HAL Portal UPHF (Université Polytechnique Hauts-de-France)
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Pyrochlore nanomineralogy questions the immobility of niobium during tropical weathering
No full textInternational audienceThe origin of the mobility of low-solubility elements such as niobium during supergene weathering remains elusive, despite growing evidence, especially in strongly weathered regolith. In the critical zone, the low mobility of these high field strength elements (HFSEs) is assigned to the resistance of their mineral hosts. This property led to their use as geochemical invariants to quantify mass transfer during alteration. Pyrochlore is a quintessential weathering-resistant mineral accommodating most HFSEs. Here, we investigate the surface alteration layers of pyrochlore using a nanoscale approach within a well-characterized regolith subject to intense weathering over millions of years. In the profile, porous pyrochlore rims exhibit distinct chemistry with higher Th contents. Sharp chemical and textural nanometer-scale interfaces between Pb-rich and Th-rich pyrochlore provide compelling evidence for interface-coupled dissolution-reprecipitation. This demonstrates the alterability of the Nb octahedral framework in pyrochlore and the relative stability of its Th-rich counterpart during tropical weathering. Nanoscale analyses of Ce-rich pyrochlores in two additional lateritic horizons confirm the generality of this mechanism across the deposit. The presence of Nb in secondary nano-cerianite found in manganiferous veins establishes its transport during weathering. These nanomineralogical studies of secondary assemblages provide the first direct evidence of Nb mobility during weathering at the deposit scale, underscoring the need to reappraise HFSE geochemical cycles at Earth’s surface, especially in light of growing mining volumes. This paves the way for further investigations into the alteration of weathering-resistant minerals at the nanoscale, providing new models of mobilization and sequestration through previously undocumented processes
Sierpiński carpet-inspired hierarchical patterning of porous materials for sound absorption
No full textInternational audienceEfficient sound absorption at low and mid frequencies is essential for mitigating anthropogenic noise but remains a scientific and engineering challenge. While porous materials are effective at medium and high frequencies, they typically require considerable thickness to absorb low-frequency sound. Inspired by the Sierpiński carpet, we propose a hierarchical pattern of through-thickness holes in porous materials to enhance low-and mid-frequency absorption. Numerical simulations and impedance tube measurements demonstrate significant performance gains (up to a 46% of improvement in absorption at 500 Hz using 10% less material for a thickness of 80 mm). This enhancement is attributed to the presence of small-scale geometric features that spatially localize the acoustic pressure field, increasing energy dissipation within the porous medium. This hierarchical approach holds promise for developing lightweight, high-performance panels for sound insulation applications, particularly where improved low-and mid-frequency absorption is required without increasing material bulk
A robust determination of the effective thermal conductivity of a multilayer Si3N4/SiO2 stack using multiple heater geometries in the 3-omega method
No full textInternational audienceMultilayer dielectric thin films are fundamental components in modern microelectronic and photonic devices where thermal management is critical. This work presents a robust methodological framework for accurately determining the cross-plane effective thermal conductivity (κeff,⊥) of such complex systems using the 3-omega method. We use a five-layer Si3N4/SiO2 stack fabricated by plasma-enhanced chemical vapor deposition as a case study. The analysis combines experimental data from multiple heater geometries with a 3D finite element method based inverse analysis. We first demonstrate, through a frequency-dependent sensitivity analysis, that a direct multi-parameter fit for intrinsic layer properties is an ill-posed problem. This analysis provides a clear, quantitative justification for adopting a simpler and more robust effective medium model (EMA). The validity and application boundaries of the EMA are then rigorously established through a numerical study on a series of “virtual samples.” Finally, applying this validated framework to our experimental data, the thermal conductivity of a fused silica substrate was determined to be 1.287 ± 0.030 W/(m K), and the effective thermal conductivity of the 1288 nm thick stack was reliably determined to be 0.621 ± 0.008 W/(m K). This work provides not only a key thermophysical property for Si3N4/SiO2 multilayers but also a comprehensive and validated workflow for reliably characterizing complex thin film systems where standard analytical solutions fail
