Vol 55, No 1 (2019)

Materials based on the titanium carbosilicide Ti₃SiC₂ (MAX phase), prepared by spark plasma sintering (SPS), have been subjected to high-temperature tests. The high-temperature behavior of samples containing the carbosilicide phase and carbosilicide-free samples were studied. The results demonstrate that the presence of the MAX phase improves the heat resistance of the material, preventing oxidation. The oxidation products of the titanium carbosilicide-based materials are titanium dioxide, silicon monoxide, and carbon dioxide.

This paper reports DFT calculations of the electron density in pure and imperfect silicon carbide clusters. The local levels produced in the band gap by doping are shown to be determined predominantly by intrinsic states of the silicon and carbon.

We have examined the effect of iron oxalate (Fe₂(C₂O₄)₃ · 5H₂O) additions on the phase composition of combustion products of silicon–carbon mixtures in nitrogen. The phase composition of the combustion products has been determined as a function of starting-mixture composition and nitrogen pressure. We have investigated the microstructure of the synthesized powders. The addition of SiO₂ to the starting mixture has been shown to increase the percentage of Si₂N₂O in the combustion products.

MoO₃ samples have been prepared by vapor transport deposition under different process conditions: at two distinct hot-zone temperatures in a tube furnace (800 and 1100°C) and with different additives to argon as a major carrier gas: O₂, H₂O vapor, or N₂O gas. According to X-ray diffraction, electron-microscopic, and optical characterization results, the layered structure of microcrystals and band gap of MoO₃ are sensitive to not only vapor transport deposition conditions (synthesis temperature and vapor phase composition) but also mechanical treatment (grinding). At a synthesis temperature of 800°C, the distortion of the MoO₃ crystal lattice by incorporated H₂O molecules causes MoO₃ to transform from the major, orthorhombic α-phase (space group Pbnm) into a monoclinic phase (P2₁/n), which is accompanied by a reduction in band gap from 2.85 to 2.68 eV. At a higher synthesis temperature of 1100°C, neither hydrogen or oxygen impurities originating from water (H₂O) vapor nor nitrogen or oxygen impurities originating from N₂O change the layered orthorhombic structure of the microcrystals, but the addition of N₂O to the vapor transport medium reduces the band gap of MoO₃ to 2.51 eV. Mechanical treatment (grinding) of the MoO₃ microcrystals synthesized at the higher temperature (1100°C) with the addition of water vapor or N₂O to argon carrier gas produces a monoclinic phase (space group P2₁/n) in addition to the major, stable, orthorhombic phase (Pbnm). The MoO₃ microcrystals synthesized at a temperature of 800°C are more resistant to mechanical treatment: after grinding, they consist of only one phase: orthorhombic (Pbnm) in the case of synthesis in an argon–oxygen vapor transport medium or monoclinic (P2₁/n) in the case of the addition of water vapor to argon as a major carrier gas.

We have studied the luminescence of europium(III)-doped materials based on yttria and yttrium oxyfluorides. The materials were prepared through heat treatment of fluorine-containing yttrium and europium(III) thioacetamide complexes at temperatures of 400, 600, and 800°C for 2 to 6 h. The products were shown to consist of (Eu_хY_{1 –х})F₃, (Eu_хY_{1 –х})₅O₄F₇, (Eu_хY_{1 –х})OF, and (Eu_хY_{1 –х})₂O₃ phases. We have proposed a scheme that reflects the formation sequence of the main components of the materials in the above temperature range. Narrow-band luminescence of the materials has been shown to arise from Eu³⁺⁵D₀ → ⁷F_j electronic transitions. The origin of the observed broad luminescence band peaking in the range 480–500 nm can be understood in terms of surface structural defects, the particle size, and the broad particle size distribution. The luminescence spectrum has been shown to be influenced by the starting-mixture composition, synthesis conditions, host composition, and excitation wavelength. Some of the observed luminescence features have been shown to be due to the effect of S^2– ions in the composition of europium-containing activator centers.

This paper presents results of physical and computational experiments aimed at gaining insight into the behavior of silica impurity particles in two flowing contacting glass melts (core and cladding melts in the arsenic sulfide fiber drawing process). It has been shown that the particles present in the cladding melt are uniformly distributed, without passing into the core melt or concentrating at the interface between the two layers of the fiber. Theoretical calculations have been confirmed by experimental data.

The basic concepts of the delocalized-atom model have been shown to be applicable to low-melting-point sulfophosphate glasses and parameters of the model have been calculated. The parameters have been used to gain insight into the “atom delocalization” process in the glasses and assess their properties (viscosity, microhardness, glass transition temperature, and internal pressure). The results demonstrate that there is a linear correlation between the atom delocalization energy and glass transition temperature. The melt–glass transition is accompanied by the freezing of the atom delocalization process.