A many-body interatomic potential for ionic systems: application to MgO

An analytic representation of the short-range repulsion energy in ionic systems is described that allows for the fact that ions may change their size and shape depending on their environment. This function is extremely efficient to evaluate relative to previous methods of modeling the same physical effects. Using a well-defined parametrization procedure we have obtained parameter sets for this energy function that reproduce closely the density functional theory potential energy surface of bulk MgO. We show how excellent agreement can be obtained with experimental measurements of phonon frequencies and temperature and pressure dependences of the density by using this effective potential in conjunction with ab initio parametrization.Comment: To appear in Journal of Chemical Physics (Oct 15th 2003

Cross-walks, fire geography, and GIS at Mesa Verde National Park : a watershed approach

Land use and land cover (LULC) data are valuable resources to fire managers making important decisions regarding the future of America's National Parks and forests. The choice of available LULC GIS products varies greatly by the provider. The National Land Cover Database (NLCD) and Landscape Fire and Resource Management Planning Tools (LANDFIRE) Program each represent vegetation at different scales for different reasons. This study utilizes a process known as cross-walking to make the data comparable at a one-to-one scale. The cross-walking process is described in detail. An overlay analysis is conducted which explores the decline, growth, and stationarity of evergreen forest, shrub/scrub, and grassland/herbaceous cover types from 2001 to 2019/2020. The results are analyzed to provide geographic and cartographic comparisons regarding the fire geography of Mesa Verde National Park. Findings include geographic and cartographic differences between datasets, especially where fires larger than approximately 12 hectares (or approximately 30 acres) have previously occurred. This information is valuable to decision-makers regarding the fire geography of Mesa Verde National Park and their GIS knowledge of LULC data such as NLCD and LANDFIRE.Includes bibliographical references

First-principles thermoelasticity of bcc iron under pressure

We investigate the elastic and isotropic aggregate properties of ferromagnetic bcc iron as a function of temperature and pressure by computing the Helmholtz free energies for the volume-conserving strained structures using the first-principles linear response linear-muffin-tin-orbital method and the generalized-gradient approximation. We include the electronic excitation contributions to the free energy from the band structures, and phonon contributions from quasi-harmonic lattice dynamics. We make detailed comparisons between our calculated elastic moduli and their temperature and pressure dependences with available experimental and theoretical data.Comment: 5 figures, 2 table

A polar prediction model for learning to represent visual transformations

All organisms make temporal predictions, and their evolutionary fitness level depends on the accuracy of these predictions. In the context of visual perception, the motions of both the observer and objects in the scene structure the dynamics of sensory signals, allowing for partial prediction of future signals based on past ones. Here, we propose a self-supervised representation-learning framework that extracts and exploits the regularities of natural videos to compute accurate predictions. We motivate the polar architecture by appealing to the Fourier shift theorem and its group-theoretic generalization, and we optimize its parameters on next-frame prediction. Through controlled experiments, we demonstrate that this approach can discover the representation of simple transformation groups acting in data. When trained on natural video datasets, our framework achieves better prediction performance than traditional motion compensation and rivals conventional deep networks, while maintaining interpretability and speed. Furthermore, the polar computations can be restructured into components resembling normalized simple and direction-selective complex cell models of primate V1 neurons. Thus, polar prediction offers a principled framework for understanding how the visual system represents sensory inputs in a form that simplifies temporal prediction

Structures and stability of calcium and magnesium carbonates at mantle pressures

Ab initio random structure searching (AIRSS) and density functional theory methods are used to predict structures of calcium and magnesium carbonate (CaCO$_3$ and MgCO$_3$) at high pressures. We find a previously unknown CaCO$_3$ structure which is more stable than the aragonite and "post aragonite" phases in the range 32--48 GPa. At pressures from 67 GPa to well over 100 GPa the most stable phase is a previously unknown CaCO$_3$ structure of the pyroxene type with fourfold coordinated carbon atoms. We also predict a stable structure of MgCO$_3$ in the range 85--101 GPa. Our results lead to a revision of the phase diagram of CaCO$_3$ over more than half the pressure range encountered within the Earth's mantle, and smaller changes to the phase diagram of MgCO$_3$. We predict CaCO$_3$ to be more stable than MgCO$_3$ in the Earth's mantle above 100 GPa, and that CO$_2$ is not a thermodynamically stable compound under deep mantle conditions. Our results have significant implications for understanding the Earth's deep carbon cycle.We acknowledge financial support from the Engineering and Physical Sciences Research Council United Kingdom (EPSRC) of the United Kingdom.This is the author accepted manuscript. The final version is available from APS via http://dx.doi.org/10.1103/PhysRevB.91.10410

Pratiques du titrement dans les villes en développement : trois cas d'étude (Inde, Ethiopie, Mauritanie) : rapport de recherche présenté dans le cadre de l'appel à projets "La sécurisation du droit de propriété dans les pays en voie de développement"