Conception et développement d'étalons de paramètres S pour la caractérisation des nanodispositifs haute fréquence
No full textThis thesis arises from work undertaking to establish reliable and traceable methods for S-parameter measurements of planar circuits. The LNE, in partnership with IEMN, is committed to continually meeting the growing needs of the RF and microwave industry, with a particular focus on the miniaturization of electronic devices.While reducing component size is essential for technological advancement, it also impacts electrical performance. These performance characteristics are assessed using measurement instruments such as vector network analyzers (VNAs), which typically operate with a nominal impedance of 50 Ω. These instruments are optimized for this impedance and are especially sensitive to variations introduced by the device under test.Miniaturization can significantly alter the impedance range, sometimes resulting in extreme values. Measuring a nanocomponent with a VNA therefore introduces challenges related to impedance matching and measurement sensitivity. For example, a high impedance nanodevice might be interpreted by the instrument as either a short or open circuit, or something close to these conditions, making accurate detection difficult and leading to highly noisy measurements.In response to these challenges, LNE is expanding its research on VNA measurement methodologies. The work aims to cover a wider frequency range, target specific impedance domains, and operate at the nanoscale. The objective is to design and develop traceable coplanar nanostructures for vector calibration, supporting an extended impedance range including extreme, low, and high values and to establish associated calibration methods that ensure the metrological verification of the measurement system.This thesis is devoted entirely to the design of new coplanar nanocomponents in a Ground–Signal–Ground (GSG) configuration for VNA calibration, specifically for on-wafer S-parameter measurements.The work addresses the key challenges and advances in RF and microwave on-wafer measurements, with special emphasis on the precise characterization of nanodevices up to 110 GHz. It covers the overall scientific context, motivations, and objectives, the state of the art in RF metrology including the fundamentals of transmission lines and S-parameters RF measurement systems, calibration and de-embedding methods, and uncertainty assessment according to the GUM. A particular focus is placed on on-wafer measurement techniques, the influence of device miniaturization, and issues related to high-impedance measurements.The thesis details the steps involved in designing and fabricating dedicated calibration kits, including the choice of technology, materials, and dimensions, electromagnetic simulations, and validation of calibration algorithms. It also presents the experimental setup for characterizing devices from DC to 110 GHz, measurement protocols, data correction procedures, reproducibility studies, and uncertainty analysis using the TRL method based on both measured and modeled standards.Furthermore, the work focuses on developing new approaches for high-impedance measurements. These approaches are experimentally validated using different calibration kits based on the multiline-TRL method. The new approaches include the well-known Short-Open-Load-Thru (SOLT) method, as well as derivative methods that integrate offsets into the standards, including Short-Short-Short-Thru (SSST), Open-Open-Open-Thru (OOOT), and Highimpedance-Highimpedance-Highimpedance-Thru (HHHT).The thesis concludes by outlining prospects for extending these methods to new applications. Overall, this research makes both theoretical and experimental contributions aimed at improving the accuracy, repeatability, and applicability of RF and microwave on-wafer measurements, particularly for increasingly miniaturized and high-impedance devices.Cette thèse s’inscrit dans le cadre de travaux visant à établir des méthodes fiables et traçables pour la mesure des paramètres S de circuits planaires. Le Laboratoire National de Métrologie et d’Essai (LNE), en partenariat avec l’Institut d’Électronique, de Microélectronique et de Nanotechnologie (IEMN), répond aux besoins croissants de l’industrie des radiofréquences (RF) et des micro-ondes, avec une attention particulière à la miniaturisation des dispositifs