Yttrium Aluminum Garnet Nanopowders Produced by Liquid-Feed Flame Spray Pyrolysis (LF-FSP) of Metalloorganic Precursors

We report here the synthesis of yttrium-aluminum garnet oxide (Y3Al5O12) nanopowders by liquid-feed flame spray pyrolysis (LF-FSP) of combinations of yttrium and aluminum precursors dissolved in EtOH, nBuOH, and\or THF. These include solutions of the following: yttrium and aluminum nitrates in EtOH or nBuOH; yttrium 2-ethylhexanoate and alumatrane [N (CH2CH2O)3Al] in THF or EtOH; yttrium methoxyacetate and alumatrane in EtOH; yttrium acetylacetonate and alumatrane in EtOH, and yttrium propionate and aluminum acetylacetonate in EtOH or THF. Each precursor system was aerosolized with O2 and subsequently ignited. Following combustion, the resulting powders were collected by electrostatic precipitation at rates of 50 g\h. Surprisingly, the precursor choice strongly influences both the initial phase composition and morphology of the LF-FSP powder, as well as the phase changes that occur during annealing. As-collected LF-FSP nanopowders, average particle size (APS) 100 nm, had the YAG composition of the precursor feed; but XRD shows an apparent mixture of hexagonal YAlO3 I and some Y4Al2O9 (YAM). The remaining Al2O3 exists either as nanosegregated, amorphous alumina or in defect structures. However, the most homogeneous powders exhibit FTIR, TGA\DTA, TEM, and XRD data that suggest a new phase with a modified YAlO3 I crystal structure and a YAG composition. Powders annealed at 900-1000 C (7-10 d) transform without grain growth or necking to freeflowing YAG phase powders. The activation energy for this phase transformation was 100 kJ\mol, much lower than values reported for amorphous Y3Al5O12.