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Overview - Nanoparticulate Dry (Flame) Synthesis & Applications

S.E. Pratsinis
ETH Zurich, CH

flame synthesis, nanoparticles, catalysis, sensors, biomaterials

Flame synthesis is a scalable technology that was developed largely by valiant evolutionary research for manufacture of commodities (e.g. carbon blacks, fumed silica and pigments). In the last 5-10 years, however, major advances in the scientific understanding of combustion and aerosol formation and growth allow now optimal reactor design and flame production of sophisticated inorganic nanoparticles with controlled composition, size and morphology. This leads to a series of new products such as highly selective (TiO2-spot-coated SiO2) epoxidation catalysts or V2O5-coated TiO2 for selective catalytic reduction of NOx with NH3 at lower temperatures than conventional catalysts that can lead to better fuel utilization and/or effective pollutant (e.g. Hg) removal during incineration.
Furthermore, this understanding allows now the creation of new processes like liquid-fed flame reactors making possible the manufacture of a much broader spectrum of products such as efficient Pt/Al2O3 catalysts for synthesis of chiral molecules for pharmaceuticals. Specialty oxides like stable CeO2 for catalytic or planarization applications can be readily made as well as stable ZnO quantum dots (1-8 nm in diameter) that exhibit the blue shift of UV light. Carbon-coated silica or titania nanoparticles have been made that could either better blend in a polymer without requiring functionalization with surfactants or facilitate the formation of electrodes for a Li-battery. Very recently, biomaterials have been made by these processes creating some unprecedented opportunities for orthopedics. Non-agglomerated fumed silica (50-90 nm in diameter) that are blended with dimethylacrylate contributes to the development of nanocomposites for novel dental fillings that could replace current ones based on polymeric resins. A new flame-nozzle process is developed that freezes particle growth and allows formation of non-agglomerated and even blue titania (5-10 nm in diameter). Scale-up relationships for flame aerosol reactors are developed and validated with a large body of data allowing a systematic design and operation of industrial units. With this technology inorganic nanoparticles with closely controlled morphology and composition are made exhibiting unique performance in a field that has been dominated by wet chemistry for years.
Summarizing, dry (flame) aerosol technology offers a proven and rather inexpensive route for large-scale production of nanoparticles. The field is expanding from synthesis of simple oxides to more complex, functional nanoparticles. The extension of classical flame aerosol synthesis to new processes allows the manufacture of new, high value products. Flame-made materials emerge into other engineering areas such as heterogeneous catalysis, biomaterials, dental materials, fuel cell membrane production and electroceramics fabrication. Early results in heterogeneous catalysis, in particular, indicate the potential of flame-made catalysts. Microelectronics and even medical applications will profit from these developments. New questions arise and underline the need for basic research in synthesis of mixed oxides with precisely controlled characteristics. Some of them include: scale-up for synthesis of particles with controlled functionalities; mesoscopic chemistry relationships that can be verified & used and nanothin coated nanoparticles made in mass. Whatever the direction, however, abundant new discoveries are awaiting on this rather unexplored interface of material and engineering science.
Aside from this potential of nanoparticles, there is significant concern for their health effects. Do the advanced material properties come with adverse health effects? Scattered data at academic and industrial laboratories imply a rather vague answer. Some public opinion groups seem eager to treat this technology as another “GMO” and even imply a moratorium on nanoparticle research. There is, however, plenty of information on the effects of nanoparticles on human health that has been completely overlooked. Humans have been in contact with nanoparticles for centuries and even with their manufacturing for over a hundred years. Clearly there is a great need to place the health effects of nanoparticles on a firm scientific basis to better protect the national investment in this field and, most importantly, guide researchers and even investors. Given the current advances of aerosol science in characterizing nanoparticles and the large body of anecdotal data regarding exposure to nanoparticle commodities (carbon black, fumed silica, pigmentary titania), there is also a great opportunity for systematic research on the health effects of nanoparticles at an international level.

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