V. V. Golubkov

On the phase separation and crystallization of glasses in the MgO-Al2O3-SiO2-TiO2 system

The changes in the structure and phase composition of glasses in the MgO–Al2O3–SiO2–TiO2 system (at different TiO2 contents and ratios MgO : Al2O3) upon their heat treatment in the temperature range 700–960°C are investigated by small-angle X-ray scattering (SAXS) technique and X-ray powder diffraction analysis. The influence of gallium oxide additives on the phase separation and crystallization is analyzed. It is demonstrated that the heat treatment results in the phase separation, which occurs through the spinodal decomposition mechanism. A regular structure formed upon phase separation is retained after the completion of crystallization in inhomogeneity regions. The interference effects due to the regularity in the distribution of nanocrystals in the vitreous matrix bring about a decrease in the light scattering intensity and provide transparency of glass-ceramic materials.

A Raman Spectroscopic Study of Phase Transformations in Titanium-Containing Magnesium Aluminosilicate Glasses

The evolution of the Raman spectra of the 20MgO · 20Al2O3 · 60SiO2 glass with 10 mol % TiO2 and glasses doped with 0.5 wt % NiO or 2.5 and 5.0 mol % Ga2O3 is investigated in the course of heat treatment resulting in phase separation and crystallization. The composition and structure of the amorphous and crystalline phases (precipitating in the course of phase separation) and the residual glass are analyzed on the basis of Raman scattering, small-angle X-ray scattering (SAXS), and X-ray powder diffraction data. The size of nanoparticles of the precipitated phases is determined by low-frequency Raman spectroscopy, and the results obtained are compared with the nanoparticle size derived from the SAXS data.

Low-frequency Raman scattering of magnesium aluminosilicate glasses and glass-ceramics

Abstract The evolution of low-frequency ( −1 ) Raman scattering of titania-containing magnesium aluminosilicate (20MgO:20Al 2 O 3 :60SiO 2 with 10 mol% TiO 2 ) glasses with heat-treatment has been studied. It was found that in spectra of heat-treated glasses the intensity of the boson peak is rapidly diminished. Under certain conditions of heat-treatment schedules the boson peak disappears while the sample remains amorphous. It implies that sometimes the boson peak is missing from spectra of amorphous materials. In Raman spectra of heat-treated glasses that show the disappearance of the boson peak a new narrow highly polarized low-frequency band appears and increases in intensity. This band has been associated with a spheroidal mode of elastic vibrations in nanometer sized particles of aluminomagnesium phase formed upon phase decomposition of the glass. Its frequency changes with heat-treatment in the temperature range 750–950°C, from 50 to 17 cm −1 reflecting an increase in the sizes of corresponding nanometer particles. The composition and structure of amorphous and crystalline phases formed upon heat-treatment and also of residual glass as well were analyzed using the Raman scattering data, small angle X-ray scattering (SAXS) and X-ray diffraction (XRD) analysis.

Kinetic regularities of the precipitation of PbS nanocrystals in sodium zinc silicate glasses

The kinetics of formation of nanostructured glass-ceramic materials based on silicate glass doped with lead sulfide PbS is investigated using small-angle X-ray scattering and X-ray powder diffraction. The influence of two-stage heat treatment on the properties of the material is studied. It is established that, in the initial stages of the precipitation during secondary heat treatment, the volume of the lead sulfide phase increases proportionally with the cube of the nanocrystal size; i.e., the number of growing particles is constant and does not depend on the temperature of the secondary heat treatment. At high temperatures of the secondary heat treatment, an increase in the heat treatment time leads to a decrease in the volume of the crystalline phase. The inference is made that this effect is possibly caused by the reaction PbS + Na2O = PbO + Na2S, which should bring about a decrease in the lead sulfide concentration in the matrix and the dissolution of precipitated nanocrystals. The observed kinetics of crystal growth has defied description in terms of the known mechanisms. This is explained by the occurrence of a competing process of dissolution of the crystalline phase that is responsible for the absence of the Ostwald ripening stage, which is governed by the universal kinetics of diffusion phase decomposition at the late stages in supersaturated solid solutions.

