Electrical, Thermal and Optical Diagnostics of an Atmospheric Plasma Jet System

Plasma diagnostics of atmospheric plasmas is a key tool in helping to understand processing performance issues. This paper presents an electrical, optical and thermographic imaging study of the PlasmaStream atmospheric plasma jet system. The system was found to exhibit three operating modes; one constricted/localized plasma and two extended volume plasmas. At low power and helium flows the plasma is localized at the electrodes and has the electrical properties of a corona/filamentary discharge with electrical chaotic temporal structure. With increasing discharge power and helium flow the plasma expands into the volume of the tube, becoming regular and homogeneous in appearance. Emission spectra show evidence of atomic oxygen, nitric oxide and the hydroxyl radical production. Plasma activated gas temperature deduced from the rotational temperature of nitrogen molecules was found to be of order of 400 K: whereas thermographic imaging of the quartz tube yielded surface temperatures between 319 and 347 K.<br/

Atmospheric pressure plasmas for aerosols processes in materials and environment

International audienceThe paper highlights applications of some atmospheric pressure plasmas (dc-corona, streamer and spark and ac-Dielectric Barrier Discharges) to aerosol processes for Materials and Environment (filtration, diagnostics). The production of vapor i.e. condensable gaseous species, leads to nano-sized particles by physical and chemical routes of nucleation in these AP plasmas: (i) when dc streamer and spark filamentary discharges as well as ac filamentary dielectric barrier discharges interact with metal or dielectric surfaces, and (ii) when discharges induce reactions with gaseous precursors in volume. It is shown how composition, size and structure of primary nano-particles are related to plasma parameters (energy, number per unit surface and time and thermal gradients). Then the growth by coagulation controls the final size of agglomerates versus plasma parameters and transit time in and after the plasma. Charging and electro-thermal collection are depicted to account for the related potential applications of controlled kinematics of charged aerosol

Temperature profiles in filamentary dielectric barrier discharges at atmospheric pressure

International audienceThe physico-chemical properties of atmospheric pressure filamentary Dielectric Barrier Discharge (f-DBD) depend on its electrical characteristics and thermal profile. In this paper, a method for separating thermal and electrical effects is developed. Therefore, thermal profiles of f-DBD are studied for well defined electrical characteristics of filaments: all filaments are quasi identical with a controlled spatio-temporal density. The temperatures of gas, dielectric surface and plasma depend on the surface density and the temporal frequency of the filaments and can be altered by modifying the heat transfer. Different methods to control these temperatures are depicted. Moreover, heat transfer through conduction and convection from dielectric surface is shown to be the dominant heating mechanism for the flowing gas in the reactor. Finally, experimental results show that the temperature gradient around filaments can be controlled by the frequency of the applied voltage. Actually, the temperature difference between the filament and surrounding gas is independent of the applied frequency below 10 kHz. However, above 10 kHz, it increases linearly with the frequency. At high frequency, the time between two successive filaments occurring at the same position becomes smaller than the relaxation time constant of the thermal exchanges (~ 0.1 ms). Thus, this rise in temperature can be attributed to inefficient heat transfer from the filament formation zone

Nano-particle size-dependant charging and electro-deposition in Dielectric Barrier Discharges at Atmospheric Pressure for thin SiO x film deposition

International audienceThis paper focuses on charging and electro-deposition of nano-particles produced in a mixture of Silane and nitrous oxide diluted in N 2 , by Dielectric Barrier Discharge (DBD) at atmospheric pressure for SiO x film deposition. Townsend Discharge (TD) and Filamentary Discharge (FD) are compared with and without SiH 4. Without SiH 4 , particles are produced by filament-surface interaction. Both filament-surface and plasma-Silane interactions lead to bimodal particle size distributions from nucleation and agglomeration. With SiH 4 , particle formation and growth imply the same mechanisms in TD and FD. Faster dynamics in FD are related to higher local volume energy density than in TD. From scanning electron microscope images of the film and measurements downstream of the DBD reactor, the diameter of the particle produced is below 50 nm. An analytical model of electro-collection in ac electric field is used to investigate nano-particle charging. To account for the selective electro-deposition leading to particles smaller than 50 nm being included in the layer and to the particle size distribution measured downstream of the DBD, the same size-dependent charging and electrodeposition of particle are involved, with different charging dynamics in TD and FD

DBD as a post-discharge bipolar ions source and selective ioninduced nucleation versus ions polarity

International audienceIons densities and mobilities in post-dielectric barrier discharge (post-DBD) are presented here. To extract ions from DBD and perform post-discharge measurements the best functioning conditions are low overpressure, high frequency (> 25 kHz, to avoid electro-collection) and low flow rate (1 lpm, to reduce dilution). Besides, ions densities in post-discharge increase with electrode temperature and, at low flow rates, with the number of discharge filaments by time and surface unit (controlled by voltage at fixed frequency). In both cases, the reinforcement of dielectric material surface polarization reduces the local electro-collection of ions inside DBD or increases the production of ions by subcritical avalanches outside filaments. Concerning mobility measurements, it is shown that for low saturation, vapours emitted from post-DBD polymer tubes only affect positive ions mobility due to selective ion-induced nucleation on positive ions. When metal post-DBD tubes are used, positive ions keep the same range of electric mobility at any temperature while there is a drop in negative ions mobility around 100 °C, probably related to a chemical transition between O3 and NOx

Post-corona unipolar chargers with tuneable aerosol size-charge relations: Parameters affecting ion dispersion and particle trajectories for charger designs

International audienceThis paper focusses on the mean charge per particle of monodisperse submicron aerosol, charged by diffusion of unipolar ions in post-corona discharge. It aims to confirm and discuss the limits of considering a single value of Ni·t to describe aerosol charging and then to present methods to control the size-charge relation. Three aerosol chargers, with different mixings of ion and aerosol flows are investigated. Despite comparable ion sources with discharge currents of a few tens of µA, the size-charge relations differs from one charger to another due to different ion-aerosol mixing conditions and subsequent ion density along particles trajectories. Discrepancies are even more noticeable as the particle size increases. Discharge current, velocities of ion and aerosol flows and electric field control post-discharge ion density in each point of the charging volume. The control of particle trajectory in expanding unipolar ion cloud, leads to tuneable size-charge relations. Aerosol inertia and charging dynamics, that both depends on particle size, affects the Ni·t experienced by the particle and thus the final charge of the particle. Operating conditions to reach a constant mean charge for particles larger than 200 nm are reported. Conclusions provide a basis to design aerosol chargers devoted to electric mobility selection for aerosol deposition, separation or electrical measurements especially to overcome the limits of mobility-to-size data inversion due to multiple charge ambiguity using diffusion chargers