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. / Graduation date: 2004
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/30356 |
| Date | 27 May 2003 |
| Creators | Vongpayabal, Panut |
| Contributors | Kimura, Shoichi |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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