Les élèves à risque dans des situations problèmes statistiques : stratégies de résolution et obstacles cognitifs

Dans cet article, nous présentons les résultats d’une recherche visant à explorer les raisonnements statistiques d’élèves à risque de la fin du primaire pendant la résolution de situations problèmes. Les démarches des élèves ont été analysées en mettant en relation leurs stratégies avec les caractéristiques des situations. Les élèves à risque ont proposé des stratégies originales et variées en mobilisant des savoirs pertinents. Quelques passages cruciaux dans l’évolution des raisonnements statistiques sont mis en évidence. Cet article permet également d’envisager le potentiel des situations et de discuter des conditions d’engagement d’élèves à risque dans une tâche de résolution de problèmes statistiques.This article presents the results of a research to explore statistical reasoning of at-risk students at the end of primary school as they solve various problems. The processes used by the students were analyzed by relating their strategies with characteristics of each situation proposed. The at-risk students proposed various and original strategies while mobilizing pertinent knowledge. The authors describe some crucial passages that show the evolution of statistical thinking processes. The authors envisage the potential of situations presented and present a discussion of the conditions in which at-risk students engage in a task requiring the resolution of statistical problems.En este artículo, presentamos los resultados de una investigación que tiene por propósito explorar los razonamientos estadísticos de alumnos de riesgo de final de primaria durante la resolución de situaciones problemas. Las gestiones de aprendizaje de los alumnos fueron analizadas relacionando sus estrategias con las características de las situaciones. Los alumnos de riesgo propusieron estrategias originales y variadas movilizando saberes pertinentes. Se destacan algunos momentos cruciales en la evolución de los razonamientos estadísticos. Asimismo este artículo permite contemplar el potencial de las situaciones y discutir las condiciones de implicación de alumnos de riesgo en una tarea de resolución de problemas estadísticos

Analysis of Multivalley and Multibandgap Absorption and Enhancement of Free Carriers Related to Exciton Screening in Hybrid Perovskites

International audienceSolution-processable metal-halide perovskites recently opened a new route toward low-cost manufacture of photovoltaic cells. Converting sunlight into electrical energy depends on several factors among which a broad absorption across the solar spectrum and attractive charge transport properties are of primary importance. Hybrid perovskites meet such prerequisites, but despite foremost experimental research efforts, their understanding remains scanty. Here we show that in these materials the appropriate absorption and transport properties are afforded by the multibandgap and multivalley nature of their band structure. We also investigate the nature of the photoexcited species. Our analysis suggests exciton screening by collective orientational motion of the organic cations at room temperature, leading to almost free carriers. Molecular collective motion is also expected to couple to carrier diffusion at room temperature. In mixed halides, our interpretation indicates that doping might hinder collective molecular motions, leading to good transport properties despite alloying and local lattice strain

On the entanglement of electrostriction and non-linear piezoelectricity in non-centrosymmetric materials

International audienceAn extended and complete thermodynamical model of third-order electro-elastic coupling is proposed with symmetry analyses and density functional theory (DFT) calculations to evaluate consistently the various linear and non-linear coefficients. It is shown that in non-centrosymmetric materials, electrostrictive and non-linear piezoelectric phenomena are strongly coupled, except for materials crystallizing in a cubic lattice associated to the 432 point group. Thorough numerical results are given for GaN and AlN compounds in the Würtzite structure. Electrostriction dominates, but non-linear elasticity and non-linear piezoelectricity must be taken into account for strain evaluation whereas non-linear piezoelectricity yields a significant correction for electric field

Theoretical insights into multibandgap hybrid perovskites for photovoltaic applications

Copyright 2014 Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.International audienceFollowing pioneering works, the 3D hybrid lead-halide perovskites CH3NH3PbX3 (X=Cl, Br, I) have recently been shown to drastically improve the efficiency of Dye Sensitized Solar Cells (DSSC). It is predicted to open "a new era and a new avenue of research and development for low-cost solar cells ... likely to push the absolute power conversion efficiency toward that of CIGS (20%) and then toward and beyond that of crystalline silicon (25%)" (Snaith, H. J. Phys Chem. Lett. 4, 3623-3630 (2013).). Here, we investigate theoretically the crystalline phases of one of the hybrids relevant for photovoltaic applications, namely CH3NH3PbCl3. Critical electronic states and optical absorption are thoroughly investigated both in the low and high temperature phases. Our findings reveal the dramatic effect of spin orbit coupling on their multiple band gaps. Their physical properties are compared to those of conventional semiconductors, evidencing inversion of band edge states

Comment on “Density functional theory analysis of structural and electronic properties of orthorhombic perovskite CH3NH3PbI3” by Y. Wang et al., Phys. Chem. Chem. Phys., 2014, 16, 1424–1429

International audienceYun Wang et al. used density functional theory (DFT) to investigate the orthorhombic phase of CH3NH3PbI3, which has recently shown outstanding properties for photovoltaic applications. Whereas their analysis of ground state properties may represent a valuable contribution to understanding this class of materials, effects of spin–orbit coupling (SOC) cannot be overlooked as was shown in earlier studies. Moreover, their discussion on optical properties may be misleading for non-DFT-experts, and the nice agreement between experimental and calculated band gap is fortuitous, stemming from error cancellations between SOC and many-body effects. Lastly, Bader charges suggest potential problems during crystal structure optimization

EMA space: a collaborative workspace as collaborative urban ecosystem generator?

