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) e100 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 .

Liquid-feed flame spray pyrolysis (LF-FSP) is a general aerosol combustion route to unagglomerated and often single crystal mixed-metal oxide nanopowders with exact control of composition. LF-FSP of xNi(O 2 CCH 2 CH 3 ) 2 /yAl(OCH 2 CH 2 ) 3 N EtOH solutions at selected x:y ratios provides mixed-metal oxide nanopowders with compositions covering much of the Al 2 O 3 -NiO phase space. All powders were characterized by XRD, BET, FTIR, SEM, TEM, and TGA-DTA. With the exception of pure NiO (specific surface area, SSA, ∼7 m 2 /g), all product powders offer SSAs g 45 m 2 /g (average particle sizes e 30 nm) without microporosity. At NiO/Al 2 O 3 ratios near 1:1, the LF-FSP nanopowders are single phase, bright blue NiAl 2 O 4 inverse spinel. The blue color of these materials is typical of Ni spinels. At higher NiO contents, NiO is the dominant phase with some δ-alumina and intermediate spinels. At low NiO contents, blue powders form but the δ-alumina phase predominates, suggesting incorporation of Ni 2+ in the alumina lattice or formation of traces of NiAl 2 O 4 . Compositions near 20:80 mol NiO/Al 2 O 3 generate an inverse spinel structure, per XRD with peaks shifted ≈4°2θ to higher values from those of pure NiAl 2 O 4 . This contrasts with the published phase diagram, which suggests a mixture of NiAl 2 O 4 spinel, and corundum should form at this composition. This material resists transformation to the expected phases on heating to 1400 °C, indicating a single stable phase which contrasts with the known phase diagram and, therefore, is a new material in NiO-Al 2 O 3 phase space with potential value as a new catalyst.

Nanometre-sized particles of transition (t)-aluminas are important for the fabrication of high-quality alumina ceramics. Multiple tons are produced each year using a variety of gas-phase processes. The nanoparticles produced by these methods consist mainly of the undesired delta phase with some gamma- and theta-Al(2)O(3). Nano-t-aluminas should provide access to dense nano/submicrometre-grained alpha-Al(2)O(3) shapes offering significant advantages over micrometre-grained shapes. Unfortunately, polymorphism coupled with the high activation energy for nucleating alpha-Al(2)O(3) greatly impedes efforts to process dense alpha-Al(2)O(3) with controlled grain sizes, especially for submicrometre materials. Typically alpha-Al(2)O(3) nucleation within t-aluminas is sporadic rather than uniform, leading to exaggerated grain growth and vermicular microstructures without full densification (5). Thus, production of quantities of nano-alpha-Al(2)O(3) from multiple nano-t-aluminas for seeding or direct processing of alpha-Al(2)O(3) monoliths could greatly change how alpha-Al(2)O(3) components are processed. We report here that liquid-feed flame spray pyrolysis of nano-t-aluminas converts them to dispersible 30-80 nm alpha-Al(2)O(3) powders (50-85% phase transformed). Surprisingly, the powder surfaces are fully dehydrated. These powders pressureless sinter to more than 99.5% dense alpha-Al(2)O(3) with final grain sizes < or =500 nm without sintering aids.

