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Energy Transfer Mechanisms (energy + transfer_mechanism)

Distribution by Scientific Domains
Physics and Astronomy40%

Selected Abstracts

Polymer characterization by ultrasonic wave propagation

ADVANCES IN POLYMER TECHNOLOGY, Issue 2 2008
Francesca Lionetto
Abstract The propagation of low-intensity ultrasound in polymers, acting as a high-frequency dynamic mechanical deformation, can be successfully used to monitor changes in the modulus of polymers associated with glass transition, crystallization, cross-linking, and other chemical and physical phenomena related to changes in the viscoelastic behavior, such as gelation phenomena. The velocity of sound is related to the polymer storage modulus and density, whereas the absorption of ultrasonic waves is related to the energy dissipation in the material and, therefore, to the loss modulus. Accordingly, ultrasonic measurements have been used by several authors to monitor the evolution of the viscoelastic moduli of polymers as a function of time or temperature and, recently, become a characterization technique of its own right, generally known as ultrasonic dynamic mechanical analysis (UDMA). Often the technique is used in conjunction with rheological methods as a means of providing a better insight into the viscoelastic behavior of polymer systems. As yet UDMA is underutilized primarily because of the low operating temperatures (usually below 100,C) of commercially available ultrasonic transducers, and also due to the requirement of a coupling medium to ensure an efficient energy transfer mechanism between the transducer and the test material. Despite these limitations, this paper shows that the use of ultrasonics is potentially a powerful method for the characterization of polymers, particularly as a tool for online monitoring of events occurring during polymer processing and in the manufacture of polymer matrix composites. The aim of this paper is to review the progress made in recent years, highlighting the potential and reliability of UDMA for monitoring physical transitions in polymers such as glass transition, melting, crystallization, as well as physical changes taking place during curing of thermosetting resins. © 2009 Wiley Periodicals, Inc. Adv Polym Techn 27:63,73, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20124 [source]

Photophysical Properties of Terbium Molecular-based Hybrids Assembled with Novel Ureasil Linkages

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2007
Bing Yan
Three silica-based organic,inorganic hybrid systems composed of hydroxyl aromatic derivatives (2-acetylphenol [HAP], 2-hydroxy-3-methylbenzoic acid [HMBA], 3-hydroxy-meta-phthalic acid [HMPHTH] complexes) were prepared via a sol,gel process. The active hydroxyl groups of the three ligands grafted by 3-(triethoxysilyl)-propyl isocyanate (TESPIC) through hydrogen transfer addition reaction were used as multi-functional bridge components, which can coordinate to Tb3+ with carbonyl groups, strongly absorb ultraviolet light and effectively transfer energy to Tb3+ through their triplet excited state, as well as undergo polymerization or crosslinking reactions with tetraethoxysilane (TEOS), for anchoring terbium ions to silica backbone. NMR, FT-IR, UV,vis absorption, luminescence spectroscopy was used to investigate the obtained hybrid material. UV excitation in the organic component resulted in strong green emission from Tb3+ ions due to an efficient ligand-to-metal energy transfer mechanism. [source]

Large eddy simulation of turbulent channel flow using an algebraic model

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 5 2005
S. Bhushan
Abstract In this paper an algebraic model from the constitutive equations of the subgrid stresses has been developed. This model has an additional term in comparison with the mixed model, which represents the backscatter of energy explicitly. The proposed model thus provides independent modelling of the different energy transfer mechanisms, thereby capturing the effect of subgrid scales more accurately. The model is also found to depict the flow anisotropy better than the linear and mixed models. The energy transfer capability of the model is analysed for the isotropic decay and the forced isotropic turbulence. The turbulent plane channel flow simulation is performed over three Reynolds numbers, Re,=180, 395 and 590, and the results are compared with that of the dynamic model, Smagorinsky model, and the DNS data. Both the algebraic and dynamic models are in good agreement with the DNS data for the mean flow quantities. However, the algebraic model is found to be more accurate for the turbulence intensities and the higher-order statistics. The capability of the algebraic model to represent backscatter is also demonstrated. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Photophysics of organic emissive semiconductors under hydrostatic pressure

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 14 2004
S. Guha
Abstract The impact of hydrostatic pressure on conjugated organic semiconductors has been studied using optical spectroscopic techniques. This work focuses on three blue emitters with distinct backbone conformations: planar methylated ladder-type poly para phenylene, semi-planar polyfluorene (PF), and nonplanar para hexaphenyl. The effect of pressure on the backbone emission and defect related emission allows an understanding of the localized, delocalized electronic states and energy transfer mechanisms of the singlet excitons into defect states. The Raman spectrum of the organic emitters under pressure demonstrates changes in the backbone conformation and mesomorphic phases. The Raman peaks in PF exhibit asymmetric Fano-type line shapes beyond 40 kbar, where the defect emission is enhanced. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

UV light induced luminescence processes in AlN nanotips and ceramics

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2005
L. Trinkler
Abstract UV induced luminescence properties of AlN (Eg = 6.2 eV) have been studied for AlN nanotips and AlN ceramics, using methods of photoluminescence, optically stimulated luminescence and thermoluminescence. In both types of objects the main luminescence band, which appears in prompt and stimulated emission spectra around 400 nm, arises due to presence of oxygen-related defects. The main difference between AlN nanotips and AlN ceramics is observed in excitation spectra and TL properties. Basing on the experimental results it is assumed that several different energy transfer mechanisms occur in AlN. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]