Diagnostics and Modeling of Nanopowder Synthesis in Low Pressure Flames
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Diagnostics and modeling of nanopowder synthesis in low pressure flames N. G. Glumac and Y-J. Chen Mechanical and Aerospace Engineering Department, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08855
G. Skandan Nanopowder Enterprises Inc., an SMT company, Piscataway, New Jersey 08854 (Received 27 May 1997; accepted 15 December 1997)
Laser-induced fluorescence, thermophoretic sampling, laser light scattering, and emission spectroscopy have been used to probe low pressure hydrogen/oxygen flames in which 3–50 nm, loosely agglomerated oxide nanopowders have been synthesized at high production rates by the pyrolysis of precursor vapors, followed by condensation in the gas phase. These measurements have enabled the identification of pyrolysis, condensations, and particle growth regions in the flame. Flame simulations using a one-dimensional stagnation flow model, with complex chemistry, demonstrate that the chemical and thermal flame structure can be accurately predicted for flames without a precursor. Furthermore, some flame structure changes induced by the addition of a precursor can be simulated by addition of analogous species to the chemical mechanism.
I. INTRODUCTION
Development of potential applications for nanopowders with diameters in the 3–50 nm range has been limited by the lack of economic large-scale production methods. While a variety of techniques exist for nanopowder synthesis in this particle size range, few are scalable enough to yield kgyh or greater powder quantities in a single reactor. However, recent work has demonstrated that combustion flame-chemical vapor condensation (CF-CVC) is capable of producing loosely agglomerated oxide nanoparticles at high production rates in a single unit.1,2 The technique has been applied to synthesize a wide variety of oxide powders. Furthermore, process parameters have been varied to control powder characteristics such as particle size, crystal structure, and morphology.3 While CF-CVC offers a promising production route for large scale, nonagglomerated nanopowder production, very little is known about the kinetic processes occurring in these flames. Flame synthesis of oxide particles has been extensively studied in the past, and topics such as cluster formation and growth, the chemical flame structure, and particle formation and dynamics have been investigated (e.g., Refs. 4–9). However, the bulk of these studies were performed at atmospheric pressure (more than 30 times the CF-CVC pressure). At one atmosphere, the chemical and thermal flame structure is markedly different from that at low pressure where equivalent radical concentrations are produced at much lower temperatures. The one study performed at low pressure10 yielded some important information, but 2572
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J. Mater. Res., Vol. 13, No. 9, Sep 1998
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focused on the regime of very low precursor concentrations in which the chemical flame structure is minimally affected by the precursor ad
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