Self-Organization of Colloidal PbS Quantum Dots into Highly Ordered Superlattices

X-ray structural analysis, together with steady-state and transient optical spectroscopy, is used for studying the morphology and optical properties of quantum dot superlattices (QDSLs) formed on glass substrates by the self-organization of PbS quantum dots with a variety of surface ligands. The diameter of the PbS QDs varies from 2.8 to 8.9 nm. The QDSLs period is proportional to the dot diameter, increasing slightly with dot size due to the increase in ligand layer thickness. Removal of the ligands has a number of effects on the morphology of QDSLs formed from the dots of different sizes: for small QDs the reduction in the amount of ligands obstructs the self-organization process, impairing the ordering of the QDSLs, while for large QDs the ordering of the superlattice structure is improved, with an interdot distance as low as 0.4 nm allowing rapid charge carrier transport through the QDSLs. QDSL formation does not induce significant changes to the absorption and photoluminescence spectra of the QDs. However, the luminescence decay time is reduced dramatically, due to the appearance of nonradiative relaxation channels.

Spectroscopic and X-ray Diffraction Investigations into the Specific Features of Crystallization of Potassium Niobium Silicate Glasses

The Raman spectra and X-ray diffraction patterns of initial and crystallized glasses of the composition 25K2O · 27.5Nb2O5 · 47.5SiO2 (mol %) are investigated. It is demonstrated that, upon heat treatment at temperatures in the range 740–900°C, nanocrystals similar in composition to the initial glass precipitate in the glass. The sizes of nanocrystals are estimated with the use of both methods.

Sol-Gel synthesis of an optical silica glass doped with rare-earth elements

An optical quality silica glass doped with Er3+, Yb3+ and Lu3+ ions is synthesized by the sol-gel method. Virtually unavoidable clusterization of rare-earth elements that occurs upon introduction of lanthanide salts into a sol-gel system and leads to concentration quenching of luminescence of the glass is eliminated through the use of rare-earth phosphate complexes that are chemically bound to the SiO2 network. This makes it possible to obtain a nearly molecular distribution of rare-earth elements in the glass matrix.

Influence of heat treatment conditions on the precipitation and dissolution of lead sulfide nanocrystals in sodium zinc silicate glasses

The influence of the temperature of the primary heat treatment in the case of two-stage heat treatment of glasses on the kinetics of precipitation of PbS nanocrystals and the structure of the glass-ceramic materials prepared has been investigated. The main structural parameters of the glass-ceramic material, such as the radius of lead sulfide nanocrystals, relative volume of the crystalline phase, concentration of nanocrystals, and dispersion of nanocrystal sizes, have been determined under different heat treatment conditions. It has been demonstrated that a variation in the temperature of the primary heat treatment in the range below the glass transition temperature of the glass matrix makes it possible to control the nanocrystal sizes and to obtain close-to-monodisperse size distributions of nanocrystals.

Morphology of silicate glasses with lead sulfide nanocrystals

The specific features in the structure of nanostructured glass-ceramic materials based on sodium zinc silicate glasses doped with lead sulfide are investigated using small-angle X-ray scattering and X-ray powder diffraction. It is established that a close-to-monodisperse size distribution of PbS nanocrystals is an important structural feature of the system under investigation. An analysis of the X-ray powder diffraction data demonstrates that the shape of the lead sulfide nanocrystals precipitated depends on the heat treatment conditions and can differ from a spherical shape.

Crystallization of Glasses in the K2O–Nb2O5–SiO2System

The changes in the structure and phase composition of glasses in the K2O–Nb2O5–SiO2system upon their heat treatment in the temperature range 700–800°C are studied by the small-angle X-ray scattering (SAXS) technique and X-ray powder diffraction. It is demonstrated that the crystallization is the primary process giving rise to microinhomogeneities in glasses due to heat treatment. Nanocrystals of an unidentified niobium-containing phase precipitate in glasses with the formation of regions with a decreased content of potassium and niobium oxides. An increase in the duration of heat treatment at the studied temperatures results in an increase in the size of nanocrystals without change in their phase composition. This is accompanied by the disappearance of diffusion zones, which leads to a decrease in the SAXS intensity in the range of small scattering angles and, correspondingly, to a decrease in the light scattering intensity.