Inorganic Materials (v.54, #11)

Preparation of Dielectric Films via Thermal Oxidation of MnO2/GaAs by I. Ya. Mittova; B. V. Sladkopevtsev; E. V. Tomina; A. A. Samsonov; N. N. Tretyakov; S. V. Ponomarenko (1085-1092).
MnO2 nanolayers (~29 nm) produced on the surface of single-crystal GaAs wafers by magnetron sputtering have been shown to act as oxygen transfer agents for the thermal oxidation of the semiconductor. The presence of MnO2 increases the oxide film growth rate by three to nine times relative to stimulator-free oxidation of GaAs. The films thus grown range in thickness from 35 to 200 nm and possess good dielectric properties (resistivity on the order of ~1010 Ω cm and dielectric strength in the range (5–8) × 106 V/cm). According to X-ray diffraction data, the films are enriched in oxidized arsenic and have a regular grain structure in the surface layer, with a roughness height within 30 nm (according to atomic force microscopy data).
Keywords: gallium arsenide; thermal oxidation; nanofilms; oxides

This paper presents a theoretical study of the effects of crystal structure and Mn and Co substitutions for Cd on the electronic structure of the topological material Cd3As2. We have carried out density functional theory calculations of the band structure and density of states in tetragonal and cubic Cd3As2, as well as in Cd3 – x Mn x As2 and Cd3 – x Co x As2 crystals. The results indicate that the band structure of the cubic Cd3As2 phase differs significantly from that of a Dirac semimetal, characteristic of the tetragonal phase. It has also been shown that, after Co substitution for 1/24 of the Cd atoms, the structure of the density of 3d electron states is similar to that of the density of states in the magnetic semiconductor Cd3 – x Mn x As2, with a characteristic minimum at the Fermi energy. At the same time, in the case of analogous Mn substitution for Cd, the density of d-electron states has no such minimum.
Keywords: topological materials; Dirac semimetal; Cd3As2 ; Mn; Co; magnetic semiconductors; density functional theory (DFT) calculations; band structure; density of states (DOS)

Growth of Gallium Nitride Micro- and Nanocrystallites on the Surface of Gallium Arsenide by Yu. Ya. Tomashpolsky; V. M. Matyuk; N. V. Sadovskaya (1099-1102).
As a result of heterosegregation, we have obtained a gallium nitride phase, including oriented micro- and nanocrystallites, on the surface of gallium arsenide. The mechanism underlying the growth of crystallites of the surface phase has been analyzed.
Keywords: surface heterosegregation; gallium nitride film; gallium arsenide

Thermodynamic Analysis of the Growth of Silicon Carbide Ingots in a Reducing Atmosphere by D. D. Avrov; A. O. Lebedev; Yu. M. Tairov (1103-1108).
We have analyzed phase equilibria in the Si–C–H–Ar system during silicon carbide ingot growth by the modified Lely method. The results indicate that the addition of even small amounts of hydrogen to an inert carrier gas leads to a sharp increase in the rate of carbon transport through the gas phase, mediated by volatile hydrocarbons, primarily acetylene, and prevents carbon enrichment in the solid source and corrosion of the graphite equipment in the single-crystal seed region.
Keywords: silicon carbide; hydrogen; modified Lely method; thermodynamic analysis

Preparation of Indium Phosphide Substrates for Epilayer Growth by M. G. Vasil’ev; A. M. Vasil’ev; A. D. Izotov; A. A. Shelyakin (1109-1112).
We have carried out an integrated study of technological steps in the preparation of indium phosphide substrates for the epitaxial growth of heterostructures. We have investigated the surface morphology and condition of indium phosphide in (100)-oriented substrates and tested various chemical etchants for final chemical surface processing. Our results demonstrate that an optimal substrate preparation process is two-step chemical–mechanical polishing on both sides using zeolite slurries, with chemical polishing in a mixture of bromine and isopropanol as the final step.
Keywords: semiconductor heterostructures; indium phosphide; epitaxy; etching

Activation Energy and Mechanism of the Molybdenum Disilicide Sintering Process by D. D. Titov; Yu. F. Kargin; A. S. Lysenkov; P. A. Miloserdov; S. N. Perevislov; N. V. Petrakova; O. S. Antonova; A. A. Konovalov; A. A. Ashmarin; A. V. Shokod’ko (1113-1118).
This paper presents dilatometric analysis data for the sintering of off-the-shelf molybdenum disilicide (MoSi2) powder prepared by magnesiothermic synthesis. We have obtained continuous shrinkage curves for MoSi2 powder compacts with an initial relative density of 70% at different heating rates: 5, 10, 20, and 30°C/min. From a quantitative analysis of densification curves for the compacts, the activation energy for the initial stage of sintering has been determined to be Q = 695 kJ/mol. It has been shown that the dominant process in the initial stage of MoSi2 powder sintering is volume diffusion from grain boundaries and surfaces.
Keywords: molybdenum disilicide; MoSi2 ; dilatometry; activation energy; sintering mechanism

