Vol 55, No 6 (2019)

Heterophase equilibria in the Al_xGa_yIn_{1– x– y}As_zP_{1 –z}/InP heterosystem have been analyzed in terms of a simple solution model in the temperature range 813–1023 K. We have assessed the stability limits of AlGaInAsP solid solutions synthesized on indium phosphide substrates and investigated kinetic features of the crystallization of constant lattice parameter AlGaInAsP solid solutions on InP substrates. The crystallization rate of AlGaInAsP solid solutions on InP substrates has been shown to depend on the synthesis temperature, the molten zone composition and length, and the temperature gradient. In addition, it has been shown to decrease with increasing aluminum concentration, independent of the molten zone length, owing to the decrease in the diffusion coefficient in the liquid phase. We have obtained composition dependences of component distribution coefficients in the Al_xGa_yIn_{1– x– y}As_zP_{1 –z} /InP heterosystem and shown that raising the synthesis temperature leads to a reduction in K_Al and K_P. The effect of synthesis conditions on the structural and luminescence properties of AlGaInAsP solid solutions on InP has been examined.

We have carried out a comparative study of the catalytic activity of nanostructured M–Ru (M = Pt, Pd, Rh) bimetallic catalysts supported on detonation nanodiamond (DND) for methanol steam reforming (MSR) and ethanol steam reforming (ESR) processes in a conventional and a membrane reactor. The catalysts have been characterized by X-ray diffraction, transmission electron microscopy, and BET measurements. In the ESR process, the highest hydrogen yield is ensured by the Pt–Ru/DND catalyst, whereas the Ru–Rh/DND catalyst exhibits the highest activity for the MSR reaction. Our results demonstrate that, if the processes in question are run in a membrane reactor with a Pd–Ru membrane, there is a stable hydrogen flow, free of CO and other impurities, in the permeate zone even at temperatures on the order of 400°C.

We have studied parameters of the magnesium reduction of Nb₂O₅ and Mg₄Nb₂O₉ in the combustion regime and characteristics of niobium powders thus prepared. The combustion speed and temperature have been determined as functions of parameters of the starting mixture. The use of magnesium niobate as a precursor for the reduction process has been shown to increase the degree of reduction and the specific surface area of the niobium powders by 15 to 25 times.

We have separately assessed the contributions of isolated Fe³⁺ ions and the ferrimagnetic subsystem to the total magnetization of an aligned magnetite (Fe₃O₄) nanowire array grown in pores of an anodized alumina membrane and evaluated the magnetic anisotropy field of the nanowires, which has been found to be an order of magnitude weaker than the expected shape anisotropy field. The reduction in magnetic anisotropy in the nanowires can be accounted for by dipole–dipole interaction between individual nanowires in the array. In electron spin resonance spectra of the nanowires, we have identified a phase-inverted line, corresponding to their microwave magnetoresistance. The Verwey transition in the magnetite nanowires has been shown to be suppressed due to deviations from stoichiometry and size effects.

The thermolysis of hydrated antimony pentoxide (HAP) has been studied in the temperature range from 25 to 1000°C using a variety of experimental techniques: thermogravimetry, X-ray diffraction, pycnometry, elemental microanalysis, and mass spectrometry. The composition and structure of the forming phases have been determined. It has been shown that the initial stages of HAP thermolysis, in the temperature range 24–500°C, include dehydration processes and the formation of anhydrous antimony pentoxide, Sb₂O₅. At temperatures above 500°C, the process includes oxygen removal, the reduction of Sb(V) to Sb(III), and the formation of Sb₆O₁₃ and Sb₂O₄. The pyrochlore structure of HAP (sp. gr. Fd3m) has been shown to persist in the course of the phase transformations at temperatures below 700°C. Analysis of our experimental data has allowed us to propose a model for crystallographic site occupancies in the pyrochlore-type structure of the phases obtained and determine their temperature stability ranges.

We have studied the structural and optical characteristics and radiation hardness of LiNbO₃:ZnO crystals containing ~0.15–5.9 mol % ZnO. The results demonstrate that anomalies in composition dependences of their structural and optical characteristics and radiation hardness are closely interrelated with threshold effects in the LiNbO₃:ZnO crystals. We have confirmed the presence and determined the exact position of two concentration thresholds: at low dopant concentrations (~1.17 mol % ZnO in the crystals) and intermediate ones (~4.8 mol % ZnO in the crystals). Composition dependences of structural parameters for the crystals near the main concentration threshold (~6.8 mol % ZnO in the melt) have been determined with appreciably higher accuracy than before.

This paper presents an experimental study of the effect of doping with manganese on the vapor phase growth of lead(II) iodide crystals in a closed system and their properties. The manganese concentration in the grown crystals and the mass transport rate in the PbI₂–I₂ system are determined by the Mn concentration in the growth charge and the temperature of the source zone. We have obtained single crystals of the following morphological types: elongated ribbons, ribbons with sharp ends, plates, and their combinations and intergrowths. Doping with manganese has been shown to influence low-temperature (T = 5 K) photoluminescence spectra of PbI₂.

The boundaries of glass-forming regions in the Sm₂S₃–Ga₂S₃–EuS system have been located using simultaneous thermal analysis, X-ray diffraction, and X-ray microanalysis. We have studied IR spectra of glass with the composition (Ga₂S₃)_0.80(Sm₂S₃)_0.10(EuS)_0.10 and its stability in an inert atmosphere. The glass has been shown to soften at 985 K (T_g), which is accompanied by an endothermic peak. It crystallizes at 1112 K (T_x). Exothermic peaks observed in the range 1203–1222 K suggest that this glass has a tendency to decompose in several steps. In the IR spectra of the glasses, the bands due to the M–S (M = Sm, Ga, Eu) bonds have higher intensity and are shifted to higher frequencies relative to those in the spectra of their crystalline constituent components, which is probably due to the increase in M–S bond covalence.

We have studied the thermal shrinkage of ytterbium-doped Lu₂O₃ nanopowder compacts during heating to 1550°C and examined the effect of the intermediate microstructure of the material on the characteristics of transparent ceramics after hot isostatic pressing at a temperature of 1700°C. The results demonstrate that the optical properties of the ceramics depend on the structural features formed in the presintering step. It has been shown for the first time that rate-controlled densification of powder compacts allows the extinction coefficient of transparent Yb:Lu₂O₃ ceramics to be reduced by a factor of ~1.8 in comparison with other sintering modes.