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Oxygen removal from a hydrocarbon containing gas stream by plasma catalysis

Hydrocarbon exhaust gases containing residual amounts of oxygen may pose challenges for their conversion into value added chemicals downstream, because oxygen may affect the process. This could be avoided by plasma treating the exhaust to convert O2 in presence of hydrocarbons into CO or CO2 on demand. The underlying reaction mechanisms of plasma conversion of O2 in the presence of hydrocarbons are
analysed in a model experiment using a radio frequency atmospheric pressure helium plasma in a plug flow design with admixtures of O2 and of CH4. The plasma process is analysed with infrared absorption spectroscopy to monitor CH4 as well as the reaction products CO, CO2 and H2O. It is shown that the plasma reaction for oxygen (or methane removal) is triggered by the formation of oxygen atoms from O2 by electron.
Oxygen atoms are eciently converted into CO, CO2 and H2O with CO being an intermediate in that reaction sequence. However, at very high oxygen admixtures to the gas stream, the conversion efficiency saturates because electron induced O2 dissociation in the plasma seems to be counterbalanced by a reduction of the efficiency of electron heating at high admixtures of O2. The impact of a typical industrial manganese oxide catalyst is evaluated for methane conversion. It is shown that the conversion effciency is enhanced by 15% to 20% already at temperatures of 430 K.

Publisher: 
EP2
Project: 
SFB 1316
Authors: 
Theresa Urbanietz
Christoph Stewig
Marc Böke
Achim von Keudell

Dedicated setup to isolate plasma catalysis mechanisms

Plasma catalysis is the combination of plasma and catalysis to reach an efficient conversion of molecules for flexible operating parameters and flexible feed gases. By combining plasmas with conventional thermal catalysis, the temperature windows may be different and the process may be insensitive to catalyst poisoning. The understanding of plasma catalysis mechanisms, however, is an extremely difficult task due to the strong coupling between plasma, gas phase chemistry and surface. A multitude of reaction pathways may be enhanced or reduced by the presence of a plasma that provides excited species as reaction partners. We developed a robust setup to analyse those processes based on a parallel plate atmospheric pressure plasma jet that allows a plug flow design. The plasma chemistry is analysed by Fourier transform infrared absorption spectroscopy and mass spectrometry. The electrodes in contact with the plasma are temperature controlled and can easily be replaced to apply a catalyst on top of them. The basic characteristics of the setup are discussed as well as three examples for its application are given (i) the analysis of methane oxidation using the plug flow scheme, (ii) the plasma catalytic conversion of CO2, and (iii) the plasma catalytic conversion of methane in methane oxygen mixtures.

Publisher: 
EP2
Project: 
SFB 1316
Authors: 
Christoph Stewig
Theresa Urbanietz
Laura Chauvet
Marc Böke
Achim von Keudell

Velocity distribution of metal ions in the target region of HiPIMS: the role of Coulomb collisions

High power impulse magnetron sputtering (HiPIMS) discharges have become an important tool for the deposition of thin, hard coatings. Such discharges are operated at a very low working gas pressure in the order of 1 Pa. Therefore, elastic collisions between ions and other heavy particles are often calculated to occur with low frequency, using the hard sphere approximation. However, inside the magnetic trap region of the discharge, a very dense plasma is created and Coulomb collisions become the dominant collision process for ions. In this article, we show that Coulomb collisions are a necessary part of a complete description of ion movement in the magnetic trap region of HiPIMS. To this end, the velocity distribution function (VDF) of chromium and titanium ions is measured using high-resolution optical emission spectroscopy. The VDF of those ions is then described using a simple simulation which employs a direct simulation Monte Carlo scheme. The simulation describes the self-relaxation of the VDF that is initially a Thompson distribution as being created during the sputtering process. Measurement positions inside the discharge are matched to the simulation results choosing an appropriate relaxation time. In this fashion, excellent agreement between simulation and measurement is obtained. We find, that the distribution quickly becomes mostly Maxwellian with a temperature of 9 eV for titanium ions and 4.5 eV in the case of chromium ions. Only the high energy tail of the VDF retains the shape of the initial Thompson distribution. The observed high temperature is explained with an energy redistribution from the highly energetic Thompson distribution into an partly-thermalized Maxwell-like distribution. Finally, the temperature resulting from this energy redistribution is calculated using a simple analytical model which shows good agreement with the measurements.

