Olivier Levasseur

Determination of the electron temperature in plane-to-plane He dielectric barrier discharges at atmospheric pressure

Optical emission spectroscopy (OES) measurements coupled with a collisional-radiative model were used to characterize a plane-to-plane dielectric barrier discharge at atmospheric pressure operated in nominally pure helium. The model predicts the population densities for the n = 3 levels of He excited by electron impact processes from either ground or metastable states and takes into account excitation transfer processes between He n = 3 levels as well as all relevant radiative decays and quenching reactions. Time-resolved OES measurements indicate that line ratios from He n = 3 triplet states (for example, 587.5 nm-to-706.5 nm) and singlet states (for example, 667.8 nm-to-728.1 nm) first sharply rise as the discharge ignites and then slowly decrease as it extinguishes. Assuming that n = 3 levels are first populated only by electron impact on ground state He atoms and then only by electron impact on metastable He atoms as the discharge current and thus the metastable number density rise, triplet and singlet line ratios predicted by the model become in each opposite case solely dependent on the electron temperature T e (assuming Maxwellian electron energy distribution function). The values of T e deduced from the analysis of both ratios were relatively high early in the discharge cycle (around 1.0–1.4 eV) and then much lower near discharge extinction (around 0.15 eV). For analysis of time-integrated (or cycle-averaged) OES measurements, the electron temperatures were closer to the 0.15 eV values near the end of the discharge cycle, in good agreement with the values expected from theoretical predictions in the positive columns of He glow discharges at atmospheric pressure.

Influence of the voltage waveform during nanocomposite layer deposition by aerosol-assisted atmospheric pressure Townsend discharge

This work examines the growth dynamics of TiO2-SiO2 nanocomposite coatings in plane-to-plane Dielectric Barrier Discharges (DBDs) at atmospheric pressure operated in a Townsend regime using nebulized TiO2 colloidal suspension in hexamethyldisiloxane as the growth precursors. For low-frequency (LF) sinusoidal voltages applied to the DBD cell, with voltage amplitudes lower than the one required for discharge breakdown, Scanning Electron Microscopy of silicon substrates placed on the bottom DBD electrode reveals significant deposition of TiO2 nanoparticles (NPs) close to the discharge entrance. On the other hand, at higher frequencies (HF), the number of TiO2 NPs deposited strongly decreases due to their “trapping” in the oscillating voltage and their transport along the gas flow lines. Based on these findings, a combined LF-HF voltage waveform is proposed and used to achieve significant and spatially uniform deposition of TiO2 NPs across the whole substrate surface. For higher voltage amplitudes, in the pre...

Experimental and modelling study of organization phenomena in dielectric barrier discharges with structurally inhomogeneous wood substrates

The spatial organization of dielectric barrier discharges operating at atmospheric pressure in the presence of complex wood substrates was analysed using optical imaging, current?voltage (I?V) characteristics, and optical emission spectroscopy combined with a collisional?radiative model to extract the average electron energy. The structural inhomogeneities of selected wood species produced non-uniform light emission patterns while maintaining homogeneous-like I?V characteristics and spatially uniform average electron energy. Based on a simple electrical model of the discharge, this localization was ascribed, at least partially, to a spatial modulation of the relative dielectric permittivity on ?early? versus ?late? wood affecting the local voltage applied to the gas, and thus the local discharge current.

Organization of Dielectric Barrier Discharges in the Presence of Structurally Inhomogeneous Wood Substrates

The spatial organization of atmospheric-pressure plasmas controlled by dielectric barriers in the presence of complex wood substrates was analyzed using optical imaging and current-voltage (I-V) characteristics. While all wood species investigated yielded homogeneous-like I-V characteristics, the structural inhomogeneities of selected wood species produced nonuniform light emission patterns with less intense emission on early versus late wood sections.