électroniques.La réduction de la taille des composants constitue un facteur clé du progrès technologique, mais influence aussi fortement les performances électriques. Celles-ci sont évaluées grâce à des instruments comme les analyseurs de réseaux vectoriels (VNA), optimisés pour une impédance nominale de 50 Ω, et sensibles aux variations introduites par le composant testé.La miniaturisation peut entraîner des plages d’impédance extrêmes. Mesurer un nanocomposant avec un VNA pose donc des défis majeurs d’adaptation et de sensibilité. Ainsi, un dispositif à haute impédance peut être interprété comme un court-circuit, un circuit ouvert ou une condition similaire, compliquant la détection et générant du bruit de mesure.Pour relever ces défis, le LNE développe de nouvelles méthodologies par VNA, visant à élargir la bande de fréquences, cibler des domaines d’impédance spécifiques et travailler à l’échelle nanométrique. Ce travail conçoit et développe des nanostructures coplanaires traçables pour l’étalonnage vectoriel, capables de couvrir une large gamme d’impédances, y compris extrêmes. Il s’agit aussi de mettre en place des méthodes d’étalonnage assurant la vérification métrologique du système de mesure.La thèse est consacrée à la conception de nanocomposants coplanaires en configuration Ground–Signal–Ground (GSG) pour l’étalonnage des VNAs, spécifiquement pour les mesures de paramètres S sur wafer. Elle traite des principaux défis et avancées dans la mesure RF et micro-ondes sur wafer, avec un accent sur la caractérisation précise des nanodispositifs jusqu’à 110 GHz.Le document présente le contexte scientifique, les motivations et objectifs, l’état de l’art en métrologie RF (lignes de transmission, systèmes de mesure de paramètres S), les méthodes d’étalonnage et de de-embedding, ainsi que l’évaluation des incertitudes selon le GUM. Une attention particulière est portée aux techniques sur wafer, à l’influence de la miniaturisation et aux problématiques liées aux mesures à haute impédance.La thèse détaille la conception et la fabrication de kits d’étalonnage : choix technologiques, matériaux, dimensions, simulations électromagnétiques, et validation des algorithmes. Elle présente aussi le dispositif expérimental utilisé pour caractériser les dispositifs de DC à 110 GHz, les protocoles de mesure, les corrections de données, les études de reproductibilité et l’analyse des incertitudes via la méthode TRL, sur la base de standards mesurés et modélisés.Le travail introduit également de nouvelles approches pour les mesures à haute impédance. Validées expérimentalement avec divers kits basés sur la méthode multiline-TRL, elles incluent la méthode Short-Open-Load-Thru (SOLT) et des variantes intégrant des offsets, telles que Short-Short-Short-Thru (SSST), Open-Open-Open-Thru (OOOT) et Highimpedance-Highimpedance-Highimpedance-Thru (HHHT).La thèse se conclut par des perspectives d’extension de ces méthodes à de nouvelles applications. Globalement, cette recherche apporte des contributions théoriques et expérimentales pour améliorer la précision, la répétabilité et l’applicabilité des mesures RF et micro-ondes sur wafer, notamment pour des dispositifs miniaturisés et à haute impédance
On the origin and suppression of parasitic channel in AlN/GaN/AlGaN back barrier RF devices
No full textInternational audienceThis work investigates the origin and physical location of a parasitic conduction path in GaN devices incorporating an AlGaN back barrier (BB) through a combined approach of calibrated Technology Computer-Aided Design (TCAD) simulations and experimental C–V and I–V measurements on Schottky diodes and high electron mobility transistors (HEMTs). The phenomenon was observed to directly depend on the carbon doping in the AlGaN BB, which significantly affects its resistivity. Insufficient doping leads to an uncompensated n-type BB, enabling electron accumulation at the GaN channel/BB interface and the formation of a parasitic charge sheet. Conversely, high carbon concentrations result in a semi-insulating behavior, preventing vertical leakage current and thus effectively suppressing the conduction path. This mechanism, which is not intrinsic to the heterostructure but emerges from inadequate doping compensation, contrasts with previous hypotheses invoking defect-mediated hopping. This study highlights the critical role of BB resistivity, not just polarization design, in determining vertical confinement. These findings offer valuable insights into the optimization of BB doping strategies for leakage control and enhanced electron confinement in next-generation GaN HEMTs
Alcibiade transpose ! De l'épistémologie à la pratique en situation-classe.
