This study investigates the temperature and solid-liquid-vapor phase transitions of iron and iron-oxide particles during oxidation and reduction, respectively. In iron combustion, particles are injected into a laminar premixed methane/air flame, and their temperature history is measured by high-speed multi-band RGB pyrometry and near-infrared hyperspectral emission spectroscopy. The agreement between the two temperature measurements obtained in distinct spectral regions supports the validity of the gray-body assumption, i.e., spectrally and temperature-independent emissivity, under the present conditions. The RGB pyrometry diagnostic is applied in an optically accessible argon/hydrogen microwave plasma reactor to monitor iron-oxide particles during inflight reduction. The particles exhibit rapid heating to temperatures exceeding the boiling point of iron, accompanied by luminous vapor clouds indicative of partial vaporization and plasma-driven reactions. A comparison with the plasma emission spectra reveals that the gas-phase luminescence originates from atomic iron vapor. The average particle temperature decreases rapidly both with radial distance from the reactor centerline and in the downstream direction. During subsequent cooling, the temperature exhibits a plateau associated with the solid-liquid or crystal phase formation. In this stage, particles occasionally undergo thermomechanical breakup. The particle size and feed-gas composition strongly influence the particle temperature history and must therefore be carefully controlled to achieve efficient reduction while avoiding excessive vaporization and mass loss. Novelty and significance statement: While most studies in the metal-fuel community focus on iron particle oxidation, the reverse pathway of iron-oxide reduction has received far less attention. This work combines optical pyrometry and spectroscopic diagnostics to resolve the in-flight thermal evolution of individual particles in both a premixed flame and a hydrogen microwave plasma. The measurements enable a detailed analysis of particle temperature evolution, vaporization and associated luminescent gas-phase iron species, particle breakup events, and temperature plateaus indicating phase transitions, as well as the influence of operating parameters on particle behavior. These results provide direct insight into plasma-particle interactions and the thermal mechanisms governing plasma-assisted iron-oxide reduction, relevant to metal-fuel cycles and hydrogen-based low-carbon metallurgy.

The ion energy distribution functions (IEDFs) have been measured for a helium atmospheric pressure dielectric barrier discharge jet expanding into the air and impacting a metal or ceramic surface. The plasma jet produces ionization waves as guided positive streamers that reach the surface. Molecular beam mass spectrometry (MBMS) with an energy filter has been used to monitor the IEDFs at a distance of 1.5 cm from the dielectric barrier discharge plasma jet exit. The species are sampled from the supersonic expanding helium beam passing into the MBMS through a 40 mu m (metal) or a 50 mu m (ceramic) diameter orifice. N 2+ , O 2+ , NO+, O 3+ and water cluster ions (H2O)nH+ (n = 1...4) are abundantly produced in the discharge. The analysis of the time-resolved IEDFs reveals that all ions are predominantly sampled at a reference energy Ebeam when using the metallic orifice. This energy Ebeam is determined by the seeding of the ions into the supersonic expanding helium beam into the MBMS. After the impact of the streamer, an afterglow of 10 mu s is observed when ions are continuously sampled at an energy higher than Ebeam by a few 0.1 eVs. This is resolved by postulating a positive space-charge region in front of a positively charged surface. The temporal sequence of the ion impact is consistent with reaction schemes in air plasmas, where O 2+ and N 2+ are created before the formation of NO and O3, as well as larger water cluster ions.

Non-thermal plasma-based technologies have emerged as versatile tools for various industrial processes due to their ability to induce chemical reactions efficiently under ambient conditions. In particular, dielectric barrier discharges (DBDs) are of interest because of their robust and reliable design and scalability. This study investigates the role of pressure in tuning conversion, plasma parameters, and flow patterns in a plasma-assisted chemical reaction using a surface DBD (SDBD) reactor. The removal of O2 traces in H2 was used as a model reaction, where an unexpected increased conversion at elevated pressure was observed at high powers. This effect was studied using high-speed photography to analyze streamer dynamics and optical emission spectroscopy to determine plasma parameters. With increasing pressure, both the plasma area and the number of individual streamers decreased, and the electron density decreased as well. Fluid simulations were conducted to examine the impact of increased pressure on mass transport pointing to an enhanced contact time as the origin of the increased conversion at high dissipated powers. The findings highlight the importance of optimizing pressure and power conditions to maximize the efficiency of plasma-based chemical processes.

The elementary processes during the fixation of nitrogen by plasma catalysis are studied in a low-pressure plasma experiment with N _2 and O _2 as source gases. The formation of surface groups on an iron oxide foil is monitored with infrared reflection absorption spectroscopy. Surface nitrates (NO _3^- ) are formed when the substrate is exposed to a 1:1 N _2 :O _2 plasma, as well as N _2 O(g), NO(g), NO _2 (g), and O _3 (g) in the gas phase. It is postulated that NO _1,2 (g) species created by the plasma, adsorb at the surface and create these nitrates. This constitutes an intermediate step for nitrogen oxidation by plasma catalysis.

