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Comparison of bulk vs. surface dislocation elastic fields in GaN: a study for non-destructive near-surface electron diffraction techniques
No full textInternational audienceDislocations are linear crystalline defects that induce long-range elastic fields inside materials. Dislocations can significantly alter the positions of atoms in their neighborhood, enabling their detection through near surface electron diffraction techniques, such as electron backscatter diffraction (EBSD), High-Resolution Electron Backscatter Diffraction (HR-EBSD), or electron channeling contrast imaging (ECCI). In this contribution, we explore the effect of free surfaces on dislocation elastic fields, by comparison with bulk signatures, in order to assess the possible influence of free surfaces on dislocation detection and characterization using near surface electron diffraction techniques. To this end, we employ a field dislocation mechanics (FDM) model numerically approximated by a fast Fourier transform (FFT) algorithm, which allows to model any dislocation configuration with the effect of free surfaces. In addition to strain, rotations and stress fields, the model is further used to generate virtual curvature and associated geometrically necessary dislocation (GND) density maps that can be typically measured in EBSD to characterize dislocations. We apply the workflow to threading dislocations (TDs) in GaN semiconductor [0001] layers deposited on a Si substrate. As a model material system featuring mostly straight dislocations, GaN allows for the assessment of the possible applicability of EBSD, HR-EBSD and ECCI to characterize dislocations near the surface. Our findings reveal that the simulated fields can differ significantly between the surface and the bulk, which can lead to potential misinterpretation in dislocation characterization when using experimental surfacebased techniques like EBSD. Additionally, the necessary metrics required for the unambiguous characterization and classification of dislocations are established.[1] BERTIN. N., International Journal of Plasticity, (2019), vol. 122, p. 268-284.[2] DJAKA. K.S et al., Computer Methods in Applied Mechanics and Engineering, (2017), vol. 315, p. 921-942.[3] MANDAL. A et al., Microscopy and Microanalysis, (2024)
A plasticity-induced internal length mean field model based on statistical analyses of EBSD and nanoindentation data
No full textInternational audienceA new plasticity-induced internal length mean field model (ILMF) is developed, based on statistical analyses of geometrically necessary dislocation (GND) densities and total dislocation densities estimated from EBSD and nanoindentation data, respectively. It is applied to a single phase ferritic Al-killed steel, which plastically deforms with the occurrence of heterogeneous intra-granular fields. During tensile tests up to 18 % of overall plastic strain, the deformation maps of GND densities due to intra-granular plastic strain gradients are obtained together with nano-hardness maps. The Nye tensor (or dislocation density tensor) is calculated from the 2D EBSD orientations to estimate the intragranular GND density, while a mechanistic model is used to estimate the intragranular total dislocation density from nano-hardness measurements. These data are quantified as a function of the distance to grain boundaries (GBs) to study the development of such plastic strain gradients in the vicinity of GBs. The novel methodology lies in extracting the evolution law of a single plasticity-induced internal length, denoted , from the statistical analysis of GND and total dislocation densities spatial distribution. Hence, it is introduced as an evolving variable in an elastoviscoplastic self-consistent model (EVPSC) for a two-phase composite as a new internal mean field (ILMF) approach. Both experimentally quantified microstructural internal lengths defined by the mean grain size and the evolving layer , are considered to more realistically describe the macroscopic and phase response in terms of stress, GND density evolution and total dislocation density in each phase. An experiment/model comparison is also discussed regarding GND density evolution with plastic deformation
BUSINESS ENVIRONMENT FAILURES AND FIRM INNOVATIVENESS: DOES FAMILY INVOLVEMENT PLAY A ROLE?
