Vol 55, No 11 (2019)

We have demonstrated the feasibility of doping CdCr₂Se₄ single crystals with Group III elements (Ga, In, and Al) during annealing and identified relationships between annealing conditions, the composition of annealing mixtures, and properties of the crystals. Annealing mixture compositions have been found that ensure the possibility of preparing CdCr₂Se₄ single crystals with a considerably increased Curie temperature. We have performed multistep annealing of particular samples, which has made it possible to understand the role of each mixture component.

We have studied the phase composition and structure of titanium carbide with a nickel binder prepared by self-propagating high-temperature synthesis in a cocurrent inert or reactive gas stream using granulated mixtures containing different grades of titanium. The results demonstrate that, unlike in the case of powder mixtures with a loose bulk density, the products of combustion of a granulated Ti + C + 25% Ni mixture in flowing nitrogen or without it retain their structure and granule size and can readily be ground into powder. In the case of the powder mixture both in a flowing gas and without it and in the case of the granulated mixture in flowing argon, the combustion products have the form of unbreakable sinter cakes, independent of the grade of titanium. Microstructural analysis of the combustion products points to spontaneous dispersion of the titanium particles surrounded by the nickel binder, independent of the starting mixture (granules or powder with a loose bulk density). Moreover, the phase composition of the synthesis products depends on the size and morphology of the titanium particles. In the case of PTM titanium, the final synthesis product consists of titanium carbide and nickel phases. After the combustion of mixtures based on PTM-1 titanium powder or a 50% PTM + 50% PTM-1 mixture, the final product consists of TiC, Ni, and Ti_xNi_y intermetallic phases. Synthesis in flowing nitrogen has been shown to change the phase composition of the combustion products of the mixtures based on PTM-1 titanium powder and a 50% PTM + 50% PTM-1 mixture, causing the intermetallic phases to disappear. To account for the combustion behavior of the mixtures, we have proposed a two-step mechanism of interaction in the Ti + C + 25% Ni system.

We have studied the feasibility of preparing tantalum and niobium nitrides by reducing Та₂O₅, Nb₂O₅, Mg₄Та₂O₉, and Mg₄Nb₂O₉ with magnesium vapor. The process was run in two steps: (1) reduction of the oxides at a temperature of 820°C and a residual argon pressure of 5 kPa in the reactor for 4 h and (2) nitridation of the reduction products in a nitrogen atmosphere at 820 and 900°C for 1–12 h. The phase composition of the powders after leaching out the magnesium oxide was determined on DRF-2 and DRON-4 X‑ray diffractometers. The specific surface area was determined by BET adsorption measurements on a Micromeritics TriStar II 3020 analyzer. When Ta₂O₅ was used as precursor, the nitridation product contained θ-TaN, ε-TaN, and Ta₂N. The use of Nb₂O₅ as a precursor led to the formation of face-centered cubic NbN, hexagonal close-packed NbN, Nb₂N, and Nb₄N₃. The Mg₄Ta₂O₉ and Mg₄Nb₂O₉ reduction products did not absorb nitrogen. The results obtained are discussed in detail.

We have synthesized silica-based sorbents with MCM-41 and MCM-48 pore structures, modified with hydrazide groups based on tret-carboxylic acids of Versatic 10 and Versatic 1519 fractions, and investigated textural and structural properties of the silicas. Conditions for tungsten(VI) sorption from acid solutions by the silicas have been studied in relation to the pH of the medium and grafting density, and conditions have been found for tungsten(VI) and molybdenum(VI) separation from hydrochloric acid solutions using MCM-41 modified silica.

We have prepared a composite material based on a mixture of calcium phosphates and sodium alginate and investigated their composition, morphology, and dynamic dissolution behavior. The addition of a powder material to the sodium alginate matrix causes no changes in its composition, but increases the specific surface area and resorption rate of the sample. We have optimized the synthesis conditions of the composite material: the filler/matrix ratio and the drying temperature and time.

Ceramic Sr₂Ce_0.95Yb_0.05О₄ samples have been synthesized by the Pechini process and their electrical conductivity has been determined as a function of temperature (300–900°C), and (40–2500 Pa) using four-probe and impedance measurements. In air at high temperatures, the material is a mixed oxygen–hole conductor, with predominantly oxygen-ion conductivity. At reduced temperatures, proton conductivity prevails, with σ ~

We have studied luminescence and electron paramagnetic resonance (EPR) spectra of manganese- and europium-activated fluorozirconate glasses in the ZrF₄–BaF₂–LaF₃–AlF₃–NaF (ZBLAN) system and fluorohafnate glasses in the HfF₄–BaF₂–LaF₃–AlF₃–NaF (HBLAN) system in order to assess the oxidation state and spatial distribution of the activator ions. The manganese luminescence band has been shown to shift from the green (545 nm) to the red (610 nm) spectral region, and additional lines in the EPR spectrum of manganese have been observed to emerge in the EPR spectrum of manganese as a result of BaCl₂ substitution for BaF₂ in the ZBLAN glass. The ratio of the concentration of free activator ions to that of clustered ions has been estimated quantitatively. It has been shown that, at high doping levels, the manganese and europium ions in the glasses are predominantly clustered, and only a small part of them are present as isolated ions.

The subject of this study is olivine dolerite, a holocrystalline variety of basic volcanic and dike rocks of the normally alkaline rock subclass of the basalt family. Based on innovative methods combining physicochemical computer simulation and experimental studies, we have developed processes for preparing melts and modifying the composition of this magmatic rock for the fabrication of glass-ceramic materials. Such melts offer mineralogical and chemical homogeneity and high flowability, and their crystallization behavior meets requirements for stone casting. The mineralogical composition of such melts can be modified by melting in different atmospheres (oxidizing, reducing, and inert).