Search results for "methods"
showing 10 items of 4526 documents
On the origin of the sigmoid shape in the UO2 oxidation weight gain curves
2009
International audience; Cracking and spalling are known to occur during the oxidation of UO2. However, these phenomena are not considered by the existing kinetic models of the oxidation of UO2 into U3O8. In this study the oxidation of UO2 samples of various sizes from the single crystal to nanopowders, has been followed by isothermal and isobaric thermogravimetry, environmental scanning electron microscopy and in situ X-ray diffraction at temperatures ranging from 250 to 370°C in air. It has been shown that cracking occurs once a critical layer thickness of intermediate oxide has been reached, which corresponds to the beginning of the sigmoid kinetic curve. Cracking contribution to the sigm…
High-resolution Raman spectroscopy of the ν1 region and Raman-Raman double resonance spectroscopy of the 2ν1-ν 1 band of 32SF6 and 34SF 6. Determinat…
2004
The ν1 region of 32SF6 and 34SF6 has been studied by stimulated Raman spectroscopy. For both isotopomers, a detailed analysis has been performed. Several hot bands (ν1+ν6-ν6, ν1+2ν 6-2ν6, ν1+ν5-ν 5) have been taken into account to calculate synthetic spectra that satisfactorily reproduce the experimental data. These results, together with the previous studies of the other fundamental bands have allowed us to determine the equilibrium bond length of sulfur hexafluoride as r e=1.5560(1) Å, in very good agreement with recent ab initio calculations. The 2ν1-ν1 band has also been studied for both isotopomers by Raman-Raman double resonance spectroscopy and the resulting spectra have been analyze…
Comparing different processing methods in apple slice drying. Part 1. Performance of microwave, hot air and hybrid methods at constant temperatures
2019
The drying curves, energy efficiency and colour variations of sliced apples dried at various temperatures and using different techniques were evaluated, i.e. microwave (35, 55 and 65 °C), hot air (35, 55, 65 and 75 °C) and combination of both (65 °C). The microwave (MW) tests also included comparison between air recirculation and continuous inlet of fresh air. Each drying method was evaluated at a fixed temperature level by means of a control system based on infrared thermography. The time required to complete the drying process at 65 °C varied from about 44 min for MW with fresh air ventilation (double with air recirculation), to 122 min for hybrid heating and 238 min for hot air. Drying k…
Ab initio hybrid DFT calculations of BaTiO3 bulk and BaO-terminated (001) surface F-centers
2017
Using a supercell model and a hybrid B3PW exchange-correlation functional, we have performed first principles calculations for the F-center in the BaTiO3 bulk and on the BaO-terminated (001) surface. We find that two Ti atoms nearest to the bulk F-center are repulsed, while nearest eight oxygen and four barium atoms relax toward the oxygen vacancy (by 1.06, 0.71 and 0.08% of the lattice constant [Formula: see text], respectively). The magnitudes of atomic displacements around the F-center located on the BaO-terminated (001) surface in most cases (except for Ti) are larger than those around the bulk F-center (0.1, 1.4 and 1.0% of [Formula: see text], respectively). Our calculated BaTiO3 bul…
Structural and electronic properties ofβ-FeSi2nanoparticles: The role of stacking fault domains
2014
We use conventional and aberration-corrected transmission electron microscopy (TEM) and ab initio calculations to investigate the structural and electronic properties of \ensuremath{\beta}-FeSi${}_{2}$ nanoparticles, which are a promising material for photovoltaic applications due to a band gap of 1 eV and a high absorption coefficient. The nanoparticles have average sizes of \ensuremath{\sim}20 nm, form aggregates, and are prepared by gas-phase synthesis. Amorphous SiO${}_{x}$ shells with thicknesses of \ensuremath{\sim}1.7 nm around \ensuremath{\beta}-FeSi${}_{2}$ cores are identified on individual nanoparticles using electron energy-loss spectroscopy, while stacking fault domains in the …
Ab initio calculations of MgF2 (001) and (011) surface structure
2010
We present the results of calculations of surface relaxations, rumplings, and charge distribution for the MgF2 (0 0 1) and (0 1 1) surfaces using ab initio code Crystal-2003 and the hybrid exchange-correlation B3PW functional. These two neutral and polar surfaces show very small relaxation and negligible increase of covalent contribution to the chemical bonding thus remaining considerably ionic. The calculated bulk optical band gap is in a good agreement with the experimental value, whereas optical band gap for the polar (0 1 1) surface is reduced by 0.6 eV compared with the calculated bulk value, in contrast to the (0 0 1) surface gap which remains very close to the bulk.
Ab initio hybrid DFT calculations of BaTiO3, PbTiO3, SrZrO3 and PbZrO3 (111) surfaces
2015
Abstract The results of ab initio calculations for polar BaTiO 3 , PbTiO 3 , SrZrO 3 and PbZrO 3 (111) surfaces using the CRYSTAL code are presented. By means of the hybrid B3LYP approach, the surface relaxation has been calculated for two possible B (B = Ti or Zr) or AO 3 (A = Ba, Pb or Sr) BaTiO 3 , PbTiO 3 , SrZrO 3 and PbZrO 3 (111) surface terminations. According to performed B3LYP calculations, all atoms of the first surface layer, for both terminations, relax inwards. The only exception is a small outward relaxation of the PbO 3 -terminated PbTiO 3 (111) surface upper layer Pb atom. B3LYP calculated surface energies for BaO 3 , PbO 3 , SrO 3 and PbO 3 -terminated BaTiO 3 , PbTiO 3 , …
Ab initio calculations of the atomic and electronic structure of MgF2 (011) and (111) surfaces
2011
Abstract The results of ab initio slab calculations of surface relaxations, rumplings and charge distribution for the different terminations of the MgF2 (011) and (111) polar surfaces are presented and discussed. We have employed the computer code CRYSTAL with the Gaussian basis set and the hybrid B3PW exchange-correlation functional. Despite the ionic nature of the chemical bonding at both surfaces, a considerable decrease of the optical band gap is predicted (1.3 eV or 10%) for the (111) surface as compared to the bulk.
Random Structural Modification of a Low-Band-Gap BODIPY-Based Polymer
2017
International audience; A BODIPY thiophene polymer modified by extending conjugation of the BODIPY chromophore is reported. This modification induces tunability of energy levels and therefore absorption wavelengths in order to target lower energies.
Effect of melting on ionization potential of sodium clusters
2003
The effect of melting transition on the ionization potential has been studied for sodium clusters with 40, 55, 142, and 147 atoms, using ab initio and classical molecular dynamics. Classical and ab initio simulations were performed to determine the ionization potential of Na142 and Na147 for solid, partly melted, and liquid structures. The results reveal no correlation between the vertical ionization potential and the degree of surface disorder, melting, or the total energy of the cluster obtained with the ab initio method. However, in the case of 40 and 55 atom clusters, the ionization potential seems to decrease when the cluster melts.