In-situ high-temperature EXAFS measurements on radioactive and air-sensitive molten salt materials

The development at the Delft University of Technology (TU Delft, The Netherlands) of an experimental set-up dedicated to high-temperature in situ EXAFS measurements of radioactive, air-sensitive and corrosive fluoride salts is reported. A detailed description of the sample containment cell, of the furnace design, and of the measurement geometry allowing simultaneous transmission and fluorescence measurements is given herein. The performance of the equipment is tested with the room-temperature measurement of thorium tetrafluoride, and the Th—F and Th—Th bond distances obtained by fitting of the EXAFS data are compared with the ones extracted from a refinement of neutron diffraction data collected at the PEARL beamline at TU Delft. The adequacy of the sample confinement is checked with a mapping of the thorium concentration profile of molten salt material. Finally, a few selected salt mixtures (LiF:ThF4) = (0.9:0.1), (0.75:0.25), (0.5:0.5) and (NaF:ThF4) = (0.67:0.33), (0.5:0.5) are measured in the molten state. Qualitative trends along the series are discussed, and the experimental data for the (LiF:ThF4) = (0.5:0.5) composition are compared with the EXAFS spectrum generated from molecular dynamics simulations

Thermal transport properties of multiphase sintered metals microstructures. The copper-tungsten system: Experiments and modeling

International audienceThe thermal diffusivity of Cu-W sintered alloys microstructures is measured at room temperature at different compositions, using rear face flash experiments. The samples are synthesized with the Spark Plasma Sintering technique. The resulting microstructures are slightly porous and consist of angular nanoscale grains of tungsten with medium sphericity in a copper matrix. The tungsten particles are at the nanoscale with an average grain size of 250 nm in contrast to the copper matrix for which the average grain size lies in the range 20 mu m-30 mu m; this is large enough to avoid the grains boundary effect upon the thermal transport. The overall porosity of the microstructures lies within the range: 6% <= P <= 12%. Along with the experimental work, a predictive model describing the effective thermal conductivity of multiphasic macrostructures is proposed in order to explain the obtained experimental results. The model was developed based only on physical considerations and contains no empirical parameters; it takes into account the type of microstructure and the microstructure parameters: porosity, grain shape, grain size, and grain size distribution. The agreement between the experiments and the model is found to be excellent. (C) 2016 AIP Publishing LLC

Evaluation of machine learning interpolation techniques for prediction of physical properties

A knowledge of the physical properties of materials as a function of temperature, composition, applied external stresses, etc. is an important consideration in materials and process design. For new systems, such properties may be unknown and hard to measure or estimate from numerical simulations such as molecular dynamics. Engineers rely on machine learning to employ existing data in order to predict properties for new systems. Several techniques are currently used for such purposes. These include neural network, polynomial interpolation and Gaussian processes as well as the more recent dynamic trees and scalable Gaussian processes. In this paper we compare these approaches for three sets of materials sciences data: molar volume, electrical conductivity and Martensite start temperature. We make recommendations depending on the nature of the data. We demonstrate that a thorough knowledge of the problem beforehand is critical in selecting the most successful machine learning technique. Our findings show that the Gaussian process regression technique gives very good predictions for all three sets of tested data. Typically, Gaussian process is very slow with a computational complexity of typically where n is the number of data points. In this paper, we found that the scalable Gaussian process approach was able to maintain the high accuracy of the predictions while improving speed considerably, make on-line learning possible

Pourquoi certains carbones graphitent ou ne graphitent pas ? Une interprétation phénoménologique inspirée de la thermodynamique

National audienc

Why some carbons may or may not graphitize? Interpreting graphitization data with a phenomenological model inspired from thermodynamics

International audienc

Why some carbons may or may not graphitize? The point of view of thermodynamics.

