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Abstract: Photonic crystals obtained by the modified Stöber method with the natural sedimentation of nanospheres is investigated. The surfaces of the silica nanospheres are investigated using atomic-force microscopy. The surfaces of all samples have a cauliflower structure. The structure of the photonic crystals is studied by the method of small-angle synchrotron radiation scattering (SASRS). The SASRS curves exhibit two or three regions with different scattering intensities. Three original techniques for analyzing the data obtained are proposed. Periodicity in the dependence of the logarithm of intensity on the scattering vector at the smallest wavenumbers, related to scattering from nanospheres, is found. The influence of the water/TEOS molar ratio on the size and concentration of these particles is revealed. Particles of two types (nanospheres and subglobules) are observed at low water concentrations (6–8 mol/L), whereas at water concentrations of 13–23 mol/L the particles are of three types (nanospheres, subglobules, and grains). The morphology of the detected particles is estimated.

Abstract: Nanocrystalline CaSi films with thicknesses from 80 to 130 nm were grown on high-resistance silicon substrates with orientations (111) and (100) by the methods of low-temperature (190–330°C) molecular-beam epitaxy and low-temperature (330°C) solid-phase epitaxy, for which the microstructure, phase composition, and crystal structures were studied. It is found that the polycrystalline, nanocrystalline (NC), and amorphous CaSi and CaSi2films are characterized by preferential contribution of holes in the range 1.4–300 K. In magnetic fields 1–4 T and at temperatures 40–100 K, a giant linear magnetoresistive effect (MRE) (to 500%) was observed for the first time in CaSi films with the contribution of another CaSi2phase. In CaSi2film containing another phase (CaSi), peaks are detected on the temperature dependences of the resistivity and the Hall coefficient that correspond to a phase transition. In addition, in this film, the transition from the positive MRE to negative MRE is observed at Т= 120–200 K. This effect is not observed in the single-phase CaSi2film, which corresponds to a certain reconstruction of carrier flows in a magnetic field only in the two-phase system. The study of the thermoelectric properties of CaSi and CaSi2films shows that the semimetallic type of the conduction in them leads to the independence of the positive Seebeck coefficient Т= 330–450 K. It is found that the maximum contribution to the Seebeck coefficient and the power factor are observed in the amorphous CaSi film in the case of the presence of some fraction of NC Ca2Si phase. In the single-phase CaSi2films, the Seebeck coefficient and the power factor are halved due to an increase in the hole concentration as compared to the CaSi films.

Measurements have been made of the radiative properties of porous silicon obtained by anodic etching of plasma-processed p-type single-crystal silicon (100). It is found that the photoluminescence intensity of the test samples is significantly greater than that of porous silicon obtained on the surface of single-crystal silicon with no plasma preprocessing.

Optimization of growth parameters has permitted to create monolithic nanocomposites with buried nanocrystals (NCs) of iron and chromium disilicides and polycrystalline nanocomposites with buried Mg2Si NCs (3-40 nm) on the base of reactive deposition epitaxy (RDE), solid phase epitaxy(SPE) and molecular beam epitaxy (MBE), which demonstrated wonderful thermoelectrical properties and the possibility of strong light emission with wavelength of 1.2 - 1.6 microns in the mesa-diode structures with p-n junction at direct and back bias. Doping process of Mg2Si nanocrystals inside nanocomposite layers was developed on the base of ordered surface phases of metals on a silicon substrate. Nanocomposite layers with n- and p-type conductivity have been successfully grown. The huge increase of Zeebeck coefficient in the Si-p/-FeSi2NCs/Si-p and Si/Mg2Si NCs/Si nanocomposites has been found.

Abstract: This paper reports the results of the study of hydrothermal precious opals. It was established that, in terms of their nanostructure and formation conditions, the hydrothermal opals differ from exogenic precious opals. They are made up of small globules and do not show any structuring of their nanoparticles on the basis of closest packing, which is typical of exogenic precious opals. During their formation, these opals were subjected to pneumatolytic annealing—they experienced the impact of a high-temperature vapor under elevated pressure. The influence of the thermal effect led to the formation of two-dimensional photonic band gaps in the chaotic opal matrix. These bands are composed of sheets whose cells were formed owing to the thermal effects according to the principle of “Benard cells.” Precisely these structured blocks and thin films are responsible for the spectral dispersion of light and iridescence.

