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 (Y 3 Al 5 O 12 ) nanopowders by liquid-feed flame spray pyrolysis (LF-FSP) of combinations of yttrium and aluminum precursors dissolved in EtOH, n BuOH, and/or THF. These include solutions of the following: yttrium and aluminum nitrates in EtOH or n BuOH; yttrium 2-ethylhexanoate and alumatrane [N(CH 2 CH 2 O) 3 Al] 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 O 2 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 YAlO 3 I and some Y 4 Al 2 O 9 (YAM). The remaining Al 2 O 3 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 YAlO 3 I crystal structure and a YAG composition. Powders annealed at 900−1000 °C (7−10 d) transform without grain growth or necking to free-flowing YAG phase powders. The activation energy for this phase transformation was ≈100 kJ/mol, much lower than values reported for amorphous Y 3 Al 5 O 12 .