Vol 55, No 12 (2019)

We have studied the effect of low-melting-point salt additives on the morphology and phase composition of silicon nitride prepared by combustion synthesis. The additives have been shown to influence the structure formation mechanism. Based on equilibrium compositions calculated for particular synthesis conditions, we have proposed mechanisms underlying the influence of the additives on the structure formation process. The synthesis temperature and additives have been shown to influence the phase composition of the synthesis products. Conditions have been found for the preparation of alpha-silicon nitride powders consisting of equiaxed particles.

We have studied magnetic parameters of monocrystalline manganese spinel ferrite films differing in composition and structural perfection and assessed the effect of dislocation distribution on the frequency dependence of the ferromagnetic resonance (FMR) linewidth in the microwave range. The results demonstrate that, if an external magnetic field is perpendicular to the film surface, electron paramagnetic resonance (EPR) absorption lines correspond to ferromagnetic and spin wave resonances (SWRs). SWR is due to nanostructural surface inhomogeneities. The EPR spectrum of a tangently magnetized film represents a single dipole exchange oscillation spectrum. Parameters of dipole and exchange modes have been determined.

We report an experimental study concerned with the synthesis and luminescence properties of Al₅O₆N aluminum oxynitride doped with Tb³⁺ in the concentration range 0.025–0.5 at %. All of the samples, prepared by firing appropriate mixtures in a nitrogen atmosphere at 1600°C, consisted predominantly of phase-pure γ-ALON (Al₅O₆N), with small amounts of impurity phases: Al₇O₃N₅ aluminum oxynitride and Tb₃Al₅O₁₂ terbium aluminate. Their pulsed cathodoluminescence and photoluminescence spectra contain emission bands typical of Tb³⁺ ions. The concentration threshold for luminescence quenching corresponds to a Tb³⁺ concentration at a level of 0.4 at %. A Tb³⁺ cross relaxation effect found in Al₅O₆N:Tb³⁺ is discussed and photoluminescence spectra are shown to be inhomogeneous at low Tb³⁺ concentrations in Al₅O₆N.

The structural and phase changes induced by heat treatment (HT) and brief lamp processing (LP) in 70- and 1500-nm-thick Li–Nb–O films similar in elemental composition to LiNbO₃, grown on the surface of unheated substrates (monocrystalline Si wafer and NaCl single crystal) by rf magnetron sputtering and ion-beam sputtering of a LiNbO₃ target have been studied by reflection high-energy electron diffraction and X‑ray diffraction. The as-grown films were extremely nanostructured and consisted of crystalline lithium oxides, niobium oxides, and LiNbO₃ as the product of reaction between them. We have demonstrated the conceptual feasibility of driving the synthesis of polycrystalline LiNbO₃ films to completion using HT or brief LP. In the case of LP, LiNbO₃ synthesis is three orders of magnitude faster than in the case of HT.

A novel process has been developed for the preparation of Bi₄Ge₃O₁₂ (BGO) by self-propagating high-temperature synthesis. In this method, microwave radiation is used for the first time to heat organic fuel (urea), which allows its decomposition to be uniformly activated throughout the reaction mixture. The key synthesis parameters are the pH of the medium, the volume of the liquid phase in the system, and the way in which the starting mixture is homogenized. Varying these parameters has made it possible to reduce the synthesis time to 3 min. We have obtained BGO samples consisting of quasi-spherical particles, potentially attractive for the fabrication of scintillating ceramics. In addition, we have synthesized powders that have an order of magnitude better kinetic scintillation characteristics in comparison with single-crystal BGO and are thus interesting for designing scintillating materials for highly sensitive detectors.

This paper summarizes results of studies aimed at developing physicochemical and scientific methodological principles of the preparation of high-purity substances by integrated technological processes. We rely on the identification and chemical transformations of impurities in the recovery from raw materials, synthesis, and ultrapurification of substances by various methods. A scientific methodological approach has been developed for choosing and substantiating integrated processes for the preparation of extrapure-grade (6N or 7N) compounds with a total content of trace impurities from 0.1 to 1 ppm by weight from standard and unconventional raw materials. We have obtained batches of high-purity arsenic- and gallium-containing substances and tested them in the technology of advanced microelectronic and optical materials.

The structure of ultrathin sections of dense submicrocrystalline hydroxyapatite (HA) (Ca₁₀(PO₄)₆(OH)₂) ceramics has been studied by X-ray diffraction and high-resolution transmission electron microscopy (TEM). The results demonstrate that high-energy electron irradiation can lead to surface modification with thin CaO, α-Ca₃(PO₄)₂, and HA nanoparticles. Two-level interference contrast and double Fourier transformation of TEM images have been shown to be effective tools for revealing and identifying nanoparticles, respectively.

The structure and electrical transport properties of Bi₄V_1.5Fe_0.5O_{11 – δ} (BIFEVOX) and Bi₄V_1.5Fe_0.5O_{11 – δ} + xBi₂O₃ composites have been studied as functions of composition and temperature. X‑ray diffraction data have been used to evaluate crystal-chemical parameters of individual compounds. The elemental and phase compositions of the materials have been determined by scanning electron microscopy and X-ray diffraction, and phase transitions of individual phases have been identified. The electrical conductivity of the materials has been determined by impedance spectroscopy.