Nanosecond and microsecond plasma-in liquid systems are explored to oxidize or regenerate a copper oxide surface in situ to serve as a catalyst for electrochemical CO_2 conversion. The plasma excitation generates H_2O_2 in the liquid, which induces the dissolution of Cu into Cu(OH)_2 and the recrystallization into Cu_2O nanocubes at the interface. The plasma performance of the two excitation schemes is analyzed, showing that the H_2O_2 production of nanosecond plasma is more efficient than of microsecond plasmas. The nature of the Cu_2O nanocubes is evaluated using electron microscopy and electrochemical characterization.

Surface dielectric barrier discharges (sDBD) are efficient and scalable plasma sources for plasma-based gas conversion. One prominent feature of an sDBD is the generation of an ion wind, which exerts a force on the neutrals, thus leading to an efficient mixing of plasma and a passing gas stream. This becomes apparent by the creation of upstream and downstream vortices in the vicinity of the plasma. In this study, these vortices are generated by high voltage burst pulses consisting of two half cycles of an almost sinusoidal voltage shape. The vortices are monitored by Schlieren imaging diagnostic to benchmark and connect two simulations of the sDBD: a plasma model simulating a streamer for 25 ns starting from the electrode and propagating along a dielectric surface followed by a decay. The streamer is the source of electrical charges accelerated as ion wind by the applied electric field from the sDBD power supply. A second flow simulation models this ion wind as a time-averaged thrust acting on the passing gas stream. The conversion of the time-resolved forces from the nanosecond plasma simulation into the steady state thrust in the flow simulation indicates that the force from the plasma lasts much longer than the actual streamer propagation phase. This is explained by the fact that the charges in the streamer channel remain present for almost 100 ns, and the voltage from the power supply lasts for a few microseconds being applied to the electrode so that ions in the streamer channel are still accelerated even after a streamer stops to propagate after a few ns. The thrust generated during the streamer phase, including the relaxation phase, agrees well with predictions from flow simulation. Additionally, properly converting the time-resolved forces from the plasma simulation into a time-averaged thrust for the flow simulation yields exactly the synthetic Schlieren images as measured in the experiments.

Hydrogen is often envisioned as a green fuel and energy carrier. However, a greenhouse gas-free production method is still needed for this purpose. A promising method is the pyrolysis of methane. In this study, we present the three-dimensional simulation of methane pyrolysis in a microwave plasma torch operated in an argon-methane mixture. Due to the high gas temperatures, the chemistry is dominated by reactions between neutral particles. Therefore, the plasma is treated only as a heat source using space-resolved gas temperature measurements as input. The end-of-pipe results of the simulation are compared to measured values, showing good agreement for methane conversion. Methane pyrolysis in a microwave plasma torch is simulated using three-dimensional computational fluid dynamics. Due to the high gas temperatures, neutral particle reactions dominate the chemistry. Therefore, the plasma is only treated as a heat source. The end-of-pipe results are compared to measured values, showing good agreement for methane conversion. image

Copper oxide surfaces are commonly used as the catalyst for the CO2 reduction reaction towards hydrocarbons. However, the lifetime of these catalyst surfaces is limited. In this paper, a method of production of copper oxides through in-liquid plasma is explored, which may be a suitable reactivation method in such applications. The influence of the plasma, ignited in distilled water, with copper and its oxides is monitored in - situ using infrared spectroscopy and ex-situ using scanning electron spectroscop and x-ray photoelectron spectroscopy of the samples. It is shown that the interaction of the plasma with the samples causes a reduction of the copper oxide on a fast time scale and an oxidation on a longer time scale. The formation of preferentially oriented copper nanocubes is observed.

Three-dimensional (3D) etching of materials by plasmas is an ultimate challenge in microstructuring applications. A method is proposed to reach a controllable 3D structure by using masks in front of the surface in a plasma etch reactor in combination with local magnetic fields to steer the incident ions in the plasma sheath region towards the surface to reach 3D directionality during etching and deposition. This effect can be controlled by modifying the magnetic field and/or plasma properties to adjust the relationship between sheath thickness and mask feature size. Since the guiding length scale is the plasma sheath thickness, which for typical plasma densities is at least 10s of microns or larger, controlled directional etching and deposition target the field of microstructuring, e.g. of solids for sensors, optics, or microfluidics. In this proof-of-concept study, it is shown that $\vec{E}\times\vec{B}$ drifts tailor the local sheath expansion, thereby controlling the plasma density distribution and the transport when the plasma penetrates the mask during an RF cycle. This modified local plasma creates a 3D etch profile. This is shown experimentally as well as using 2d3v Particle-In-Cell/Monte Carlo collisions simulation.

