BIOPLASMAS & PLASMAS WITH LIQUIDS - Joint Conference of COST ACTIONS TD1208 “Electrical discharges with liquids for future applications” & MP1101 Biomedical Applications of Atmospheric Pressure Plasma Technology, Bertinoro, Italy, 13 th -17 th September 2015
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Non-equilibrium discharges operating in organic liquids and/or vapours of these liquids have become subjects of great interest both from the point of fundamental research and due to the possibility of their various applications. For instance, organic molecules (containing carbon and hydrogen atoms) are important in manufacturing processes of graphene, carbon nanotubes and carbon nanowalls [1, 2]. Moreover, discharges involving organic liquids have also found applications in biomedical field and environmental remediation [3, 4]. Although plasma sources differ in sizes and geometry, as well as pressure and operating environment (some of them produce plasma inside liquid while others operate in vapour above liquid) common issue for all of them is how to resolve complicated phenomena taking place in these discharges.
This research requires extensive and realistic models which need complete and reliable data sets including, among else, transport parameters [5]. Unfortunately, these parameters have seldom been measured systematically in vapours over the past few decades [6].
In this work we show measurements of breakdown properties of several organic vapours i.e. primary and secondary alcohols at low pressure. Alcohol vapours are obtained by evaporation of 99 % or higher purity liquid samples of alcohols using the same procedure as for water vapour [7]. The breakdown is achieved in a simple centimetre-size parallel-plate geometry electrode system, with copper cathode and quartz anode with transparent and conductive platinum film, placed inside a cylindrical quartz tube. The electrical circuit with high resistance provided highly reproducible and reliable determination of breakdown voltages [5]. High reproducibility of measurements was also facilitated by cathode surface conditioning in moderate-current hydrogen discharge prior to the measurements and tested by repeated measurements. Together with measurement of breakdown voltages in a wide range of pd (pressure x electrode gap) parameter, we recorded spatially and spectrally resolved light emission coming from low-current discharge. Time-integrated axial profiles of emission in visible spectra coming from the vapour discharge running in steady state regime reveal information on overall particle kinetics. The profiles show which elementary processes are important at different breakdown conditions i.e. at different reduced electric fields (E/N). Spectrally resolved emission recordings allow to identify dominant excited species produced in the discharge at different E/N conditions.
Additionally, profiles at lower E/N where dominant emission comes from electron-induced excitation are employed to obtain ionization coefficient and consequently the effective secondary electron yield. Even though these coefficients are obtained for narrow range of conditions, for some vapours they are obtained for the first time.
The data obtained for several organic vapours is complete set of reliable breakdown data necessary for modelling. Further studies will include time resolved measurements of breakdown, to follow kinetics of the main elementary processes.
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