International audienceFollowing the industrial background, the necessity of the innovation is found at the level of the urban systems. The innovations are not only researched at the level of the results of the urban projects but also in the processes which see them born and develop. In addition, both the societal evolutions, described by the social and human sciences, and the results of multidisciplinary researches wonder about the involvement of the citizens / users in the process of output of the city. The emergence, for few years, of the Living Labs probably gives new opportunities to lead a reflection on the involvement of the citizens in the urban projects. From then on, it consists in mixing the questionings held by the user-driven open innovation and the stakes of "democratization" of the urban projects. It is in particular what suggests the Lorraine (NIT) Smart Cities Living Lab

Water ice deuteration: a tracer of the chemical history of protostars

Context. Millimetric observations have measured high degrees of molecular deuteration in several species seen around low-mass protostars. The Herschel Space Telescope, launched in 2009, is now providing new measures of the deuterium fractionation of water, the main constituent of interstellar ices. Aims. We aim at theoretically studying the formation and the deuteration of water, which is believed to be formed on interstellar grain surfaces in molecular clouds. Methods. We used our gas-grain astrochemical model GRAINOBLE, which considers the multilayer formation of interstellar ices. We varied several input parameters to study their impact on water deuteration. We included the treatment of ortho- and para-states of key species, including H2, which affects the deuterium fractionation of all molecules. The model also includes relevant laboratory and theoretical works on the water formation and deuteration on grain surfaces. In particular, we computed the transmission probabilities of surface reactions using the Eckart model, and we considered ice photodissociation following molecular dynamics simulations. Results. The use of a multilayer approach allowed us to study the influence of various parameters on the abundance and the deuteration of water. Deuteration of water is found to be very sensitive to the ortho-to-para ratio of H2 and to the total density, but it also depends on the gas/grain temperatures and the visual extinction of the cloud. Since the deuteration is very sensitive to the physical conditions, the comparison with sub-millimetric observation towards the low-mass protostar IRAS 16293 allows us to suggest that water ice is formed together with CO2 in molecular clouds with limited density, whilst formaldehyde and methanol are mainly formed in a later phase, where the condensation becomes denser and colder.Comment: Accepted for publication to A&A. 24 pages, 10 figure

Facial exposure to ultraviolet radiation: predicted sun protection effectiveness of various hat styles

Solar ultraviolet radiation (UVR) doses received by individuals are highly influenced by behavioural and environmental factors. This study aimed at quantifying hats' sun protection effectiveness in various exposure conditions, by predicting UVR exposure doses and their anatomical distributions. A well-defined 3-dimensional head morphology and 4 hat styles (a cap, a helmet, a middle- and a wide-brimmed hat) were added to a previously published model. Midday (12:00-14:00) and daily (08:00-17:00) seasonal UVR doses were estimated at various facial skin zones, with and without hat wear, accounting for each UVR component. Protection effectiveness was calculated by the relative reduction in predicted UVR dose, expressed as a predictive protection factor (PPF). The unprotected entire face received 2.5 times higher UVR doses during a summer midday compared to a winter midday (3.3 vs 1.3 standard erythema dose [SED]) with highest doses received at the nose (6.1 SED). During a cloudless summer day, the lowest mean UVR dose is received by the entire face protected by a wide-brimmed hat (1.7 SED). No hat reached 100% protection at any facial skin zone (PPF <sub>max</sub> : 76%). Hats' sun protection effectiveness varied highly with environmental conditions and was mainly limited by the high contribution of diffuse UVR, irrespective of hat style. Larger brim sizes afforded greater facial protection than smaller brim sizes except around midday when the sun position is high. Consideration of diffuse and reflected UVR in sun educational messages could improve sun protection effectiveness

Quantum confinement and dielectric profiles of colloidal nanoplatelets of halide inorganic and hybrid organic-inorganic perovskites

International audienceQuantum confinement as well as high frequency ε∞ and static εs dielectric profiles are described for nanoplatelets of halide inorganic perovskites CsPbX3 (X = I, Br, Cl) and hybrid organic-inorganic perovskites (HOP) in two-dimensional (2D) and three-dimensional (3D) structures. 3D HOP are currently being sought for their impressive photovoltaic ability. Prior to this sudden popularity, 2D HOP materials were driving intense activity in the field of optoelectronics. Such developments have been enriched by the recent ability to synthesize colloidal nanostructures of controlled size of 2D and 3D HOP. This raises the need to achieve a thorough description of the electronic structure and dielectric properties of these systems. In this work, we go beyond the abrupt dielectric interface model and reach atomic scale description. We examine the influence of the nature of the halogen and of the cation on the band structure and dielectric constants. Similarly, we survey the effect of dimensionality and shape of the perovskite. In agreement with recent experimental results, we show an increase of the band gap and a decrease of ε∞ when the size of a nanoplatelet reduces. By inspecting 2D HOP, we find that it cannot be described as a simple superposition of independent inorganic and organic layers. Finally, the dramatic impact of ionic contributions on the dielectric constant εs is analysed