Flame synthesis of functional nanostructured materials and devices: Surface growth and aggregation

Combustion is essential to the manufacture of carbon black, fumed oxides, optical fibers and, recently, new high-value products like carbon nanotubes, nanosilver and biomagnetic nanofluids that are driven to market predominantly by start-ups. This technology is attractive for material synthesis for its proven scalability as it does not involve the tedious steps of wet chemistry and can readily form stably metastable compositions and high purity products. Recent advances in aerosol and combustion sciences reveal that coagulation and sintering and/or surface growth control product particle size and morphology through the high temperature particle residence time, self-preserving size distribution and power laws for fractal-like particles. This motivates synthesis of an array of unique particle compositions and morphologies primarily by spray combustion leading to new catalysts, gas sensors, bio-materials and, most recently, to hand-held devices such as breath analysis sensors for monitoring chronic illnesses. In particular, multi-scale process design integrating mesoscale and molecular dynamics facilitates understanding of combustion product development. The latter also contributes to understanding of aggregation and surface growth of nascent soot, a bona fide nanostructured material! So here nascent soot dynamics, after nucleation or inception, are investigated through accounting of soot agglomeration and surface growth by acetylene pyrolysis. Neglecting the fractal-like nature of soot underestimates its mobility diameter and polydispersity up to 40%. The evolution of nascent soot structure from spheres to aggregates is quantified by the mass fractal dimension and mass-mobility exponent, in excellent agreement with microscopic and mass-mobility measurements in a standard burner-stabilized stagnation ethylene flame. Surface growth chemically bonds the constituent primary particles of these aggregates, while the effect of soot volume fraction on soot morphology is elucidated. Based on aggregate projected area, a scaling law is derived for determining the primary particle size of nascent soot aggregates from mass-mobility measurements rather than tedious image counting.