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rather conditions on the formation of yttrium aluminum garnet, Y 3 Al 5 O 12 (YAG), was investigated using "single-source" pre-than YAG when reacting a mixture of precursors with 3:5 cursors (cohydrolyzed yttrium and aluminum alkoxides Y:Al molar ratio. 11-14 Second, the stiffness of YAG (Young's and yttrium aluminum glycolates) and "multiple-source"modulus, E, of 333 GPa) creates a dichotomy in refractory precursors (mixtures of metal nitrates and mixtures of sepaceramic applications, because a high temperature is necessary rately hydrolyzed yttrium and aluminum alkoxides). Phasefor sintering, which can result in increased grain size and consepure YAG was formed only in the solid-state thermal quential loss of structural integrity. The latter problem may be decomposition experiments. The lack of formation of YAG addressed by using powder synthesis techniques, such as spray in all the spray-pyrolysis experiments was ascribed to the pyrolysis, that produce highly dispersed powders that can, in short heating times and fast heating rates, which resulted in many cases, be sintered at lower temperatures. However, the the formation of kinetic products. In the case of the metal high heating rates and short heating times inherent to the spraynitrates, an additional factor that influenced product forpyrolysis process 15 may lead to the formation of kinetically mation was the difference in thermal reactivity of the prestable phases such as YAlO 3 and YAM, rather than the target cursors. It was concluded that the formation of complex thermodynamically stable phase. In fact, despite some attempts, metal oxide materials by conventional or aerosol routes is YAG has never been produced by spray-pyrolysis techniques. 14 not necessarily achieved by the use of a chemically homoge-We have investigated the potential of forming phase-pure neous precursor, such as a single-source precursor. It also YAG powders via the following set of experiments: was necessary to ensure that the precursors and intermedi-(1) Yttrium aluminate powders are synthesized by the ates have similar thermal decomposition temperatures to solid-state thermal decomposition of several 3:5 Y:Al precursor avoid phase segregation in the initial stages of thermal systems, to determine the influence of precursor homogeneity decomposition.on the phase content of metal oxide powders that are obtained by the thermal decomposition of "single-source" and "multiplesource" precursors. A single-source precursor for a heterometal I. Introduction oxide, A x B y O z , is a single molecule that contains metals A and D. W. Johnson Jr.-contributing editor zation suppressant. The error in all AA measurements was approximately Ϯ5 at.%. Electron microprobe analysis (EMPA) of the powder samples was conducted using an electron micro-Manuscript No. 192526.

We apply a number of complementary characterization techniques including electron paramagnetic resonance, optical absorption, and photoluminescence spectroscopies to characterize a wide range of different ZnO phosphor powders. We generally observe a good correlation between the 510-nm green emission intensity and the density of paramagnetic isolated oxygen vacancies. In addition, both quantities are found to peak at a free-carrier concentration ne, of about 1.4 × 1018 cm-3. We also find that the green emission intensity can be strongly influenced by free-carrier depletion at the particle surface, especially for small particles and/or low doping. Our data suggest that the green PL in ZnO phosphors is due to the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band.

Crystalline, submicrometer strontium ferrite powders, including SrFeO2.97, SrFe2O4, Sr2FeO4, Sr3Fe2O6.16, and SrFe12O19, were prepared by spray pyrolysis of an aqueous solution of mixed metal nitrates. The Sr:Fe mole ratio in the precursor solution was retained in the final products. Phase‐pure materials were typically obtained only at the highest temperatures investigated (>1100°C) and powders prepared at lower temperatures frequently contained crystalline Fe2O3. The as‐prepared particles were unagglomerated, polycrystalline, and hollow at lower temperatures, but densified in the gas phase at higher temperatures to give solid particles. The strontium ferrite (SrFe12O19) system was studied in detail as a representative example of the Sr‐Fe‐O system. At temperatures of 1200°C, dense, phase‐pure magnetoplumbite‐structure material, SrFe12O19, was obtained, while at lower temperatures, small amounts of Fe2O3 were observed. The particles prepared at 800° and 1100°C were 0.1‐1.0 μm in diameter, and consisted of crystallites <100 nm, and were nearly solid. The difficulty in forming phase‐pure SrFe12O19 was the different thermal decomposition temperatures of Sr(NO3)2 (725°C) and Fe(NO3)39H2O (125°C) as demonstrated by thermogravimetric analysis in the SrFe12O19 system.

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