Measurements of reactive species in an atmospheric pressure non-thermal plasma operating in siloxane: Mechanistic understanding

In this study molecular beam mass spectrometry measurements are conducted to measure the ionic species during plasma reformation of siloxane. A tubular configuration of an atmospheric pressure dielectric barrier discharge (DBD) is used in the experiments. Helium-siloxane gas mixture is used where the siloxane is maintained at ~ 100 ppm. We have studied two different linear siloxanes – hexamethyldisiloxane (HMDSO) and octametyltrisiloxane (OMTSO). Experiments are conducted over a range of plasma power (~0.5 – 15 W) and residence time to determine their role on the overall chemical kinetic process. For HMDSO, measurements show that at higher power higher mass species are formed at larger amounts; a repetitive sequence 73/74 mass units apart (most likely C₂H₅SiO and/or C₂H₆SiO) which are building blocks to form polydimetylsiloxane (PDMS). Experiments also show significant deposition of PDMS on the reactor walls. At lower power the mass growth is less significant. Unique small fragments – SiH₃O and CSiH₅ are observed over the entire power range. Smaller hydrocarbon fragments – CH, CH₂, CH₃, C₂H₅ are in abundance. However, formation of CH₄, C₂H₂, C₂H₄ and C₂H₆ are only observed at higher powers. A comparison between HMDSO and OMTSO measurements shows resemblance between the species suggesting the presence of common building block chemical kinetics for linear siloxane compounds. A chemical kinetic model is constructed to provide insight on the key reaction steps that drive the plasma reformation of siloxane. The model contains a simple four-step polymerization of dimethyloxosilane (C₂H₆SiO) extending to C₈H₂₄Si₄O₄ which is representative of PDMS.