Graduate Thesis Or Dissertation
 

Flow characterization of multiple-tube reactors for synthesis of nano-sized silicon nitride powder via silicon monoxide ammonolysis

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  • Concentrically arranged multiple-tube reactors with different dimensions were built for synthesizing nano-sized silicon nitride powder via the ammonolysis of SiO vapor. The reaction was operated at temperatures ranging from 1350 to 1400°C and pressure slightly above atmospheric pressure. Silicon monoxide (SiO) vapor was generated by charging argon gas through a bed of SiO particles packed in the innermost feeding tube and reacted with ammonia gas introduced through a separate feeding tube into the concentrical arrangement. Additional argon was fed into the annular space between SiO feeder and ammonia feeder to prevent the two reactant gases from reacting instantly at the feeder outlets and forming whiskers. None of the reactors with micro-scale dimensions (less than 1 mm) could be operated stably due to whisker formation at the SiO feeders. On the other hand, a meso-scale reactor with a 3.25 mm ID innermost tube for feeding SiO vapor was successfully operated for synthesizing nano-sized powder at measurable quantities. The velocity feed ratio of argon through a bed of SiO particles/NH₃ was maintained at 8.5 for effective production, maximizing nano-sized powder production and minimizing whisker formation. The arrangement of the feeding tubes of reactor was one of the most important parameters that have a direct influence on nano-sized powder formation. When a SiO generating tube was extended out of the annular Ar feeder, the highest efficiency of nano-sized powder production was obtained. The mean residence time of the reactants in the reacting zone was adjusted by the length of product collection tube placed in the uniform temperature zone and set to be about 0.04 seconds. High reaction temperature promoted the SiO utilization, leading to more nano- sized Si₃N₄ powder as well as Si₃N₄ whiskers at the outlet of the SiO generator. The apparent activation energy for the formation of nano-sized powder was 211 kJ/mol, based on an assumption of first-order with respect to the SiO concentration. The average particle size of nano-sized powder decreased with an increase in the reaction temperature, lying in the same range as obtained in the preceding work. The flow of a gaseous mixture in the reactor was simulated with a math software as an effective aid for identifying a reactor structure and operating variables suitable for synthesizing nano-sized silicon nitride powders. The simulated results suggested the optimal configuration of reactor and operating conditions to be employed in the experimental runs. It also revealed that locations where whiskers form corresponded to those appeared in the experimental runs.
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