187 research outputs found
Modélisation et simulation de la crise d'ébullition dans les REP à l'échelle CFD
Dans un RĂ©acteur Ă Eau PressurisĂ©e (REP), la chaleur dĂ©gagĂ©e par le combustible nuclĂ©aire est transfĂ©rĂ©e Ă lâeau du circuit primaire, pressurisĂ©e Ă 150 bars pour Ă©viter son Ă©bullition. Cependant, en situation accidentelle, elle peut entrer en rĂ©gime dâĂ©bullition nuclĂ©Ă©e pouvant sâintensifier jusquâĂ atteindre la crise dâĂ©bullition. Ce point de transition quasi instantanĂ© entre lâĂ©bullition nuclĂ©Ă©e et lâĂ©bullition en film entraĂźne la formation dâune couche de vapeur stable sur les crayons combustible, associĂ©e Ă une forte augmentation de leur tempĂ©rature pariĂ©tale crĂ©ant un risque de rupture de leur gaine. La prĂ©diction du flux critique (flux de chaleur auquel se produit la crise dâĂ©bullition) reprĂ©sente donc un enjeu de sĂ»retĂ© majeur et est actuellement rĂ©alisĂ©e Ă lâaide de corrĂ©lations expĂ©rimentales spĂ©cifiques Ă une configuration, nâincluant pas de reprĂ©sentation fine de la physique de lâĂ©bullition. Cette thĂšse sâintĂ©resse Ă la modĂ©lisation de la physique de lâĂ©bullition Ă lâĂ©chelle locale dite « CFD » (Computational Fluid Dynamics), Ă laquelle il est possible de rĂ©aliser des simulations dâĂ©coulements bouillants avec une discrĂ©tisation spatiale de lâordre du millimĂštre. Le code maison NEPTUNE_CFD, proposant une description eulĂ©rienne des Ă©coulements multiphasiques Ă changement de phase, est lâoutil de rĂ©fĂ©rence de EDF R&D pour enquĂȘter sur ces problĂ©matiques aux Ă©chelles locales. Dans un premier temps, des simulations dâĂ©coulements bouillants convectifs en tube vertical sont rĂ©alisĂ©es avec NEPTUNE_CFD. Des comparaisons avec lâexpĂ©rience DEBORA (Ă©coulement bouillant de rĂ©frigĂ©rant R12 en similitude REP sur plusieurs adimensionnels) ont permis une Ă©valuation du code dans des conditions similaires au cas industriel. Les rĂ©sultats obtenus sont globalement en accord avec lâexpĂ©rience, mais prĂ©sentent des Ă©carts notables sur le diamĂštre des bulles et la tempĂ©rature paroi. Cette derniĂšre est calculĂ©e au travers du modĂšle dâĂ©bullition en paroi de NEPTUNE_CFD dit à « Partition du Flux PariĂ©tal » (Heat Flux Partitioning), oĂč le flux appliquĂ© est dĂ©coupĂ© entre plusieurs mĂ©canismes de transfert de chaleur (convection, Ă©vaporation, conduction instationnaire, etc.). Le cĆur des travaux de thĂšse a alors consistĂ© en la construction dâun nouveau modĂšle de Partition du Flux, avec objectif une prise en compte plus fine de la phĂ©nomĂ©nologie de lâĂ©bullition en considĂ©rant notamment le glissement des bulles. Une modĂ©lisation de la dynamique des bulles en paroi a Ă©tĂ© dĂ©veloppĂ©e par une approche mĂ©caniste dĂ©crivant les forces appliquĂ©es sur la bulle. Les formulations de certaines forces (masse ajoutĂ©e, traĂźnĂ©e, etc.) ont Ă©tĂ© rĂ©Ă©valuĂ©es et permettent une prĂ©diction satisfaisante des diamĂštres de dĂ©tachement et des vitesses de glissement Ă basse et haute pression. Le modĂšle de Partition du Flux a Ă©tĂ© complĂ©tĂ© par une Ă©valuation des nombreuses lois de fermetures requises (temps dâattente, densitĂ© de sites de nuclĂ©ation, etc.) par comparaison avec des mesures expĂ©rimentales tirĂ©es de la littĂ©rature. Le nouveau modĂšle ainsi dĂ©veloppĂ© a ensuite Ă©tĂ© validĂ© par comparaison avec des mesures de tempĂ©rature de paroi et implĂ©mentĂ© dans NEPTUNE_CFD. La prĂ©diction du flux critique sâancre en perspective de ces dĂ©veloppements. Des observations expĂ©rimentales rĂ©centes dĂ©crivent la crise dâĂ©bullition Ă lâaide de paramĂštres physiques inclus dans le modĂšle de Partition du Flux. Un critĂšre basĂ© sur la proportion de surface occupĂ©e par les bulles a Ă©tĂ© testĂ© avec lâancien modĂšle de NEPTUNE_CFD et semble proposer un comportement qualitativement cohĂ©rent. Enfin, on sâintĂ©resse Ă une configuration de type tube avec des ailettes de mĂ©lange similaires Ă celles prĂ©sentes en cĆur de REP. Les simulations NEPTUNE_CFD montrent des Ă©carts significatifs Ă lâexpĂ©rience sur la prĂ©diction du taux de vide Ă coeur. Des simulations monophasiques montrent une surestimation de la rotation du liquide, pouvant expliquer la trop grande accumulation de vapeur dans le cas bouillant
High-sensitive MIS structures with silicon nanocrystals grown via solid-state dewetting of silicon-on-insulator for solar cell and photodetector applications
This work reports an original method for the fabrication of
Metal-Isulator-Semiconductor (MIS) structures with silicon nanocrystals (Si
NCs) based active layers embedded in the insulating SiO 2 oxide, for high
performance solar cell and photodetector applications. The Si NCs are produced
via the in situ solid-state dewetting of ultra-pure amorphous
silicon-oninsulator (a-SOI) grown by solid source molecular beam epitaxy
(SSMBE). The size and density of Si NCs are precisely tuned by varying the
deposited thickness of silicon. The morphological characterization carried out
by using atomic force microscopy (AFM) and scanning electron microscopy (SEM)
shows that the Si NCs have homogeneous size with welldefined spherical shape
and densities up to ~10 12 /cm 2 (inversely proportional to the square of
nominal a-Si thickness). The structural investigations by high resolution
transmission electron microscopy (HR-TEM) show that the ultra-small Si NCs
(with mean diameter ~7 nm) are monocrystalline and free of structural defects.
