T. V. Antropova

Size-driven ferroelectric–paraelectric phase transition in TGS nanocomposites

Dielectric properties of porous glass nanocomposites with TGS crystals embedded into six porous matrices with average pore size from 5 to 312xa0nm were investigated in the temperature range from 280 to 380xa0K at selected frequencies. The results are discussed based on the effect of the particle size on the phase transition temperature of TGS nanocomposites. Temperature–size phase diagram of TGS composites was derived. Non-monotonic character of the temperature-driven phase transition (Tp) with the decreasing particle size was determined. The nature of the Tp variation can be ascribed to the size-effect theoretically predicted by Zhong et al. (Phys Rev B 50:698–703, 1994).

Influence of the acid concentration on the morphology of micropores and mesopores in porous glasses

The polymodal microporosity and mesoporosity of porous glasses prepared by through leaching of phase-separated alkali borosilicate glasses in hydrochloric acid solutions of different concentrations are investigated by two new methods based on an analysis of the kinetic and equilibrium isotherms of gas desorption at a temperature of 77.5 K under low partial pressures, as well as by a modified classical adsorption method under medium and high pressures. It is established that an increase in the concentration of hydrochloric acid in the leaching solution brings about transformations in the microporous and mesoporous structures of the porous glasses. The structural transformations can be summarized as follows: (i) a considerable decrease in the specific surface of mesopores and the corresponding increase in the diameter of secondary silica globules, (ii) a decrease in the mesopore volume and the corresponding increase in the coordination number of the packing of secondary silica globules in channels formed upon phase separation, (iii) an increase in the diameter of mesopores arising from gaps between secondary silica globules in channels formed upon phase separation, (iv) an increase in the diameter of regions in these channels that are partially or completely free from secondary silica globules, and (v) the formation of ultramicropores with a molecular size of 0.4–0.5 nm.

Size effects in KDP-porous glass ferroelectric nanocomposites

An influence of the size effect on the phase transition temperature in potassium dihydrogen phosphate (KDP)-porous glass nanocomposites was studied in this work. Results of the dielectric measurements in a wide temperature range for KDP crystals embedded in porous glasses are presented. The phase transition temperature shift toward higher temperatures was observed for KDP embedded in porous glasses with the mean value of the pore size of 312 and 160u2009nm, whereas for smaller pore sizes (71 and 23u2009nm) the phase transition temperature decreased. The phase transition temperature dependence of the mean values of the pores dimensions of KDP-porous glass nanocomposites is the experimental evidence showing the non-monotonical character of the size effect on the phase transition temperature in ferroelectrics particles. It was found that the dielectric permittivity of KDP embedded into the porous glasses with the mean values of pore sizes of 160u2009nm and 312u2009nm practically did not depend on frequency, whereas for KDP embedded into the pores with dimensions of 71u2009nm and 23u2009nm a dispersion of the dielectric permittivity is observed in both para- and ferro-electric phases.

Microporosity of Porous Glasses: New Investigation Techniques

Two new methods are developed for investigating the microporosity of porous materials. The first method, namely, diffusion diagnostics, is based on a computer simulation of the kinetics of gas desorption from a porous material into high vacuum. In this method, the kinetics of gas desorption is measured using mass spectrometry. Another method is based on the analysis of the equilibrium isotherms of gas desorption at low pressures. The microporous and mesoporous substructures of the porous glasses prepared by leaching of two-phase alkali borosilicate glasses in a 3 M HCl solution at 100°C are investigated using both the new methods and the classical adsorption methods. A correlation between the porous structure of glasses and their synthesis and phase separation conditions is revealed. It is demonstrated that porous glasses have a through mesoporous substructure with mean pore diameters ranging from 4 to 15 nm and a polymodal microporous substructure with pore sizes ranging from 0.35 to 2 nm. This microporous substructure is formed by ultramicropores of molecular sizes and also medium- and large-sized micropores. It is found that porous glasses contain bottle-shaped pores with sizes corresponding to the micropore range. An increase in the temperature of heat treatment of glasses from 550 to 700°C and an increase in the boron oxide content in the silica phase lead to an increase in the volume of micropores in porous glasses.

Peculiarities of the formation of planar micro-optic elements on porous glass substrates under the effect of laser radiation followed by sintering

The theoretical and practical approaches to the formation of micro-optic elements (MOE) for the integrated optical communication on substrates made of different materials, including silicate glasses, have been reviewed. The applicability of high-silica porous glasses (PG) fabricated by a homogeneous acid leaching of two-phase alkali borosilicate glasses as basic matrices for laser formation of MOE of different types (microlenses and microchannels) has been discussed. It has been demonstrated that sintering laser-modified PGs at temperatures ensuring the collapse of the pore can be used to stabilize MOE optical properties. The results of choosing the time-temperature mode for sintering PG substrates with a planar MOE-strip waveguide (SW) providing the SW fixation on a completely sintered substrate were presented.

Specific Features of the Formation of a Porous Structure in Products of Leaching of Two-Phase Sodium Borosilicate Glasses in Acid-Salt Solutions

This paper reports on the results of an investigation into the influence of the addition of a KCl salt to an HCl acid solution on the leaching kinetics of two-phase sodium borosilicate glasses and on the structural parameters of the leaching products (porous glasses). The contributions from different molecular species of silica to its total amount in leaching solutions are evaluated using the kinetic spectrophotometric technique for determining silicon in the form of β-silicomolybdic heteropoly acid. It is revealed that the addition of the salt to the leaching acid solution leads to a fast polymerization process with the formation of weakly structurized silica gel.

Femtosecond laser-induced stress-free ultra-densification inside porous glass

Unusually high densification ≤26% was obtained without lateral residual stresses within the laser beam waist inside porous glass during its multi-shot femtosecond laser irradiation, which may induce in the glass the related high refractive index change ~0.1. Corresponding laser irradiation regimes, resulting in such ultra-densification, decompaction and voids inside the glass, were revealed as a function of laser pulse energy and scanning rate, and were discussed in terms of thermal and hydrodynamic processes in the silica network.

Surface Sintering of Porous Glass Plates under Laser Radiation

The kinetics of surface sintering of flat plates of the 96SiO2 · 3.8B2O3 · 0.2Na2O (mol %) porous glass exposed to CO2 laser radiation is investigated under conditions when either the irradiation time is varied at a constant radiation power density or the radiation power density is varied at a constant irradiation time. It is established that the irradiation time is the most appropriate controlled parameter in the process of sintering. A decrease in the thickness of the porous layer sintered on the plate surface is accompanied by an increase in the radii of curvature of the interfaces of this layer.

Local laser-induced change of optical properties in the bulk of photochromic porous silicate glass doped by silver and copper halides

The principles of the formation of modified areas of a spherical shape and complex structure in porous glass plates doped by silver and copper halides under the continuous radiation of a fiber laser focused inside the plate have been formulated. The results of the study of the obtained modified areas are given.

Laser induced structural changes in porous glass due to hot and cold compaction

A comparison is presented of the structural changes occurring in porous glass during laser-induced hot and cold compaction. The changes occurring in modified regions of different compaction processes when a porous-glass plate with a micro-region is sintered in a furnace are also compared.