Morphology of TiO2 Layers with a Well-Developed Acicular–Fibrous Structure in VT-20 Titanium by E. A. Drobakha; G. S. Drobakha; S. V. Shevtsov; A. A. Konovalov; K. A. Solntsev (1119-1123).
We have studied the effect of size factors of substrates on the formation of TiO2 layers with a well-developed acicular–fibrous structure on VT-20 titanium. The proposed technique allows a TiO2 layer with a large surface area to be produced by etching in a KOH solution (without using hydrothermal synthesis conditions), followed by heat treatment. The layers thus produced consist of fibers 20–100 nm in diameter and up to 1 μm in length, which are strongly bonded to the substrate surface. Their phase composition is dominated by rutile. The fiber growth process is shown to be influenced by substrate etching and heat treatment conditions, weakly influenced by the curvature of the substrate, and independent of its dimensions.
Keywords: titania; large surface area; acicular structure; morphology

Silicon-substituted hydroxyapatite (Si-HA) materials have been synthesized from a model SBF solution with different concentrations of silicon ions. All of the samples prepared from an extracellular fluid solution at different concentrations of silicate ions have been shown to be single-phase and consist of hydroxyapatite. The nature of the silicon-containing reagent (Na2SiO3 or tetraethyl orthosilicate) has no effect on the structure of hydroxyapatite. Analysis of silicon-modified phosphate coatings on titanium substrates indicates that more complete Si-HA deposition occurs on etched samples during the first three days of holding in the model solution. After exposure of Si-HA-coated titanium substrates to a high-power ion beam, further crystal growth and surface regeneration are possible.
Keywords: hydroxyapatite; extracellular fluid solution; titanium substrates; silicon ions; coatings; surface

Catalytic Properties of Zinc Manganites for Carbon Oxidation by P. G. Chigrin; E. A. Kirichenko (1131-1135).
This paper presents a comparative study of the catalytic properties of the ZnMn2O4 and ZnMnO3 manganites for carbon oxidation. The manganites have been prepared by a sol–gel process using reaction mixtures with different ratios of the constituent oxides. The particles of the two manganites were comparable in size (200–500 nm) and specific surface area (34–42 m2/g) and ensured similar behaviors of the catalytic reaction in the temperature range 260–470°C. The catalytic activity of the perovskite phase ZnMnO3 can be accounted for by the high absolute oxygen nonstoichiometry, δ = 0.29, as determined by iodometry. Contact interaction between spinel ZnMn2O4 and carbon leads to the reduction of Mn3+ to Mn2+ and is accompanied by Mn3O4 formation. The change in the relationship between the constituent oxides in the surface layer of the ZnMn2O4 particles leads to the formation of the active perovskite phase ZnMnO3, which initiates catalytic reaction.
Keywords: mechanism of heterogeneous reactions; heterogeneous catalysis; perovskites; spinels, oxygen nonstoichiometry

Oxidative Dehydrogenation of Ethane on Oxide Materials in a Pulsed Microcatalytic and a Membrane Reactor by N. A. Zhilyaeva; M. M. Ermilova; N. V. Orekhova; N. L. Basov; S. V. Mikhailovskii; A. A. Malygin; A. B. Yaroslavtsev (1136-1143).
This paper presents a study of oxidative dehydrogenation of ethane (ODHE) on nanostructured single-component and multicomponent catalysts based on V5+, Mo5+, and Nb5+ oxides and produced on alumina by molecular layering. It has been shown that the molybdenum-containing catalysts exhibit higher activity for ODHE in both single-component and binary samples and ensure higher ethylene selectivity. We have demonstrated advantages of ODHE on membrane–catalyst systems in the form of asymmetric ceramic tubes, with catalysts placed inside of them. The membrane–catalyst systems produced by molecular layering offer considerably higher activity and selectivity in ODHE.
Keywords: oxidative dehydrogenation; ethane; ethylene; membrane reactor; composite membranes; Mo–V–Nb–Te–O catalysts

Using the Pechini process in the presence of citric acid and mannitol, followed by annealing in the range 800–1000°C, we have synthesized nanostructured rare-earth borate tungstates crystallizing in hexagonal symmetry (sp. gr. P63): undoped Gd3BWO9 and Gd3 – x – y Yb x Er y BWO9, containing Yb3+ and Er3+ active ions. We have measured upconversion luminescence spectra of the Er3+ ions in Gd3 – x – y Yb x Er y BWO9 for the 2 H 11/2, 4 S 3/24 I 15/2 and 4 F 9/24 I 15/2 transitions under excitation by an IR laser (λ = 974 nm). The spectra consist of two groups of broadened bands in the red and green spectral regions. Brighter luminescence is observed in the green spectral region, which contains two bands corresponding to the Er3+ 2 H 11/2, 4 S 3/24 I 15/2 and 4 F 9/24 I 15/2 transitions. We have analyzed the effect of annealing temperature on the upconversion luminescence intensity of Gd3 – x – y Yb x Er y BWO9 and determined the energy yield of upconversion luminescence in the Gd3 – x – y Yb x Er y BWO9 borate tungstates. The highest value, B en = 0.19%, has been obtained for the phosphor with Yb : Er = 7 : 1.
Keywords: rare-earth borate tungstates; upconversion phosphors; nanophosphors

