Search results for "silica"

Investigating the cooling rate dependence of amorphous silica: A computer simulation study

1996

We use molecular dynamics computer simulations to study the dependence of the properties of amorphous silica on the cooling rate with which the glass has been produced. In particular we show that the density, the glass transition temperature, the radial distribution function and the distribution of the size of the rings depend on the cooling rate.

INN2000 - Intesa Natura 2000: valori e patrimoni nel settore natura.

2014

Synthesis of large-pore ordered mesoporous silicas containing aminopropyl groups

2005

Ordered mesoporous silicas with large-pore diameters incorporating aminopropyl groups in variable quantity have been synthesized via the co-condensation of tetraethyl orthosilicate (TEOS) and 3-tert-butyloxycarbonylaminopropyltriethoxysilane templated with nonionic surfactant P123 under acidic conditions. The deprotection of amino groups was then quantitatively achieved either by thermal treatment or acid hydrolysis followed by Et3N treatment, both routes leading to exactly the same materials. We showed that the free amino centers are fully accessible, by using the condensation of the amine function with benzaldehyde.

Structure and transport properties of amorphous aluminium silicates: computer simulation studies

2005

The structure and transport properties of SiO2-Al2O3 melts containing 13 mol% and 47 mol% Al2O3 are investigated by means of large scale molecular dynamics computer simulations. The interactions between the atoms are modelled by a pair potential which is a modified version of the one proposed by Kramer et al. [J. Am. Chem. Soc. 64, 6435 (1991)]. Fully equilibrated melts in the temperature range 6000 K >= T > 2000 K are considered as well as glass configurations, that were obtained by a rapid quench from the lowest melt temperatures. Each system is simulated at two different densities in order to study the effect of pressure on structural and dynamic properties. We find that the Al ato…

Spectroscopic parameters related to non bridging oxygen hole centers in amorphous-SiO2

2005

The relationship between the luminescence at 1.9 eV and the absorption bands at 2.0 eV and at 4.8 eV were investigated in a wide variety of synthetic silica samples exposed to different gamma- and beta-ray irradiation doses. We found that the intensities of these optical bands are linearly correlated in agreement with the model in which they are assigned to a single defect. This finding allows to determine spectroscopic parameters related to optical transitions efficiency: the oscillator strength of the 4.8 eV results ~200 times higher than that of the 2.0 eV; the 1.9 eV luminescence quantum yield under 4.8 eV excitation is lower (by a factor ~3) than that under 2.0 eV excitation. These res…

Optical absorption induced by UV laser radiation in Ge-doped amorphous silica probed by in situ spectroscopy

2007

We studied the optical absorption induced by 4.7eV pulsed laser radiation on Ge-doped a-SiO2 synthesized by a sol-gel technique. The absorption spectra in the ultraviolet spectral range were measured during and after the end of irradiation with an in situ technique, evidencing the growth of an absorption signal whose profile is characterized by two main peaks near 4.5eV and 5.7eV and whose shape depends on time. Electron spin resonance measurements performed ex situ a few hours after the end of exposure permit to complete the information acquired by optical absorption by detection of the paramagnetic Ge(1) and Ge-E' centers laser-induced in the samples.

Optical properties of Ge-oxygen defect center embedded in silica films

2007

The photo-luminescence features of Ge-oxygen defect centers in a 100nm thick Ge-doped silica film on a pure silica substrate were investigated by looking at the emission spectra and time decay detected under synchrotron radiation excitation in the 10-300 K temperature range. This center exhibits two luminescence bands centered at 4.3eV and 3.2eV associated with its de-excitation from singlet (S1) and triplet (T1) states, respectively, that are linked by an intersystem crossing process. The comparison with results obtained from a bulk Ge-doped silica sample evidences that the efficiency of the intersystem crossing rate depends on the properties of the matrix embedding the Ge-oxygen defect ce…

Radiation Effects on Silica-Based Preforms and Optical Fibers - II: Coupling Ab Initio Simulations and Experiments

2008

International audience; Both experimental and theoretical approaches are combined to study the nature of precursor sites and radiation-induced point defects in pure and germanium-doped amorphous silica-based glasses.

Selective Encapsulation and Enhancement of the Emission Properties of a Luminescent Cu(I) Complex in Mesoporous Silica

2018

Thermally Induced Structural Modification of Silica Nanoparticles Investigated by Raman and Infrared Absorption Spectroscopies

2010

We report an experimental investigation by Raman and infrared (IR) absorption spectroscopies on the structural modifications induced by isochronal thermal treatments on amorphous SiO2 nanoparticles (fumed silica). In particular, three different commercial types of this material, characterized by particle mean diameters of 7, 14, and 40 nm, were subjected to thermal treatments from 100 up to 1000 °C. We found that some properties of fumed silica, such as the SiOSi mean bond angle, ring size distribution, and surface adsorbed water content, are drastically different from those of common bulk silica materials and intimately related to the particles' dimension. The SiOSi mean bond angle, probed…