Vol 54, No 6 (2018)

We have studied the formation of the Ti4SiC3 MAX phase during the vacuum carbosilicothermic reduction of TiO₂ with a combined reducing agent consisting of SiC and elemental Si and analyzed the effects of the synthesis temperature, heat treatment time, and percentage of elemental silicon in the starting mixture on the Ti₄SiC₃ yield. Optimal Ti₄SiC₃ synthesis conditions are as follows: temperature from 1550 to 1650°C, isothermal holding time of 360 min, and the starting-mixture composition TiO₂ + 1.2SiC + 0.6Si. The Ti₄SiC₃ yield then reaches 92 wt %.

Size effects in phase transformations of nanoscale systems show up as significant changes in their phase diagram. Here, using a thermodynamic approach we demonstrate how the immiscibility region of Cr–W solid solutions is influenced by their particle size. For 40- and 70-nm-diameter particles, we consider two thermodynamically stable states with a core–shell configuration, differing in the composition of the core phase. It is shown that, in the nanometer range, one of these states becomes metastable and that the phase diagrams of the stable and metastable states differ significantly. Reducing the particle size leads to a decrease in the upper critical dissolution temperature (UCDP) by 300–400 K and marked changes in the mutual solubility of the components at temperatures comparable to the UCDP.

We have studied contact interaction effects and wettability in M–SiO₂ (M = metal catalyst for nanowhisker growth) systems. The results demonstrate that the wettability of SiO₂ by the metal catalysts for nanowhisker growth (Au, Ni, Cu, and Sn) improves with a decrease in the first ionization potential of their atoms and is determined by dispersion interaction with the oxide surface. The interaction of the metal atoms with the O^2– anions of the oxide is responsible for the development of cohesion between the liquid metal and SiO₂ on the interface.

A Ti₉O₁₀ nanophase (sp. gr. Immm) has been synthesized via high-energy milling followed by prolonged high-temperature vacuum heat treatment of nonstoichiometric TiO_y titanium monoxide. The atomic structure and microstructure of Ti₉O₁₀ have been investigated by X-ray powder diffraction and high-resolution transmission electron microscopy. According to quantum-chemical calculation results, macrocrystalline Ti₉O₁₀ is less energetically favorable than a disordered cubic phase of the same composition, TiO_10/9, so the formation of the former phase is due to size effects.

Photoinduced light scattering and optical spectroscopy have been used to study the photorefractive effect and determine the band gap in nominally undoped congruent and stoichiometric lithium niobate crystals, as well as in a series of congruent LiNbO₃ crystals doped with Mg, Zn, B, Gd, Y, and Er cations and LiNbO₃ single crystals codoped with Mg:Gd, Mg:Fe, Mg:Y, and Mg:Ta.

Ca_{0.5(1 + x)}Zr_2–xFe_x(PO₄)₃ phosphates have been synthesized by a sol–gel process. The individual compounds and solid solutions obtained crystallize in the NaZr₂(PO₄)₃ structure (trigonal symmetry, sp. gr. R\(\bar 3\)). Using high-temperature X-ray diffraction, we have determined their thermal expansion parameters in the temperature range from 25 to 800°C. With increasing x, the magnitudes of their linear thermal expansion coefficients and thermal expansion anisotropy decrease. Most of the synthesized phosphates can be rated as low-thermal-expansion compounds and can be regarded as materials capable of withstanding thermal “stress.”

We have prepared samples of carbon reinforced alumina ceramics with different volume fractions of synthetic graphite (0–20%) and graphene (0–4%) and measured their electrical conductivity. It has been shown that increasing the volume percent of the conductive component increases the electrical conductivity of the samples from 10^–8 to 2 × 10^–3 S/cm. The results have been analyzed in terms of percolation theory and tunneling conduction theory. The synthetic graphite-based samples show linear current–voltage behavior and their electrical conductivity increases by a factor of 1.4 to 2.8 in the temperature range 300–550 K, with a sharp rise above 550 K. The temperature dependences of their electrical conductivity are analyzed in terms of hopping transport and thermally induced tunneling conduction mechanisms. The conclusion is made that the conduction mechanism in the corundum–carbon ceramics differs significantly from that in polymer composite materials.

We have developed a process for the synthesis of fine bismuth orthogermanate, Bi₄Ge₃O₁₂ (BGO), powders in a NaCl/KCl melt and examined the effect of synthesis conditions on the phase composition and morphology of the powders. Our results demonstrate that the synthesized materials compare well in scintillation performance with single-crystal BGO: their scintillation efficiency is 90% relative to single-crystal BGO, with a decay time of 300 ns.