High-power air plasma diagnostics by optical emission spectroscopy for subsecond continuous carbon fibre surface modification

This paper aims to elucidate the interplay between surface heating, plasma emission, and treatment time in a subsecond continuous unsized CF plasma surface oxidation process. Optical emission spectroscopy, highspeed imaging, infrared thermography, scanning electron microscopy, and X-ray photoelectron spectroscopy were utilised to characterise the plasma species, discharge mode, CF surface temperature, morphology, and functionality, respectively. It was shown that plasma transition from diffuse to filamentary discharge for a nozzle in close proximity to CF changes the surface modification trend. Specifically, increased treatment time was needed for O/C composition enhancement in the filamentary discharge, potentially resulting from elevated treatment heterogeneity. The diffuse discharge achieved a maximum O/C for the minimum residence time of 0.41 s. The transitional discharge mode with the maximum O emission intensity produced the overall maximum O/C enhancement of 57% in 0.61 s with a non-monotonic temporal trend. It was revealed that for diffuse and transitional plasma, C - OH, C - O - C surface content is explained well by the maximum center dot NO2 luminescence emission intensity divided by treatment time, enabling the use of center dot NO2 emission as an indicator for atmospheric pressure air plasma reactivity. While high temperatures, exceeding 650 degrees C for filamentary plasma, can influence the oxidation rate, only energetic and highly concentrated oxidative plasma species such as O, O3, center dot NO2 can realise the subsecond functionalisation. Overall, this work shows that characterising the discharge mode and measuring the plasma species using readily available tools provides valuable information to commission plasma sources for fibre functionalisation.