Synthesis Carbon Aerosol Gels in Inverted Flames
This approach for synthesis of structured nanomaterials (NM) is an alternative to the current liquid-based aerogel synthesis. The approach replaces the complex liquid aerogel process with Inverted Flame Reactor (IFR) approach, with no need for extracting liquid component. The traditional combustion aerosol gel synthesis does not provide the ideal conditions for aerosol gelation; aerosol gels cannot be produced unless extremely sooting fuels are used and/or the gelation occurs as a results of explosion; neither approach is practical for the continuous production of NMs. The inverted flame has previously used for synthesis of amorphous carbon-based aerosol gels using highly sooting fuels. In the traditional (up-right) flame configuration the particle growth process ends shortly after the precursor particles exit the flame or their concentration becomes too dilute (few milliseconds). In the inverted flame, the recirculation zone traps the precursors, extending their residence time to several seconds, which allows gelation and leads to the formation of aerosol gels.
The extended lifetime of precursor allows gelation to occur and leads to the formation of high porosity aerosol gels and coatings. The inverted flame has been used for synthesis of amorphous carbon-based aerosol gels. Though these carbon gels may be of interest in some application they cannot be used in the grafts due to their cytotoxicity. TiO2 and other flame produced NPs (e.g. fumed silica, etc.) are used in the variety of applications. These particles do not have properties that are required for improving biocompatibility: the interface between the graft and tissue must stimulate bone regeneration and endothelial cells must form a network inside the scaffold to form blood vessels. Recently we have demonstrated synthesis of structured TiO2. Figure 1 shows the dual inverted flame structure in H2 – TiCl4 system. The precursor TiCl4 was injected into the inverted H2 flame reacting with water vapor from hydrogen combustion. The exothermic reaction of TiCl4 and water (at temperatures above 2000K) resulted in the high flame temperature synthesis of TiO2 crystalline (rutile) structure with high surface area. The residence time in flame was in the order of seconds (100-1000 times greater than in the typical flame) allowing crystal growth from primary NPs.
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 Dhaubhadel, R., F. Pierce, A. Chakrabarti, and C. M. Sorensen (2006), Hybrid Superaggregate Morphology as a Result of Aggregation in a Cluster-Dense Aerosol, Physical Review E, 73(1), 011404
 Chakrabarty, R., Novosselov, I., Beres, N.,Moosmuller, H., Sorenson, C.M., and Christopher Stipe,C. Gravity-induced Trapping and Aerogelation of Nanoparticles in Hydrocarbon Flames”, Appl. Phys. Lett. 104, 243103 (2014)