Manufacturing of nanostructured materials and functional devices offers many exciting opportunities for scientists and engineers to contribute substantially in renewable energy utilization, environmental compliance, and product development. In the past two decades, gas-phase flame synthesis has not only proved to be one of the most scalable and economical technologies for producing well-controlled nanostructured materials, including single metal-oxide, mixed-oxide nanocomposite, and carbon nanostructures, but also has been recognized as a new low-cost fabrication method of nano-devices. In this paper, we focus our review mainly on the recent trends in specific applications of flame aerosol synthesis in the last decade, e.g., usage of a substrate in stagnation geometry with controlled particle temperature-time history, application of external fields to control particle characteristics, development of advanced spray technique for doping synthesis of nanocomposites of multicomponent metal oxides or carbon-metal oxides, and fabrication of nanomaterial-based functional devices. For the possibility to improve the design and operation of flame aerosol reactors, in situ optical diagnostics for either gas phase or particle phase in flame field, along with multi-scale modeling and simulation employing gas-phase chemistry, population balance method, molecular dynamics, and nanoscale particle dynamics are summarized.

Recognizing that previous experimental studies on constant-pressure, outwardly propagating, spherical flames with imaging capability were limited to pressures less than about 5 atm, and that pressures within internal combustion engines are substantially higher, a novel experimental apparatus was designed to extend the environmental pressure to 60 atm. Results substantiate previous observations of the propensity of cell formation over the flame surface due to hydrodynamic and diffusive-thermal instabilities and provide convincing evidence that wrinkled flame is the preferred mode of propagation in hydrogen/air mixtures in environments with pressures above only a few atmospheres. It is further shown that, by using helium as the diluent, and by reducing the oxygen concentration of the combustible, diffusional-thermal instability can be mostly suppressed and the hydrodynamic instability delayed. Stretch-free laminar flame speeds were subsequently determined for such smooth flames up to 20 atm and were compared with the calculated values, allowing for detailed chemistry and transport.

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Flame spray pyrolysis is an established technique for synthesizing nanoparticles in the gas phase through aerosol combustion of precursor/solvent droplets. The combustion characteristics of isolated micron-sized precursor/solvent droplets are investigated experimentally. Pure solvent droplets burn uniformly and classically quasisteady, whereas precursor/ solvent droplets manifest disruptive combustion behavior. The fast onset of droplet disruption, which occurs only for solutions with dissolved metal precursors, is not due to solid-particle precipitation within the droplet. Instead, the mechanism of disruptive droplet burning is similar to that of slurry droplets, consisting of three main steps: (1) diffusioncontrolled burning of the high-volatile solvent, (2) viscous-shell formation due to decomposition of the low-volatile metal precursor, and (3) subsequent disruption due to heterogeneous nucleation. The time sequence of the three steps depends on the concentration and decomposition characteristics of the metal precursor, shortening with increased concentration and higher incremental decomposition temperature.

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Aligned single-crystal WO2.9 nanowires are grown directly from tungsten substrates at high rates using a flame synthesis method. The nanowires have diameters of 20–50nm, lengths >10μm, coverage density of 109–1010cm−2, and growth rates >1μm∕min. Growth occurs by the vapor-solid mechanism, with local gas-phase temperature (∼1720K) and chemical species (O2, H2O, and H2) strategically specified at the substrate for self-synthesis. Advantages of this synthesis method are reduced processing times, absence of necessity for substrate pretreatment or catalysts, scalability for large-area surface coverage, high purity and yield of oriented nanowires, and continuous processing conditions.