52.80.Pi High-frequency and RF discharges
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Institute
Because of the vital role of the liquid as interface in plasma medicine, this work is focused on the elucidation of the interaction of plasmas with biologically relevant liquids. The results of this thesis are an important step in the direction of the applications to real biological liquids such as blood and wound secretion ex vivo as well as in vivo. In this thesis the following questions are investigated and answered with the special focus on the free radicals as highly reactive and, therefore, hard to detect relevant group of chemical species: What is the impact of the atmospheric-pressure argon plasma jet on biologically relevant solutions? Which species are generated due to the plasma treatment of liquids? What is an appropriate detection procedure for the qualification and quantification of the short-lived species? Does the surrounding conditions influence the formation of liquid-phase reactive species and can this influence be used to tailor a desired liquid composition? What is the influence of the plasma surroundings? What is the influence of feed gas manipulation regarding the reactive species generation? Can these impacts be used for a selected reactive species composition generation? Does the treated liquid medium affect the plasma-generated reactive species output and in what way? Which are the underlying mechanisms and origins of the plasma-caused chemical changes in the solutions? Do reactive species exist, which origin is located in the gaseous phase? What is the impact of the plasma jet radiation?
In this work the mechanisms leading to the generation of the various reactive oxygen and nitrogen species (RONS) in a cold atmospheric plasma (CAP) jet and means to control their composition were studied. The investigated CAP jet kinpen is typically operated with Ar feed gas (pure or with molecular admixtures), driven at a frequency of approximately 1 MHz and features fast ionization waves or guided streamers, traveling at velocities of several km/s. The complex reaction networks were investigated by numerical and experimental techniques. Detailed experimental, analytical and computational investigations on the mass and heat transport in the plasma plume were performed: A novel analytical approach to diffusion in jet flows, the non-dispersive path mapping approximation (NDPM) was developed. The method for the first time allows for an estimation of the ambient species density in the near-field of jets that feature a non-homogeneous flow-field. The NDPM approximation was employed for the evaluation of laser induced fluorescence measurements on OH. Through combining measurements and NDPM approximation, this approach yielded an estimation for the ambient species density at the position of the guided streamers, not only in the laminar, but also in the (standard) turbulent operating regime. Accurate measurements of the temporally averaged ambient species density and temperature in the plasma plume were obtained by quantitative Schlieren measurements. The method yields temperature values with sub-Kelvin accuracy and, through combination with computational fluid dynamics (CFD) simulations, allowed for an estimation of the calorimetric power of the jet. In order to obtain a defined environment for the jet to operate in, a shielding gas device was designed in this work, which creates a gas curtain of defined composition around the plasma plume. The plasma dynamics on the ns timescale was investigated by phase resolved optical measurements. The effect of different shielding compositions ranging from pure N2 to pure O2 on guided streamer propagation was investigated. An electrostatic focusing mechanisms was discovered, which promotes the propagation of guided streamers along the channels formed by a noble gas in the plume of plasma jets operating in electronegative gases (such as air or O2). Two zero-dimensional (volume averaged) models were developed: First, the local processes in the guided streamer were modeled using an electron impact reaction kinetic model, which is closely correlated to densities of metastable argon (Ar*) obtained by laser atom absorption measurements. This first model shows that Ar* is the species which dominantly drives the plasma chemistry in the plasma plume. This is exploited in the second plug-flow reaction kinetics model, which is employed to investigate the formation of long-living RONS and uses an Ar* source term as sole energy input. The model uses the previous experimental data on mass and heat transport and temporal dynamics as input and is in turn verified by quantitative FTIR absorption measurements on O3, NO2, N2O, HNO3 and N2O5 in the far-field of the jet, where large absorption lengths can be achieved using a multi pass cell. For the evaluation of the zero-dimensional model, the time-of-flight of RONS from their generation to reaching the multi pass cell was determined using CFD simulations. The insight gained through this combined experimental-modeling approach on the reaction networks revealed relevant control parameters and enabled adjusting the plasma chemistry towards a desired RONS output. Through choosing appropriate feed-gas admixtures and shielding gas compositions, it is possible to generate an NOx-dominated plasma chemistry, although the jet usually produces a strongly O/O3-dominated chemistry. Understanding and controlling the plasma chemistry of cold atmospheric plasma sources for medical applications is not only essential for research, but is also the key for designing future plasma sources for specific medical applications that yield an optimum efficacy and avoid potential side effects of plasma treatment.
