577 research outputs found
Structure of velocity distributions in shock waves in granular gases with extension to molecular gases
International audienceVelocity distributions in normal shock waves obtained in dilute granular flows are studied. These distributions cannot be described by a simple functional shape and are believed to be bimodal. Our results show that these distributions are not strictly bimodal but a trimodal distribution is shown to be sufficient. The usual Mott-Smith bimodal description of these distributions, developed for molecular gases, and based on the coexistence of two subpopulations (a supersonic and a subsonic population) in the shock front, can be modified by adding a third subpopulation. Our experiments show that this additional population results from collisions between the supersonic and subsonic subpopulations. We propose a simple approach incorporating the role of this third intermediate population to model the measured probability distributions and apply it to granular shocks as well as shocks in molecular gases
Structure des ondes de choc dans les gaz granulaires
In different materials such as gases, plasmas and granular material, an object, moving at supersonic speed,compresses and heats the fluid ahead. The shock front is the out-of-equilibrium area, where violent changesin temperature, pressure and density occur. It has a particular structure with notably strongly non-Gaussianparticle velocity distributions, which are difficult to observe. In an important breakthrough in 1951, Mott-Smithdescribes the shock front as a superposition of two states: the initial supersonic gas and the compressed andheated subsonic gas, implying existence of bimodal velocity distributions. Several experiences at high Machnumbers show this overall bimodal structure. However this model does not explain the existence of a surplusof particles with intermediate velocities, between the supersonic and the subsonic gas.This thesis focuses on shock waves in granular gases, where particles undergo only inelastic binary collisions.In these dissipative gases, the granular temperature, reflecting the particle random motion, allows to definethe equivalent to the speed of sound by analogy with molecular gases. The low values of this speed of soundpermit to generate easily shock waves in which each particle can be tracked, unlike molecular gases. The firstpart of this work focuses on the effect of the energy dissipation, due to inelastic collisions, on the shock frontstructure in granular gases. Modifications induced on temperature, density and mean velocity, are captured bya model based on the bimodal hypothesis of Mott-Smith and including energy dissipation. The second part isdevoted to the study of velocity distributions in the shock front. From experiences in granular gases, a trimodaldescription, including an additional intermediate state, is proposed and successfully extended to the velocitydistributions in molecular gases.Dans des milieux tels que les gaz, les plasmas et les milieux granulaires, un objet se dĂ©plaçant Ă des vitessessupersoniques, compresse et chauffe le fluide devant lui, formant ainsi une onde de choc. La zone hors-Ă©quilibreappelĂ©e front dâonde, oĂč ont lieu de brusques variations de tempĂ©rature, pression et densitĂ©, prĂ©sente unestructure particuliĂšre, avec notamment des distributions des vitesses des particules fortement non-gaussienneset difficiles Ă visualiser. Dans une avancĂ©e importante en 1951, Mott-Smith dĂ©crit le front dâonde comme lasuperposition des deux Ă©tats que sont le gaz supersonique initial et le gaz subsonique compressĂ© et chauffĂ©,impliquant ainsi lâexistence de distributions des vitesses bimodales. Des expĂ©riences Ă grands nombres de Machont confirmĂ© cette structure globalement bimodale. Ce modĂšle nâexplique cependant pas la prĂ©sence dâun surplusde particules Ă des vitesses intermĂ©diaires, entre le gaz supersonique et le gaz subsonique.Ce travail de thĂšse porte sur lâĂ©tude des ondes de choc dans les gaz granulaires, oĂč les particules interagissentuniquement par des collisions binaires inĂ©lastiques. Dans ces gaz dissipatifs, la tempĂ©rature granulaire, traduisantlâagitation des particules, permet de dĂ©finir lâĂ©quivalent dâune vitesse du son par analogie aux gaz molĂ©culaires.Les basses valeurs de ces vitesses du son dans les gaz granulaires, permettent de gĂ©nĂ©rer facilement des ondes dechoc dans lesquelles chaque particule peut ĂȘtre suivie, contrairement aux gaz molĂ©culaires. La premiĂšre partie decette Ă©tude porte sur lâeffet de la dissipation dâĂ©nergie, due aux collisions inĂ©lastiques, sur la structure des ondesde choc dans les gaz granulaires. Les modifications induites sur la tempĂ©rature, la densitĂ© et la vitesse moyennemesurĂ©es, sont interprĂ©tĂ©es Ă lâaide dâun modĂšle basĂ© sur lâhypothĂšse bimodale de Mott-Smith et intĂ©grant ladissipation dâĂ©nergie. La deuxiĂšme partie est consacrĂ©e Ă lâinterprĂ©tation des distributions des vitesses dans lefront dâonde. Ă partir des expĂ©riences rĂ©alisĂ©es dans les gaz granulaires, une description trimodale, incluant unĂ©tat intermĂ©diaire supplĂ©mentaire, est proposĂ©e et Ă©tendue avec succĂšs aux distributions des vitesses dans lesgaz molĂ©culaires
Characterization of ferroelectric hafnium/zirconium oxide solid solutions deposited by reactive magnetron sputtering
International audienceThe room temperature deposition of 10 nm-thick ferroelectric hafnium/zirconium oxide, (Hf, Zr)O 2 , thin solid films is achieved with a single hafnium/zirconium, Hf/Zr, alloy target by reactive magnetron sputtering. After rapid thermal annealing (RTA), crystallization of our samples is analyzed by grazing incidence x-ray diffraction. Changing the pressure inside the chamber during deposition leads to grow amorphous or monoclinic phase (m-phase). The authors demonstrate that if the (Hf, Zr)O 2 films are crystallized in the m-phase after deposition, no ferroelectric/orthorhombic phase can be obtained further. On the contrary, when the as-deposited film is amorphous, the ferroelectric/orthorhombic phase appears after the RTA. Published by the AVS. https://doi.org/10.1116/1.506064
Time-resolved PhotoEmission Spectroscopy on a Metal/Ferroelectric Heterostructure
In thin film ferroelectric capacitor the chemical and electronic structure of
the electrode/FE interface can play a crucial role in determining the kinetics
of polarization switching. We investigate the electronic structure of a
Pt/BaTiO3/SrTiO3:Nb capacitor using time-resolved photoemission spectroscopy.