International audienc

Why some carbons may or may not graphitize? Interpreting graphitization data with a phenomenological model inspired from thermodynamics

International audienc

Experimental study of the thermal conductivity of sintered tungsten: Evidence of a critical behaviour with porosity

International audienceRear face flash experiments were performed in order to determine the thermal conductivity of sintered tungsten at room temperature. Ten different samples were synthesized with the spark plasma sintering technique. The microstructure obtained from the sintering is porous and consists of angular grains with medium sphericity. The average grain size (d) and the porosity (P) of the samples lie within the ranges of 2 mu m <= d <= 7 mu m and 0 <= P <= 0.35. We show that the dependence of the thermal conductivity of the sintered tungsten samples on the porosity shows a critical behaviour. A theoretical explanation of this behaviour and a predictive model for this porosity dependence are proposed. (C) 2015 AIP Publishing LLC

Influence des propriétés des grains sur la conductivité thermique d’un lit de poudre d’alumine α\alpha-Al2_2O3_3

International audienceThe effective thermal conductivity of powder beds depends on a large number of parameters. In this paper, the apparent influence of grain size and geometry on this parameter of the medium is presented, based on a thermal conductivity model of unconnected porous media in air at atmospheric pressure, which we have developed. This model is based on the theory of percolation to describe the heat transfer in this kind of material. The medium is described by the following microstructural parameters: intra-granular and inter-granular porosity, as well as parameters specific to the material and to the arrangement of the particles. The first two parameters are measured for some samples through a characterization of the bed microstructure by an X-ray micro-tomograph. The effective bed conductivities predicted by this model are compared with measurements made using the unsteady hot plane method. The results show that the thermal conductivity of a powder bed appears to be very sensitive to the porosity distribution between inter-granular and intra-granular porosity.La conductivité thermique effective de lits de poudres dépend d’un nombre important de paramètres. Dans cet article est présentée l’influence apparente de la taille des grains et de leurs géométries sur ce paramètre du milieu, en s’appuyant sur un modèle de conductivité thermique de milieux poreux non-connectés dans l’air à pression atmosphérique, que nous avons développé. Ce modèle se base sur la théorie de la percolation pour décrire le transfert thermique dans ce type de matériaux. Le milieu est décrit par les paramètres microstructuraux suivants: la porosité intra-granulaire et inter-granulaire, ainsi que des paramètres propres au matériau et à l’arrangement des particules. Les deux premiers paramètres sont mesurés pour certains échantillons grâce à une caractérisation de la microstructure du lit par un micro-tomographe à rayons X. Les valeurs de conductivités effectives du lit prédites par ce modèle sont comparées avec des mesures effectuées à l’aide de la méthode du plan chaud instationnaire. Les résultats montrent que la conductivité thermique d’un lit de poudre semble très sensible à la répartition des porosités entre la porosité inter-granulaire et intra-granulaire

Experimental and Computational Exploration of the NaF-ThF₄Fuel System: Structure and Thermochemistry

The structural, thermochemical, and thermophysical properties of the NaF-ThF4 fuel system were studied with experimental methods and molecular dynamics (MD) simulations. Equilibrium MD (EMD) simulations using the polarizable ion model were performed to calculate the density, molar volume, thermal expansion, mixing enthalpy, heat capacity, and distribution of [ThFn]m- complexes in the (Na,Th)Fx melt over the full concentration range at various temperatures. The phase equilibria in the 10-50 mol % ThF4 and 85-95 mol % ThF4 regions of the NaF-ThF4 phase diagram were measured using differential scanning calorimetry, as were the mixing enthalpies at 1266 K of (NaF/ThF4) = (0.8:0.2), (0.7:0.3) mixtures. Furthermore, the β-Na2ThF6 and NaTh2F9 compounds were synthesized and subsequently analyzed with the use of X-ray diffraction. The heat capacities of both compounds were measured in the temperature ranges (2-271 K) and (2-294 K), respectively, by thermal relaxation calorimetry. Finally, a CALPHAD model coupling the structural and thermodynamic data was developed using both EMD and experimental data as input and a quasichemical formalism in the quadruplet approximation. Here, 7- and 8-coordinated Th4+ cations were introduced on the cationic sublattice alongside a 13-coordinated dimeric species to reproduce the chemical speciation, as calculated by EMD simulations and to provide a physical description of the melt.RST/Reactor Physics and Nuclear Material