German Science Foundation (DFG)

Atomic oxygen generation in atmospheric pressure RF plasma jets driven by tailored voltage waveforms in mixtures of He and O2

Absolute atomic oxygen densities measured space resolved in the active plasma volume of a COST microplasma reference jet operated in He/O2 and driven by tailored voltage waveforms are presented. The measurements are performed for different shapes of the driving voltage waveform, oxygen admixture concentrations, and peak-to-peak voltages. Peaks- and valleys-waveforms constructed based on different numbers of consecutive harmonics, N, of the fundamental frequency f0 = 13.56 MHz their relative phases and amplitudes are used. The results show that the density of atomic oxygen can be controlled and optimized by voltage waveform tailoring (VWT). It is significantly enhanced by increasing the number of consecutive driving harmonics at fixed peak-to-peak voltage. The shape of the measured density profiles in the direction perpendicular to the electrodes can be controlled by VWT as well. For N > 1 and peaks-/valleys-waveforms, it exhibits a strong spatial asymmetry with a maximum at one of the electrodes due to the spatially asymmetric electron power absorption dynamics. Thus, the atomic oxygen flux can be directed primarily towards one of the electrodes. The generation of atomic oxygen can be further optimized by changing the reactive gas admixture and by tuning the peak-to-peak voltage amplitude. The obtained results are understood based on a detailed analysis of the spatio-temporal dynamics of energetic electrons revealed by phase resolved optical emission spectroscopy (PROES).

Publisher: 
AEPT
Project: 
SFB 1316
Authors: 
I. Korolov
D. Steuer
L. Bischoff
G. Hübner
Y. Liu
V. Schultz-von der Gathen
M. Böke
T. Mussenbrock
J. Schulze

Dissipated electrical power and electron density in an RF atmsopheric pressure helium plasma jet

Here we present a method for the operando determination of absolute absorbed power in an RF atmospheric pressure helium plasma discharge using miniaturized probes. A detailed error analysis demonstrates the reliability of the measured power values. With the help of a global model, the sheath width and electron density (4 × 1016^(–11) × 1016 m^(−3)) are derived from these power measurements and compared to literature. The results and thus the validity of the electrical model are confirmed by a second, independent characterization method using optical emission spectroscopy and time-averaged imaging.

Publisher: 
PIP
Project: 
SFB 1316
Authors: 
J. Golda
F. Kogelheide
P. Awakowicz
V. Schultz-von der Gathen

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

Plasma‐driven in situ production of hydrogen peroxide for biocatalysis

Peroxidases and peroxygenases are promising classes of enzymes for biocatalysis because of their ability to carry out one‐electron oxidation reactions and stereoselective oxyfunctionalizations. However, industrial application is limited, as the major drawback is the sensitivity toward the required peroxide substrates. Herein, we report a novel biocatalysis approach to circumvent this shortcoming: in situ production of H2O2 by dielectric barrier discharge plasma. The discharge plasma can be controlled to produce hydrogen peroxide at desired rates, yielding desired concentrations. Using horseradish peroxidase, it is demonstrated that hydrogen peroxide produced by plasma treatment can drive the enzymatic oxidation of model substrates. Fungal peroxygenase is then employed to convert ethylbenzene to (R)‐1‐phenylethanol with an ee of >96 % using plasma‐generated hydrogen peroxide. As direct treatment of the reaction solution with plasma results in reduced enzyme activity, the use of plasma‐treated liquid and protection strategies are investigated to increase total turnover. Technical plasmas present a noninvasive means to drive peroxide‐based biotransformations.

Publisher: 
Applied Microbiology
Project: 
SFB 1316
Authors: 
Abdulkadir Yayci
Alvaro Gomez Baraibar
Marco Krewing
Elena Fernandez Fueyo
Frank Hollmann
Miguel Alcalde
Robert Kourist
Julia E. Bandow

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

Experimental and computational investigations of electron dynamics in micro atmospheric pressure radio-frequency plasma jets operated in He/N_2 mixture

The electron power absorption dynamics in radio frequency driven micro atmospheric pressure capacitive plasma jets are studied based on experimental phase resolved optical emission spectroscopy (PROES) and computational (PIC/MCC) simulations. The jet is operated at 13.56 MHz in He with different admixture concentrations of N_2 and at several driving voltage amplitudes. We find the spatiotemporal dynamics of the light emission of the plasma at various wavelengths to be markedly different. This is understood by revealing the population dynamics of the upper levels of selected emission lines/bands based on comparisons between experimental and simulation results. The populations of these excited states are sensitive to different parts of the electron energy distribution function and to contributions from other excited states. Mode transitions of the electron power absorption dynamics from the Ω- to the Penning-mode are found to be induced by changing the N_2 admixture concentration and the driving voltage amplitude. Our numerical simulations reveal details of this mode transition and provide novel insights into the operation details of the Penning-mode. The characteristic excitation/emission maximum at the time of maximum sheath voltage at each electrode is found to be based on two mechanisms: (i) a direct channel, i.e. excitation/emission caused by electrons generated by Penning ionization inside the sheaths and (ii) an indirect channel, i.e. secondary electrons emitted from the electrode due to the impact of positive ions generated by Penning ionization at the electrodes.

