G. B. Tel’nova

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.

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.

Phase Transformations during Plasma Synthesis of Sodium Polyaluminates

The synthesis of sodium beta-alumina with the use of a low-temperature nitrogen plasma is described. The major plasma synthesis product is the sodium polyaluminate Na2O·nAl2O3 with an unidentified structure and n=3.5–9.5. The phase transformations of the plasma synthesis products are studied during heating in the range 600–1400°C. The phase with the rhombohedral beta-alumina structure (β″-Al2O3) is shown to form in several steps, through metastable sodium polyaluminates. The results are analyzed in relation to the phase equilibria in the Na2O-Al2O3 system.

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.

Structure and ionic conductivity of a beta-alumina-based solid electrolyte prepared from sodium polyaluminate nanopowders

This paper examines the structure and transport properties of Na-β″-Al2O3-based ceramic electrolytes synthesized from plasma synthesis products. The chemical composition of sodium polyaluminate nanopowders was varied in the ranges 6.5–9.3 wt % Na2O, 0.7–0.8 wt % Li2O, and 89.9–92.8 wt % Al2O3, at a phase ratio β″/(β″ + β) = 0.83–1.00. Data are presented on the formation of a highly conductive β″-phase with a rhombohedral structure, equilibrium chemical composition, and hexagonal unit cell parameters a = 0.5587(8) nm and c = 3.342(3) nm in the system in question during sintering and annealing. The phase formation dynamics are shown to be determined by the initial composition of the powders. The 573-K ionic conductivity of β″-alumina solid electrolytes with the compositions Na(1.53–1.73)Li(0.28–0.32)Al(10.66–10.72)O17 and a relative density of at least 99% ranges from 25 to 33 S/m, with an activation energy for conduction ΔEa = 0.15–0.20 eV.

Mechanisms of formation of solid solutions and optical spectroscopic properties of transparent ceramic materials based on Y2O3:Yb

It has been shown that, during vacuum sintering of a material based on the 0.804Y2O3 · 0.196Yb2O3 cubic solid solution, ytterbium is partially reduced to the divalent state and forms with yttria cubic solid solutions with heterovalent cation substitution, which is accompanied by the appearance of oxygen vacancies and the introduction of the doubly charged cation into crystal lattice voids. The concomitant decrease in the content of oxygen-occupied tetrahedral sites leads to the expansion of the cubic close packing of the metal so that the cubic unit cell parameter a increases to 10.5745 Å. After the oxidative annealing of the ceramics in air, the a value decreases to 10.5718 Å. The optical spectroscopic properties of transparent Y2O3:Yb ceramic materials after treatment in different media have been studied.

Study of the thermomechanical properties of a ceramic based on sodium polyaluminate with the structure of Na-β″-alumina

Results are presented from a study of how the thermomechanical properties of specimens of a ceramic based on sodium polyaluminate with the structure of Na-β″-alumina are affected by the ceramic’s microstructure, chemical composition, and phase composition. The range of critical pressure gradients that sharply reduce the strength and service characteristics of specimens of the solid electrolyte is determined. Quantitative estimates are made of the relative change in fracture toughness after thermal shock, the sensitivity of the material to defects, and the degree to which defects accumulate at the root of a notch during thermal shock.

Synthesis and sintering of submicron Nd:YAG particles prepared from carbonate precursors

Using high-temperature X-ray diffraction, differential scanning calorimetry, and electron microscopy, we have studied the formation of yttrium aluminates and Nd:YAG (YAG) activated garnet nanoparticles during the thermal decomposition of a poorly crystallized carbonate precursor prepared in the NH4Al(OH)2CO3–(Y,Nd)(ОН)CO3 nanosystem and the development of the morphological structure of powders during heating to a temperature of 1350°C. The results demonstrate that heat treatment in the temperature range 850–950°C leads to the formation of metastable nonstoichiometric YAlO3 with a garnet-like structure, which reacts with Al2O3 at a temperature of 1000°C to form YAG. The cubic cell parameter a and X-ray density of YAG crystals with the composition Y2.97Nd0.03Al5O12 synthesized at 1200°C are 1.2009 nm and 4.565 g/cm3, respectively, and the average particle size is 108 nm. Using carbonate route, we prepared transparent Nd:YAG ceramics with a relative density of 99.7%, X-ray density of 4.562 g/cm3, and crystallite size in the range 1–7 μm.