Aerosol nanotechnology has rapidly evolved in the past years. This fascinating technology has resulted in the development of functional nanomaterials providing novel solutions in industrial applications. The extensive research on the physical understanding of gas phase processes has strongly contributed to the present industrial use of single and mixed oxides and the design of industrial aerosol reactors.
The Functional Materials Laboratory of the ETH Zurich deals among other things with gas phase processes, namely with flame spray synthesis.
Chemical aerosol engineering allowed considering flame spray synthesis as chemical reactors
For nearly a century this method has been used for the large-scale preparation of carbon black of over 8 million metric tons per year, mostly covering application in the production of tires and rubber. The flame technology was later adapted for the production of silica and titania by the ocidation of chloriedes in high temperature flames. Today this is the majaor sytnesis route for the manufacturing aof pigmentary titania (~ 5 Mt/year) and fumed silica nanoparticles (~ 4 Mt/year), which are used as powder flowing aids and in cosmetics and in the fabrication of optical fibres for telecommunication.
A typical sequence of the basic steps illustrating particle formation in a gas stream.
The product composition can be controlled by the combustion and reaction conditions, resulting in a wide variety of nanoparticles (present example: iron).
A metal loaded precursor is injected as a gas or liquid spray into the flow, the sprayed precursor is evaporated due to rapid heating through surrounding gases depending on the process (source of the thermal energy: laser, plasma or flame). Nanoparticles grow from monomers or nuclei by fast agglomeration forming fractal aggregates. Sintering and coalescence then leads to the formation of spherical particles that continue to collide by Brownian motion. If the temperature is insufficient for full coalescence to speheres, hard agglomerates are formed. In the cold zone of the process and during filtraion soft agglomerates are formed, which are only interconnected by Van-der Waals forces.
Reducing flames: From oxides to metal nanoparticles
Depending on the reaction and combustion conditions in the flame a complex gas mixture can be altered fron CO2/H2O (oxidizing, traditional flame) to CO/H2/H2O (reducing conditions). Noble metal nanoparticles such as Pt, Au, Ag or their alloys can be obtained readily in oxygen-rich flames but the production of non-noble metals requires reducing conditions.
The flame spray pyrolysis is surronded by a porous tube and placed into a glove box with an inert atmosphere. controlling the gas flow rates allows highly reducing conditions (O2 smaller then 100 ppm). While traditional reactors mostly yield oxides, this modified process allows the one-step synthesis of metal or alloy or carbon coated metal nanoparticles, or metal/ceramic composites.
E. K. Athanassiou, R. N. Grass, W. J. Stark, Chemical Aerosol Engineering as a Novel Tool for Material Science: From oxides to Salt and Metal Nanoparticles, Aerosol. Sci. Tech., 44(2), 161-72, (2010).
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