CH3 Radical Generation in Microplasmas for Up-Conversion of Methane.

The conversion of methane, CH4, into higher value chemicals using low temperature plasmas is challenged by both improving efficiency and selectivity. One path toward selectivity is capturing plasma-produced methyl radicals, CH3, in a solvent for aqueous processing. Due to the rapid reactions of methyl radicals in the gas phase, the transport distance from the production of the CH3 to its solvation should be short, which then motivates the use of microplasmas. The generation of CH3 in Ar/CH4/H2O plasmas produced in nanosecond pulsed dielectric barrier discharge microplasmas is discussed using results from a computational investigation. The microplasma is sustained in the channel of a microfluidic chip in which the solvent flows along one wall or in droplets. CH3 is primarily produced by electron-impact of and dissociative excitation transfer to CH4, as well as CH2 reacting with CH4. CH3 is rapidly consumed to form C2H6 which, in spite of being subject to these same dissociative processes, accumulates over time, as do other stable products including C3H8 and CH3OH. The gas mixture and electrical properties were varied to assess their effects on CH3 production. CH3 production is largest with 5% CH4 in the Ar/CH4/H2O mixture due to an optimal balance of electron-impact dissociation, which increases with CH4 percentage, and dissociative excitation transfer and CH2 reacting with CH4, which decreases with CH4 percentage. Design parameters of the microchannels were also investigated. Increasing the permittivity of the dielectrics in contact with the plasma increased the ionization wave intensity, which increased CH3 production. Increased energy deposition per pulse generally increases CH3 production as does lengthening pulse length up to a certain point. The arrangement of the solvent flow in the microchannel can also affect the CH3 density and fluence to the solvent. The fluence of CH3 to the liquid solvent is increased if the liquid is immersed in the plasma as a droplet or is a layer on the wall where the ionization wave terminates. The solvation dynamics of CH3 with varying numbers of droplets was also examined. The maximum density of solvated methyl radicals CH3aq occurs with a large number of droplets in the plasma. However, the solvated CH3aq density can rapidly decrease due to desolvation, emphasizing the need to quickly react with the solvated species in the solvent.

Atmospheric pressure plasma treatment of skin: penetration into hair follicles.

Sterilization of skin prior to surgery is challenged by the reservoir of bacteria that resides in hair follicles. Atmospheric pressure plasma jets (APPJs) have been proposed as a method to treat and deactivate these bacteria as atmospheric plasmas are able to penetrate into structures and crevices with dimensions similar to those found in hair follicles. In this paper, we discuss results from a computational investigation of an APPJ sustained in helium flowing into ambient air, and incident onto a layered dielectric similar to human skin in which there are idealized hair follicles. We found that, depending on the location of the follicle, the bulk ionization wave (IW) incident onto the skin, or the surface IW on the skin, are able to launch IWs into the follicle. The uniformity of treatment of the follicle depends on the location of the first entry of the plasma into the follicle on the top of the skin. Typically, only one side of the follicle is treated on for a given plasma pulse, with uniform treatment resulting from rastering the plasma jet across the follicle over many pulses. Plasma treatment of the follicle is sensitive to the angle of the follicle with respect to the skin, width of the follicle pocket, conductivity of the dermis and thickness of the underlying subcutaneous fat layer, the latter due to the change in capacitance of the tissue.

Chemical kinetics in an atmospheric pressure helium plasma containing humidity.

Atmospheric pressure plasmas are sources of biologically active oxygen and nitrogen species, which makes them potentially suitable for the use as biomedical devices. Here, experiments and simulations are combined to investigate the formation of the key reactive oxygen species, atomic oxygen (O) and hydroxyl radicals (OH), in a radio-frequency driven atmospheric pressure plasma jet operated in humidified helium. Vacuum ultra-violet high-resolution Fourier-transform absorption spectroscopy and ultra-violet broad-band absorption spectroscopy are used to measure absolute densities of O and OH. These densities increase with increasing H2O content in the feed gas, and approach saturation values at higher admixtures on the order of 3 × 1014 cm-3 for OH and 3 × 1013 cm-3 for O. Experimental results are used to benchmark densities obtained from zero-dimensional plasma chemical kinetics simulations, which reveal the dominant formation pathways. At low humidity content, O is formed from OH+ by proton transfer to H2O, which also initiates the formation of large cluster ions. At higher humidity content, O is created by reactions between OH radicals, and lost by recombination with OH. OH is produced mainly from H2O+ by proton transfer to H2O and by electron impact dissociation of H2O. It is lost by reactions with other OH molecules to form either H2O + O or H2O2. Formation pathways change as a function of humidity content and position in the plasma channel. The understanding of the chemical kinetics of O and OH gained in this work will help in the development of plasma tailoring strategies to optimise their densities in applications.

Electron collisions with atoms, ions, molecules, and surfaces: Fundamental science empowering advances in technology.

Electron collisions with atoms, ions, molecules, and surfaces are critically important to the understanding and modeling of low-temperature plasmas (LTPs), and so in the development of technologies based on LTPs. Recent progress in obtaining experimental benchmark data and the development of highly sophisticated computational methods is highlighted. With the cesium-based diode-pumped alkali laser and remote plasma etching of Si3N4 as examples, we demonstrate how accurate and comprehensive datasets for electron collisions enable complex modeling of plasma-using technologies that empower our high-technology-based society.

Fundamentals of gas phase plasmas for treatment of human tissue.

The use of gas phase plasmas for treating human tissue is at the intersection of two disciplines - plasma physics and engineering, and medicine. In this paper, a primer will be provided for the medical practitioner on the fundamentals of generating gas phase plasmas at atmospheric pressure in air for the treatment of human tissue. The mechanisms for gas phase plasmas interacting with tissue and biological fluids will also be discussed using results from computer modeling.

Electron energy distributions and anomalous skin depth effects in high-plasma-density inductively coupled discharges.

Electron transport in low pressure (<10s mTorr), moderate frequency (<10s MHz) inductively coupled plasmas (ICPs) displays a variety of nonequilibrium characteristics due to their operation in a regime where the mean free paths of electrons are significant fractions of the cell dimensions and the skin depth is anomalous. Proper analysis of transport for these conditions requires a kinetic approach to resolve the dynamics of the electron energy distribution (EED) and its non-Maxwellian character. To facilitate such an investigation, a method was developed for modeling electron-electron collisions in a Monte Carlo simulation and the method was incorporated into a two-dimensional plasma equipment model. Electron temperatures, electron densities, and EEDs obtained using the model were compared with measurements for ICPs sustained in argon. It was found that EEDs were significantly depleted at low energies in regimes dominated by noncollisional heating, typically within the classical electromagnetic skin depth. Regions of positive and negative power deposition were observed for conditions where the absorption of the electric field was both monotonic and nonmonotonic.