Hypersonic plasma particle deposition of nanostructured silicon and silicon carbide

A new process for the production of nanostructured materials, hypersonic plasma particle deposition (HPPD), is experimentally investigated. In HPPD, vapor phase precursors are injected into a flowing plasma generated by a DC arc. The plasma undergoes a supersonic expansion into a deposition chamber, with the pressure dropping across the nozzle from approximately 500 Torr to approximately 2 Torr. Ultrafine particles nucleate in the nozzle, accelerate in the hypersonic free jet downstream of the nozzle, and deposit by inertial impaction onto a temperature-controlled substrate. The low particle residence time (approximately 50 μs) minimizes particle agglomeration, while the high particle deposition velocity (approximately 1 km s^-1) results in the formation of a partially consolidated coating. We have characterized silicon, carbon and silicon carbide coatings produced by injecting vapor-phase SiCl₄ and hydrocarbon (CH₄ and C₂H₂) precursors into an Ar-H₂ plasma. The silicon coatings are polycrystalline, while the carbon and silicon carbide deposits are amorphous and hydrogenated. Both Si and SiC coatings had nanostructured regions with grain sizes on the order of 20-30 nm, reasonably close to the diameters of impacting particles measured using an extractive aerosol probe coupled to a scanning electrical mobility analyzer.