Biomedical subjects

S N Yannopoulos

Publications and source records attributed to S N Yannopoulos.

7 recordsLinked to original sources

PubMed2006

A high-temperature Raman spectroscopic investigation of the potassium tetrasilicate in glassy, supercooled, and liquid states.

Raman spectra of K2Si4O9 were measured over a broad temperature range including the glassy, supercooled, and molten states in an effort to follow the structural changes caused by temperature variation. Potassium tetrasilicate glass has been prepared using a containerless method and a CO2 laser for heating and melting the samples and thus avoiding contamination induced by the walls of the crucibles. Systematic Raman intensity measurements caused by temperature variation have been performed in order to elucidate the induced structural changes in the high-frequency stretching and in the three- and four-membered ring breathing vibration regions. The high-frequency symmetric stretching vibrations of the nonbridging Si-O bond are associated to the presence of two distinct types of tetrahedral units with terminal oxygen atoms. The low-frequency Raman spectra reveal the, well resolved, presence of the boson peak at temperatures above the melting point. The temperature dependence of the boson peak energy has also been determined and compared with that of the sound velocities of potassium tetrasilicate. The results are discussed in the context of recent experimental and theoretical works.

Journal Article↗

PubMed2006

On the extent of polymerization of liquid sulfur at very high temperatures.

The extent of polymerization of liquid sulfur up to 731 K has been obtained using Raman scattering. The data reveal the absence of a maximum in the polymerization curve. The obtained results are discussed in the spirit of computer simulations that suggest the existence of a maximum which depends on the bond breaking energy.

Journal Article↗

PubMed2006

Temperature-induced structural changes in glassy, supercooled, and molten silica from 77 to 2150 K.

In situ polarized and depolarized Raman spectra of glassy, supercooled, and molten SiO2 have been measured over the broad temperature range 77-2150 K in an effort to examine possible structural changes caused by temperature variation. A new experimental setup using a CO2 laser for heating the sample has been designed allowing measurement with controllable blackbody radiation background at temperatures up to 2200 K. Careful and systematic relative intensity measurements and the use of the isotropic and anisotropic Raman representation of the spectra revealed hidden bands in the bending mode region and resolved bands in the stretching region of the spectra. Overall the spectra behavior shows similarities with the spectra of the recently studied tetrahedral glasses/melts of ZnCl2 and ZnBr2. Increasing temperature causes subtle changes of the relative intensities within the silicon-oxygen stretching region at approximately 750-850 cm(-1) and gives rise to a new band at approximately 930 cm(-1). The spectral behavior is interpreted to indicate that the "SiO42" tetrahedra are bound to each other to form the network by apex-bridging and partly by edge-bridging oxygens. The network structure of the glass/melt is formed by mixing a variety of tetrahedra participating in "open" (cristobalitelike), "cluster" (supertetrahedra), and "chain" edge-bridged substructures bound to each other by bridging oxygens. A weak in intensity but strongly polarized composite band is resolved at approximately 1400 cm(-1) and is assigned to Si[Double Bond]O terminal bond frequency. Temperature rise increases the concentration of the terminal bonds by breaking up the network. These structural changes are reminiscent of the polyamorphic transformations occurring in silica as has recently been predicted by computer simulations. At low frequencies the Raman spectra reveal the presence of the Boson peak at approximately 60 cm(-1) which is well resolved even above melting temperature up to 2150 K.

Journal Article↗

PubMed2004

Raman scattering study on structural and dynamical features of noncrystalline selenium.

We report on a detailed, temperature-dependent, off-resonant Raman scattering study of glassy and supercooled selenium. Raman spectra in the frequency regime of the first-order scattering (5-450 cm(-1)) have been recorded over a wide temperature range, i.e., 143-353 K. To facilitate the analysis, the spectra have intuitively been divided in three spectral regions. The analysis of the high frequency region (bond-stretching vibrational modes) yielded information on the rings-chains equilibrium. In particular, the polymer content was found to amount to more than 85% around the glass transition temperature, exhibiting a weak temperature dependence, which extrapolates nicely to the high-temperature dissolution data. The intermediate frequency range (representative of the medium-range structural order) was treated together with the low frequency regime (where low-energy excitations, i.e., the quasielastic line and the Boson peak are the dominant contributions) owing to their strong overlap. The study of the bond-bending regime revealed information which made it possible to clarify the role of ringlike and chainlike fragments incorporated in polymeric molecules. The temperature evolution of the Boson peak and the frequency dependence of the Raman coupling coefficient Comega were also determined. An attempt to decompose the partial contribution of the pure Boson peak to Comega revealed valuable information concerning the limiting (omega-->0) behavior of the coupling coefficient.

Journal Article↗

PubMed2004

High frequency dynamics in a monatomic glass.

The high frequency dynamics of glassy selenium has been studied by inelastic x-ray scattering at beam line BL35XU (SPring-8). The high quality of the data allows one to pinpoint the existence of a dispersing acoustic mode for wave vectors (Q) of 1.5<Q<12.5 nm(-1), helping to clarify a previ-ous contradiction between experimental and numerical results. The sound velocity shows a positive dispersion, exceeding the hydrodynamic value by approximately 10% at Q<3.5 nm(-1). The Q2 dependence of the sound attenuation Gamma(Q), reported for other glasses, is found to be the low-Q limit of a more general Gamma(Q) proportional, variant Omega(Q)(2) law, which applies also to the higher Q region, where Omega(Q) proportional, variant Q no longer holds.

Journal Article↗

PubMed2002

Spectral features of the quasielastic line in amorphous solids and supercooled liquids: a detailed low-frequency Raman scattering study.

The spectral features of the quasielastic light scattering in amorphous solids and supercooled liquids are investigated through a combined Stokes and antiStokes low-frequency Raman scattering study. Emphasis is given on the specific spectral details of the quasielastic line rather than on elucidating its microscopic origin. Our approach is quite general since it includes glass formers with a strong, an intermediate, and a fragile dynamic character. The results suggest that the quasielastic contribution is a symmetric spectral feature around the laser line. This finding makes possible the separation of the quasielastic line and the Boson peak. It further raises certain skepticism concerning reduction schemes followed up in the literature for the analysis of low-frequency Raman data and for models that combine these two contributions. The limiting (omega-->0) behavior of the product of the vibrational density of states and the Raman coupling coefficient has also been extracted. The validity of some phenomenological approaches is also discussed in light of the experimental facts presented in this paper and some suggestions are being advanced.

Journal Article↗

PubMed2001

Dynamic light scattering study of the liquid <--> glass transition for the GdCl3-3AlCl3 glass-forming mixture.

The dynamics of density and orientation fluctuations for a member of a family of inorganic glass-forming molten salts based on rare-earth halide-aluminum halide mixtures, namely, the GdCl3-3AlCl3, has been investigated in the supercooled state by means of dynamic light scattering. The temperature dependence of the structural relaxation time near the glass transition temperature was found to be rather strong, indicating the "fragile" character of this liquid. The microscopic origin of the structural relaxation has been rationalized by employing structural information that has been independently extracted from other spectroscopic techniques.

Journal Article↗