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III-Nitride MEMS drum resonators on flexible metal substrates
No full textInternational audienceWe present a simple and efficient process for fabricating III-Nitride (III-N) mechanical resonators on flexible metal substrates. This method combines Van der Waals epitaxy of III-N epilayers with the deposition of a thick metal stressor atop the III-N layers. During thermal treatment, the 30 μm thick metal stressor deposited on a 300 nm AlGaN/500 nm GaN layer grown on a 3 nm two-dimensional hexagonal-Boron Nitride (2D h-BN) release layer, initiates a one-step Self-Lift-Off and Transfer (SLOT) process. This process effectively transfers the III-N heterostructure from the h-BN/Sapphire growth wafer to the flexible metal stressor substrate. Additional local etching of the metal stressor and deposition of front electrodes allow for releasing self-standing III-N layers with integrated actuation. Fabricated III-N MEMS drum resonators were analyzed using optical profilometry and laser Doppler vibrometer, enabling the observation of static deflections and distinct vibration modes. Finite element method (FEM) simulations were also performed to further understand experimental observations and assess the mechanical properties of the released III-N layers, particularly enabling the estimation of stress in the GaN and AlGaN released layers. This straightforward approach not only provides a practical solution for cost-effective III-N MEMS resonators but also ensures flexibility, and crack-free structures
Robust Vision-Based Runway Detection through Conformal Prediction and Conformal mAP
No full textInternational audienceWe explore the use of conformal prediction to provide statistical uncertainty guarantees for runway detection in vision-based landing systems (VLS). Using fine-tuned YOLOv5 and YOLOv6 models on aerial imagery, we apply conformal prediction to quantify localization reliability under user-defined risk levels. We also introduce Conformal mean Average Precision (C-mAP), a novel metric aligning object detection performance with conformal guarantees. Our results show that conformal prediction can improve the reliability of runway detection by quantifying uncertainty in a statistically sound way, increasing safety on-board and paving the way for certification of ML system in the aerospace domain
Hybridization in van der Waals Epitaxy of PtSe 2 /h-BN and PtSe 2 /Graphene Heterostructures
No full textInternational audiencevan der Waals (vdW) heterostructures, which combine bidimensional materials of different properties, enable a range of quantum phenomena. Herein, we present a comparative study between the electronic properties of mono- and bilayer of platinum diselenide (PtSe2) grown on hexagonal boron nitride (h-BN) and graphene substrates using molecular beam epitaxy (MBE). Using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), the electronic structure of PtSe2/graphene and PtSe2/h-BN vdW heterostructures is investigated in a systematic manner. In contrast to PtSe2/h-BN, the electronic structure of PtSe2/graphene reveals the presence of interlayer hybridization between PtSe2 and graphene, which is evidenced by minigap openings in the π-band of graphene. Furthermore, our measurements show that the valence band maximum (VBM) of monolayer PtSe2 is located at the Γ point with different binding energies of about −0.9 and −0.55 eV relative to the Fermi level on h-BN and graphene and substrates, respectively. Our results represent a significant advance in the understanding of electronic hybridization between TMDs and different substrates, and they reaffirm the crucial role of the substrate in any nanoelectronic applications based on van der Waals heterostructures
Technical and environmental assessment of new green iron production strategies using hydrogen
No full textInternational audienceIn order to assess the decarbonization potential and overall environmental benefits of new reduction pathways in the ironmaking industry, using hydrogen to produce Direct Reduced Iron (DRI), a coupled approach combining process simulation for rigorous technical and energy evaluation of iron ore conversion and Life Cycle Assessment (LCA) for environmental assessment was developed and extended to two alternative renewable heating strategies: (i) electric gas heating and (ii) solar reactor heating. The entire hydrogen-based ironmaking process, including conversion in a shaft reactor, gas and solid heating, gas recycling, and electrolysis, was therefore simulated. The hydrogen-based reduction of iron ores in the shaft reactor was modelled using a rigorous reactor model describing the reduction of multi-layer iron ore pellets in countercurrent gas-solid moving beds, with the particularity of representing the dual influence of particle size and temperature on conversion. The remainder of the process including gas recycling and hydrogen production, was simulated using ProSim software. The hydrogen-based green ironmaking scenarios were then compared to MIDREX NG, a leading natural gas-based reduction technology. Hydrogen-based scenarios, powered by the French electricity mix, reduce carbon footprints by 53% for electric gas heating and 57% for solar reactor heating, potentially reaching 82% (-0.79 kgCO2-eq/kgDRI) with low-carbon electricity (hydro, nuclear). Compared to MIDREX NG, the energy requirements of both hydrogen-based scenarios are primarily determined by the use of electricity for hydrogen production, illustrating the importance of hydrogen production for the assessment of future hydrogen-based green ironmaking.</div
Residual Prophet Inequalities