No full textInternational audienceThis study examines the impact of business environment failures on the innovation of family businesses in Morocco, analyzingthe moderating role of family involvement in the company. Based on a sample of 281 family businesses that participated in the 2019 Enterprise Survey conducted by the European Bank for Reconstruction and Development (EBRD), the European Investment Bank (EIB), and the World Bank Group (WBG), the results reveal that institutional failures, such as regulatory constraints and limited access to financial resources, significantly hinder the innovation of family businesses. However, active family involvement in management fosters innovation, while family ownership has no significant effect. The study highlights that family businesses managed by family members develop adaptive and resilient strategies, enabling them to better overcome institutional constraints and seize innovation opportunities in a challenging environment.Cette étude examine l’impact des défaillances de l’environnement des affaires sur l’innovation des entreprises familiales au Maroc, en analysant le rôle modérateur de l’implication familiale dans l’entreprise. À partir d’un échantillon de 281 entreprises familiales ayant participé à l’Enquête Entreprise réalisée en 2019 par la Banque Européenne pour la Reconstruction et le Développement (BERD), la Banque Européenne d’Investissement (BEI) et le Groupe de la Banque Mondiale (WBG), les résultats révèlent que les défaillances institutionnelles, telles que les contraintes réglementaires et l’accès limité aux ressources financières, entravent significativement l’innovation des entreprises familiales. Toutefois, l’implication active des familles dans le management favorise l’innovation, tandis que la propriété familiale n’a pas d’effet significatif. L’étude met en évidence que les entreprises familiales dirigées par des membres de la famille développent des stratégies adaptatives et résilientes leur permettant de mieux surmonter les contraintes institutionnelles et d’exploiter les opportunités d’innovation dans un environnement difficile
De l’Arsenal à l’Innovation. Regards croisés sur le management de l’innovation dans le domaine naval de défense
No full textLiaisons de données tactiques et systèmes de commandement et de contrôle naval : conséquences sur l'organisation du commandement
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Investigation of a constitutive law for the prediction of the mechanical behavior of WEEE recycled polymer blends
No full textInternational audienceThis research focuses on a mechanical study of an acrylonitrile–butadiene–styrene (ABS)/ polycarbonate (PC) blend totally derived from Waste Electrical and Electronic Equipment (WEEE) recycling. First, an experimental work was developed in laboratory for the preparation of different mixtures of ABS/PC blend. Then, mechanical tensile tests were performed on the injected specimens and the stress/strain experimental data were gathered to be used in the modelling part. In order to enable the prediction of the mechanical response of the blend, G’Sell and Jonas constitutive law was considered for this purpose. An optimization method based on the Generalized Reduced Gradient (GRG) nonlinear algorithm was developed to identify the input parameters governing the mechanical model. In addition, an uncertainty parametric study was assessed to qualitatively and quantitatively evaluate the constitutive law sensitivity versus the parameter uncertainty. Monte Carlo simulations were performed and the convergence of the numerical model was proved in terms of means and standard deviation statistical data. The results showed an excellent agreement between the numerical approach and the experiments. Besides, it was highlighted the crucial role of coupling uncertainty parametric study with modelling for accurately describing the mechanical behavior of the blend
Analysis of the Timetable Impact on Energy Consumption of a Subway Line
No full textInternational audienceThis paper analyzes the timetable impact on the energy consumption of a subway line. In most timetable studies, simplified models are used and can lead to misestimation of the braking energy and thus the energy transfer between braking vehicles and accelerating vehicles. In this paper, specific attention is paid to the models of the vehicles, the traction power substation, and the rail supply network to enable an accurate estimation of the energy consumption. The energetic macroscopic representation formalism is used to organize the models of the subsystems so they have the right interactions. The developed model is validated by experimental tests on a real subway line. The error on the global energy consumption is lower than 2.2%. The model is then employed to examine the influence of the vehicle time interval on energy consumption. A 10-second adjustment in this time interval can result in a substantial 22% decrease in energy consumption for the analyzed real subway line
Electromagnetic and Efficiency Trade-offs in GaN and SiC-Based PFC Boost Converters for High-Performance LED Drivers
No full textInternational audienceThe rise of wide bandgap (WBG) semiconductor technologies, such as GaN HEMTs and SiC MOSFETs, is revolutionizing power converter design for LED lighting applications. These devices enable faster switching speeds, enhanced energy efficiency, and more compact converter designs, addressing the growing demands for performance and miniaturization. However, their high-speed switching introduces significant electromagnetic compatibility (EMC) challenges, resulting in increased conducted and radiated emissions. This paper presents a comparative analysis of GaN and SiC technologies implemented within a power factor correction (PFC) boost converter tailored for LED drivers. Using PSIM simulations, the study evaluates both overall efficiency and EMC performance. The results highlight key technology trade-offs and offer design guidance for selecting appropriate WBG components based on EMC constraints and efficiency targets.</div
Large-scale streak instabilities of transitional channel flow
No full textInternational audienceThe emergence of large-scale spatial modulations of turbulent channel flow, as the Reynolds number is decreased, is addressed numerically using the framework of linear stability analysis. Such modulations are known as the precursors of laminar–turbulent patterns found near the onset of relaminarisation. A synthetic two-dimensional base flow is constructed by adding finite-amplitude streaks to the turbulent mean flow. The streak mode is chosen as the leading resolvent mode from linear response theory. In addition, turbulent fluctuations can be taken into account or not by using a simple Cess eddy viscosity model. The linear stability of the base flow is considered by searching for unstable eigenmodes with wavelengths larger than the base flow streaks. As the streak amplitude is increased in the presence of the turbulent closure, the base flow loses its stability to a large-scale modulation below a critical Reynolds-number value. The structure of the corresponding eigenmode, its critical Reynolds number, its critical angle and its wavelengths are all fully consistent with the onset of turbulent modulations from the literature. The existence of a threshold value of the Reynolds number is directly related to the presence of an eddy viscosity, and is justified using an energy budget. The values of the critical streak amplitudes are discussed in relation with those relevant to turbulent flows