The aim of this thesis is to concentrate on the investigation of these ROS&RNS composition distribution and their production pathways in the gas phase produced by a plasma jet. By understanding the physical mechanisms behind the generation of the ROS&RNS a precise tuning and design of the composition distribution in the gas phase can be achieved. One crucial physical parameter is the dissipated power inside the plasma. Only if this parameter is known a meaningful comparison of different feed gas settings is possible. Therefore, a concept for measuring the dissipated power inside the plasma for the modified micro-scaled atmospheric pressure plasma jet( µAPPJ) is designed. Additionally, due to achievements within this thesis it is now possible to ignite a homogeneous discharge in argon and helium within the geometry of the µAPPJ. The used feed gas is a determining factor concerning the electron energy distribution function and consequently influencing the production mechanism of the ROS&RNS. First of all, the electrical characterisation of the modified µAPPJ was performed including the alpha-to-gamma transition. It is shown that the alpha-to-gamma transition power is increasing with increasing frequency. For the first time it is now feasible to investigate the influence of the dissipated power on the neutral gas temperature, the metastable atom densities and the ROS&RNS production for the modified µAPPJ with argon and helium as feed gas. Due to the possibility of changing the feed gas and controlling the dissipated power a fundamental insight into the production mechanism of the ROS&RNS generated by the plasma jet is achieved. With rising dissipated power the temperature and the metastable densities as well as the ozone and nitrogen dioxide concentrations are increasing. By adding molecular oxygen and nitrogen to the feed gas of a plasma jet the ROS&RNS composition can be tuned. However, also the dissipated power is changed by the small amount of admixtures. Due to the developed dissipated power measurements within this thesis it was possible to disentangle the influence of the admixture on the power and on the ROS&RNS production. If the dissipated power is fixed for the µAPPJ with argon and helium feed gas, respectively, the highest amount of ozone was measured with oxygen admixture in an argon discharge, the highest amount of dinitrogen pentoxide with nitrogen admixture in an argon discharge and the highest amount of nitrogen dioxide with nitrogen admixture in a helium discharge. Beyond the influence of the dissipated power and the molecular admixture on the ROS&RNS production the feed gas temperature is a crucial parameter for the corresponding chemical reactions. By changing this parameter the distribution of ozone and nitrogen dioxide can be tuned precisely in such a way that with increasing temperature the ozone density goes down and the nitrogen dioxide density rises. Another determinant for the ROS&RNS composition produced by an atmospheric pressure plasma jet is the influence of ambient air. If the ambient air is changing from pure nitrogen to pure oxygen atmosphere the ozone density produced by the plasma jet is increasing. For the same conditions the nitrogen dioxide has a maximum at an oxygen-to-nitrogen ratio of 1:1. To avoid the influence of the ambient air on the reactive species production the afterglow of the µAPPJ was prolonged with a glass tube. By increasing the amount of molecular admixtures to the feed gas with each in equal quantities a totally different ROS&RNS composition can be obtained compared without the glass tube. It figures out that for small molecular admixtures the reactive species composition is nitrogen dominated and for higher admixtures it is oxygen dominated. Consequently, by shielding the ambient air from the active effluent and by admixing molecular oxygen and nitrogen the ROS&RNS composition can be designed.
Diese Dissertation beschäftigt sich mit der Erzeugung von edelmetallfreien Katalysatoren für die Sauerstoffreduktion in Brennstoffzellen. Dabei wird ein neuartiger, dualer Plasmaprozess entwickelt, aufgebaut und die so-erzeugten Schichten mit verschiedenen elektrochemischen (CV, RDE und RRDE) und strukturanalytischen Methoden (SEM, EDX, IR, XPS, Leitfähigkeit, XRD, NEXAFS, EXAFS und TEM) untersucht. Auf diese Weise ist es erstmalig gelungen edelmetallfreie Katalysatoren mit einem Plasmaprozess herzustellen, ohne dass eine zusätzliche Pyrolyse benötigt wird. Die katalytische Aktivität der Schichten ist außerdem deutlich höher als die von rein chemisch hergestellten Metall–Polypyrrol-Schichten.