Sorption of Cs+ Ions from Seawater by a Nanostructured Aluminosilicate Sorbent by P. S. Gordienko; I. A. Shabalin; S. B. Yarusova; I. G. Zhevtun; S. B. Bulanova (1151-1156).
We have studied the Cs+ sorption properties of a synthetic nanostructured X-ray amorphous KAlSi3O8 ⋅ 1.5H2O potassium aluminosilicate (SPA) with a specific surface area of 105.0 m2/g under static conditions in cesium carbonate solutions in seawater at Cs+ concentrations from 0.05 to 23.07 mmol/L. The results demonstrate that, in this Cs+ concentration range, the sorption capacity of the SPA reaches 0.45 mmol/g.
Keywords: synthetic potassium aluminosilicate; sorption; cesium ions; seawater

Optical Properties and Electron Paramagnetic Resonance of MnO2-Doped Fluorozirconate Glasses by M. N. Brekhovskikh; S. Kh. Batygov; L. V. Moiseeva; S. P. Solodovnikov; I. A. Zhidkova; G. L. Denisov; V. A. Fedorov (1157-1161).
We have studied luminescence and electron paramagnetic resonance (EPR) spectra of manganese-activated fluorozirconate glasses. In ZrF4–BaF2–LaF3–AlF3–NaF (ZBLAN) glasses, manganese gives a green luminescence band peaking at 545 nm. Partial chlorine substitution for fluorine in the glass leads to a shift and broadening of the luminescence band. Complete BaCl2 substitution for BaF2 in ZBLAN〈Cl〉 fluoride chloride glass increases the peak emission wavelength to 610 nm. The observed distinctions between the EPR spectra of the Mn-doped ZBLAN and ZBLAN〈Cl〉 glasses correlate with those between their luminescence spectra. The effect of chlorine substitution for fluorine on the luminescence and EPR spectra of the fluorozirconate glass is tentatively attributed to a change in the oxidation state of the manganese ions.
Keywords: fluorozirconate glasses; luminescence; electron paramagnetic resonance; manganese ion; doping

A Thermokinetic Study of a Polycarbosilane- and Oligovinylsilazane-Based Ceramic-Forming Composition by M. A. Khaskov; E. A. Davydova; M. I. Valueva; A. M. Shestakov (1162-1167).
The thermokinetic behavior of a polycarbosilane- and oligovinylsilazane-based ceramic-forming binder during curing and pyrolysis has been studied by differential scanning calorimetry and infrared spectroscopy. The rate equations for the curing and pyrolysis reactions have been derived under the assumption that the reaction rate, weight loss, and heat flux are proportional to the heating rate. Curing and pyrolysis have been shown to involve at least two competing processes. Based on the kinetic models obtained, we have proposed heat treatment conditions that ensure a constant rate of the curing and pyrolysis reactions.
Keywords: ceramic-matrix composite materials; polycarbosilane; oligovinylsilazane; curing; pyrolysis; thermokinetic study

A process has been proposed for the fabrication of reaction-bonded dicalcium phosphate dihydrate-based composite materials possessing rather high strength owing to reinforcement with discrete glass fiber and compaction. We have studied the effects of the heat treatment temperature of tricalcium phosphate, a precursor to dicalcium phosphate dihydrate, and filler concentration on the chemical and physicomechanical properties of the composite materials thus produced.
Keywords: brushite; cement; glass fiber; strength

Effect of Mechanical Activation on Thermally and Shock Wave Initiated Reactions of Refractory Metals with Teflon by M. I. Alymov; S. G. Vadchenko; I. S. Gordopolova; I. V. Saikov; I. V. Milyukova (1175-1182).
We have studied ignition and structure formation processes in powder mixtures of refractory metals, Teflon, and energetic additives. Tungsten or tantalum has been used as a component of the mixtures in order to obtain high-density condensed products. Aluminum and zirconium have been used as energetic additives that lower the ignition temperature and raise the combustion temperature of the mixtures. The compositions studied have been chosen using thermodynamic calculations in order to maximize the amount of condensed products with a high combustion temperature. The mixtures were pressed into pellets. The heating rate of the crucible was varied. Experiments with the mechanically activated mixtures have shown an appreciable increase in the magnitude of the effect for both thermal and shock wave reaction initiation.
Keywords: high-energy metal–Teflon mixtures; mechanical activation; ignition; explosion and shock wave impacts