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Combinatorial Nanopowder Synthesis Along the ZnO–Al2O3 Tie Line Using Liquid-Feed Flame Spray Pyrolysis


  • R. Riedel—contributing editor

  • This work was supported by the Air Force Office of Scientific Research, through contracts No. F49620-03-1-0389 and subcontract from UES Inc. F074-009-0041/AF07-T009, Diamond Innovations Inc., the National Science Foundation (IGERT grant DGE-9972776 and DMR 9975542), and Maxit Corp., Oslo, Norway.

†Author to whom correspondence should be addressed. e-mail:


Liquid-feed flame spray pyrolysis (LF-FSP) of mixtures of alumatrane [Al(OCH2CH2)3N]/zinc acetate dihydrate [Zn(O2CCH3)2·2(H2O)] or zinc propionate [Zn(O2CCH2CH3)2]/aluminum acetylacetonate [Al(Acac)3] dissolved in EtOH in known molar ratios can be used to combinatorially generate nanopowders along the ZnO–Al2O3 tie-line. LF-FSP was used to produce (ZnO)x(Al2O3)1−x powders with x=0–1.0. Powders were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared, thermal gravimetric analysis, differential thermal analysis, and BET. The resulting powders had average particle sizes (APSs) <100 nm with the majority being <50 nm. Analytical data suggest that at concentrations of interest for transparent conducting oxides, <10 mol% Al2O3 the particle morphologies are combinations of plates and rods that grow with c/a ratios close to 1. The spinel phase dominates at (ZnO)x(Al2O3)1−x (x=0.5 and 0.3). In the latter case, the currently accepted phase diagram for the ZnO–Al2O3 couple indicates that phase separation should occur to form zinc spinel (ZnAl2O4) and α-alumina. It appears that the rapid quenching during LF-FSP helps to preserve the spinel phase at ambient temperature giving rise to kinetic nanopowder products along the ZnO2–Al2O3 tie-line. Finally, the solubility of ZnO in Al2O3 and vice versa in the materials produced by LF-FSP suggest apparent flame temperatures reached before quenching are 1700°–1800°C. Efforts to re-pass the spinel phase powders, (ZnO)x(Al2O3)1−x, x=0.5 and 0.3 through the LF-FSP system were made with the hope of generating core shell materials. However, instead the x=0.5 material generated materials closer to the x=0.3 composition and pure ZnO nanoparticles that coat the former materials. These results suggest that at LF-FSP flame temperatures ZnO remains in the vapor phase for sufficient times that Al3+ oxy-ions generated promote nucleation of finer particles leaving essentially phase pure ZnO still in the vapor phase to condense giving the two distinct particle morphologies observed.