V. V. Zyryanov

Morphology and structure of magnetic spheres based on hematite or spinel and glass

Magnetic spheres of various origins, isolated from fly ashes of the main Russian thermal coals, have been studied by electron microscopy, optical microscopy, X-ray diffraction, and Mössbauer spectroscopy. The magnetic spheres differ in morphology and have the form of multilevel core-shell nanocomposites, with a hematite or spinel Fe-rich core and a Fe-containing silicate glass shell. We have derived structural formulas of the spinels, which are consistent with all our experimental data. The most promising application fields of such magnetic spheres, after proper selection and modification, are catalysis and composites.

Porous supports for conducting ceramic membranes

We describe glass-ceramic materials produced using several kaolins for the fabrication of inert porous supports by a casting process. Optimizing the slurry composition and preparing quality slurries, we obtained a support material which had an open porosity of ≃34% after firing at 1380°C. Modification of the support material fired at 850–950°C through leaching, impregnation, and treatment with oxide sols insures effective control over its sintering shrinkage curve, which enables a reduction in the shrinkage mismatch between the support and ceramic layers to an acceptable level. To improve the chemical compatibility between the support material and ceramics, we formulated a number of effective solutions within the proposed approach.

Structure of SrCo0.5Fe0.5O3 − δ-based composites prepared by sol-gel and mechanochemical processes

X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and Mössbauer spectroscopy have been used to investigate the structural phase transformations of SrCo0.5Fe0.5O3 − δ-based mixed-oxide nanocomposites containing fine iron oxide particles. The nanocomposites have been prepared using sol-gel and mechanochemical processes. The addition of iron(III) oxide sol to SrCo0.5Fe0.5O3 − δ xerogel is shown to enhance the thermal stability of the resultant cubic perovskite phase. The stabilization is due to partial shielding of the perovskite surface with a thin Fe2O3 layer, which hinders cobalt diffusion to the surface, preventing Co3O4 formation.

Preparation, structure, and electrical conductivity of calcium-antimonate-based materials

Metastable antimony(V) oxide compounds are prepared via mechanochemical reactions in mixtures of CaO and Sb2O3, followed by mild oxidizing heat treatment and calcium leaching in a hydrochloric acid solution, and are characterized by x-ray diffraction and thermal analysis. The synthesized compounds have the pyrochlore structure or a monoclinic pyrochlore-like structure (Ca16Sb3+8Sb5+8O48). Doping with fluorine slows down subsequent oxidation, while doping with lanthanum impedes calcium leaching. The composition and morphology of the powders (nanopores and core-shell microstructure of the grains) determine their properties: water release in a broad temperature range, proton conductivity, and low absorption capacity for Na+ ions. In hydrogen atmosphere, the reduction of antimony decreases their conductivity.

Mechanochemical synthesis, phase composition, and properties of Pb(Zn1/3Nb2/3)O3-based ferroelectric ceramics

Pb-containing relaxor ferroelectric ceramics are prepared by mechanochemical ceramic processing. Mechanochemical reactions in binary and ternary mixtures of the PbO-ZnO-Nb2O5 system are studied by x-ray diffraction. Disordered compounds with the columbite, changbaiite, and pyrochlore structures are prepared. The perovskite and pyrochlore phases in 0.9Pb(Zn1/3Nb2/3)O3 + 0.1ABO3 morphotropic phase boundary materials are shown to be in mechanochemical equilibrium. Among the ABO3 additives studied, BaMnO3 is the most effective for stabilizing the perovskite structure. The mechanochemical synthesis path has a strong effect on the phase composition of the resulting material. Conventional synthesis through a columbite phase leads to the predominant formation of a pyrochlore phase. Firing conditions also have a profound effect on the phase composition of the ceramics, but the disordered perovskite phase retains cubic symmetry.

High-shrinkage-potential porous glass-ceramic supports for high-temperature oxide ceramic membranes

We have optimized the composition of porous glass-ceramic supports compatible with conducting ceramics and have developed a procedure for the fabrication of such supports. High-shrinkage-potential porous supports in the form of dead-end tubes have been shown to be best suited for designing multilayered selective ceramic membranes. We have assessed the effects of various modifiers on the shrinkage curve of such supports. The factors that play a central role in determining the properties of supports have been identified: the amount of burnout additives determines the macroporosity, the addition of low-melting clay influences the shrinkage potential, leaching influences the microporosity, impregnation with a lanthanum salt determines the shape of the shrinkage curve and reactivity, and modification with Al2O3 sol determines the heat resistance of the supports.

Planar multilayer membranes on metallic supports

We have tested a number of approaches to producing oxygen-selective planar multilayer membranes with mixed conductivity of functional layers on metallic gauze as a key component of a porous support. The results demonstrate that the shrinkage mismatch can be considerably reduced via pressure sintering. The conditions for firing metallic gauzes and applying ceramic coatings have been established. We describe the optimal structure and processes for applying ceramic layers with the aim of producing planar multilayer membranes of catalytic membrane reactors for methane conversion. Mechanochemical ceramic processing was successfully used to produce ceramic nanopowders and modifying paste.

Preparation, crystal structure, and electrical conductivity of the solid electrolyte Zr0.86Sc0.12Y0.02O1.93

The solid electrolyte Zr0.88Sc0.12Y0.02O1.93 for reduced-temperature SOFCs has been characterized by Rietveld X-ray powder diffraction analysis and conductivity measurements in the temperature range 295–970 K. Gas-tight nanostructured ceramic composites consisting of cubic, rhombohedral, and monoclinic phases have been produced by reaction sintering of mechanochemically prepared powders. The oxygen ion conductivity of the ceramic prepared by sintering at 1630 K, with a relative density of 94%, is three times lower than that of ceramics fabricated from DKKK Zr0.89Sc0.1Ce0.01O1.95 powder, but raising the sintering temperature to 1670 K increases the density of the ceramic to 99%, and its conductivity reaches the level of the DKKK ceramics. The core-shell ceramic nanocomposite obtained in this study possesses high mechanical strength and a reduced activation energy for grain-boundary conduction.

Polymorphism of ZrO2 nanopowders and mechanochemical synthesis of Zr0.88Sc0.1Ce0.01Y0.01O1.955

The structure of ZrO2 powders prepared by dehydration of zirconium hydroxide and milling (including techniques with the introduction of grinding additives, such as NaF, CaF2, diamond, and Cu) was investigated using x-ray powder diffraction and Raman spectroscopy. Samples containing crystallites with the smallest size were synthesized in the presence of copper additives. Ceramic powders of the composition Zr0.88Sc0.1Ce0.01Y0.01O1.955 with an improved quality for the use as solid electrolytes in fuel cells were prepared by the mechanochemical synthesis from nanoprecursors and then were characterized. An analysis of the X-ray powder diffraction patterns revealed that the symmetry of the structure of strongly aggregated nanopowders of metastable zirconia increases as a result of twinning, which is favored by a high concentration of vacancies.

Effect of surface modification with Au, Pd, and Pt on the morphology of δ-Bi2O3/Ag-based nanocermets

We have studied the effect of surface modification with noble metals on the morphology of δ-Bi2O3/Ag-based nanocermets as potential materials for medium-temperature oxygen membranes. Gold has been found to have a strong effect on silver recrystallization on the surface of the composites. The observed changes in morphology are due to the effect of the modifiers on the thermodynamic and kinetic factors of silver recrystallization. The most important factor for improving thermal stability of the nanocermets is homo- and heterogeneous doping with silver, but it is also necessary to optimize their composition and ensure a multilevel architecture of the material with an interpenetrating structure.