Vol 68, No 1 (2023)

Ni–Al layered double hydroxides (LDHs) are of interest as functional materials. The effects of preparation methods on the dielectric properties of Ni–Al layered double hydroxides were studied on samples prepared from solution (by coprecipitation and a hydrothermal process) and by plasma technology. The prepared layered structures were characterized by advanced analytical methods. The high ζ potentials of the particles prepared in suspensions evidence their high aggregation stability. X-ray powder diffraction and IR spectroscopy were used to determine the phase composition of samples and to identify the interlayer anion. The plasma between Al and Ni electrodes in distilled bulk water gives rise to the formation of Ni–Al LDHs with hydroxide ion as the interlayer anion. Thermal properties of the structures prepared were studied by thermal analysis. The results of dielectric measurements are presented.

A new promising method for the synthesis of NASICON (Na3Zr2Si2PO12) by pyrolysis of organic solutions has been developed. Sodium oleate, zirconyl oleate, tributyl phosphate, and tetraethoxysilane have been used as precursors. The molar ratios of the components of the mixture for the formation of NASICON have been established. The effect of sodium on the formation of the zirconium dioxide phase has been proven. A finely dispersed material with an average grain size of 0.2 µm has been obtained. Changes in morphology and composition depending on the time and temperature of firing the sample are studied. The results have been confirmed by X-ray powder diffraction and scanning electron microscopy. To refine the parameters of the crystal lattice, a full-profile analysis has been performed by the Rietveld method. The process of obtaining NASICON takes about 9 h, i.e. it is the least time consuming of all the alternative ways of synthesis. The advantages of this method are the possibility of lowering the sintering temperature, the absence of the need to control many parameters during synthesis, and minimizing the duration and multi-stage process. The method contributes to the development and production of more promising ion-substituted structures.

Single crystals of the Pb5WO8 phase were grown in the PbO–WO3 system by crystallization of (1 – x)PbO·xWO3 (x = 0.15–0.20) mixed melts. Thermogravimetric, X-ray diffraction, and dielectric studies of the single crystals were carried out. The phase melts at 712°С with decomposition to PbO and a liquid. The Pb5WO8 crystal structure is monoclinic (space group P21/n, 293 K) with the unit cell parameters a = 7.4430(1) Å, b = 12.1156(2) Å, c = 10.6284(2) Å, β = 90.658(1)°. The Pb5WO8 structure is retained at 100 K; the minor alterations in unit cell parameters are associated only with thermal expansion. The Pb5WO8 structure has a pronounced layered character; it appears as an alternation of layers formed of WO6 octahedra and strongly distorted PbO4 and PbO5 polyhedra in the direction [010]. A detailed crystal-chemical analysis of the structure was carried out. An important role of the Pb lone pair in the formation of characteristic voids in the structure was noted. The temperature-dependent dielectric permittivity and dielectric loss tangent feature relaxation peaks associated with lead and oxygen vacancies in the structure.

The dependence of the stability of Mn(C5H7O2)3 modifications on the properties of the solvent chosen for recrystallization is considered. Low-polarity solvents with a low dielectric permittivity enhance intermolecular interactions, which leads to the formation of the β-Mn(C5H7O2)3 modification during the synthesis of Mn(C5H7O2)3 from chloroform solutions. The use of mixtures of chloroform with petroleum ether makes it possible to control supersaturation, the rate of formation, and growth of phase nuclei due to the evaporation of chloroform under isothermal conditions. The use of polar solvents for recrystallization favors the formation of γ-Mn(C5H7O2)3. The composition of the thermal decomposition products of β‑Mn(C5H7O2)3 in a dry inert atmosphere has been determined by X-ray powder diffraction, IR spectroscopy, thermogravimetric and mass spectral analysis, and differential scanning calorimetry. In the temperature range 140–240°C, β-Mn(C5H7O2)3 melts to form Mn(C5H7O2)2. At temperatures of 500–550°С, Mn(C5H7O2)2 decomposes to a mixture of MnO, Mn3O4, Mn2O3, and carbon.

The crystalline form of sodium-doped hexagonal borophene (B2Na2) has been studied using DFT calculations. The calculations predict the dynamic stability of B2Na2 whose structure is a flat honeycomb boron sheet sandwiched between two sodium layers. According to estimated electronic and mechanical properties, B2Na2 is a rather soft material with metallic characteristics. Evaluation of thermal stability by the molecular dynamics method indicates sufficient stability of the predicted material, which makes it possible to observe it experimentally at temperatures below 200 K.

