Synthesis, characterization, and microstructural evolution of ultrafine oxide powders

Phase-pure, homogeneous, single- and multi-component ultrafine ceramic oxide particles (UFPs) were synthesized via flame spray pyrolysis (FSP) of combustible metalloorganic precursors. A variety of single source, atomically mixed, inexpensive, and air stable chemical precursors were used to (1) demonstrate the versatility of FSP to produce high purity UFPs and (2) control their phase and composition.; Ultrafine {dollar}rm TiOsb2, CeOsb2, 3Alsb2Osb3{lcub}cdot{rcub}2SiOsb2{dollar} (mullite), and {dollar}rm Ysb3Alsb5Osb{lcub}12{rcub}{dollar} (YAG) composition powders were produced by injecting a metalloorganic precursor/ethanol solution into a flame. UFP production occurs by combustion of the aerosol droplets of this solution to produce molecular M-O monomers, which then coalesce to form molecular clusters, which in turn form larger particles. These particles then grow via vapor condensation and/or coagulation. The semi-continuous FSP process used 50-100 mL of precursor/min to produce powders at 300-500 g/h.; The resulting UFPs were characterized with gas sorption, CHN, TGA, DTA, DRIFTS, XRD, and TEM techniques. Ultrafine TiO{dollar}sb2{dollar} particle sizes were 40-60 nm with surface areas of 35 m{dollar}sp2{dollar}/g and were mostly unagglomerated, single crystals. Mullite particles were mostly unagglomerated amorphous particles with some interparticle necking. Ultrafine mullite particle sizes ranged from 40-80 nm (due to necking) with surface areas = 45 m{dollar}sp2{dollar}/g. Ultrafine CeO{dollar}sb2{dollar} particles averaged 80 nm in diameter and were unagglomerated single crystals with surface areas = 10 m{dollar}sp2{dollar}/g.; The compaction and sintering behavior of UFPs were traced with XRD, DRIFTS, and SEM techniques. UFP compacts sinter and densify at lower temperatures than typical of coarser powder compacts. This work reports significant results for which there are no literature precedents. UFP-TiO{dollar}sb2{dollar} densified to 97% of theory with final average grain sizes of {dollar}<{dollar}200 nm. UFP-mullite composition compacts densified to 92% of theory with {dollar}<{dollar}400 nm average grain sizes. In contrast, compacted UFP-CeO{dollar}sb2{dollar} densified to 90% with average grain sizes of {dollar}>{dollar}2 {dollar}mu{dollar}m. CeO{dollar}sb2{dollar} inherent oxygen vacancies render it thermally unstable above 800{dollar}spcirc{dollar}C, where extensive grain growth occurs due to rapid oxygen diffusion. Doping may further inhibit grain growth in all compacts of UFP powders. Future work entails doping in the precursor state as well as solid-state doping.