Alexandre Chmel

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 .

The role of driving rate in scaling characteristics of rock fracture

We present the chronological series of acoustic emission (AE) signals detected in samples of Westerly granite loaded in different regimes, one of which included a feedback loop between the axial stress and AE activity. In both regimes, the fracturing system evolves without a characteristic energy scale and with the same scaling exponent independently of the applied feedback. The scaling properties of the waiting-time distribution are more sensitive to both the drive control and the stage of fracture process. The revealed period of a lack of time correlation during continuous loading is explained by the transition between the hierarchic levels in the heterogeneous material.

Interaction of dopants with intrinsic structural defects in vitreous silica

Abstract The interactions of germanium, boron, phosphorus and fluorine dopants with the defect sites of vitreous silica were studied by the Raman spectroscopy method. The scattering cross sections of Raman lines associated with SiOGe, SiOB, P=0 and SiF structural groups were found, which made it possible to determine directly from spectra the absolute values of concentrations of the dopants listed. The incorporation of dopants into silica was shown to occur mainly over the defect sites of a silica network. The defects play the most significant role when germanium is incorporated, which is explained, apparently, by the structural similarity of silicate and germanate glasses. The generation of chemical groups of the GeO 2 phase (GeOGe) in place of defect sites was noticed. A similar effect was not observed for boron and phosphorus groups. The nature of defect sites responsible for 606 and 490 cm −1 Raman lines is discussed.

Laser-induced generation of structural defects in vitreous silica and in activated silicate glass

Abstract Ruptures of a silica network in a 1.06 μm-laser irradiated vitreous silica and activated silicate glass were observed by infrared and Raman spectroscopy. Irradiation conditions were so chosen that the laser beam should not produce visible traces of damage in the sample bulk or on the surface. The formation of broken chemical bonds was found not to be of threshold character, but the effect increases sharply as the power density in a laser pulse grows. The dependence of the concentration of newly formed ruptures on the number of repeated pulses exhibits a saturation and represents a kinetic curve of chemical bond destruction in a glass. It is assumed that the accumulation process is the cause of macroscopic breakdown of transparent dielectrics under repated laser irradiation.

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.

Microcracking in impact-damaged granites heated up to 600 °C

The acoustic emission (AE) technique was applied for monitoring the high-speed cracking process in three kinds of impact-damaged granites at temperatures 20 °C to 600 °C. The analysis of the amplitude-frequency response of the AE time series showed the presence of a few populations of cracks differing in size in the range 0.25 mm to 2.3 mm. The dependence of the appearance/growth/decay of any given population on temperature and physical and mechanical properties of granites evidenced the interplay between inter- and intragranular cracking under various thermal conditions. At room temperature (pure mechanical forcing), the intergranular damages along boundaries between the same minerals prevail; at 200 °C (impact forcing plus thermoelastic stress), the cracking between grains of quartz and feldspar emerge; finally, in the range 500 °C to 600 °C, the intragranular damages appear.

Acoustic emission accumulation stage in compression and impact rupture of granite

Specimens of granite were exposed to uniaxial compression and impacted by a dropped weight in order to obtain time sequences of acoustic emission signals generated due to micro-cracking. Time resolution in the impact fracturing was 10 ns. Both under compression and impact, the macroscale failure stage was preceded by micro-scale damages accumulation stage. The accumulation stage duration under compression is far much longer than the failure stage, while the situation is opposite in case of failure under impact.

Nonextensive statistical analysis of the data on the high-speed impact fracture of solids

The high-speed detection of impact-induced cracks in samples of materials that differ strongly in their degree of homogeneity (fused quartz and granite) has been performed by the acoustic emission method. The experimental energy distributions of the acoustic emission signals that correspond to the energy distribution in the events of the nucleation, growth, and coalescence of the microcracks have been interpreted in terms of the Tsallis statistics, which has been developed to generalize classical thermodynamics over the case of nonequilibrium systems. This allowed us to estimate the degree of correlation in the process of crack formation and to compare the energy release densities in various materials and at various stages of the impact fracture.

The fractality of sea-ice drift dynamics as revealed from the ‘North Pole 32’ monitoring

The O(n) vector model with logarithmic action on a lattice of coordination 3 is related to a gas of self-avoiding loops on the lattice. This formulation allows for analytical continuation in n: critical behaviour is found in the real interval [-2,2]. The solution of the model on random planar lattices, recovered by random matrices, also involves an analytic continuation in the number n of auxiliary matrices. Here we show that, in the two cases n=-1, -2, a combinatorial reformulation of the loop gas problem allows to achieve the random matrix solution with no need of this analytical continuation.

Scaling response of the amorphous alloy surface on loading and extrinsic crystallization

The surface nano-topography of the amorphous alloy Fe77Ni1Si9B13 was found to be anisotropic and fractal with the absolute values of Hurst exponent and fractal dimension close to those known for fracture surfaces of both conventional and amorphous metals. The tensile stress application causes the formation of shear bands (SBs), the highly prevailing orientation of which is normal to the stress applied. However, in very few cases, the SBs oriented at 45° to the stress direction detected. The latter SB orientation is typical for conventional (crystalline) metals where a specific dislocation mechanism determines the SBs orientation; the occurrence of 45°-oriented SBs in the amorphous alloy was attributed to the local, stress-induced extrinsic crystallization at the sample surface. In the region of heterogeneity of this kind, the scaling properties of surface relief were fully disturbed. The mechanism of the fractal roughness formation was related to the fractal geometry of thermal vibrations of walls of primary cavities during the new surface formation under critical conditions, such as that take place at fracturing or ultra high cooling.