Publisher: 
EP2
Project: 
SFB TR 87
Authors: 
Julian Held
Sascha Thiemann-Monjé
Achim von Keudell
Volker Schulz-von der Gathen

Fast charge exchange ions in high power impulse magnetron sputtering of titanium as probes for the electrical potential

High power impulse magnetron sputtering (HiPIMS) plasmas exhibit a high ionization fraction of the sputtered material and ions with high kinetic energies, which produce thin films with superior quality. These ion energy distribution functions (IEDF) contain energetic peaks, which are believed to be linked to a distinct electrical potential hump (ionization zone) inside rotating localized ionization zones, so called spokes, at target power densities above 1kW/cm^−2. Any direct measurement of this electrical potential structure is, however, very difficult due to the dynamic nature of the spokes and the very high local power density, which hampers the use of conventional emissive probes. Instead, we use a careful analysis of the IEDFs for singly and doubly charged titanium ions from a HiPIMS plasma at varying target power density. The energy peaks in the IEDFs measured at the substrate depend on the point of ionization and any charge exchange collisions on the path between ionization and impact at the substrate. Thereby, the IEDFs contain a convoluted information about the electrical potential structure inside the plasma. The analysis of these IEDFs reveal that higher ionization states originate at high target power densities from the central part of the plasma spoke, whereas singly charged ions originate from the perimeter of the plasma spoke. Consequently, we observe different absolute ion energies with the energy of Ti2+ being slightly higher than two times the energy of Ti+. Additional peaks are observed in the IEDFs of Ti+ originating from charge exchange reactions from Ti2+ and Ti3+ with titanium neutrals. Based on this analysis of the IEDFs, the structure of the electrical potential inside a spoke is inferred yielding Delta Phi_(ionization zone)= 25 V above the plasma potential, irrespective of target power density.

Publisher: 
EP2
Project: 
SFB TR 87
Authors: 
Wolfgang Breilmann
Christian Maszl
Achim von Keudell

Excitation and dissociation of CO2 heavily diluted in noble gas atmospheric pressure plasmas

The excitation and dissociation of CO2 admixed to argon and helium atmospheric pressure radio frequency plasmas is analyzed. The absorbed plasma power is determined by voltage and current probe measurements and the excitation and dissociation of CO2 and CO by transmission mode Fourier-transform infrared spectroscopy (FTIR). It is shown that the vibrational temperatures of CO2 and CO are significantly higher in an argon compared to a helium plasma. The rotational temperatures remain in both cases close to room temperature. The conversion efficiency, expressed as a critical plasma power to reach almost complete depletion, is four times higher in the argon case. This is explained by the lower threshold for the generation of energetic particles (electrons or metastables) in argon as the main reactive collision partner, promoting excitation and dissociation of CO2, by the less efficient quenching of vibrational excited states of CO and CO2 by argon compared to helium and by a possible contribution of more energetic electrons in an argon plasma compared to helium.

Publisher: 
EP2
Project: 
SFB 1316
Authors: 
Christoph Stewig
Steffen Schüttler
Theresa Urbanietz
Marc Böke
Achim von Keudell

Pattern Formation in HiPIMS Plasmas

High power impulse magnetron sputtering (HiPIMS) plasmas produce a very energetic growth flux for the synthesis of thin films with superior properties. High power densities in the range of a few kW/cm2 are applied to a metal target electrode in short pulses with a length of 10 to 400 microseconds and duty cycles of a few percent or less in an argon plasma gas. Fast camera and probe measurements revealed the formation of very characteristic plasma patterns that become visible as rotating localized ionization zones, so called spokes. The appearance of these spokes at high plasma powers is believed to be essential for the good performance of HiPIMS plasmas. The rotation direction of the spokes is in ExB direction at high plasma powers, but in retrograde ExB direction at low plasma powers. This characteristic behavior is explained by applying a simple drift wave model from literature and comparing the dispersion relation of those waves with measured data. The pronounced rotation reversal is explained by either a change in the governing density gradient in the plasma or by the change in the direction of the streaming ions during the transition from an argon dominated regime at low powers to a metal dominated regime at high powers.

Publisher: 
EP2
Authors: 
Julian Held
Achim von Keudell