N. P. Simonenko

Promising ultra-high-temperature ceramic materials for aerospace applications

Some aspects of heat transfer upon the interaction between components with a sharp leading edge and high-enthalpy high-speed flow of dissociated air have been considered; some material characteristics, which should be primarily taken into account when prognosticating the behavior of materials that are promising for using as components of hypersonic flight vehicles, have been substantiated; specific features of the oxidation of materials based on zirconium and hafnium diborides have been touched briefly; the methods of increasing oxidation resistance of these materials that have been developed by various groups of researchers have been demonstrate; some works concerning the behavior of samples under the effect of high-enthalpy flows of dissociated air have been described, including those that simulate sharp leading domes and edges of wings of hypersonic flight vehicles.

Synthesis of ultrafine yttrium aluminum garnet using sol-gel technology

Mesoporous yttrium aluminum garnet Y3Al5O12 powders were prepared using sol-gel technology proceeding from solutions of metal alkoxoacetylacetonates. Xerogel microstructure was studied by SEM, and the fact of mesopores being formed was established. The temperature range within which Y3Al5O12 crystallizes in a dynamic mode from the xerogel was determined to be 850–950°C using an SDT Q600 TGA/DTA/DSC analyzer. A 1-h isothermal treatment of the xerogel was shown to reduce the garnet phase formation temperature to 800°C. At lower temperatures (400, 450 or 500°C), even long-term (6-h) calcination yielded X-ray amorphous powders with developed surfaces (specific surface areas were 230–350 m2/g). Powder particle coarsening was studied upon sintering for 2 and 4 h at 1000, 1200, and 1400°C.

Synthesis, vaporization, and thermodynamics of ultrafine Nd2Hf2O7 powders

Ultrafine Nd2Hf2O7 powders with the pyrochlore structure are prepared by self-propagating hightemperature synthesis (in the Pechini version). The elemental and phase composition of the powder are studied. Microstructure is studied by scanning electron microscopy. BET specific surface areas are determined. Thermal behavior is studied by TGA/DSC/DTA up to 1473 K. The sintering kinetics of as-synthesized Nd2Hf2O7 powder at various temperatures is studied. Nd2Hf2O7 vaporization in the range 2400–2600 K is studied by Knudsen effusion/mass spectrometry, and the thermodynamic characteristics of this compound are determined.

Synthesis of nanocrystalline silicon carbide using the sol-gel technique

Highly disperse silicon carbide is synthesized using a hybrid method comprising the sol-gel step to provide the SiO2-C starting mixture involving the formation of a transparent gel and carbothermal synthesis under relatively soft conditions, namely, at temperatures of 1200–1500°C under a dynamic vacuum. The elemental and phase compositions of the products and their thermal behavior in air are studied. A relationship is found to exist between the microstructure of the product, on the one hand, and the temperature and time of heat treatment, on the other.

Production of 8%Y2O3-92%ZrO2 (8YSZ) thin films by sol-gel technology

Hydrolytically active heteroligand coordination compounds [M(C5H7O2)x(iOC5H11)y] (M = Zr, Y) were synthesized from zirconium and yttrium acetylacetonates, and the kinetics of gelation of their solutions by hydrolysis and polycondensation was studied. Silicon substrates were dip-coated with thin films of solutions of these precursors, and their bulk hydrolysis was investigated. In heat treatment of xerogel coatings under various conditions, the crystallization of nanostructured films of oxide of the composition 8%Y2O3-92%ZrO2 (8YSZ) was examined, and the dependence of the particle phase composition, microstructure, and size on the synthesis conditions was described.

Preparation of nanostructured thin films of yttrium iron garnet (Y3Fe5O12) by sol–gel technology

The synthesis of hydrolytically active heteroligand coordination compounds [M(C5H7O2)3–x(C5H11Oi)x] (where M = Al3+ and Y3+) using aluminum and yttrium acetylacetonates has been studied. The gel formation kinetics in their solutions upon hydrolysis and polycondensation has also been studied. Thin films of a solution of these precursors have been applied to polished sapphire substrates by dip coating. The crystallization of nanostructured yttrium aluminum garnet (Y3Al5O12) films during heat treatment of xerogel coatings under various conditions has been studied. How the phase composition, microstructure, and particle size depend on the synthesis parameters has been recognized.

Gel formation during sol–gel synthesis of silicon dioxide

The effects of water and catalyst (formic acid) concentrations and temperature on tetraethoxysilane hydrolysis, followed by gel formation, have been studied. The catalyst concentration is found to have a stronger effect on the variation rate of dynamic viscosity of the system compared to the effect of water concentration. The thermal behavior of the thus-prepared xerogels has been studied in an air flow; distinctions have been noticed for samples whose initial dispersion-bound systems were formed at room temperature.

Thin films of the composition 8% Y 2 O 3 –92% ZrO 2 (8YSZ) as gas-sensing materials for oxygen detection

The synthesis of hydrolytically active heteroligand precursors of the composition [M(O2C5H7)x(iOC5H11)y] (M = Zr4+ and Y3+) using zirconium and yttrium acetylacetonates was investigated. It was shown that the reactivity of the obtained precursors in hydrolysis and polycondensation depends on the composition of the coordination sphere. It was studied how thin films of their solutions are applied by dip coating to the surface of Al2O3 substrates with platinum interdigital electrodes and a microheater. A study was made of the effect of the crystallization conditions and thickness of the oxide coatings of the composition 8 mol % Y2O3–ZrO2 on their microstructure and sensor characteristics in oxygen detection.

Influence of the composition of [Ti(OC4H9)4 – x(O2C5H7)x] complexes and hydrolysis conditions on the synthesis of titania by sol–gel technology

The influence of the coordination sphere of [Ti(OC4H9)4–x(O2C5H7)x] complexes, concentration in solution, and the ratio n(H2O)/n(Ti4+) on the hydrolysis and polycondensation kinetics of the complexes, the stability of the resulting gels, the thermal behavior of then-formed xerogels, the shift of anatase–rutile phase transition temperature, and the microstructure of the product, was determined.

Preparation of nanostructured titania thin films by sol–gel technology

This study is concerned with the preparation of hydrolytically active heteroligand complex [Ti(OC4H9)3.61(O2C5H7)0.39] from titanium butoxide and acetylacetone and with the gel formation kinetics in a solution of this complex upon hydrolysis and polycondensation. Single-layer and double-layer thin films of a solution of this precursor were coated on polished silicon substrates using the dip-coating method. The crystallization of nanostructured titania films during the heat treatment of these xerogel coatings was studied using various protocols; the anatase–rutile phase transition temperature was found to depend on the film thickness. The effects of the precursor solution viscosity on the film thickness and crystallite size were determined.