Publisher: 
AEPT
Project: 
SFB 1316
Authors: 
L. Bischoff
G. Hübner
I. Korolov
Z. Donkó
P. Hartmann
T. Gans
J. Held
V. Schultz-von der Gathen
Y. Liu
T. Mussenbrock
J. Schulze

Helium metastable species generation in atmospheric pressure RF plasma jets driven by tailored voltage waveforms in mixtures of He and N2

Spatially resolved tunable diode-laser absorption measurements of the absolute densities of He-I (23S1) metastables in a micro atmospheric pressure plasma jet operated in He/N_2 and driven by ‘peaks’- and ‘valleys’-type tailored voltage waveforms are presented. The measurements are performed at different nitrogen admixture concentrations and peak-to-peak voltages with waveforms that consist of up to four consecutive harmonics of the fundamental frequency of 13.56 MHz. Comparisons of the measured metastable densities with those obtained from particle-in-cell/Monte Carlo collision simulations show a good quantitative agreement. The density of helium metastables is found to be significantly enhanced by increasing the number of consecutive driving harmonics. Their generation can be further optimized by tuning the peak-to-peak voltage amplitude and the concentration of the reactive gas admixture. These findings are understood based on detailed fundamental insights into the spatio-temporal electron dynamics gained from the simulations, which show that voltage waveform tailoring allows to control the electron energy distribution function to optimize the metastable generation. A high degree of correlation between the metastable creation rate and the electron impact excitation rate from the helium ground state into the He-I ((3s)3S1) level is observed for some conditions which may facilitate an estimation of the metastable densities.

Publisher: 
AEPT
Project: 
SFB 1316
Authors: 
I. Korolov
M. Leimkühler
M. Böke
Z. Donkó
V. Schultz-von der Gathen
L. Bischoff
G. Hübner
P. Hartmann
T. Gans
Y. Liu
T. Mussenbrock
J. Schulze

Control of electron dynamics, radical and metastable species generation in atmospheric pressure RF plasma jets by Voltage Waveform Tailoring

Atmospheric pressure capacitively coupled radio frequency discharges operated in He/N2 mixtures and driven by tailored voltage waveforms are investigated experimentally using a COST microplasma reference jet and by means of kinetic simulations as a function of the reactive gas admixture and the number of consecutive harmonics used to drive the plasma. Pulse-type ‘peaks’-waveforms, that consist of up to four consecutive harmonics of the fundamental frequency (f_0=13.56MHz), are used at a fixed peak-to-peak voltage of 400V. Based on an excellent agreement between experimental and simulation results with respect to the DC self-bias and the spatio-temporal electron impact excitation dynamics,we demonstrate that Voltage Waveform Tailoring allows for the control of the dynamics of energetic electrons, the electron energy distribution function in distinct spatio-temporal regions of interest, and, thus, the generation of atomic nitrogen as well as helium metastables, which are highly relevant for a variety of technological and biomedical applications. By tuning the number of driving frequencies and the reactive gas admixture, the generation of these important species can be optimised. The behavior of the DC self-bias, which is different compared to that in low pressure capacitive radio frequency plasmas, is understood based on an analytical model.

Publisher: 
AEPT
Project: 
SFB 1316
Authors: 
I. Korolov
Z. Donkó
G. Hübner
L. Bischoff
P. Hartmann
T. Gans
Y. Liu
T. Mussenbrock
J. Schulze

Non-equilibrium excitation of CO2 in an atmospheric pressure helium plasma jet

The energy ecient excitation of CO2 in atmospheric pressure plasmas
may be a method to generate solar fuels from renewable energies. This energy eciency
can be very high, if only specic states of the molecules in the plasma are populated
creating a strong non-equilibrium. This requires a specic design of the plasma source,
method of plasma excitation and choice of gases and admixtures. In this paper, nonequilibrium
excitation and dissociation of CO2 in an atmospheric pressure helium RF
plasma jet is analysed for varying absorbed plasma power and admixture levels of
CO2. The concentrations of CO2 and of CO, as well as the vibrational and rotational
temperatures of the possible degrees of freedom of the molecules are evaluated by
Fourier transform infrared spectroscopy (FTIR). The molecular rotational vibrational
spectra are modelled based on Maxwell-Boltzmann state populations using individual
temperatures for each degree of freedom. A strong non-equilibrium excitation of CO2
and CO has been found. Whereas the rotational temperatures are 400 K or below, the
vibrational temperature for CO reaches values up to 1600 K and that of the asymmetric
vibration of CO2 of 700 K. The dependence of these excitation temperatures on plasma
power and admixture level is rather weak. The mass balance, the energy and conversion
eciency are consistent with a very simple chemistry model that is dominated by CO2
dissociation via Penning collisions with helium metastables. A conversion effciency up
to 30 % and an energy effciency up to 10 % is observed in the parameter range of the
experiment.

Publisher: 
EP2
Project: 
SFB 1316
Authors: 
Theresa Urbanietz
Marc Böke
Volker Schulz-von der Gathen
Achim von Keudell