K. A. Solntsev

Titanium-to-rutile oxidation kinetics in the direct-oxidation fabrication of thin-wall ceramics

The oxidation kinetics of titanium preforms for the direct-oxidation fabrication of thin-wall ceramics have been studied in the temperature range 750–1100°C using titanium plates up to 5 mm in thickness. Empirical relations for the kinetics of these processes have been obtained. The preform weight has been shown for the first time to influence the oxidation rate of titanium and the amount of forming rutile, which is inconsistent with the known patterns of metal oxide formation. One possible mechanism of rutile formation during the direct-oxidation fabrication of thin-wall ceramics is considered.

Synthesis and cathodoluminescence characteristics of europium-doped Ca-sialons

Using solid-state reactions and a combination of carbothermal reduction-nitridation with solgel processing, we have synthesized a number of oxynitride phases: Eu2+-doped Ca-sialons. We have measured the pulsed cathodoluminescence spectra of the synthesized powders under pulsed electron beam excitation and determined their emission parameters, including temporal emission characteristics.

Phase transformations during the synthesis and sintering of Y2 − xYbxO3 nanopowders

The formation of an Y2O3-Yb2O3 solid solution (8 mol % Yb2O3) during the thermal decomposition of (Y,Yb)2(CO3)3 · 2H2O mixed carbonates obtained by coprecipitation from a nitrate solution has been studied by X-ray diffraction, thermal analysis, and optical microscopy. The results demonstrate that the formation of a cubic yttria-based solid solution begins in the range 470–500°C and reaches completion at temperatures above 1100°C. The unit-cell parameter a of the cubic solid solution and the X-ray density of the corresponding ceramic are 10.5910 Å and 5.349 g/cm3, which corresponds to the intended chemical composition of the isovalent substitutional solid solution: Y1.84Yb0.16O3.

Si3N4/TiN composites produced from TiO2-Modified Si3N4 powders

Si3N4/TiN composites have been produced by hot pressing at temperatures from 1600 to 1800°C in a nitrogen atmosphere, using silicon nitride powders prepared by self-propagating high-temperature synthesis and surface-modified with titanium dioxide nanoparticles. We examined the effect of TiO2 content on the microstructure, phase composition, and mechanical strength of the ceramics. It is shown that titanium nitride can be formed by the reaction Si3N4 + TiO2 → TiN + NO + N2O + 3Si. The Si3N4/TiN composites containing 5–20% TiN have a low density, high porosity, and a bending strength of 60 MPa or lower. In Si3N4/TiN ceramics produced using calcium aluminates as sintering aids, the silicon nitride grains are densely packed, which ensures an increase in strength to 650 MPa.

Microstructure and properties of silicon nitride ceramics with calcium aluminate additions

Silicon nitride ceramics containing calcium aluminates as sintering aids have been prepared by hot pressing at 1650°C in a nitrogen atmosphere, and the effect of sintering aid content on their microstructure, phase composition, mechanical strength, and air oxidation resistance has been studied. The results demonstrate that the Si3N4 ceramic containing 10 wt % calcium aluminates has a uniform distribution of intergranular multicomponent oxide phases and consists of densely packed silicon nitride grains. Owing to this, it offers the maximum mechanical strength (850 MPa) and is stable to air oxidation up to 1300°C.

Hydrogen permeability of thin condensed Pd–Cu foil: Dependence on temperature and phase composition

The hydrogen permeability of thin (about 4 μm thick) magnetron-sputtered Pd–Cu foil and structural transformations during temperature cycling (heating–cooling process) are studied. It is found that the hydrogen permeability is maximal when the content of the β-phase is 100%. Upon annealing of Pd–Cu alloy in hydrogen, the temperature range where a regular structure exists expands.

Phase transformations upon the synthesis of Y3Al5O12:Nd

Methods of X-ray diffraction and thermal analysis have been used to study the processes of the formation of activated yttrium aluminum garnet Y3Al5O12:Nd (YAG:Nd) upon the thermal decomposition of carbonate precursors (two-phase nanocrystalline (Y,Nd)(OH)CO3 · nH2O-NH4Al(OH)2CO3 and an amorphous material) obtained by the coprecipitation from a solution of inorganic salts. It has been shown that the cubic structure of the garnet begins forming at a temperature of 900°C and is developed from the hexagonal metastable yttrium aluminate YAlO3, which starts forming (as an intermediate phase) at 800°C. The values of the lattice parameter a of the cubic structure and the X-ray density of the synthesized garnet Y2.97Nd0.03Al5O12 for the sample of the transparent phase-pure ceramics obtained are 1.2011 nm and 4.562 g/cm3, respectively.

Structure and hardness of ceramics produced through high-temperature nitridation of zirconium foil

We have studied the structure and microstructure of ZrN prepared by high-temperature nitridation of zirconium foil and estimated its hardness. The results demonstrate the feasibility of producing a composite heterostructure with the composition ZrN-(α-solid solution of nitrogen in zirconium/ZrN)-ZrN in the conditions of the process under consideration by heating to a temperature above the peritectic reaction temperature.

Orientation, substructure, and optical properties of rutile films

The orientation, optical properties, and substructure of rutile films prepared by thermal and pulsed photon-assisted oxidation of single-crystal Ti films were investigated by transmission electron microscopy, optical spectroscopy, and high-energy electron diffraction. Crystallographic orientation relationships at the contact between titanium and rutile were established, and the energy band gap was determined. It was shown that the luminescence intensity decreased significantly with decreasing grain size from the submicrolevel to nanolevel.

Effect of carbonate precursor synthesis conditions on the formation of monodisperse Nd:YAG nanopowders

Using electroacoustic spectroscopy and X-ray diffraction, we have studied the formation of monodisperse Y3Al5O12:Nd3+ doped garnet nanopowders (1 at % Nd) from carbonate precursors in the form of a system of two basic carbonates, Y(OH)CO3 · nH2O and NH4Al(OH)2CO3, in amorphous and crystalline states. The results demonstrate that monodisperse YAG nanopowders can be obtained from weakly agglomerated carbonate precursors with a nearly unimodal size distribution of particle conglomerates, produced by chemical precipitation from a dilute (0.15 M Al3+) aqueous chloride solution with ammonium bicarbonate and subsequent aging of the precipitate at room temperature and pH 7.86–7.89. The zeta potential of the disperse system for the carbonate precipitate suspension in its dynamic equilibrium is 4.3 ± 0.5 mV at an average nanoparticle conglomerate size of 165 ± 5 nm. After heat treatment of the carbonate precursors at temperatures from 1100 to 1200°C, we obtained single-phase Nd:YAG doped garnet nanopowders. The cubic cell parameter a of the Nd:YAG varies from 1.2022 to 1.2025 nm, depending on the synthesis temperature, and its crystallite size varies from 35 to 50 nm.