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27. Structural Modifications in Silica Sonogels Prepared with Additions of Poly(vinyl alcohol)

Author

A. Portella, J., A. Donatti, D., Ibañez Ruiz, A., and R. Vollet, D.

Abstract

Silica wet gels were prepared from acid sonohydrolysis of tetraethoxysilane (TEOS) and additions of poly(vinyl alcohol) (PVA)−water solution. Aerogels were obtained from supercritical CO2extraction. The samples were studied by thermal gravimetric (TG) analysis, small-angle X-ray scattering (SAXS), and nitrogen adsorption. The structure of wet gels can be described as a mass fractal with dimension Dequal to 2.0 on the whole length scale experimentally probed by SAXS, from ∼0.3 to ∼15 nm. Pure and low-PVA-addition wet gels exhibit an upper cutoff accounting for a finite characteristic length of the mass fractal structure. Additions of PVA increase without modifying D, which was attributed to a steric effect of the polymer in the structure. The pore volume fraction of the aerogels diminishes typically about 11% with respect to that of the wet gels, although nitrogen adsorption could be underestimating some porosity. The pore size distribution of the aerogels is shifted toward the mesopore region with the additions of PVA, in a straight relationship with the increase of in the wet gels. The thermal stability of the pore size distribution of the aerogels was studied up to 1000 °C.

Published
2008
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28. Elasto- and electro-optical dipole properties of off-center Ag+defects in RbI

Author

Donatti, D. A., Li, M. S., and Aegerter, Michel A.

Subjects
Diffusion, Iodide, Dipol, General Engineering, ddc:500, ddc:620, Rubidium, Absorption
Abstract

Electric dipole properties of off-center Ag+ defects, introduced by a diffusion technique, were investigated in RbI. After rapid quenching and cooling the absorption spectrum consists of 4 UV bands at 4.82, 4.92, 5.12 and 5.47 eV. Their oscillator strength slightly decreases then rises with increasing temperature. Weak zero moment changes in all bands were observed when high electric field was applied along the and directions only. No effect occurs along . The sign of the effect allows the determination of the main state of polarization of the bands: perpendicular to the dipole axis for the 4.82 and 5.12 eV, parallel to it for the 4.92 and 5.47 eV. Two models which fit equally well the results are discussed: a dipole model and a restrictive model with 90° reorientation only. They lead to a dipole moment of 1.16 eÅ, respectively 1.34 eÅ if polarization of the bands is assumed. Several arguments favor the model with deep off-center wells for Ag+ in RbI.

Published
1983

34. Elasto- and electro-optical dipole properties of off-center Ag defects in RbI

Author

Donatti, D., Li, M. S., and Aegerter, M. A.

Abstract

Electric dipole properties of off-center Ag+ defects, introduced by a difusion technique, are investigated in RbI. After rapid quenching and cooling the absorption spectrum consists of 4 UV bands at 4.82, 4.92, 5.12 and 5.47 eV. Their oscillator strength slightly decreases then rises with increasing temperature. Weak zero moment changes in all bands are observed when high electric field is applied along the <110> and <111> directions only. No effect occurs along <100>. The sign of the effect allows the determination of the main state of polarization of the bands: perpendicular to the dipole axis for the 4.82 and 5.12 eV, parallel to it for the 4.92 and 5.47 eV. Two models which fit equally well the results are discussed: a <111> dipole model and a restrictive <110> model with 90° reorientation only. They lead to a dipole moment of 1.16 eÅ, respectively 1.34 eÅ if we assume full polarization of the bands. Several arguments favor the <111> model with deep off-center wells for Ag+ in RbI.

Published
1983
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