Abstract: Growth and the optical properties of epitaxial heterostructures Si(111)/(CrSi2 nanocrystallites)/Si(111) based on nanosized islands of chromium disilicide (CrSi2) on Si(111) were studied using low-energy electron diffraction, atomic-force microscopy, and optical reflection and transmission spectroscopy. The heterostructures with thicknesses of 0.1, 0.3, 0.6, 1.0, and 1.5 nm were formed by reactive epitaxy at a temperature of 500°C followed by the epitaxial growth of silicon at 750°C. The specific features of changes in the density and sizes of CrSi2 islands on the silicon surface were determined at T = 750°C as the chromium layer thickness was increased. It was established that, in the heterostructures with chromium layer thicknesses exceeding 0.6 nm, a small part of faceted Cr2Si2 nanocrystallites (NCs) emerge into near-surface region of the silicon, which is confirmed by the data from optical reflectance spectroscopy and an analysis of the spectral dependence of the absorption coefficient. A critical size of NCs is shown to exist above which their shift to the silicon surface is hampered. The decreased density of emerging NCs at chromium layer thicknesses of 1.0–1.5 nm is associated with the formation of coarser NCs within a silicon layer, which is confirmed by the data from differential reflection spectroscopy.

Abstract: The intraband absorption of electromagnetic radiation by two types of nanostructures of cylindrical symmetry—by a quantum cylinder (ring) and a quantum wire—is investigated. Analytical expressions for the coefficients of absorption of high-frequency electromagnetic radiation by the electron gas of nanostructures are obtained. It is shown that the absorption curve exhibits resonance peaks and that, in the case of a degenerate gas, these peaks have breaks.

This article gives a calculation of the solubility product of the hydroxide of tetravalent uranium, a knowledge of which is essential for the efficient operation of many processes in the treatment of uranium and methods for determining it. The calculation is based on experimental data obtained in the potentiometric titration of a chloride solution of tetravalent uranium by alkali. The activity of the hydrogen ions was determined by a tube potentiometer LP-5; the active concentration of the quadruple charged ion of tetravalent uranium was calculated from the analytically found concentration of tetravalent uranium with an allowance for hydrolysis and the value of the ionic strength. It is shown that the required solubility product is equal to (1.10±0.72)·10-52.

This review gives general information on the history of the development and the modern state of uranium metallurgy outside the Soviet Union. There is a brief description based on practice in the USA) of methods for producing metallic uranium. The article deals with the production of the starting materials (UO2, UF4), the theory and practice of magnesium-thermic reduction and the process for refining the heat. There is a discussion of the problem of obtaining metal from uranium hexafluoride, enriched and poor in U235 isotope. In conclusion, there is a discussion of the possibilities for development in uranium metallurgy.

On the basis of published material the present article gives brief characteristics of different methods for reducing smelting. Values are given for the free energy of some investigated reactions of the reduction of uranium hexafluoride. The effect of various factors is considered on the purity and the granular specific gravity of the obtained uranium tetrafluoride. The advantages and disadvantages of different schemes for reducing uranium hexafluoride are discussed.

Studies have been made of the solubility and kinetics of solution of uranium hexafluoride in carbon tetrachloride, chloroform, dichloromethane, asymmetrical dichloroethane,symmetrical tetrachloroethane, pentachloroethane, trifluorotrichloroethane, symmetrical trichloropropane,and tetrachloropropane. It has been shown that solutions of uranium hexafluoride in carbon tetrachloride, tetrachloroethane, pentachloroethane,and trifluorotrichloroethane are completely stable for two weeks at 20 °C; at this temperature solutions of uranium hexafluoride in chloroform, dichloroethane,and dichloromethane are unstable. It has been shown that reactions of uranium hexafluoride with these studied organic solvents at 60–100 °C occur in the following way: at first uranium pentafluoride is formed which is reduced at first to the intermediate uranium fluor ides, containing a large amount of tetravalent uranium.and then to uranium tetrafluoride.

In the literature there is a reference to the use of NH4HCO3 in a technological system for the purification of chemical uranium concentrates [1]. However, the conditions in which this process is carried out have not been described in detail. Moreover, it is known that during the analysis of acid solutions containing uranium, the addition of ammonium carbonate makes it possible to separate uranium from iron and aluminum [2]. It is of interest to study this method of separating uranium from impurities in greater detail. In the present work an investigation was made of the dissolution of pure (NH4)2U2O7 in solutions of (NH4)2CO3 and NH4HCO3, the separation of uranium from the solution in the form of (NH4)4UO2(CO3)3] and the behavior of certain impurities in the salting-out of crystals of this carbonate complex.

Abstract: This paper reports on a theoretical study of intraband resonances arising in the absorption of electromagnetic radiation by a quantum nanotube both with and without involvement of optical phonons. Explicit relations are derived for the absorption coefficients. The relative intensities of the resonance peaks are analyzed.