Vol 44, No 6 (2018)

A combinational-topological analysis and simulation of the crystal structure of Na₂₆K₈Ga₁₀₄-hR414, R\(\bar {3}\)m, V = 8109 ų are performed by computer methods (the TOPOS software package). The framework-forming 124-atom three-layered icosahedral nanocluster ico-K124 = 0@12(Ga₁₂)@32(Ga₁₂Na₂₀)@80(Ga₆₀Na₆K₁₄) with a diameter of 17 Å and the symmetry g = \(\bar {3}\) is established by the method of the complete decomposition of a 3D atomic lattice into cluster structures. The second shell corresponds to the Bergman cluster with 32, 90, and 60 peaks, edges, and faces, respectively. The chemical composition of the shell is Ga₁₂Na₂₀. The third shell consisting of 80 atoms is characterized by 80, 210, and 132 peaks, edges, and faces, respectively. The chemical composition of the shell is made up of Ga₆₀Na₆K₁₄. Sixty atoms of Ga in the shell are located in the same manner as 60 carbon C atoms in the fullerene С₆₀. All 20 positions above the Ga₆ hexagons are occupied by 6 Na atoms and 14 K atoms. The symmetry and topological code of the self-assembly of 3D structures from iсo-K143 nanoclusters-precursors are simulated in the following form: primary chain → layer → framework. iсo-K124 nanoclusters form densely packed 2D layers 3⁶. The large voids in the 3D framework are occupied by 10-atom GaGa₉ clusters with the symmetry g = 3m.

The glass ceramic with the crystals of lithium disilicate Li₂O · 2SiO₂ (LS₂) as the main crystal phase is one of the prospective materials in the field of restorative dentistry. In this study, the crystallization kinetics of LS₂ glass ceramic obtained from the base glass of the system SiO₂–Li₂O–Al₂O₃–K₂O–P₂O₅ were investigated by the non-isothermal method using differential thermal analysis at four different heating rates. The DTA curves showed different exothermic crystallization peaks over the temperature ranges of 645–683 and 807–845°C. The lithium metasilicate, Li₂O · SiO₂ (LS), and the lithium disilicate, crystallized over these respective temperature ranges, was established by XRD technique. The crystallization kinetic parameters were calculated by the Kissinger plot and Augis-Bennett equations for non-isothermal analysis. The calculated activation energy of crystal growth, E_C1= 236 kJ/mol, E_C2 = 340 kJ/mol, and the Avrami parameters, n₁ = 1.46–1.67, n₂ = 2.73–2.91, together with the results from SEM observations, indicated that the crystallization mechanism of LS was substantial surface crystallization while the crystallization mechanism of LS₂ was dominant bulk crystallization. The calculated activation energy of glass transition was also determined E_V = 516 kJ/mol.

The experimental results are presented for a laser-induced modification of the structure of porous glass (PG) preliminarily doped with an aqueous nitric-acid solution of Bi(NO₃)₃. It is established that the laser-induced local stabilization of bismuth active centers (BACs) in the bulk of PG plates is possible, which is confirmed by the change in the luminescence in the irradiation zone.

IR absorption spectra of quartz glass with varying contents of hydroxyl OH groups, namely, KU1 (1000 ppm OH) and KS-4V (≤0.1 ppm OH) are compared prior to and after exposure to fast neutron fluences of 10¹⁸–5 × 10¹⁹ cm^–2. The appearance of the IR absorption band of 2260 cm^–1 regardless of the large difference in the content of OH groups in nonirradiated KU1 and KS-4V glass is related to the bending vibrations of the bridging ≡Si−O−Si≡ bonds. A decrease in the amplitude of the band at 2260 cm^–1 and shift of its maximum by 20 cm^–1 to the IR range is caused by the decrease in the bridging ≡Si−O−Si≡ bond angle with the growth of the neutron fluence. The asymmetric band at 3674 cm^–1, which is related to OH groups, is observed only in nonirradiated KU1 glass. An increase in the number of OH groups in KU1 glass and the formation of OH groups in KS-4V glass after the accumulation of the neutron fluence of ≥10¹⁹ cm^–2 rationalizes the slow accumulation of the intrinsic defects such as nonbridging oxygen atoms and oxygen vacancies in both brands of the glass in the range of the neutron fluence.

Shrinkage during the sintering of porous glass (PG) of different backgrounds and the viscosity of quartz-like glass (QG) sintered until the pores collapse are measured. The samples are obtained as a result of acid leaching of phase-separated sodium borosilicate glass. The initial PG and the samples that are doped with bismuth nitrate from a water-salt solution are studied. It is found that the heat treatment of the matrices at 750°C leads to an increase in their thermal stability. The viscosity of the matrices sintered at 870°C is practically independent of their pretreatment temperature. The viscosity of bismuth-containing quartz-like glass (BQG) decreases with an increase in the matrices’ treatment temperature.

