An In Situ FTIR Study of DBD Plasma Parameters for Accelerated Germination of Arabidopsis thaliana Seeds

Current agricultural practices are not sustainable; however, the non-thermal plasma treatment of seeds may be an eco-friendly alternative to alter macroscopic plant growth parameters. Despite numerous successful results of plasma-seed treatments reported in the literature, the plasma-treatment parameters required to improve plant growth remain elusive due to the plethora of physical, chemical, and biological variables. In this study, we investigate the optimal conditions in our surface dielectric barrier discharge (SDBD) setup, using a parametric study, and attempt to understand relevant species in the plasma treatment using in situ Fourier transform infrared (FTIR) absorption spectroscopy. Our results suggest that treatment time and voltage are key parameters for accelerated germination; however, no clear conclusion on causative agents can be drawn.

Etude de l'origine et de la croissance de particules submicrométriques dans les plasmas radiofréquence réactifs

Christian Deschenaux

The formation of sub-micron sized particulates has been studied in a low pressure (0.1 mbar) radio-frequency (13.56 MHz) capacitively coupled plasma discharge. Particles are studied in situ by infrared absorption spectroscopy, laser illumination and Cavity Ring Down Absorption Spectroscopy. Transmission Electron Microscopy was applied ex situ for particulates collected during the plasma. We propose that two principal mechanisms may be involved in the formation of these particulates: formation and trapping of negative ions in the gaseous phase, and the recycling of material deposited on the electrodes during the discharge, by film erosion or delamination. The relative importance of these two mechanisms was studied in silane, methane, ethylene and acetylene plasmas. The growth of the particulates in silane plasmas, monitored with Infrared and Cavity Ring Down Absorption Spectroscopy, allowed us to distinguish an increasing volume growth phase followed by an agglomeration phase at constant volume, in agreement with the literature. A slight powder and film recycling effect is detected as the experiments are repeated, as observed by an increasing particle size. The hydrogen content of powder formed in silane plasmas was estimated in situ by infrared absorption spectroscopy and changes in the infrared absorption spectrum could be observed for silane diluted with hydrogen and argon. In particular, the porous structure of the powder was observed from the high content of surface related vibration bonds relative to bulk related bonds. Photoluminescence was detected for the particles at 2.15eV that could possibly be attributed to the crystalline structure of the particles. Infrared Absorption Spectroscopy and mass spectrometry applied in situ showed that acetylenic compounds are preferentially produced in methane, ethylene and acetylene plasmas and that the hydrogen content of the other species produced in these plasmas depends on the initial gas hydrogen content. Negative ions were evidenced in these plasmas and a polymerization reaction is proposed between acetylene and C2nHx- negative ions to explain the formation of particle's precursors. The formation of aromatic structures is suggested for n>3. The different relative abundance of C2Hx- ions is proposed to explain the different powder production rates with different initial gases or different fluxes. Hydrogen may inhibit the formation of high-mass negative ions and particles. Fast growth of spherical amorphous carbon particles (∼100s) was observed for a low pressure (0.1 mbar) acetylene plasma. The sp3 structure was confirmed in situ by infrared absorption spectroscopy. Slow growth (∼1000s) of irregularly shaped particles is observed for methane plasmas at the same pressure and power. Particle heterogeneity was studied with Transmission Electron Microscopy and showed graphitic onions, multiwalled nanotubes, flakes and agglomerates attributed to the erosion or the delamination of the deposited carbon layer. The influence of the recycling of the deposited layer on the powder formation was observed in situ from enhanced particle formation as the experiments were repeated. A similar behavior was observed for discharges containing mixtures of hexamethyldisiloxane, oxygen and helium. Keeping in mind that the control of powder homogeneity implies a control over the reactor contamination and its history, we conclude that an important part of the particulates formed in the discharges studied depends on the gas phase reactions, determined by the plasma chemistry.

EPFL2002

Investigations of CH4, C2H2 and C2H4 dusty RF plasmas by means of FTIR absorption spectroscopy and mass spectrometry

Christian Deschenaux, Christoph Hollenstein

Infrared (IR) absorption spectroscopy and mass spectrometry have been simultaneously applied to dusty radiofrequency (RF) plasmas in methane, acetylene and ethylene. The combination of IR absorption spectroscopy and mass spectrometry allows the chemical composition and structure of the most relevant plasma-produced neutral species, the ionic plasma composition and the chemical composition of the nanometer-sized particles to be precisely identified. The production of acetylenic compounds (C2Hx) seems to be a key mechanism for the powder formation in all the investigated hydrocarbon plasmas. Electron attachment to acetylenic compounds and the following ion-neutral reactions might lead to the high-mass carbon anions, which are trapped in the plasma and finally end in powder formation. The hydrogenation of the monomer strongly influences the composition of the ions. Finally the composition of the plasma-produced particles is mainly sp(3) bonded carbon and the infrared spectra show similarities to that of polyethylene.

