Graduate Thesis Or Dissertation
 

Kinetics of nano-sized Si₃N₄ powder synthesis via ammonolysis of SiO vapor

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  • An 89 mm-diameter vertical tubular-flow reactor was used to study the kinetics of nano-sized silicon nitride powder synthesis via the animonolysis of SiO vapor at temperatures ranging from 1300°C to 1400°C. The SiO generation rate was controlled by adjusting the mass of SiO particles initially charged in the SiO generator, when the flow rate of carrier gas argon was maintained unchanged. The molar feed ratio of NH₃/SiO at the feeder outlets was maintained in large excess of the stoichiometric ratio ranging from about 100 to 1200 mol NH₃/mol SiO. The SiO-NH₃ reaction yielded two different morphologies of silicon nitride products at different locations in the reactor: nano-sized powder with an averaged particle size of about 17 nm and whiskers with a variety of shapes and diameters of a few micrometers. Nano-sized powder was the dominant product in the system and its mass fraction over the total product varied from 83% to 100%, depending on operating conditions. The contact pattern between SiO vapor and NH₃ inside the reacting zone was one of the most important parameters that affected Si₃N₄ formation kinetics. When a small single tube was employed for feeding NH₃ (flow condition J), a highest efficiency of SiO vapor utilization was achieved at a high level of SiO conversion. The SiO conversion increased from 72% to 91% with an increase in the residence time from 0.17 s to 0.69 s, indicating that the SiO-NH₃ reaction was not instantaneous but was relatively fast. When the molar feed rate of NH₃ was 2-3 orders of magnitude greater than that of SiO vapor, the rate of nano-sized powder synthesis was independent of NH₃ concentration and of first order with respect to the SiO concentration. A pseudo-first order rate expression was proposed, and the apparent activation energy was determined to be 180 kJ/mol. The gas flow in the reactor simulated with a computational fluid dynamic program revealed that whisker formed where the stagnation of gas flow formed. A power law rate expression for whisker formation was proposed based on measured rates of whisker formation and simulation-predicted reactant-gas concentrations.
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