V.S. Shashkin

Vibrational spectra and deffect structure of silica prepared by non-organic sol-gel process

Abstract Samples of porous and monolithic silica glass produced by the low-temperature non-organic sol-gel process were studied by vibrational spectroscopy methods. The spectra of finely porous samples with 0.6–0.9 nm-thick inter-pore membranes reveal a set of the bands associated with all possible kinds of structural defects observable in IR and Raman spectra. The comparison of Raman spectra of the porous samples obtained under different conditions shows that the 490 cm −1 ‘defect’ line intensity grows in accordance with the intensity of the ‘surface’ silanol groups line at 980 cm −1 .

Structural correlation and structural relaxation at the final stage of gel-to-glass transition in silica

Abstract In this work more detailed information is obtained about the final stage of the gel-to-glass transition and the interrelations between the glass network structure and the mechanical properties of the material are revealed. The structural modification of silica xerogels was followed by measuring Vickers hardness and Youngs modulus in parallel with measurements of the network structure by means of IR reflection and low-frequency Raman spectroscopies. Two correlations of material properties and scales have been found: (i) at annealing temperature, Ta=1000°C, the relaxation of the silica network produces a definite correlation sphere and an increase of Youngs modulus to that for vitreous SiO2; (ii) at Ta=1160°C, a decrease of the mobility of glass-forming units causes a decrease of the correlation radius and an increase of Vickers hardness to that specific for vitreous SiO2.

Raman study of fractal voids in hot-pressed SiO2

The sensitivity of the dynamic (vibrational) properties of fractal nanocracks in hot-pressed silica to the thermal and mechanical prehistory of the material was studied by low-frequency Raman scattering. Samples were prepared from gel-derived and fused powdery products. The Raman data demonstrate the fractality of elementary continuity defects. From the frequency dependence of the Raman intensity, the values of the fracton (spectral) dimension that reflects the mid-range order in glass, and the dimensions of fractals in real space were calculated.

Déplacement du pic Boson dans le spectre Raman de la silice préparée à partir de gels

Abstract The appearance of so-called Boson peak in the low-frequency Raman spectrum of silica xerogel was observed after the heat treatment of specimens at T a =1000°C. The evolution of the peak frequency and intensity as well as Brillouin scattering data give evidence of decreasing of structure correlation radius with increasing of T a with simultaneous growing of degree of ordering inside the vibrational modes localization units.

Fracton vibrations in vitreous SiO2 as revealed from Raman spectroscopy study

Low-frequency Raman spectroscopy is used to study the sensitivity of the dynamic (vibrational) properties of the fractal structure of nanocracks in vitreous SiO2 to the mechanical and thermal previous history of the sample. The material is obtained by vacuum-compression fritting of the sol-gel synthesis products. After fritting, continuity defects having a nanostructural scale and possessing fractal geometry are left at the site of microcracks in the course of preliminary mechanical processing of the sample. The features of such defects are an absence of the boson peak in the Raman spectrum and a monotonic decrease of the intensity according to the law of light scattering from acoustic vibrations of fractals. The conditions for the emergence of the fractal structure and its dependence on the rigidity of the walls of nanocracks are determined from the scattering mode.

Effect of bound water on the fractal geometry of nanosized cracks in glasses prepared by vacuum pressure-assisted sintering

The vibrational properties of fractal nanocracks in vitreous SiO2samples prepared by vacuum pressure-assisted sintering are investigated by low-frequency Raman scattering spectroscopy. In the Raman spectrum, nanosized discontinuities with a fractal geometry manifest themselves in a specific frequency dependence of the scattering intensity: a monotonic decrease in the intensity according to the law of light scattering by acoustic vibrations localized on fractals is observed instead of the boson peak. The frequency dependence of the low-frequency Raman scattering intensity is analyzed for samples of different origins. A “smoothing” of the nanocrack profile with a decrease in the amount of bound water in the material is revealed.

Manifestation of glass-crack fractal geometry in Raman spectra

A Raman spectroscopic investigation of the fractal structure on the surface of cracks of different size scales in glassy SiO2 is reported. A study was made of the crack surface and of the distorted layer formed after grinding and polishing and comminution. The parameters of the fractal structure thus found reflect the properties of the material and the crack nature; namely, fractal dimension is of greater importance for more compact materials, and the geometric size of fractal units (i.e., the size in real space) correlates with crack dimensions.