Three-dimensional (3D) computer models of Ag–Cu–Ni and Ag–Cu–Pb isobaric phase diagrams, designed based on 23 and 31 base points, respectively, assembled from 14 and 32 surfaces, 9 and 15 phase fields, respectively, and intended to digitize information on these diagrams, are used to verify the adequacy of interpretation of published isothermal and polythermal sections, both calculated and experimentally studied ones. The geometric features of the phase diagram regions that relate to liquid–liquid miscibility gaps and solid solution decomposition are refined in the 3D models. Mistakes arising from an incorrect imaging of the decay of copper–nickel solid solutions and from discrepancies in the values of the Ag–Cu–Pb ternary eutectic temperature are shown on polythermal sections.

Complexation in systems containing iron(III) chloride and barbituric (H2BA) or 2-thiobarbituric (H2TBA) acid has been studied by spectrophotometry and pH-metry in the pH range of 1.3–3.3 (I = 0.1 (NaCl), t = 20°C). The presence of 1 : 1 complexes with mono- and deprotonated forms of the ligand has been established, and their stability constants (in log units) have been determined: 1[FeHBA]2+ (3.49 ± 0.15), [FeHTBA]2+ (2.69 ± 0.07), [FeBA]+ (12.22 ± 0.13), and [FeTBA]+ (11.05 ± 0.08). It has been shown that the higher thermodynamic stability of barbiturate complexes compared to 2-thiobarbiturate ones is due to the greater basicity of the barbiturate anion. Based on the stability constants obtained, it has been proposed to use orthophosphate, fluoride, and ethylenediaminetetraacetate ions to eliminate the interfering effect of iron(III) in the determination of malondialdehyde by the thiobarbiturate method. Orthophosphoric acid is the most convenient for practical applications, as it makes it possible to mask iron(III) and to create a strongly acidic medium necessary for the formation of a colored malondialdehyde–H2TBA adduct.

Comparative results of studying the oxidative dehydrogenation of ethane (ODE) on catalytic vanadium-phosphorus oxide systems deposited by the molecular layering method on the surface of oxide supports (Al2O3, SiO2) have been presented. It has been found that the highest activity in ODE and selectivity for ethylene are exhibited by vanadium-phosphorus-containing catalysts. The influence of the acidity of catalytic systems on the activity and selectivity of the process has been revealed. The selectivity of the ODE process for ethylene reaches 90%. An increase in the oxygen concentration in the initial mixture from 3.5 to 20% leads mainly to a decrease in the selectivity of the ODE process with respect to the ethylene yield.

An important scientific task of practical materials science is the production of metal-ceramic composites in the form of functional gradient materials (FGM) for special-purpose products. In this regard, a study was conducted on the application of spark plasma sintering (IPS) technology for the effective diffusion connection of SiC ceramics and high-alloy steel (grade X18R15) to obtain a combined FGM composite. In a comprehensive experimental study, the dynamics of consolidation and changes in the phase composition of dispersed SiC under conditions of different temperatures and heating rates, pressing pressure, and holding time were studied. As a result, the IPS conditions were optimized for obtaining SiC ceramics of high relative density (>82%) and microhardness (>500 HV) of stable phase composition. The physicochemical foundations of the formation of a strong compound of a two-component SiC-ceramic and steel system under IPS conditions without additives and using a mixture of additives in the form of a binder, a reaction binder and a damper (Ti–Ag, Ti–TiH2, Ti–Ag–TiH2 and Ti–Ag/Mo additive systems) have been studied. The structure, composition of ceramics and intermediate (binding and damping) layers, as well as the diffusion of elements at the boundary of the formed compounds in FGM composites, were studied using XRF, SEM and EMF methods. It was found that the Ti–Ag/Mo additive in the ratio of 30 wt. % Ti–70 wt. % Ag and a dense layer of Mo (thickness ~ 2 mm), acting as a damper to compensate for the temperature coefficient of linear expansion, ensure the formation of a connected FGM composite of an integral shape. The presented studies have been implemented for the first time, are promising and require further development in order to gain scientific knowledge of the manufacture of composite products for special purposes.

Indium oxide–graphene composites (containing 0–6.0 wt % graphene) were manufactured by the sol–gel process. The phase composition, microstructure, and gas-sensitive properties of the prepared materials were studied. The composites consist of isolated In2O3 and graphene phases, where graphene is predominantly adsorbed on the surfaces of indium oxide grains (the indium oxide grain sizes are 8–11 nm). The nanocomposites are distinguished by an enhanced sensitivity to both reducing gases (CH4, acetone) and oxidative gases (NO2). A far greater enhancement is in the sensory response to oxidative gases. Presumably, the major factors influencing the sensory properties of the composite are the high defectiveness of In2O3 and graphene phases, higher specific surface areas of composites compared to those of individual In2O3, and the likely formation of p–n junctions in the indium oxide and graphene contact zone. Graphene additives to indium oxide can improve the main performances (sensory response, response time, and recovery time) of single-electrode semiconductor sensors.