The structural state of metal epoxysilicate and metallosilicate nanocomposites obtained by the sol-gel technology from systems based on alkali silicate solutions is studied. Hybrid nanocomposites prepared based on liquid glass (LG), in the initial state and after modification by copper salts (CuCl₂ and CuSO₄) and/or by the second component such as ED-20 epoxy resin, are considered. Based on the data of the XRD analysis and scanning electron microscopy, it is shown that the modified xerogels are structurally inhomogeneous, whereas the change in the microwave exposure time used in the synthesis stage to homogenize the mixture and intensify the physicochemical interaction processes affects the size and character of the packing of inhomogeneities. For the studied samples, the coordination numbers, coordination sphere radii, and their variances are calculated and analyzed.

The formation of crystalline phases during cooling of the Na₂O–Al₂O₃–P₂O₅ melts obtained in corundum crucibles at a temperature of 1100°С and at atmospheric pressure from a pre-prepared glass with the composition close to that used for immobilization of highly active radioactive waste (RW) is studied. It is shown that under slow cooling, the crystallization process is accompanied by changes in the liquidus compositions of the melts leading to their transformations into glasses with compositions that, in some cases, are very different from the initial ones. A new topology of the melting diagram of a low-alumina section of three-component system is proposed, the melt’s compositions are determined, and the temperatures of phase equilibria, which play a crucial role in industrial technologies, are estimated. The formation of crystalline phases under the slow cooling of the Na₂O–Al₂O₃–P₂O₅ melts should be taken into account in the vitrification of real RW.

A synthesis of thin films consisting of hydrogenated silicon oxycarbonitride using high-frequency discharge plasma from a novel organosilicon compound methyltris(diethylamino)silane (MTDEAS) mixed with oxygen and nitrogen are studied. CVD diagrams are obtained as a result of the thermodynamic modeling of the Si–C–N–H–O system in the temperature range from 300 to 1300 K, which makes it possible to optimize the synthesis of film materials such as SiC_xN_yO_z:H. The silicon oxycarbonitride films of various compositions are experimentally obtained in the range of deposition temperatures from 373 to 973 K for different compositions of the initial MTDEAS + O₂+ xN₂ gas mixture. A change in the chemical composition of the gas mixtures leads to obtaining SiC_xN_yO_z:H films having a large range of functional properties; the refractive index varies from 1.5 to 2.21; the adjustable transparency varies from 92–99.7% in the UV visible and IR spectral regions; and the tunable band gap characteristics vary in the range from 2.5 to 4.5 eV and from 2.9 to 0.7 eV for films grown from MTDEAS + O₂+ N₂ and MTDEAS + O₂+ 2N₂ mixtures, respectively. It is shown that the films contain nanocrystals of phases belonging to structures such as Si_{3 – x}C_xN₄ embedded in the amorphous matrix of hydrogenated silicon oxycarbonitride layers.

The conditions required for the gas transport of Ti, Zr, Mn, and Cr metals on corundum powders are studied. The core-shell materials are obtained and their properties are studied.

The possibilities of creating special-purpose ceramics by the spark plasma sintering (SPS) according to the principle of the low-voltage electro-pulse consolidation of powders under the conditions of an external mechanical load is investigated. Ceramics applicable in the nuclear, medical, and electrotechnical industries are fabricated. A methodological description and special features of the presented technique are provided for the first time in terms of fabricating nuclear ceramics in the form of a pellet product of UO₂; composite bioceramics with a theoretical density exceeding 97.6% (based on ZrO₂ doped by hydroxyapatite) characterized with controlled (meso-, macro-) porosity and a high compression strength of ~400 MPa; and magnetic ceramics resistant to reverse magnetization based on nanostructured iron oxides (Fe₃O₄/α-Fe₂O₃) with a saturation magnetization of 50 emu/g.

Crystalline hydrate of Al(NO₃)₃ · Mg(NO₃)₂ · nH₂O salts is obtained by the method of the joint crystallization of the solutions of nitric acid salts of aluminum and magnesium that are taken in the required ratio for the synthesis of an (MgAl₂O₄) aluminum–magnesium spinel, which was subjected to thermal decomposition at a temperature of 500°C to form a MgAl₂O₄ compound. The effect of the mechanochemical activation (MA) on the dispersity of the MgAl₂O₄ powder is studied. The optimal regimes of activation and sintering of the synthesized MgAl₂O₄ powder for forming dense single-phase ceramics are selected. Mechanical activation makes it possible to obtain ceramics with an open porosity of ~2% and a grain size of 60 to 65 nm at 1400°C, which is lower than the temperature of the solid-phase synthesis of the aluminum–magnesium spinel (1700–1750°C). The physicochemical properties of the sintered ceramic samples of MgAl₂O₄ are determined.