Search results for "Composites"
showing 10 items of 1905 documents
Radiation induced generation of non-bridging oxygen hole center in silica: Intrinsic and extrinsic processes
2007
Abstract The generation of non-bridging oxygen hole center ( Si–O ) was investigated in a wide variety of natural (fused quartz) and synthetic silica samples exposed to different γ- and β-irradiation doses by looking at its optical bands. We distinguish two different generation processes: intrinsic associated with the cleavage of Si–O bond and characterized by a sublinear law and extrinsic due to the conversion of OH precursor characterized by a growth curve with a saturating tendency. The interplay between the two processes and the role of H are discussed.
Growth of paramagnetic defects by gamma rays irradiation in oxygen-deficient silica
2005
Abstract We report an electron paramagnetic resonance (EPR) study of gamma rays irradiation effects on an oxygen-deficient silica obtained from fused quartz. We have found that three types of E′ centers, E α ′ , E γ ′ and E δ ′ , and a paramagnetic defect in a spin triplet state (S = 1) are induced. The EPR signals dependence on the microwave power have been investigated. The concentrations of these centers as a function of gamma ray irradiation show that for high dose a limit value is reached suggesting a generation process from precursors.
Luminescence of GeO2 glass, rutile-like and α-quartz-like crystals
2006
Abstract The luminescence of GeO 2 rutile-like crystals was studied. Crystals were grown from a melt of germanium dioxide and sodium bicarbonate mixture. Luminescence of the crystal was compared with that of sodium germanate glasses produced in reduced and oxidized conditions. A luminescence band at 2.3 eV was observed under N 2 laser (337 nm). At higher excitation photon energies and X-ray excitation an additional band at 3 eV appears in luminescence. The band at 2.3 eV possesses intra-center decay time constant about 100 μs at 290 K and about 200 μs at low temperature. Analogous luminescence was obtained in reduced sodium germanate glasses. No luminescence was observed in oxidized glasses…
Effect of pressure and temperature on viscosity of a borosilicate glass
2018
International audience; During industrial glass production processes, the actual distribution of stress components in the glass during scribing remains, to date, poorly quantified, and thus continues to be challenging to model numerically. In this work, we experimentally quantified the effect of pressure and temperature on the viscosity of SCHOTT N-BK7 glass, by performing in situ deformation experiments at temperatures between 550 and 595 °C and confining pressures between 100 MPa and 300 MPa. Experiments were performed at constant displacement rates to produce almost constant strain rates between 9.70 × 10 −6 s-1 and 4.98 × 10-5 s-1. The resulting net axial stresses range from 81 MPa to 8…
Investigation of the Na2O/Ag2O ratio on the synthesis conditions and properties of the 80TeO2–10ZnO–[(10−x)Na2O–xAg2O] glasses
2019
International audience; Properties of the tellurite glasses 80TeO2–10ZnO–[(10−x)Na2O–xAg2O] are investigated as a function of the substitution ratio x between Na2O and Ag2O. One observe that the variation of glass transition temperature decreases monotonously with x and that surface crystallization mechanism is favored. The assignment of the Raman bands and their relation with the underlying glass structure is discussed. While both Na2O and Ag2O oxides act as glass network modifiers, their progressive equimolar substitution does not lead to a meaningful evolution in the structure of the TZ[Na10-xAgx] glass. The refractive index and the cut-off wavelength are found to increase with x. The co…
Reactivity of SiCl and SiF groups in SiO2 glass with mobile interstitial O2 and H2O molecules
2007
Reactions of common network-bound halogens in synthetic SiO2 glass, SiCl and SiF groups, with interstitial O2 and H2O molecules incorporated by thermally annealing were studied. It was found that the chemical properties are distinctly different between SiCl and SiF groups. SiCl groups react with interstitial O2 and H2O to form interstitial Cl2 and HCl, respectively. In contrast, formation of interstitial F2 and HF due to the reaction of SiF groups with interstitial O2 and H2O is not observed. The reactivity of SiCl and SiF groups is in accord with the properties and thermodynamic data of their respective analogous compounds, SiCl4 and SiF4.
Ni-containing spinel aluminates glass-ceramic materials obtained from cordieritic bulk glasses
2003
Monolithic glasses with compositions 2MO � 2Al2O3 � 5SiO2, being M ¼ Ni and equimolar mixtures of Ni and Mg, were prepared at 1650 C by melting mixtures of raw materials. The crystallization of monoliths was produced by heattreatment at several temperatures up to 1200 C. The crystallization sequence was followed by differential thermal analysis (DTA), X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) and diffuse reflectance spectroscopies. Surprisingly, the only crystalline phase formed after heating up to 1100 C was a nickel-containing aluminate spinel for both compositions. The microstructural characterization indicated t…
Effect of some additives on the development of spinel-based glass-ceramic glazes for floor-tiles
2005
Abstract The feasibility of developing spinel-based glass-ceramic glazes from a glass with composition in the system ZnO–MgO–B 2 O 3 –Al 2 O 3 –SiO 2 was examined. To do it additional fluxes and/or nucleants were added to a parent glass before melting. Pressed pellets of powdered glasses were submitted to standard thermal treatments up to 1200 °C. The crystallization path and the microstructural development at several temperatures were followed by several experimental techniques. The results showed that additions of TiO 2 as nucleant or additional B 2 O 3 as flux to the chosen glass favored the crystallization of cordierite or mullite as main crystalline phase. Glasses which also contained …
Biodegradable hydrogels obtained by photocrosslinking of dextran and polyaspartamide derivatives
2003
The functionalization of dextran with glycidyl methacrylate (GMA) leads to the formation of a derivative that generates hydrogels for irradiation at 365nm. The effects of various polymer concentrations and irradiation times on the yield and the properties of the obtained hydrogels are reported. The networks have been characterized by FT-IR spectra, dimensional analysis and swelling measurements carried out at different pH values. In vitro studies suggest that all samples undergo a partial chemical hydrolysis, whereas the incubation with dextranases causes a total degradation whose rate depends on the degree of crosslinking. In addition, aqueous solutions of functionalized dextran have been …
Sweet (hetero)aromatics: glycosylated templates for the construction of saccharide mimetics
2011
Mono- and diglycosylated aromatics and heteroaromatics may serve as building blocks for the construction of metabolically stable mimetics of oligosaccharides. Methods for their preparation from monosaccharidic precursors by direct C-glycosylation, dipolar cycloaddition or Larock cyclization are described.