1999

Aero Engine Flow Control by Surface Discharge Plasma

Sami Goekce, Christoph Hollenstein, Pénélope Leyland, Peter Ott, Philip Peschke

Experimental investigations and numerical modeling with the objective to study the effects that surface discharge plasmas can provoke on aerodynamic flow situations, including high speed flows present in aero-engines, are undertaken at EPFL. The actuator’s potential to influence the flow under realistic flight and engine operating conditions, largely depends on its geometric configuration and the power supply. Initially, a long-life (more than 12 hours) surface discharge actuator was designed at EPFL and proven to survive transonic flow speeds on typical compressor blades. This was the first time that such a long-term, continuously operating actuator was applied successfully. However, with AC voltage applied to the electrodes, the effects on the shock waves did not appear to be significant [1, 2]. Recent experiments conducted at EPFL focused on fast rise voltage pulse driven dielectric barrier discharge (DBD) actuators and the development of a power supply that could provide the required voltage pulses. A generic DBD actuator with two electrodes in asymmetric configuration, separated by a dielectric was chosen. High speed imaging was applied to investigate the spatial and temporal discharge development. In order to study the influence of voltage and current on the plasma, several power supply configurations were examined by varying voltage rise and fall time, maximum voltage, peak current and pulse width. Two discharge periods were observed on the positive and the negative edge of the voltage pulse. It was observed that the characteristics of the plasma differ considerably between these two periods. Furthermore, the light emission intensity and the propagation speed of the plasma along the surface of the dielectric increased with the maximum voltage and peak current. A similar effect was observed when the voltage rise time was decreased. In a next step, a technique similar to synthetic schlieren was applied to examine the interaction of the DBD actuator with air under atmospheric pressure. A micro shock wave that propagates from the electrode edge normal to the surface was observed, which agrees well with numerical results gained during a parallel investigation. In addition, the impact on the flow structure of maximum voltage and frequency is currently examined. First results suggest that the intensity of the micro shock wave increases with the voltage whereas the frequency primarily provokes electrode heating. With the objective to investigate the modelling of the fast rise pulse driven DBD discharge, a numerical tool was developed that applies emission spectroscopy to determine the electron energy distribution function (EEDF) by comparing the relative intensity of bands, as described by several authors [3-5]. The EEDF is then used to infer populations present in the gas through electron impact ionization, excitation or dissociation cross sections data [6]. Contrary to previous studies, this approach employs two naturally occurring transitions in atmospheric plasmas in conjunction with recent data on collisional quenching [7], which makes the method more reliable and does not require the adjunction of other species such as Helium. References: 1. Pavon S., Dorier J.L., Hollenstein C., Ott P., Leyland P., “Effects of high-speed airflows on a surface dielectric barrier discharge”, J. Phys. D: Appl. Phys. 40. No 6.2 1733-1741, 2007. 2. Pavon S., Sublet A., Dorier J.L., Hollenstein C., Ott P., Leyland P., “Long Lifetime system for the generation of Surface Plasmas”, International Patent no: P1883PC00/13-71, PCT/IB2009/050489, 2008. 3. Bibinov N. K., Kokh D.B., Kolokolov N.B., Kostenko V.A., Meyer D., Vinogradov I.P., Wiesemann K., “A comparative study of the electron dis-tribution function in the positive columns in N2 and N2/ He dc glow dis-charges by optical spectroscopy and probes”, Plasma Sources Science and Technology, 298-309,1998. 4. Behringer K., Fantz U., “Spectroscopic diagnostics of glow discharge plasmas with non-maxwellian electron energy distributions”, Journal of Applied Physics D, 2128-2135, 1994. 5. Isola L.M., Gómez B.J., Guerra V., “Determination of the electron tem-perature and density in the negative glow of a nitrogen pulsed discharge using optical emission spectroscopy”, Journal of Applied Physics D, 015202, 2010. 6. Itikawa Y., “Cross sections for electron collisions with nitrogen mole-cules”, Journal of Physical and Chemical Reference Data, 31-53, 2006. 7. Dilecce G., Ambrico P.F., Benedictis S., “On the collision quenching of N2plus by N2 and O2 and its influence on the measurement of E over N by intensity ratio of nitrogen spectral bands”, Journal of Applied Physics D, 195201, 2010.