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An alumina microreactor was designed and fabricated to study the synthesis of nanosized silicon nitride powder via the ammonolysis of SiO vapor at temperatures ranging from 1300°C to 1400°C. It is the first high temperature ceramic microreactor capable of operating at temperatures upto 1600°C. The microreactor was designed using 3D CAD software SolidWorks and AutoCAD. This entirely new design of the
microreactor simplifies the lamination and bonding techniques by using just 3 plates
instead of stacking a number of plates together. This design simplification was made
feasible by using a combination of advanced processing techniques such as laser
machining, extrusion, and tape casting. A CO₂ laser was used for fabricating the top
and bottom plates, while the extruded body of the microreactor was fabricated using
highly porous alumina (50-60% porosity). These porous microchannel walls simplify
the use of multiple flows in the microreactor, because it facilitates the reactant gas
stream to enter into the microreactor through the pores. The microreactor was finally
tested in a horizontal tube heating furnace to synthesize silicon nitride nanoparticles by
performing gas-phase reactions between SiO and ammonia. The SiO vapor generation
was controlled by the flow of argon gas. The ammonia dissociation at high
temperatures was taken into account by maintaining flow rate of ammonia atleast 5
times higher than the flow rate of SiO. Due to its portability, and hence reduced
reaction volume this microreactor provides a better control over the residence time and
diffusion length of the reactants in the hot reaction zone, resulting in a better control
over the particle morphology and size distribution.
The reaction between SiO and ammonia was carried out by varying the flow rates of
ammonia ranging between 1000-4000 cc/min, while the flow rate of argon was kept
constant at 240 cc/min. The reaction was carried out at a constant temperature of
1350°C for a cycle time of 1hr. The reaction yielded silicon nitride nanoparticles were
then collected on different filter papers at the exit of the microreactor.
Different powder characterization techniques such as transmission electron microscopy
(TEM) and x-ray diffraction (XRD) were used to determine the particle size, particle
size distribution, and chemical composition of the nano-sized particles. XRD analysis
indicated peaks of silicon oxy-nitride (Si₂N₂O) in all the samples except the sample
obtained after the first run. This is because Si₃N₄ nanoparticles on exposure to
atmosphere were oxidized to form Si₂N₂O. Toxicology tests were also conducted in
order to determine the toxicity effects of Si₃N₄ nanoparticles on different body parts of
zebra fish. The data obtained was then further used to discuss the advantages of the
microreactor in synthesis of Si₃N₄ nanoparticles and its integration to a post processing
system such as compaction press, injection molding, and extrusion. |
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