R. Le Parc

Thermal annealing and density fluctuations in silica glass

Abstract Silica glasses of a same composition with different annealing times above and below T g are characterized by infra-red and Raman spectroscopy in order to determine their fictive temperature. The Rayleigh scattering of these different samples is shown to decrease when the fictive temperature decreases in agreement with a decrease of density fluctuations. Some quantitative discrepancies are however shown to occur for long annealing at 950 °C.

Temperature independence of pressure-induced amorphization of the phase-change memory alloy Ge2Sb2Te5

In the temperature range from room temperature to about 150°C, the prototypic phase-change material Ge2Sb2Te5 becomes amorphous upon hydrostatic compression. In the studied temperature range, the onset of amorphization is at about 15GPa and the material completely amorphizes at 25GPa; these values do not depend on temperature. Upon decompression, the amorphous phase is stable at lower temperatures, yet at higher temperatures (145°C), the initial fcc phase is recovered upon decompression. A possible mechanism of pressure-induced amorphization and its implications for phase-change memories are discussed.

Anorthite and CaAl2Si2O8 glass: low frequency Raman spectroscopy and neutron scattering

Abstract Raman spectra of anorthite crystal and glass have been obtained on a wide frequency range from 5 to 1200 cm −1 . Medium-range order has been pointed out in this study: numerous low frequency peaks in anorthite have been observed and a comparison with the well known band around 60 cm −1 in the Raman spectrum of anorthite glass is made. In order to probe the medium-range structure of CaAl 2 Si 2 O 8 glass, a comparison between neutron inelastic scattering, Raman low frequency vibrations and heat capacity data is done for both glass and crystal.

Glass structure and light scattering

Abstract Elastic and inelastic light scattering experiments are performed on silica, SiO 2 , with OH content

Structural studies of germanium doped silica glasses: the role of the fictive temperature

Abstract We studied the influence of germanium atom introduction into the silica network and the influence of heat treatements on the glass structure by doing Raman and infra-red spectra of vitreous germanium doped silica (5% wt GeO2, 95% wt SiO2). Germanium incorporates into the network by substituting Si atoms and consequently, we observed band shifts and broadening of defect peaks D1 and D2. The peak positions are directly correlated with the structural state characterized by the glass fictive temperature TF of the different samples.

In situ measurements of density fluctuations and compressibility in silica glasses as a function of temperature and thermal history

In this paper, small-angle x-ray scattering measurements are used to determine the different compressibility contributions, as well as the isothermal compressibility {chi}{sub T}{sup 0} in thermal equilibrium in silica glasses having different thermal histories. Using two different methods of analysis, in the supercooled liquid and in the glassy state, we obtain, respectively, the temperature and fictive temperature dependences of {chi}{sub T}{sup 0}. The values obtained in the glass and supercooled liquid states are very close to each other. They agree with previous determinations of the literature. The compressibility in the glass state slightly decreases with increasing fictive temperature. The relaxational part of the compressibility is also calculated and compared to previous determinations. We discussed the small differences between the different determinations.

Influence of thermal aging on density fluctuations in oxide glasses measured by small-angle X-ray scattering

Small-angle X-ray measurements of the density fluctuations as a function of temperature and thermal history are performed in silica and float glass. Structural relaxation in the glass transition range is observed for the samples stabilized at high temperature. The amplitude of the density fluctuations in the glassy state depends on the thermal history for samples of same composition. It increases with fictive temperature for silica as well as for float glass. Our results are compared to previous results in oxide glasses or polymers and discussed in relation with the concept of fragility i.e. the temperature sensitivity of viscosity in the liquid state.

In situ high pressure and high temperature Raman studies of (1−x)SiO2xGeO2 glasses

The structure of glasses in the binary system SiO(2)-GeO(2) has been studied by Raman spectroscopy. Our results are consistent with mixing of SiO(2) and GeO(2) tetrahedra. The changes induced by temperature and by pressure on the structure are monitored by in situ measurements on the same mixed glasses. Anomalous temperature dependences are observed not only for SiO(2) glass and GeO(2) glass but also for mixed glasses. Particular attention is focused on the pressure densification mechanism in mixed glasses. Via the pressure dependence of the width of the main Raman band, we show that the compression mechanism in mixed glasses is intermediate between that of the end members.

Dynamic sound attenuation at hypersonic frequencies in silica glass

In order to clarify the origin of the dominant processes responsible for the acoustic attenuation of phonons, which is a much debated topic, we present Brillouin scattering experiments in various silica glasses of different OH impurities content. A large temperature range, from

Density and concentration fluctuations in SiO2–GeO2 optical fiber glass investigated by small angle x-ray scattering

5\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1500\phantom{\rule{0.3em}{0ex}}\mathrm{K}