The electrical measurements performed by current versus voltage (I-V) and
photocurrent spectroscopies on the Si-NCs based MIS structures prove the
efficiency of Si NCs to enhance the electrical conduction in MIS structures and
to increase (x10 times) the photocurrent (i.e. at bias voltage V =-1 V) via the
photogeneration of additional electron-hole pairs in the MIS structures. These
results evidence that the Si NCs obtained by the combination of MBE growth and
solid-state dewetting are perfectly suitable for the development of novel high
performance optoelectronic devices compatible with the CMOS technology
Universal dephasing in a chiral 1D interacting fermion system
We consider dephasing by interactions in a one-dimensional chiral fermion
system (e.g. a Quantum Hall edge state). For finite-range interactions, we
calculate the spatial decay of the Green's function at fixed energy, which sets
the contrast in a Mach-Zehnder interferometer. Using a physically transparent
semiclassical ansatz, we find a power-law decay of the coherence at high
energies and zero temperature (T=0), with a universal asymptotic exponent of 1,
independent of the interaction strength. We obtain the dephasing rate at T>0
and the fluctuation spectrum acting on an electron.Comment: 5 pages, 3 figures; minor changes, version as published
Shape relaxation of epitaxial mesa for finite-size strain-engineering
Silicon-Germanium (SiGe) layers are commonly used as stressors in
the gate of MOSFET devices. They are expected to introduce a beneficial stress
in the drift and channel regions to enhance the electron mobility. When
reducing the gate lateral size, one of the major issues is the stress
relaxation which results in a significant decrease in the electron mobility. We
report a new morphological evolution of a strained epitaxial SiGe nanolayer on
a silicon gate (mesa) driven by strain inhomogeneity due to finite-size
effects. Unlike the self-induced instability of strained films, this evolution
arises here due to the elastic inhomogeneity originating from the free
frontiers. We analyze the growth dynamics within the thermodynamic surface
diffusion framework accounting for elasticity and capillarity, the former being
solved in two dimensions thanks to the Airy formalism. The resulting dynamical
equation is solved with a decomposition on eigenmodes, and reveals different
developments depending upon the mesa geometric parameters. Mass transfer occurs
towards the relaxed areas and creates a beading at the nanolayers free surface
with either a W or V shape as a function of time and geometry. The evolution is
then controlled by the proportions of the structure as well as its scale.Comment: 10 pages, 5 figure
Magnetic anisotropy in epitaxial Mn5Ge3 films
High crystalline quality Mn 5 Ge 3 films with thicknesses ranging 4â200 nm have been grown on Ge(111) substrates by solid phase epitaxy. The basal hexagonal plane of Mn 5 Ge 3 is in epitaxy with the Ge(111) plane. Magnetic properties of the films have been investigated as a function of the film thickness and the magnetization curves have been analyzed using a theory that includes a description of magnetic domains in uniaxial thin films. The results clearly indicate the existence of a critical thickness below which the magnetic stripe phase disappears. We have determined the value of this thickness to lie between 10 and 25 nm from the analysis of experimental magnetization curves and the theoretical fit of the in-plane remanent magnetization. Although analogies can be drawn between the behavior observed in our system and that of hcp Co, we have shown that the critical thickness is considerably smaller in Mn 5 Ge 3 ; this has the potential to open new fields of applications for Mn 5 Ge 3 thin films in magnetic recording and spintronics
Hyperuniform monocrystalline structures by spinodal solid-state dewetting
Materials featuring anomalous suppression of density fluctuations over large
length scales are emerging systems known as disordered hyperuniform. The
underlying hidden order renders them appealing for several applications, such
as light management and topologically protected electronic states. These
applications require scalable fabrication, which is hard to achieve with
available top-down approaches. Theoretically, it is known that spinodal
decomposition can lead to disordered hyperuniform architectures. Spontaneous
formation of stable patterns could thus be a viable path for the bottom-up
fabrication of these materials. Here we show that mono-crystalline
semiconductor-based structures, in particular SiGe layers
deposited on silicon-on-insulator substrates, can undergo spinodal solid-state
dewetting featuring correlated disorder with an effective hyperuniform
character. Nano- to micro-metric sized structures targeting specific
morphologies and hyperuniform character can be obtained, proving the generality
of the approach and paving the way for technological applications of disordered
hyperuniform metamaterials. Phase-field simulations explain the underlying
non-linear dynamics and the physical origin of the emerging patterns.Comment: 6 pages, 3 figures, supplementary information (7 pages) enclose
Studying Potential Side Channel Leakages on an Embedded Biometric Comparison System
We study in this work the potential side channel leakages of a hardware biometric comparison system that has been designed for fingerprints.