No full textWe introduce a variant of the classic prophet inequality, called \emph{residual prophet inequality} (RPI). In the RPI problem, we consider a finite sequence of nonnegative independent random values with known distributions, and a known integer . Before the gambler observes the sequence, the top values are removed, whereas the remaining values are streamed sequentially to the gambler. For example, one can assume that the top values have already been allocated to a higher-priority agent. Upon observing a value, the gambler must decide irrevocably whether to accept or reject it, without the possibility of revisiting past values. We study two variants of RPI, according to whether the gambler learns online of the identity of the variable that he sees (FI model) or not (NI model). Our main result is a randomized algorithm in the FI model with \emph{competitive ratio} of at least , which we show is tight. Our algorithm is data-driven and requires access only to the largest values of a single sample from the input distributions. In the NI model, we provide a similar algorithm that guarantees a competitive ratio of . We further analyze independent and identically distributed instances when . We build a single-threshold algorithm with a competitive ratio of at least 0.4901, and show that no single-threshold strategy can get a competitive ratio greater than 0.5464
Thermal performance and environmental impact of a wood frame wall enhanced with clay-based plaster: an experimental and simulation analysis
No full textInternational audienceThis study evaluates the thermal performance and environmental impact of wooden frame walls with clay-based plaster by using the complementary properties of both materials. Clay’s moderate density and high thermal capacity enhance the thermal behaviour of wood structures, making it a valuable addition to lightweight constructions. At the material scale, the thermal properties of clay-based plaster were experimentally characterized and integrated into thermal dynamic simulations to assess the influence of internal and external wall configurations on the indoor temperature in the framework of overheating in summer. A life cycle assessment was conducted to compare the environmental impact of three walls: one made of concrete, a wooden frame wall, and another using wood frame wall with clay-based plaster
Impact of isomerism on the photoproduction of carbon monoxide and singlet oxygen by visible-light absorbing rhenium(I) photoCORMs
No full textInternational audiencePhotoactive rhenium(I) complexes have high therapeutic potential, particularly in the fight against cancer and antibiotic-resistant bacteria. They have the particularity of combining several modes of action, including photogeneration of carbon monoxide (CO) and singlet oxygen. However, their design is still to be improved. One limitation is that most of them absorb in the UV range, as is the case for Re-Pyta-TPP, a very attractive photochemically CO-releasing molecule (PhotoCORM). To shift absorption to the visible, an additional nitrogen atom was introduced into the pyridyltriazole bidentate ligand of this complex. The four resulting isomers Re-Ln-TPP (n = 1–4) were thus synthesized and studied from both an experimental and a theoretical point of view. Remarkably, their spectroscopic and photochemical properties strongly depend on the substitution pattern. Three out of four complexes can indeed be excited in the visible. The photodecarbonylation yield of the new complexes was lower than that of Re-Pyta-TPP. Interesting links were found between absorption and photoluminescence data, and photoreactivity. DFT and TD-DFT calculations provided rational bases to these observations, showing that the quantum yield for CO photorelease can be correlated to the potential energy profile for the complexes in their lowest triplet state. The Re-Ln-TPP complexes that absorb and emit at the longest wavelengths have the best singlet oxygen photoproduction. This shows that it is not easy to optimize all the spectroscopic and photochemical properties at once, and that a fair balance must be found between them. This study is another step towards rationally-designed photoCORMs
MAXIMUM NUMBER OF RATIONAL POINTS ON HYPERSURFACES IN WEIGHTED PROJECTIVE SPACES OVER FINITE FIELDS
No full textInternational audienceAn upper bound for the maximum number of rational points on an hypersurface in a projective space over a finite field has been conjectured by Tsfasman and proved by Serre in 1989. The analogue question for hypersurfaces on weighted projective spaces has been considered by Castryck, Ghorpade, Lachaud, O'Sullivan, Ram and the first author in 2017. A conjecture has been proposed there and proved in the particular case of the dimension 2. We prove here the conjecture in any dimension provided the second weight is also equal to one
Thermodynamics of quantum oscillators
No full textIn this work, we present a compact analytical approximation for the quantum partition function of systems composed of quantum oscillators. The proposed formula is general and applicable to an arbitrary number of oscillators described by a rather general class of potential energy functions (not necessarily polynomials). Starting from the exact path integral expression of the partition function, we introduce a time-dependent Gaussian approximation for the potential contribution and, then, invoke a principle of minimal sensitivity to minimize the error. This leads to a system of coupled nonlinear equations whose solution yields the optimal parameters of the gaussian approximation. The resulting approximate partition function accurately reproduces thermodynamic quantities such as the free energy, average energy, and specific heat -even at zero temperature-with typical errors of only a few percent. We illustrate the performance of our approximate formula with numerical results for systems of up to ten coupled anharmonic oscillators. These results are compared to "exact" numerical results obtained via Hamiltonian diagonalization for small systems and Path Integral Monte Carlo simulations for larger one
On defect in finite extensions of valued fields
No full textIn recent decades, the defect of finite extensions of valued fields has emerged as the main obstacle in several fundamental problems in algebraic geometry such as the local uniformization problem. Hence, it is important to identify defectless fields and study properties related to defect. In this paper we study the relations between the following properties of valued fields: simply defectless, immediate-defectless and algebraically maximal. The main result of the paper is an example of an algebraically maximal field that admits a simple defect extension. For this, we introduce the notion of quasi-finite elements in the generalized power series field