Acoustic Measurements in a Hexamethyldisiloxane-Loaded Low-Temperature Direct Barrier Discharge (DBD) Plasma Effluent: Nozzle Cleaning
in: Journal of Thermal Analysis and Calorimetry (2015)
Acoustic emission measurements (AE) as well as laser scattering experiments were performed during SiO2 layer deposition. SiO2 was generated in low temperature atmospheric plasma torches 50 or ≤500 W) which were seeded with hexamethyldisiloxane (HMDSO). These AE measurements can be used to detect conveniently the necessity of nozzle cleaning. Although cold plasma torches have been used already to obtain high-quality coatings, we observe under certain conditions, typically for less than 350 W, parasitic SiO2 deposition, which reduces the nozzle aperture with increasing operation time . No parasitic SiO2 deposition inside the burner nozzle was observed when the plasma source was operated with more than 350 W electrical power. The reduced nozzle aperture causes increased plasma flow rates and acoustic noise. Especially burst-like increases of this acoustic noise which could be heard and were visualized by sound tracks, sonograms, and acoustic power spectra were assumed to be correlated to the ejection of particles . This hypothesis could be confirmed by all-optical measurements recording light from a laser light sheet at 248 nm, which was scattered from the ejected particles in the plasma torch. The obtained correlations suggest to use a stand-alone, cheap microphone as a low-cost monitor for the grade of parasitic deposition inside the plasma burner’s nozzle. This online-control will objectively indicate the need for cleaning of the nozzle.