Liquid–feed flame spray pyrolysis (LF-FSP) is one of the most recent iterations in flame spray pyrolysis (FSP) powder production technology. FSP produces metal oxide powders from highly volatile gaseous metal chlorides that are decomposed/oxidized in hydrogen-oxygen flames to form nano-oxide powders. However, products made from FSP's vapor-phase process are limited to Al-, Ti-, Zr-, and Si-based oxides from their metal chlorides. Thus, interest in producing more complex materials required a new methodology, LF-FSP. LF-FSP, as invented at the University of Michigan, uses metalloorganic precursors such as metal carboxylates or alkoxides, not metal chlorides. Briefly, alcohol (typically ethanol) solutions containing 1–10 wt % loading of the target ceramic components as precursors are aerosolized with O2 into a quartz chamber and ignited with methane pilot torches. Initial combustion temperatures run 1500–2000 °C, depending on the processing conditions, generating nanopowder "soot". Temperatures drop to 300–500 °C over 1.5 m, equivalent to a 1000 °C quench in 100 ms leading to kinetic products and nanopowders that are unaggregated. Production rates can be 200 g/h when using wire-in-tube electrostatic precipitators operating at 10 kV. Typical powders have 15–100 nm average particle sizes (APS) with specific surface areas of 30–100 m2/g. LF-FSP technology can be used to produce mixed and single metal oxides easily from low-cost starting materials in a single step without forming harmful byproducts like HCl, which forms when metal chlorides are used as precursors.
Process Initially, metalloorganic precursors are dissolved in alcohol, typically ethanol, to a desired ceramic loading. For further explanation on precursors, refer to precursors section below. The mass of final ceramic oxide can be calculated with the ceramic yield and the amount of precursors used. The production process, called as "shooting", refers broadly to aerosolizing the dissolved liquid precursor solution and combusting it in the flame. Metal oxides are produced, having final stoichiometries determined by the precursor solution compositions. Production rates depend on the precursor solution's ceramic yield; this can be understood practically as the number of metal atoms injected into the flame per volume of liquid. Additionally, particle collection efficiency is important to minimize waste and loss. The collection efficiency is defined as mass of powder collected over theoretically expected mass. While "shooting", a portion of powder flows into exhaust without being deposited onto the electrostatic precipitators (ESP), and during collection of powder which is done by brushing it off, powder loss occurs which causes deviation of mass of collected powder from theoretically expected value. In laboratory settings, production rates can range from 10 to 300 g/hour, producing uniform, unaggregated nanoparticles with APS between 15 and 100 nm. Commercially, Nanocerox holds an exclusive license for LF-FSP and can produce 4 kg/hour quantities via the continuous process. Typically, the solvent serves as the fuel; thus cost and solubility issues leads to use of ethanol or other "low cost" alcohols to dissolve the precursors. The oxygen/alcohol aerosol undergoes rapid combustion within milliseconds, oxidizing all the organic components at temperatures up to 2000 °C leaving only metal-oxyanions e.g., (M-O)x in the gas phase. These oxyanions thereafter nucleate to form clusters and finally sub-100 nm particles, as seen in Figure 1.
Combustion of the precursor results in oxidation of ligands/adducts generating vapors that likely consist of gaseous metal ions and oxyanion species, which co-react to nucleate and grow to form clusters of metal oxide bonds. These clusters condense to form nuclei, which subsequently grow by consuming the vapor phase species and bonding with oxygen available in the atmosphere. In this context, the term cluster refers to the initially generated species that form as a vapor. These clusters coalesce to form nuclei, which later form stable particles. Once formed, nuclei collide to coalesce or agglomerate where temperature and species dictate the mechanism. Cooling changes the effect of collision from coalescence to agglomeration. LF-FSP's rapid drop in temperature as the particles exit the flame prevents the formation of aggregate. Definition of aggregate and its detrimental effect is discussed in advantages section. Collisions that take place after the temperature drop result in agglomerates, in which particles bond weakly by Van der Waals forces, and they can be separated easily with ultrasonication or ball-milling. While exceptions exist, most flame-made particles are nano-sized (< 100 nm) and highly crystalline. Also, neither phase separation within each particle nor composition variance among particles is observed, as the entire process is so rapid that atomically mixed particles are formed. Their properties stem from the flame temperature (up to 2000 °C) and high cooling rates (>500 °C/s). Low residence times in the flame (the amount of time metal ions spend in the flame zone) and rapid cooling lead to metastable phase formation and more importantly unaggregated particles, as they do not have the energy to coalesce and neck. The purity of the initial reactants largely drives the final powder's purity. Some carbonate species may be present on as-produced powders; however, processing techniques can minimize these impurities in final products. First, the powder is dispersed in a solvent via ultrasonication and left to sit for 8 to 12 hours, which leads to some small fraction of larger particles, mostly carbonates, settling at the bottom. The suspension is separated from the sediment and is dried in an oven before being ground into a powder. Thus, LF-FSP provides a robust, versatile route to single and mixed-metal oxide powders in the 15–100 nm size range with varying phase and morphology from relatively low-cost organic precursors.
Equipment A LF-FSP apparatus has five components: aerosol generator with fluid feed and reservoir, cylindrical quartz combustion chamber, Y-shaped quartz tube, four wire-in-cylinder electrostatic precipitators (ESPs) connected in parallel-series, and exhaust piping.
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