Development of a new aerosol reactor for the synthesis of metal nanoparticles
Scharmach, William J.
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The dissertation focuses on the development of a new flame-based aerosol reactor configuration that combines thermal decomposition and hydrogen reduction to produce metal nanoparticles. This approach uses a fuel-rich hydrogen flame as a source of low-cost energy to initiate particle synthesis, but separates the flame chemistry from the particle formation chemistry. The goals of the research described here are to develop a metal nanoparticle process that is a: (1) Scalable "bottom-up" continuous process of producing non-oxide nanoparticles (under 50 nm). (2) More economical process. Flame based energy is almost always more economical than plasma, lasers, or furnace heating methods. This is normally difficult to do as and the particle formation chemistry is intermixed with and coupled to the flame chemistry. (3) Flexible system capable of mass producing a variety of non-oxide nanoparticles using a flame technique, including monocrystalline alloys and polycrystalline materials. The system is designed so that hot combustion products pass through a nozzle to produce a high-temperature reducing jet. A liquid precursor solution is rapidly atomized, evaporated, and decomposed by the expanding jet, initiating particle formation. Particles are then quenched and collected downstream via filter. We have produced several different metal nanoparticles including pure metals such as copper, silver, and gadolinium; alloys and multi-crystal particles such as copper/silver hybrids; and semiconductors such as zinc oxide. By modifying the liquid precursor injected into the reactor we can also produce controlled coatings on the particles. Coatings include copper hydroxide on copper and carbon on silver. The reactor can also be tuned to encapsulate nanoparticles in an amorphous carbon material that serves as a protective coating to prevent oxidation and facilitates collection of the particles. Nanoparticles are characterized by aerosol mobility distribution measurements, electron microscopy, and x-ray diffraction. Copper and silver serve here as prototypes for non-oxide materials that are generally difficult to produce in flame-based reactors. This work demonstrates that such materials can be produced in substantial quantities with particle diameters below 20 nm using this new reactor technology. We also have produced silver sulfide and silver orthophosphate nanoparticles by the reaction of silver nanoparticles produced via the HTRJ process with hydrogen peroxide and a sulfur or phosphorus source. The silver sulfide and silver orthophosphate nanoparticles were characterized by electron microscopy, UV-Vis absorbance spectrometry, photoluminescence spectroscopy, and x-ray diffraction.