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Transition Mechanism (transition + mechanism)

Distribution by Scientific Domains

Physics and Astronomy	50%

Selected Abstracts

Large-scale structures at various stages of separated boundary layer transition

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 6 2002
Zhiyin Yang
Abstract Large-eddy simulation is employed to investigate separated boundary layer transition induced by a change of curvature of the surface. The geometry is a flat plate with a semi-circular leading edge. The Reynolds number based on the uniform inlet velocity and the leading edge diameter is 3450. The simulated mean and turbulence quantities compare well with the available experimental data. The LES data have been comprehensively analysed to elucidate the transition process leading to breakdown to turbulence. Large-scale structures have been observed at various stage of the transition process. Understanding the formation of these large-scale structures, their evolution and eventual break-up into smaller structures may help to shed light on the transition mechanism. Copyright © 2002 John Wiley & Sons, Ltd. [source]

Molecular dynamics of phase transitions in clusters of alkali halides

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2001
Pedro C. R. Rodrigues
Abstract Molecular dynamics simulations of unconstrained alkali halide clusters with 8, 64, 216, 512, 1000, 1728, 2744, 4096, 5832, and 8000 ions have been carried out using the Born,Mayer,Huggins potential. All the clusters exhibit first-order melting and freezing transitions. The melting temperature increases with the number of ions and approaches the melting temperature of the bulk. Clusters with a number of ions less than approximately 1000 present hysteresis cycles and practically do not have phase coexistence. Clusters with a number of ions over 1000 present phase coexistence during a significant part of the transition region and hysteresis is progressively eliminated as the clusters size increases. It is suggested that hysteresis is an intrinsic characteristic of small clusters. In the transition regions the calculations have been performed by fixing the total energy of the clusters. It is shown that such a technique provides a better way of analyzing the transition mechanism than the usual procedure of fixing the temperature by ad hoc rescaling the velocities or by using canonical molecular dynamics or Monte Carlo. A detailed analysis of the melting transition is presented. The effects of interfaces and impurities are discussed. A method based on the velocity autocorrelation functions is proposed, in order to determine the molar fraction of the ions present in the solid and liquid phases as well as to produce colored snapshots of the phases in coexistence. The overall agreement of the estimated melting points and enthalpies of melting with the experiment is fairly good. The estimated melting point and enthalpy of melting for KCl in particular are in excellent agreement with the experimental values. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 169,180, 2001 [source]

TEM study of the structural similarity between NaCl and rhombohedral phase of Te-based ternary chalcogenide materials

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2009
Changwoo Sun
Abstract Comparing the phase-change behaviour of Ge,Bi,Te (GBT) to that of Ge,Sb,Te (GST), we have tried to verify a general relationship between a metastable NaCl and a stable rhombohedral structure that is easily incurred by stretching the cube along the body diagonal. High-resolution transmission electron microscopy (TEM) images show that the NaCl as well as the rhombohedral GBT and GST structures are observed along specific directions. From the TEM images, we suggest an atomic model and a phase transition mechanism by lattice distortion. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Structural study of NaNO2 embedded into porous glass

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2007
E. Rysiakiewicz-Pasek
Abstract Temperature-dependent Raman study of NaNO2 confined within porous glass with a pore size of 46 nm has been performed. It is demonstrated that the bandwidths and frequencies of the Raman bands as a function of temperature show similar behaviour, but not identical, as those observed for bulk NaNO2. This result indicates that the first-order phase transition mechanism is only slightly modified by embedding NaNO2 into the porous glass. It seems that formation of "premelted state" is not significant in the studied material, at least up to 40-50 K above Tc. At higher temperatures, however, a significant increase of bandwidth of the 1326 cm,1 band is observed, which could indicate appearance of "premelted" state. The dielectric measurements are consistent with this view since they show large increase of dielectric permittivity above 480 K, well below the NaNO2 melting temperature (554 K). (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Soil organic matter beyond molecular structure Part II: Amorphous nature and physical aging

JOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 2 2006
Gabriele E. Schaumann
Abstract Glassy, rubbery, and crystalline phases are representatives of supramolecular structures which strongly differ in order, density, and other characteristics. In this contribution, the amorphous nature of soil organic matter (SOM) is reviewed with respect to the glassy/rubbery model, glass transition mechanisms, interactions of SOM with water, and physical aging. Glass-transition behavior and physical aging are inherent properties of amorphous solids, and numerous spectroscopic investigations give insights into different domain mobilities of humic substances (HS). The correlation between sorption nonlinearity and glassiness of polymers and HS supports a relation between sorption and amorphicity in Aldrich humic acid. Further evidence is still required for the transfer to soil HS and SOM. Sorption and differential scanning calorimetry (DSC) data suggest a correlation between aromaticity and glassiness in HS, and the available data do currently not allow to decide unambiguously between specific sorption and hole filling as explanation. This needs to be verified in future research. Although parts of the investigations have up to now only been conducted with humic substances, the collectivity of available data give strong support for the glassy/rubbery conception of SOM. They clearly indicate that amorphous characteristics cannot be excluded in SOM. This is further supported by the observation of different types of glass-transition behavior in samples of whole humous soil. In addition to classical glass transitions in water-free soil samples, water surprisingly acts in an antagonistic way as short-term plasticizer and long-term antiplasticizer in a second, nonclassical transition type. Latter is closely connected with physico-chemical interactions with water and suggests water bridges between structural elements of SOM (HBCL-model). The gradual increase of Tg* in SOM indicates physico-chemical aging processes, which are not restricted to polymers. They may be responsible for contaminant aging, changes in surface properties and increased soil compaction in agricultural soils. [source]

Recombination processes with and without momentum conservation in degenerate InN

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 1 2006
E. Valcheva
Abstract We report on a theoretical approach in which the two cases of recombination with and without momentum conservation in optically excited high carrier concentration InN are considered. The calculations are used to analyze emission spectra of n-type InN layers of electron concentrations from 7.7 × 1017 to 1.4 × 1019 cm,3 studied in the temperature range between 9 and 100 K. The spectra peak near 0.7 eV and the applicability of the two approaches with increasing carrier concentration is estimated. Different transition mechanisms are considered in order to properly account for the observed features in the spectra. Recombination processes involving acceptor- and donor-like localized states are discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]