Les trois cas d’études en Inde, en Ethiopie et en Mauritanie ont permis de voir comment est appréhendé un même cadre référentiel qui pose la sécurisation foncière au coeur de la lutte contre la pauvreté. Plusieurs types de villes (petites, secondaires, capitales) et formes d’urbain (zone centrale, quartiers périurbains, urbain diffus) ont permis de souligner quelques uns des enjeux que représente la titrisation dans les Suds. Le contexte urbain joue fortement. L’impact des politiques de titrement foncier dépend réellement de la taille de la ville et des formes urbaines. L’urbanisation généralisée et rapide dans les trois pays étudiés, longtemps considérés comme étant à dominante rurale, implique l’adaptation et la création de nouveaux régimes fonciers. Dans nos différents cas d’étude, l’accès au sol urbain fait émerger de nouveaux besoins en termes de réglementation foncière, de nouveaux types de reconnaissance, mais aussi et surtout de nouvelles pratiques de sécurisation se tissent entre les habitants et les autorités et entre habitants. La présente étude a fait ressortir l’importance de la circulation à l’échelle internationale de politiques urbaines et « bonnes pratiques », pensées depuis Washington par les Institutions internationales, et réappropriées de façon tout à fait originale localement. La comparaison des trois cas a mis en lumière l’importance des pratiques habitantes, qui, en Ethiopie, en Inde et en Mauritanie, exploitent et construisent des opportunités offertes par les normes juridiques relatives à l’accès à la propriété, plus qu’elles arrivent à en bénéficier pleinement et simplement. Bien souvent, les réformes appuyées par les bailleurs de fond ne font que se surimposer à des juridictions foncières déjà complexes, souvent coûteuses à respecter pour les habitants qui optent pour des pratiques plus informelles. La réforme sur le papier n’est que rarement suivie par la réforme en action

Oxidation of UC: an in-situ high temperature environmental scanning electron microscopy study

Uranium carbide (UC) has the potential to be used as fuel in Generation IV nuclear reactors thanks to its higher metal atom density and better thermal conductivity when compared to the most commonly used fuel: uranium dioxide (UO2) [1]. Although UC offers improved properties during operation, it needs to be converted into an oxide form after usage as it is reactive and potentially pyrophoric [2] in oxidising environments. Previous oxidation studies on UC, performed over a range of oxygen atmospheres and temperatures, suggest different mechanisms lead to the formation of either a protective or a pulverised non-protective oxide layer [3]. New experimental observations of the oxidation and self-ignition of UC were reported in our previous work [4] involving a combination of state-of-the-art techniques: high temperature environmental scanning electron microscopy (HT-ESEM), high-resolution transmission electron microscopy (HRTEM) combined with an image analysis technique (ImageJ). In situ HT-ESEM oxidation of sintered UC fragments from 723 to 848 K in 10 to 100 Pa oxygen atmosphere revealed the morphological changes to the oxide during the transformations between UC to UO2 and UO2 to U3O8. Oxidation at 723 K in a low O2 atmosphere (≤ 25 Pa O2) produced a compact UO2+x oxide layer, confirmed by post mortem HRTEM analysis. The oxide formed after an induction period and it was accompanied by an exponential followed by logarithmic sample area expansion and crack propagation. Furthermore, samples oxidised at 50 Pa O2 at 723 K and at 773-848 K in an oxygen atmosphere of 10 to 100 Pa O2 showed “explosive” oxidation (see Figure 1). Sample expansion and crack propagation are well described by an exponential law until the “explosion” occurred causing a transformation to a popcorn-like morphology which is typical for oxidation from UO2 to U3O8. HRTEM analysis on the sample powder showed the oxide to be formed of a mixture of U3O7/U3O8 with U3O8 showing preferential growth in the [001] direction. The explosive nature of the oxide is triggered by ignition of UC, which set off this reaction throughout the entire sample with a propagation speed of 150-500 ± 50 µm/s, which shows similarities to a self-propagating high-temperature synthesis reaction. Please click Additional Files below to see the full abstract

Reaction path of a way of synthetis of nepheline NaAlSiO4

International audienceThe transformation of a simulated nuclear waste into nepheline NaAlSiO4 has been studied, using thermogravimetry, mass spectrometry and X-Ray diffraction. Several mixtures have been used, which were elaborated from a simulated waste (containing mainly sodium nitrate), alumina and silica. Considering the various mass losses observed, the phases obtained after each mass loss (identified by X-Ray diffraction) and the gases released, a reaction path for the transformation has been proposed

Modeling of a pyrolysis process for the elimination of epoxy resin from embedded nuclear fuels

This paper presents the modeling of a bench-scale reactor for the pyrolysis of epoxy resin containing nuclear fuel samples. Strict operating conditions must be met to avoid nuclear fuel oxidation and the final hydrogen content in the residual char must be close to zero. By using the finite element method software, COMSOL Multiphysics®, transport phenomena and reaction kinetics can be combined to obtain a representative model of the reactor in terms of the temperature and the concentration distribution of the representative chemical species. The numerical results have been found consistent with the temperature measurements in the reactor. The model is also able to predict the distribution of permanent gases in the reactor over time. The variation in the composition and concentration of the gas near the fuel sample can then be monitored to control the oxygen potential. We have calculated the in-vessel transfers of a representative species of gases, hydrogen. The comparison of simulated and experimental values for hydrogen shows good agreement