The chemical, electronic and depth sensitivity of core level photoemission is
used to probe the transient response of different parts of the upper
electrode/ferroelectric interface to voltage pulse induced polarization
reversal. The linear response of the electronic structure agrees quantitatively
with a simple RC circuit model. The non-linear response due to the polarization
switch is demonstrated by the time-resolved response of the characteristic core
levels of the electrode and the ferroelectric. Adjustment of the RC circuit
model allows a first estimation of the Pt/BTO interface capacitance. The
experiment shows the interface capacitance is at least 100 times higher than
the bulk capacitance of the BTO film, in qualitative agreement with theoretical
predictions from the literature.Comment: 7 pages, 10 figures. Submitted to Phys. Rev.
Interface Electronic Structure in a Metal/Ferroelectric Heterostructure under Applied Bias
The effective barrier height between an electrode and a ferroelectric (FE)
depends on both macroscopic electrical properties and microscopic chemical and
electronic structure. The behavior of a prototypical electrode/FE/electrode
structure, Pt/BaTiO3/Nb-doped SrTiO3, under in-situ bias voltage is
investigated using X-Ray Photoelectron Spectroscopy. The full band alignment is
measured and is supported by transport measurements. Barrier heights depend on
interface chemistry and on the FE polarization. A differential response of the
core levels to applied bias as a function of the polarization state is
observed, consistent with Callen charge variations near the interface.Comment: 9 pages, 8 figures. Submitted to Phys. Rev.
Azimuthal instability of the radial thermocapillary flow around a hot bead trapped at the water-air interface
We investigate the radial thermocapillary flow driven by a laser-heated
microbead in partial wetting at the water-air interface. Particular attention
is paid to the evolution of the convective flow patterns surrounding the hot
sphere as the latter is increasingly heated. The flow morphology is nearly
axisymmetric at low laser power P. Increasing P leads to symmetry breaking with
the onset of counter-rotating vortex pairs. The boundary condition at the
interface, close to no-slip in the low-P regime, turns about stress-free
between the vortex pairs in the high-P regime. These observations strongly
support the view that surface-active impurities are inevitably adsorbed on the
water surface where they form an elastic layer. The onset of vortex pairs is
the signature of a hydrodynamic instability in the layer response to the
centrifugal forced flow. Interestingly, our study paves the way for the design
of active colloids able to achieve high-speed self-propulsion via vortex pair
generation at a liquid interface
Thérapie cellulaire des maladies musculaires Un avenir à l'aune d'une comparaison des progéniteurs
International audienceCell therapy approaches dedicated to the treatment of dystrophinopathies and involving essentially myoblasts and mesoangioblasts have produced mitigated clinical results. If several types of alternative progenitors have been developed, no standardized comparison has been carried out yet to investigate their regenerative efficacy in vivo, at least at a local level. A comparative study has therefore been designed recently aiming at giving a new impetus to this therapeutic field.Les approches de thĂ©rapie cellulaire des dys- trophinopathies basĂ©es sur lâutilisation de myo- blastes ou de mĂ©soangioblastes se sont traduites par des rĂ©sultats cliniques mitigĂ©s. De nombreux candidats cellulaires alternatifs ont Ă©tĂ© dĂ©crits, mais aucune comparaison standardisĂ©e nâa pu encore Ă©tablir leurs efficacitĂ©s, ne serait-ce quâen vue dâune rĂ©gĂ©nĂ©ration musculaire locali- sĂ©e. Une Ă©tude comparative a donc Ă©tĂ© dĂ©cidĂ©e rĂ©cemment et pourrait permettre de donner un nouvel Ă©lan Ă cette approche
Chemistry and structure of BaTiO3 ultra-thin films grown by different O2 plasma power
International audienceWe present a study of the chemical and atomic properties of 5 nm TiO2-terminated BaTiO3 (001) epitaxialfilms on Nb-doped SrTiO3, as a function of the atomic oxygen plasma power for film growth. Lowerplasma power produces non-stoichiometric films with oxygen vacancies and Ti3+ ions. The larger Ti3+ion radius and the in-plane clamping gives rise to an increase in the out-of-plane lattice parameter.XPS measures the Ti3+ concentration and the concomitant increase in dissociative water uptake in thefilm, giving rise to on-top OH adsorption on surface Ti, proton adsorption on surface oxygen, and a nearsurface Ba-OH environment
Evidence for the formation of two phases during the growth of SrTiO3 on silicon
International audienceEpitaxial SrTiO3 (STO)/Si templates open a unique opportunity for the integration of ferroelectric oxides, such as BaTiO3 on silicon and for the realization of new devices exploiting ferroelectricity. STO itself has been shown as ferroelectric at room temperature when deposited in thin layers on Si, while bulk STO is tetragonal and, thus, ferroelectric below 105 K. Here, we demonstrate the coexistence, at room temperature, of strained cubic and tetragonal phases in thin STO/Si layers. The tetragonal STO phase presents a pronounced tetragonality for thicknesses up to 24 ML. Above this thickness, the strained cubic STO phase starts relaxing while the tetragonal STO phase progressively transits to cubic STO. The origin of the simultaneous formation of these two phases is analyzed and is attributed to oxygen segregation at the early stages of the growth
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