V.N. Baumer

Low-agglomerated yttria nanopowders via decomposition of sulfate-doped precursor with transient morphology

Abstract The fabrication peculiarities of low-agglomerated yttria (Y2O3) nanopowders via thermal decomposition of sulfate-doped precursor with transient morphology were studied. It was determined that Y2(OH)5(NO3)x(CO2)y(SO4)z·nH2O (n=1-2) crystalline precursor underwent fragmentation and decomposition into isolated quasi-spherical Y2O3 particles upon calcination. Effect was connected with minimizing the free energy of the plate-like crystallites via reducing the contact surface until to the moment of spheroidization and attainment of isolation that occurred at T=1100 °C. Residual sulfate ions slowed down the surface diffusion during heat treatment thus retaining quasy-spherical morphology and low aggregation degree of Y2O3 nanopowders. Sulfate-doped yttria nanopowders with medium particle size of 53±13 nm possessed improved sinterability in comparison with undoped ones arising from finer particle size, narrower particle distribution and lower agglomeration degree.

Luminescence of Heavily Ce-Doped Alkaline-Earth Fluorides

Luminescent and structural properties of M1-xCexF2+x (M=Ca, Sr, Ba, 0.15<x<0.35) single crystals were studied. It was found that the solid solutions have fluorite crystal structure with the lattice constant noticeably different from that of MF2 crystal. A partial deficiency in the fluorine positions as well as excessive fluorine in interstitial sites was revealed. Crystal lattice modification does not strongly affect the basic absorption and emission properties as compared with weakly Ce-doped MF2. The spectral-kinetic parameters of solid solutions luminescence are similar to those of Ce3+ ion in perturbed site in CeF3 lattice. Low light yield of mixed compounds is connected with the small Stokes shift and reabsorption. The probable model of perturbed Ce3+ center in M1-xCexF2+x crystals assumes the presence of interstitial fluorine in the nearest or next nearest neighborhood of the cerium ion.

Growth of LGSO : Ce Crystals by the Czochralski Method

Single crystals of Lu2xGd2 − 2xSiO5: Ce (0 < x < 1) compounds with different atomic ratios Lu/(Lu + Gd) have been grown by the Czochralski method. It has been shown that a change in the spatial symmetry from P21/c to C2/c in the course of substitution of lutetium for gadolinium occurs at the ratio Lu/(Lu + Gd) = 0.1. The lattice thus formed with symmetry C2/c in the structure of Lu2xGd2 − 2xSiO5: Ce crystals favors the maximum possible incorporation of Ce3+ ions into the sevenfold-coordinated position with respect to oxygen. This explains the substantial improvement of the scintillation characteristics of the grown crystals.

Nd3+:Y3Al5O12 laser ceramics: Influence of the size of yttrium oxide particles on sintering

The influence of the size of Y2O3 powder particles on the structure formation and densification of Nd3+:Y3Al5O12 laser ceramics has been studied. It is shown that the use of 50- and 100-nm yttrium oxide particles makes it possible to synthesize single-phase yttrium aluminum garnet at temperatures of 1200 and 1500°C, respectively, whereas in the case of 5000-nm yttrium oxide particles 2-h exposure at a temperature of 1500°C yields only 80 wt % of the Nd3+:Y3Al5O12 phase. Bulk swelling of pressed samples during sintering of 2.94Y2O3-0.06Nd2O3-5Al2O3 powders with the size ratio of the initial particles R(Al2O3/Y2O3) ∼ 5 is observed. The application of different-sized powders (R ∼ 2.5) provides quantitative ratios between phases in the 3Y2O3-5Al2O3 system at which shrinkage in a temperature range of 20–1500°C is dominant. Laser ceramics 0–2 at % Nd3+:Y3Al5O12 have been obtained by the solid-phase sintering of oxide powders (R ∼ 2.5). The slope efficiency for 1 at % Nd3+:Y3Al5O12 laser ceramics is found to be 33%.

Structure and morphology of spherical crystalline (Y1 − xEux)2O3 particles

The structure and morphology of monodisperse spherical particles of (Y1 − xEux)2O3 solid solutions prepared through the low-temperature thermolysis of an amorphous precursor under isothermal conditions (t = 800°C) have been analyzed in detail in relation to the particle size and composition. The results demonstrate that, in addition to having a polycrystalline (block) structure, the spherical (Y1 − xEux)2O3 particles are porous, with a mesoscopic residual pore size. Reducing the diameter of the spherical particles from 400 to 70 nm reduces their porosity and improves their structural perfection. The addition of Eu3+ cations stimulates the growth of crystalline blocks that form a spherical particle, activating diffusion processes in the cation sublattice of the solid solution.

Influence of Time-Temperature Parameters on the Structure and Photoluminescence of (Y1−xEux)2O3 Crystalline Spheres

In this paper, sphere-like (Y1−xEux)2O3 nanocrystalline particles with (xxa0=xa00.02;0.05;0.06;0.07;0.08, and 0.09) were obtained by the urea-based homogeneous precipitation technology followed by low-temperature crystallization from 600 to 900xa0°C. The structure and morphology of the (Y1−xEux)2O3 nanocrystalline particles and thermal stability of the nanocrystalline particles were investigated by means of transmission electron microscopy and x-ray diffraction analysis. The structural evaluation was established in relation to thermal conditions of sphere crystallization to ensure maximally effective luminescence of Eu3+ ions (5D0xa0→xa07F2 transitions) with the morphological stability, the spherical shape of individual nanocrystalline particles remaining unchanged. The crystalline nanospheres were shown to have the threshold of concentration quenching of luminescence shifted toward lower concentrations of Eu3+ ions in comparison with micropowders of analogous composition. That may be caused by the formation of (Y1−xEux)2O3 composition with the concentration gradient of Eu3+, followed by accumulation of Eu3+ ions around the grain boundaries.

Specific features of the structure of ZnO nanocrystals grown in pores of Y 2 O 3 spherical matrices

A two-phase spherical Y2O3/ZnO nanocomposite particle with a diameter variable in the series of 70, 130, 180, 250, and 400 nm (variance ⩽15%) has been obtained; the first phase is a mesoporous Y2O3 matrix, while the second is a crystalline ZnO phase located in the size-limited pore volume. Specific features of ZnO nanocrystals formed on the high-curvature surface of pores with excess surface energy are studied. It is shown that the thermally activated coarsening of ZnO crystallites accompanied by an increase in the nanophase structural quality occurs during Y2O3/ZnO annealing in the temperature range t = 600–800°C. It is established that the wurtzite lattice is strained on the surfaces of small Y2O3 pores.

Fabrication and characterization of 1–4 at.% Nd 3+ :Y 3 Al 5 O 12 Laser Ceramics by solid-state reactive sintering

Peculiarities of phase formation and densification processes during reactive sintering of Nd3+:YAG laser ceramics prepared using particles with different size have been studied. It has been found that utilization of submicron powders can significantly reduce YAG formation temperature from 1700°C for coarse powder to 1200-1500°C for fine powders. Moreover, utilization of different-sized yttria and alumina particles provide a competitive advantage of shrinkage over expansion processes by changing the kinetics of YAG phase formation. Nd3+:YAG (1-4 at.%) laser ceramics with in-line optical transmission of about 81.5 % at λ=1064 nm has been produced by reactive sintering. Finally, we succeed in lasing of Nd3+:YAG (4 at.%) ceramics under pulsed laser diode pumping.