifts in protists. Heterotrophs, including diverse parasitic functional groups, dominated the warm, stratified oligotrophic period spanning spring and summer. By contrast, nutrient influx during deep convective mixing in winter triggered a shift to photoautotrophic communities dominated by a few genera of chlorophytes. Deeper winter mixing resulted in larger blooms at the onset of stratification dominated by diatoms, relative to chlorophytes that prevailed during shallower blooms. This result illustrates the impact of mixing depth on bloom formation and composition. Comparisons with oceanwide rDNA datasets indicate that the oligotrophic protist assemblages from the Gulf resemble those from warm, open oceans. This work provides a detailed assessment of the seasonal switch in dominant trophic functions in protists in phase with nutrient levels in a subtropical planktonic ecosystem