Asymmetrical capacitively coupled RF discharges in oxygen, argon and hydrogen have been experimentally investigated with the innovative technique of the phase resolved optical emission spectroscopy. This diagnostic tool allows to measure spatio-temporally resolved emission intensities of electronically excited species with a high resolution. The spatial (axial) resolution was better than 1 mm and a temporal resolution of about 1.5 ns has been achieved. Therefore the plasma induced optical emission within the RF cycle (TRF = 73.75 ns) from the RF sheath region with a typical mean sheath thickness of about 5mm has been studied. Spatio-temporally resolved optical emission patterns of the following optical transitions have been measured for a total gas pressure in the range of 20 to 100 Pa and self-bias voltages between -50 and -550 V: Oxygen plasma Emission at 777.4 nm and 844.6 nm (atomic oxygen) Argon plasma Emission at about 751 nm and 841 nm (argon) Hydrogen plasma Emission at 656.3nm (atomic hydrogen, H alpha-line) These transitions are the most prominent ones of the investigated excited species in these plasmas as could be shown from overview spectra of the plasma induced optical emission in the range from 350 to 850 nm. For the first time such extensive PROES measurements in oxygen CCRF plasmas are presented in this work. The additional investigations of argon and hydrogen plasmas serve as a reference and for a direct comparison with results from the literature. The temporal behavior of the emission intensity is influenced by the effective lifetime of the emitting states which is on the order of the nanosecond time scale of the RF cycle. Therefore, it does not represent the real temporal behavior of the excitation. A simple method has been applied to calculate relative excitation rates from the measured emission intensities to distinguish different excitation mechanisms and their correct relative temporal behavior. In a close collaboration within the framework of the Sonderforschungsbereich Transregio 24 'Fundamentals of Complex Plasmas' a newly 1d3v PIC-MCC code for simulations of capacitive RF discharges in oxygen has been developed by Matyash et al. The very close coupling of experiment and modeling allowed a really detailed and microscopic understanding of the processes and dynamics from the sheath to the bulk plasma in CCRF discharges. The spatio-temporally resolved excitation rate profiles show four different excitation structures (I-IV). Excitation processes due to the following mechanisms in CCPs could be identified and characterized: I Electrons expelled from growing sheath II Electrons detached from negative ions (collisions with neutrals) + secondary electrons from the electrode surface (ion bombardment) III Field-reversal effect, reduced mobility of electrons (electron-neutral collisions) IV Heavy-particle collisions These excitation mechanisms are characterized by different temporal and spatial behaviors of the excitation rate within the RF cycle. Additionally it has been shown that the excitation by electron impact in the investigated oxygen plasmas results mainly from dissociative electron impact excitation (O2 + e -> O + O* + e) and not from direct electron impact excitation (O + e -> O* + e). Actinometry measurements show that the results are not really credible. Thus actinometry is not applicable on the investigated oxygen RF plasma. A challenge in interpretation is the observed excitation pattern IV. Pattern IV has to be caused in connection with heavy particle collisions nearby the electrode surface and could be observed in all the three plasmas oxygen, argon and hydrogen. It is located directly in front of the powered electrode and appears during almost the whole RF cycle. The temporal modulation is nearly sinusoidal and weak in comparison to the first three patterns. This is due to the weak RF modulation of the ion flux towards the electrode surface which has been proven by a PIC simulation. It could be shown that the modulation degree of pattern IV depends on the transition time of the corresponding positive ions through the RF sheath which is influenced by the ion mass. In oxygen as well as in argon CCRF plasmas pattern IV is less modulated than in hydrogen CCRF plasmas due to the heavier ions in oxygen and argon. Additionally the modulation degree increases with increasing pressure due to the more confined plasma at higher pressures which is yielding in a stronger modulated ion current towards the powered electrode.