Atmospheric pressure plasmas, and streamers in particular, sustained in air often encounter dust or aerosol particles having sizes of a few to tens of micrometres. The dynamics of streamers intersecting such particles are of interest due to their possible use for functionalizing the surfaces of the particles. Using a 2-dimensional plasma hydrodynamics model having an unstructured mesh, the consequences of dust particles on streamer dynamics were investigated while varying the particle size, shape and material properties. We found that while small dielectric particles (< tens of micrometres) are enveloped by the streamer, larger particles can intercept and reinitiate streamers. By increasing the permittivity and capacitance of the particles, streamer interception and re-initiation may also occur on smaller particles. The presence of multiple particles in the path of a streamer can increase the speed of the avalanche front due to synergistic polarization of the particles.
The breakdown phase of the startup of metal halide lamps is typically through a cold fill of a rare gas and the ambient vapour pressure of a dose of metals. The dynamics of the breakdown stage are of interest for improving the efficiency and lifetime of lamps. A computational investigation of the breakdown of Ar/ Xe mixtures in an idealized lamp geometry was performed using global and two-dimensional (2-d) models to provide insight into the lamp ignition processes and to facilitate comparison with experiments. The experimental trends for breakdown for pressures of 10-90 Torr were qualitatively captured with the global model. Quantitative agreement required accounting for the temporal and spatial plasma dynamics included in the 2-d model. Small fractions of Xe in Ar were found to decrease the breakdown time as the ionization rates increased due to the lower ionization potential of xenon, while the electron energy distribution was not significantly affected. With higher Xe fractions the electron temperature in the ionization front decreased due to there being larger momentum transfer and inelastic losses to the Xe, and as a result the breakdown times increased. The compression of voltage ahead of the ionization front produced large electric fields at the cathode that enabled significant contributions to ionization by secondary electrons.
Plasmas generated at atmospheric pressure are used to functionalize the surfaces of polymers by creating new surface-resident chemical groups. The polymers used in textiles and biomedical applications often have non-planar surfaces whose functionalization requires penetration of plasma generated species into sometimes complex surface features. In this regard, the atmospheric pressure plasma treatment of a rough polypropylene surface was computationally investigated using a two-dimensional plasma hydrodynamics model integrated with a surface kinetics model. Repetitively pulsed discharges produced in a dielectric barrier-corona configuration in humid air were considered to affix O. Macroscopic non-uniformities in treatment result from the spatial variations in radical densities which depend on the polarity of the discharge. Microscopic non-uniformities arise due to the higher reactivity of plasma produced species, such as OH radicals, which are consumed before they can diffuse deeper into surface features. The consequences of applied voltage magnitude and polarity, and the relative humidity on discharge dynamics and radical generation leading to surface functionalization, are discussed.
Due to the difficulty of H 2 storage, development of real time H 2 generators would be advantageous for portable fuel cells. In this paper, the real time production of H 2 using microdischarge devices is discussed with results from a computational investigation. Ar/NH 3 mixtures were studied using plug flow and two-dimensional models. Dissociation of NH 3 by electron impact and thermal processes produces H atoms which recombine to form H 2 . We found that for sandwich type microdischarges with a diameter of 300 µm, dissociation of NH 3 is approximately 95% by electron impact and 5% by thermal processes for a NH 3 mole fraction of 5%. Efficiency of conversion of NH 3 to H 2 is dependent on residence time in the discharge, mole fraction and geometry, as these properties determine the eV/molecule deposited into NH 3 . Conversion efficiencies (fraction of H in NH 3 converted to H 2 ) in excess of 83% are predicted for optimum conditions.
Atmospheric pressure corona discharges are industrially employed to treat large areas of commodity polymer sheets by creating new surface functional groups. The most common processes use oxygen containing discharges to affix oxygen to hydrocarbon polymers, thereby increasing their surface energy and wettability. The process is typically continuous and is carried out in a web configuration with film speeds of tens to hundreds of cm s −1. The densities and relative abundances of functional groups depend on the gas composition, gas flow rate and residence time of the polymer in the discharge zone which ultimately determine the magnitude and mole fractions of reactive fluxes to the surface. In this paper, results are discussed from a two-dimensional computational investigation of the atmospheric pressure plasma functionalization of a moving polypropylene sheet in repetitively pulsed He/O 2 /H 2 O discharges. O and OH typically initiate surface processing by hydrogen abstraction. These species are regenerated during every plasma pulse but are also largely consumed during the inter-pulse period. Longer-lived species such as O 3 accumulate over many pulses and convect downstream with the gas flow. Optimizing the interplay between local rapid reactions, such as H abstraction which occurs dominantly in the discharge zone, and non-local slower processes, such as surface-surface reactions, may enable the customization of the relative abundance of surface functional groups.
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