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Accueil > Séminaires et conférences > Séminaires, soutenances de thèses et HDR précédents > 2014 > Séminaires 2014 > Le mardi 1er juillet à 10h30

Le mardi 1er juillet à 10h30

Igor Adamovich (Ohio State University) - Nanosecond Pulse Ionization Wave Discharges on Liquid Surfaces : Discharge Development and Plasma Chemistry

Lieu  : Palaiseau, salle de l’aile 0, exLIX

Résumé  :
Kinetics of nonequilibrium electric discharges in liquids and at liquid-vapor interfaces is of great interest from the fundamental viewpoint, and is of critical importance for development of applications such as reactive nitrogen / oxygen species generation, plasma activation of water for sterilization and wound treatment, removal of volatile organic compounds from water and aqueous solutions, and plasma chemical reforming of liquid hydrocarbons and oxygenates for portable fuel cells, proposed as an alternative to thermo-catalytic fuel reforming. In spite of extensive experimental and modeling studies of nonequilibrium plasmas generated in the presence of liquids, kinetics of plasma chemical reactions at these conditions remains poorly understood. One of the main difficulties encountered in quantitative studies of liquid / vapor phase plasma chemistry is sustaining the plasma at controlled, well-reproducible, ample optical access conditions, which would lend themselves to in situ, time- and spatially resolved optical diagnostics. Lack of plasma parameter measurements at well-characterized conditions makes difficult development and validation of kinetic models, and assessing their predictive capability.
Kinetics of energy coupling in surface ionization wave discharge plasmas remains poorly understood. In particular, experimental measurements of temperature, electric field, electron density, and coupled energy distributions in surface discharge plasmas are difficult due to close proximity to surface. Kinetic modeling calculations suggest that electron density in the near-surface plasma layer may be up to ne 1014-1015 cm-3, at the reduced electric field of E/N 102-103 Td. This would result in high discharge energy loading per molecule near the surface of an evaporating liquid, with a significant fraction of input energy going to molecular dissociation. On the other hand, plasmachemical reactions near the vapor-surface interface may occur at low temperature even at relatively high energy loadings, due to rapid evaporative cooling and large heat capacity of the liquid.
In our recent work, surface ionization wave discharges generated by high-voltage nanosecond pulses, propagating over a plane quartz surface and over liquid-vapor surfaces (distilled water and 1-butanol) have been studied in a rectangular cross section test cell. The main motivation for the use of this discharge configuration is to enhance surface plasma chemistry, due to high peak electric field expected in the surface ionization wave front, enhanced by a high dielectric constant of the liquid. The discharge was initiated using a custom-made, alternating polarity, high-voltage nanosecond pulse plasma generator. Wave speed was measured by a capacitive probe. ICCD camera images demonstrated that the ionization wave propagated predominantly over the quartz wall or over liquid surface adjacent to the grounded waveguide placed along the bottom wall of the test cell. At all experimental conditions tested, the surface plasma “sheet” was diffuse and fairly uniform, both for positive and negative polarities. No perturbation of the liquid surface by the discharge was detected. In most cases, positive polarity surface ionization wave propagated at a higher speed and over a longer distance compared to the negative polarity wave. Stable surface ionization wave discharge was also observed over liquid butanol / saturated butanol vapor interface, as well as over distilled water / saturated water vapor interface, without the buffer gas flow. Plasma emission images yield preliminary evidence of charge removal from the liquid surface between the pulses, on microsecond time scale. Products of plasma chemical reaction accumulated in the ionization wave discharge over liquid butanol / saturated butanol vapor interface were detected ex situ, using FTIR absorption spectroscopy. Reaction products identified include CO, alkanes (CH4, C2H6, C3H8), alkynes (C2H2), aldehydes (CH2O), and lighter alcohols (CH3OH). This suggests significant potential of the use of the present approach for near-surface plasmachemical reforming of evaporating liquid reactants.
For quantitative studies of this potential, we are using in situ laser diagnostics to measure radical species concentrations (calibrated OH LIF and calibrated H TALIF). Absolute, two-dimensional distributions of [OH] and [H] have been measured in a repetitively pulsed nanosecond discharge sustained near liquid water / saturated water vapor interface, operated at the conditions when the plasma is partially lifted from the surface. Figure 1 shows ICCD image of the plasma, as well as raw OH LIF and H atom TALIF line images, taken at different heights from the liquid surface. Peak [OH] and [H] at these conditions are 3∙1014 cm-3 and 2∙1016 cm-3, respectively.


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Tutelles : CNRS Ecole Polytechnique Sorbonne Université Université Paris Sud Observatoire de Paris Convention : CEA
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Exploitant du site : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Hébergeur : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Directeur de la publication : Pascal Chabert (Directeur)