An embedded biometric system for comparison aims at comparing a stored biometric data with a freshly acquired one without the need to send the stored biometric data outside the system. Here one may try to retrieve the stored data via side channel, similarly as for embedded cryptographic modules where one may try to exploit side channel for attacking the modules.
On one hand, we show that we can find partial information by the means of simple Side Channel Analysis that may help to retrieve the stored fingerprint. On the other hand, we illustrate that reconstructing the fingerprint remains not trivial and we give some simple countermeasures to protect further the comparison algorithm
Enhanced nanoscopy of individual CsPbBr3 perovskite nanocrystals using dielectric sub-micrometric antennas
We demonstrate an efficient, simple, and low-cost approach for enhanced nanoscopy in individual green emitting perovskite (CsPbBr3) nanocrystals via TiO2 dielectric nanoantenna. The observed three- to five-fold emission enhancement is attributed to near-field effects and emission steering promoted by the coupling between the perovskite nanocrystals and the dielectric sub-micrometric antennas. The dark-field scattering configuration is then exploited for surface-enhanced absorption measurements, showing a large increase in detection sensitivity, leading to the detection of individual nanocrystals. Due to the broadband spectral response of the Mie sub-micrometric antennas, the method can be easily extended to electronic transitions in other spectral regions, paving the way for absorption nanoscopy of many different quantum emitters from organic molecules to quantum dots.We demonstrate an efficient, simple, and low-cost approach for enhanced nanoscopy in individual green emitting perovskite (CsPbBr3) nanocrystals via TiO2 dielectric nanoantenna. The observed three- to five-fold emission enhancement is attributed to near-field effects and emission steering promoted by the coupling between the perovskite nanocrystals and the dielectric sub-micrometric antennas. The dark-field scattering configuration is then exploited for surface-enhanced absorption measurements, showing a large increase in detection sensitivity, leading to the detection of individual nanocrystals. Due to the broadband spectral response of the Mie sub-micrometric antennas, the method can be easily extended to electronic transitions in other spectral regions, paving the way for absorption nanoscopy of many different quantum emitters from organic molecules to quantum dots
BAFF, a Novel Ligand of the Tumor Necrosis Factor Family, Stimulates B Cell Growth
Members of the tumor necrosis factor (TNF) family induce pleiotropic biological responses, including cell growth, differentiation, and even death. Here we describe a novel member of the TNF family, designated BAFF (for B cell activating factor belonging to the TNF family), which is expressed by T cells and dendritic cells. Human BAFF was mapped to chromosome 13q32-34. Membrane-bound BAFF was processed and secreted through the action of a protease whose specificity matches that of the furin family of proprotein convertases. The expression of BAFF receptor appeared to be restricted to B cells. Both membrane-bound and soluble BAFF induced proliferation of anti-immunoglobulin Mâstimulated peripheral blood B lymphocytes. Moreover, increased amounts of immunoglobulins were found in supernatants of germinal centerâlike B cells costimulated with BAFF. These results suggest that BAFF plays an important role as costimulator of B cell proliferation and function
Design and performance of the multiplexing spectrometer CAMEA
The cold neutron multiplexing secondary spectrometer CAMEA (Continuous Angle
Multiple Energy Analysis) was commissioned at the Swiss spallation neutron
source SINQ at the Paul Scherrer Institut at the end of 2018. The spectrometer
is optimised for an efficient data collection in the horizontal scattering
plane, allowing for detailed and rapid mapping of excitations under extreme
conditions. The novel design consists of consecutive, upward scattering
analyzer arcs underneath an array of position sensitive detectors mounted
inside a low permeability stainless-steel vacuum vessel. The construction of
the world's first continuous angle multiple energy analysis instrument required
novel solutions to many technical challenges, including analyzer mounting,
vacuum connectors, and instrument movement. These were solved by extensive
prototype experiments and in-house developments. Here we present a technical
overview of the spectrometer describing in detail the engineering solutions and
present our first experimental data taken during the commissioning. Our results
demonstrate the tremendous gains in data collection rate for this novel type of
spectrometer design
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