Search results for "thermal [correlation function]"
showing 10 items of 1923 documents
Local structure of perovskites ReO3and ScF3with negative thermal expansion: interpretation beyond the quasiharmonic approximation
2016
We propose an approach beyond the quasiharmonic approximation for interpretation of EXAFS and XRD data and for ab initio calculations of electronic and vibration properties of materials with negative thermal expansion. Ab initio electronic structure and lattice dynamics calculations for cubic and distorted ScF3 were performed using the linear combination of atomic orbitals (LCAO) method. The band gap obtained in calculations for ScF3 is equal to 10.54 eV and agree well with the expected value. The calculated infrared spectra of F displaced (FD) cubic ScF3 allow us to predict that its mean Sc-F-Sc angle within NTE deviates from 180 degree.
Maximizing phonon thermal conductance for ballistic membranes
2007
At low temperatures, phonon scattering can become so weak that phonon transport becomes ballistic. We calculate the ballistic phonon conductance G for membranes using elasticity theory, considering the transition from three to two dimensions. We discuss the temperature and thickness dependence and especially concentrate on the issue of material parameters. For all membrane thicknesses, the best conductors have, counter-intuitively, the lowest speed of sound.
Effect of electrolytical hydrogenation on the thermal stability and crystallization kinetics of METGLASS MBF-50
2007
The effect of electrolytical hydrogenation on both the surface and volume crystallization kinetics and thermal stability of amorphous alloy METGLASS MBF-50 has been investigated. The surface crystallization has been investigated by the exoelectron emission (EEE) technique, whereas the volume crystallization has been followed by differential thermal analysis (DTA). It has been found that both the surface and volume crystallization of investigated material occur in two stages. The surface crystallization occurs at temperature lower and with activation energy distinctly smaller than the volume crystallization. Hydrogenation of the investigated metallic glass enhances its thermal stability by i…
Effect of electrolytical hydrogenation on the thermal stability and crystallization kinetics of metallic glass Fe79Si9B12
2011
The effect of electrolytical hydrogenation on both the surface and volume crystallization kinetics and thermal stability of amorphous alloy Fe79Si9B12 has been investigated. The parameters of the surface and volume crystallization (temperature, activation energy) have been determined applying the exoelectron emission (EEE) and differential thermal analysis (DTA) methods, respectively. It has been found that the surface crystallization of investigated material occurs at temperature much lower and with activation energy smaller than the volume crystallization. The determination of the activation energies for the volume and surface crystallization by the combination of DTA and EEE techniques e…
Effect of the milling conditions on the degree of amorphization of selenium by milling in a planetary ball mill
2007
The effect of the milling parameters (rotation speed of the milling device and duration of milling) on the phase composition of the products of milling of fully crystalline selenium has been investigated. The milling was conducted using a planetary micromill and the phase composition of the milling products was determined by differential thermal analysis. It has been found that ball milling leads to the partial amorphization of the starting crystalline material. The content of amorphous phase in the milling products depends, in a rather complicated way, on the milling parameters. At the milling parameters adopted in the present study, the milling product was never fully amorphous. The compl…
The luminescence properties of ZnO nanopowders
2007
Pure and Al3+ doped ZnO nanopowders were studied by means of time-resolved luminescence spectroscopy. The powders were synthesized by hydrothermal and plasma methods. These powders were used as a raw material for vaporization-condensation process inside the Solar reactor. The commercially available ZnO nanopowder was studied for a comparison. Exciton to defect band luminescence intensity ratio was estimated in different types of ZnO nanopowders. It was found that nanopowders with whiskers morphology show superlinear luminescence intensity depending on excitation density. The observed effect depends on the average nanoparticle size and on the powder morphology.
Parametric thermal analysis for the optimization of Double Walled Tubes layout in the Water Cooled Lithium Lead inboard blanket of DEMO fusion reactor
2019
Abstract Within the roadmap that will lead to the nuclear fusion exploitation for electric energy generation, the construction of a DEMOnstration (DEMO) reactor is, probably, the most important milestone to be reached since it will demonstrate the technological feasibility and economic competitiveness of an industrial-scale nuclear fusion reactor. In order to reach this goal, several European universities and research centres have joined their efforts in the EUROfusion action, funded by HORIZON 2020 UE programme. Within the framework of EUROfusion research activities, ENEA and University of Palermo are involved in the design of the Water-Cooled Lithium Lead Breeding Blanket (WCLL BB), that …
Kinetics of phase transitions in vitreous chalcogenide semiconductors AsxSe100m-x-yBiyas studied by the differential thermal analysis and exoelectron…
2011
Kinetics of glass transition (retrification) in chalcogenide semiconductors AsxSe100-x-yBy (x = 20 or 30, and y = 0 and 1) has been investigated by parallel differential thermal analysis (DTA) and exoelectron emission (EEE) measurements. EEE is a surface effect accompanying the structural transformations in the surface layer, whereas the DTA technique gives information about the transformations occurring in the volume of the sample. Temperature dependencies of the DTA signal and of the EEE intensity have been determined and the values of the activation energy for both the volume and the surface retrification have been determined by the Ozawa method for each of the four investigated material…
On the thermal instability in a horizontal rectangular porous channel heated from below by a constant flux
2014
Published version of an article in the journal: Journal of Physics: Conference Series. Also available from the publisher at: http://dx.doi.org/10.1088/1742-6596/501/1/012003 Open Access The onset of thermoconvective instability in a rectangular horizontal channel filled with a fluid-saturated porous medium is studied. The channel is heated from below with a constant flux. The top wall is maintained at a uniform constant temperature, while the lateral boundaries are permeable and perfectly conducting. The stability of the basic motionless state is analysed with respect to small-amplitude disturbances. The eigenvalue problem for the neutral stability condition is solved analytically for the n…
Ab initio molecular dynamics simulations of the Sc K-edge EXAFS of scandium trifluoride
2016
Scandium fluoride ScF3 has a simple cubic structure and attracts attention due to its large negative thermal expansion (NTE) over a wide range of temperatures (0-1100 K). In this study we present ab initio molecular dynamics (AIMD) simulations of ScF3 and their validation using the Sc K-edge EXAFS spectra in the temperature range from 300 K to 1000 K measured at the XAFS beamline of ELETTRA. The obtained results allow an assessement of the employed AIMD model and provide insight into the local structure and the lattice dynamics of ScF3 beyond the harmonic approximation. A strong anisotropy of the fluorine atom vibrations in the direction orthogonal to the -Sc-F-Sc- chain is observed. An exp…