No full textNational audienceThis conference interrogates two didactical subjects : Did the Ancients do didactical transpositions ? Which epistemology can we make of it ?The conference underlines the fact that didactical transpositions were used by the Ancient Greeks, thanks to Plato, by the relationship master-student between Socrates and Alcibiades. The conference tends to define de concept of "didactical transposition" and its contemporaries applications.Cette communication interroge deux éléments didactiques : les Anciens transposaient-ils ? Quelle épistémologie faire de la transposition didactique ?L'intervention permet de mettre au jour la pratique pédagogique de la transposition didactique antique, à travers l'oeuvre de Platon, via la relation maître-élève entre Socrate et Alcibiade. L'intervention permet également de définir le concept de "transposition didactique" et ses applications contemporaines
Local stability for a class of piecewise affine Filippov systems
No full textInternational audienceIn this paper the local asymptotic stability problem is investigated for a class of piecewise affine (PWA) Filippov systems. We address a particular case of PWA systems with linear switching surfaces crossing at an equilibrium resulting from sliding dynamics (Filippov/pseudo equilibrium). The PWA systems may also present other unstable equilibria. Sufficient Lyapunov-based conditions are given for characterising the local asymptotic stability of the Filippov equilibrium. Methods are proposed to estimate the domain of attraction
Hopping motion of a particle along a substrate driven by a surface acoustic wave
No full textPression de radiation, streaming, dynamique de bulles; GAPSUS - Acoustique Physique, Sous-Marine et Ultra-SonoreNational audienceManipulation of small objects (particles) lying on vibrating or acoustically excited substrates is of interest for a number of applications such as micro-positioning or acoustic cleaning of surfaces. The objective is to provide a stable directed motion of a particle along a surface supporting a traveling acoustic wave. The wave has a periodic horizontal displacement component that induces friction which, in turn, can result in a horizontal particle drift under certain conditions. These conditions depend on whether the particle stays in permanent contact with substrate or can detach. In the former case, the directed horizontal motion is possible as a sequence of alternating advancing and receding phases, in which normal particle compression is lower and higher, respectively, thus leading to a higher efficiency of tangential motion in the advancing phase. Here we focus on the latter case and look for conditions necessary for efficient tangential motion of a particle that experiences multiple bounces from the surface. This situation can occur when bounces are synchronized with the wave and happen once per wave period each time in the same phase. In this communication, we theoretically describe this effect for a Hertzian particle. To do that, we use a semi-analytical contact model called the method of memory diagrams that represents a generalization of Hertz-Mindlin mechanics for an arbitrary excitation. In the synchronous regime, a typical horizontal displacement curve as a function of time includes an accelerated fragment at the beginning followed by a constant velocity phase. This behavior is in agreement with available experimental data. The effect appears for a wide range of system parameters and is therefore promising for practical applications
Pinces acoustiques hautes fréquences pour la manipulation d'objets micrométriques
No full textPression de radiation, streaming, dynamique de bulles; GAPSUS - Acoustique Physique, Sous-Marine et Ultra-SonoreNational audienceLes pinces acoustiques sont l’équivalent des pinces optiques, elles utilisent un faisceau focalisé pour piéger en trois dimension un objet à l’aide de la pression de radiation. Ces dernières ont permis de faire des manipulation d’objets micro-métrique avec une précision nano-métrique. Néanmoins une de leurs principales applications a été la mesure de l’élasticité à l’échelle moléculaires. Elles sont aussi utilisées pour des cellule biologiques dans le cadre de la méchano-transduction. Cependant, les forces appliquées par les pinces optiques y sont souvent insuffisantes. Les pinces acoustiques devraient permettre de répondre à ce besoin si leur fréquence de fonctionnement, i.e la longueur d’onde, est compatible avec les tailles cellulaires. Pour ceci de nouvelles stratégies doivent être développées pour créer un dispositif efficace de génération du faisceau ultrasonore. En général la vitesse du son est plus rapide dans l’objet piégé que dans le fluide environnant. Dans ces conditions, le piège doit avoir un zéro de pression au centre qui peut être réalisé avec un faisceau acoustique focalisé qui comporte en plus une structure hélicoïdale. Je présenterai une stratégie pour créer un tel dispositif puis des expériences à 10 MHz, une fréquence adaptée pour sélectionner, piéger et déplacer une bille de silice de 60µm de diamètre. En optique, le déplacement du piège se fait à l’aide d’un déplacement mécanique, en général un miroir. Pour les fréquences utilisées en acoustique , il est aisé de changer la fréquence de fonctionnement qui est définie par le générateur de fonction qui alimente le piézoélectrique. Cette caractéristique offre une opportunité pour réaliser des déplacements avec une précision inégalée
