Flame synthesis of functional nano materials & devices
Sotiris Pratsinis, ETH Zurich, Zurich, Switzerland
Traditionally, combustion science and engineering is focused on energy generation. So particle or soot formation is viewed as nuisance. Typically, research focuses on chemical mechanisms that lead to soot formation and growth for its reliable monitoring for eventual mitigation, if not, suppression of its emissions. In material synthesis, the goal is the reproducible generation of soot with well-defined characteristics: primary particle size, crystallinity and morphology. In most applications, performance is determined by the primary particle size and, to some extent, the crystallinity. Aside from on-line monitoring, soot morphology is primarily a solids processing or handling issue, especially when it involves sinter-bonded agglomerates, the so-called aggregates.
I will start with a brief historic overview of flame aerosol synthesis of materials from ancient China, to Bible printing of Gutenberg and to current manufacture of carbon black, pigmentary titania, fumed silica and other simple oxides at tons/h. Combustion is attractive for material synthesis as it does not involve the many process steps of wet chemistry, can produce stably metastable phases (ε WO3) and high purity particles (e.g. for optical fibers) that can be readily separated from their carrier gases. Most importantly, combustion is an undisputably scaleable technology.
Recent advances in aerosol and combustion sciences reveal that coagulation and sintering largely control product primary particle size and morphology through the high temperature particle residence time, self-preserving size distribution and power laws for fractal-like particles. This understanding now motivates synthesis of an array of particle compositions and morphologies by liquid spray combustion that contributes to new catalysts, gas sensors, biomaterials and even nutritional supplements and, most recently, to hand-held devices such as breath analysis sensors for monitoring chronic illnesses (e.g. diabetes) and even dietary habits. Multi-scale process design facilitates a better understanding of combustion product development.