(15 coronal slices, repetition time (TR)/echo time (TE) = 635/17 ms, FOV = 20 cm 20 cm, acquisition matrix = 320 512, flip angle = 90 , slice thickness = 1.5 mm with no gap, number of averages = 3). The recent advent of transparent polycrystalline YAG lasers that outperform single-crystal YAG lasers has intensified interest in the development of very fine YAG particles that are easily sintered to full density and transparency. [11,12] We report here efforts to produce nanosized Y 3 Al 5 O 12 powders for this purpose that result in a new phase with higher densities than YAG and sinter to full density at relatively low temperatures. We recently described a new method of producing large quantities of single and mixed-metal oxide nanopowders with exceptional control of composition based on liquid-feed flame spray pyrolysis. (LF-FSP).[13] In this process, metallo-organic precursors (e.g., metal carboxylates) with the exact composition of the metals desired in the final oxide nanopowders are dissolved in an alcohol (typically ethanol) and the solutions aerosolized with oxygen. The aerosol mist is ignited to produce flame temperatures exceeding 1500 C. Rapid quenching of the gas-phase species produces nanosized oxide ªsootº with the exact metal composition of the starting solution, including any impurities or dopants. Given that nanosized oxide powders are known to sinter at temperatures well below those of micrometer-sized powders, a set of precursors designed to produce the Y 3 Al 5 O 12 composition were assessed. The resulting nanopowder was found to form a new hexagonal Y 3 Al 5 O 12 phase, which we describe here. In studies reported in detail elsewhere, [13] we found that 3:5 mixtures of Y(O 2 CCH 2 CH 3 ) 2 OH and Al(Acac) 3 (Acac = acetylacetonate) dissolved in ethanol gave nanopowders with average particle sizes (APS) below 50 nm (Fig. 1a). The as-produced particles are unnecked, easily dispersed, and single crystals, as determined by high-resolution TEM (Fig. 1b).In contrast to what we anticipated, the digital diffraction and XRD powder patterns for the as-produced powders do not match those of YAG. As shown in Figure 2, the XRD most closely resembles that of the hexagonal phase of YAlO 3 . Since this is a commonly observed kinetic phase in this system, this finding was not too surprising. However, if we had produced YAlO 3 , then the overall stoichiometry of the system would be 3YAlO 3´A l 2 O 3 . The excess alumina (25 mol-%) would be expected to be visible as a crystalline phase (not observed), an amorphous phase with an amorphous hump in the XRD powder patterns (not observed), or last (and least likely), as a component in a defect structure.On careful examination, the XRD peak intensities obtained differ from those expected for YAlO 3 . This prompted examination of the low-angle XRD pattern, revealing a peak at 2h = 8.3±8.5 corresponding to a lattice parameter of » 1.1 nm, close to the unit-cell dimensions for crystalline YAG and the (001) interplanar distance of hexagonal YAlO 3 . However, neither true Y...

We report here the use of liquid‐feed flame spray pyrolysis (LF‐FSP) to produce a series of nanopowders along the CoOx–Al2O3 tie line. The process is a general aerosol combustion synthesis route to a wide range of lightly agglomerated oxide nanopowders. The materials reported here were produced by aerosolizing ethanol solutions of alumatrane [Al(OCH2CH2)3N] and a cobalt precursor, made by reacting Co(NO3)2·6H2O crystals with propionic acid. The compositions of the as‐produced nanopowders were controlled by selecting the appropriate ratios of the precursors. Nine samples with compositions (CoO)y(Al2O3)1−y, y=0−1 along the CoOx–Al2O3 tie line were prepared and studied. The resulting nanopowders were characterized by X‐ray fluorescence, BET, scanning electron microscopy, high‐resolution transmission electron micrographs, X‐ray diffraction (XRD), thermogravimetric analysis (TGA), and FTIR. The powders typically consist of single‐crystal particles <40 nm diameter and specific surface areas (SSAs) of 20–60 m2/g. XRD studies show a gradual change in powder patterns from δ‐Al2O3 to Co3O4. The cobalt aluminate spinel phase is observed at stoichiometries (21 and 37 mol%) not seen in published phase diagrams, likely because LF‐FSP processing involves a quench of >1000°C in microseconds frequently leading to kinetic rather than thermodynamic products. Likewise, the appearance of Co3O4 rather than CoO as the end member in the tie line is thought to be a consequence of the process conditions. TGA studies combined with diffuse reflectance FTIR spectroscopic studies indicate that both physi‐ and chemi‐sorbed H2O are the principal surface species present in the as‐processed nanopowders. The only sample that differs is Co3O4, which has some carbonate species present that are detected and confirmed by a sharp mass loss event at ∼250°C. The thermal behavior of the high cobalt content samples differs greatly from the low cobalt content samples. The latter behave like most LF‐FSP‐derived nanopowders exhibiting typical 1%–4% mass losses over the 1400°C range due mostly to loss of water and some CO2. The high cobalt content samples exhibit a sharp mass loss event that can be attributed to the decomposition of Co3O4 to CoO.