Abstract The dynamic of helium metastable formation along an atmospheric pressure helium plasma channel with different bounding surfaces is analysed. The densities of He(23S1) and He2(a 3 Σ u + ) are measured using optical absorption spectroscopy. A simple model for helium metastable creation, quenching by water impurities and destruction upon surface impact, is developed. The model shows a very good agreement if one assumes that surfaces mainly control secondary electron emission, affecting the local heating at the plasma boundary sheath. This, in turn, changes the rate for He(23S1) and He2(a 3 Σ u + ) conversion. The helium metastable induced desorption of adsorbed water causes a decay of the metastable density along the plasma channel due to quenching.

Hydrogen is often envisioned as a green fuel and energy carrier. However, a greenhouse gas-free production method is still needed for this purpose. A promising method is the pyrolysis of methane. In this study, we present the three-dimensional simulation of methane pyrolysis in a microwave plasma torch operated in an argon–methane mixture. Due to the high gas temperatures, the chemistry is dominated by reactions between neutral particles. Therefore, the plasma is treated only as a heat source using space-resolved gas temperature measurements as input. The end-of-pipe results of the simulation are compared to measured values, showing good agreement for methane conversion.

Experimental investigations of n-butane oxidation under atmospheric-pressure plasma conditions and in He-dilution have provided detailed information on the power-dependence of the conversion of C_4H_10 into CO and CO_2 at 450 K surface temperature. The rf-plasma discharge has been equipped with a MnO_2 -catalyst, and a significant impact on the reaction chain due to the presence of the catalyst surface could be observed. We report on ongoing data-based model development. Recently, a reaction kinetic model has been published, which agrees well with the experimental data (Stewig et al. in Plasma Sources Sci Technol 32:105006, 2023). However, that model could not clearly identify the main mechanisms in the interaction of plasma and catalyst. We show that various models can be found that explain the data similarly well. Detailed sensitivity analysis shows that only a maximum of three parameters can be identified in all the models considered for the currently limited data. Despite this limitation, we intend to continue the data analysis using more general models and introduce possible surface effects. Such unified models simultaneously describe the experimental data from both measurements with and without catalyst using a single set of physical parameters. To evaluate the hypotheses, we present numerical results for certain ranges of experimental parameters, which, in a subsequent experimental verification, allows to exclude or confirm one or another model.

The dynamic of helium metastable formation along an atmospheric pressure helium plasma channel with different bounding surfaces is analysed. The densities of He(2(3)S(1)) and He-2(a 3 Sigma (u) (+) ) are measured using optical absorption spectroscopy. A simple model for helium metastable creation, quenching by water impurities and destruction upon surface impact, is developed. The model shows a very good agreement if one assumes that surfaces mainly control secondary electron emission, affecting the local heating at the plasma boundary sheath. This, in turn, changes the rate for He(2(3)S(1)) and He-2(a(3)Sigma(+)(u)) conversion. The helium metastable induced desorption of adsorbed water causes a decay of the metastable density along the plasma channel due to quenching.

This feature article considers the analysis of the initial states of film growth on polymer substrates. The assembled results are based on the cooperation between research groups in the field of plasma physics, chemistry, electric as well as mechanical engineering over the last years, mostly within the frame of the transregional project SFB-TR 87 ("Pulsed high power plasmas for the synthesis of nanostructured functional layers"). This feature article aims at bridging the gap between the understanding of plasma processes in the gas phase and the resulting surface and interface processes of the polymer. The results show that interfacial adhesion and initial film growth can be well controlled and even predicted based on the combination of analytical approaches.

Hydrocarbon contamination is associated with light transmission losses in modern lithography machines, which contain extreme-ultraviolet-induced plasma. A volume-averaged global and deposition/etch surface model of a reference hydrogen/methane inductive discharge is developed to investigate the plasma–surface interactions. The simulation results are validated against a wide variety of experiments and verified with respect to multiple sets of computational data. The deposition rate is calculated for a variation in methane impurity (10–10 000 ppm), power, pressure, and net mass flow. The simulations conclude that the hydrocarbon plasma deposition can be minimized by reducing methane impurity and excluding solid organic structures.

NO synthesis using plasma catalysis is analyzed in a parallel-plate atmospheric pressure RF plasma from N2/O2 admixed to helium exposed to Fe and Pt catalysts on a SiO2 support. The NO x species are measured by Fourier-transform infrared spectroscopy in a multi-pass cell. The trends in species densities can be well explained by air chemistry reactions, where NO's progressive oxidation occurs with increasing oxygen admixture and ozone generation. The sequence can be controlled by the state of the surface that preferentially quenches O3 and allows for an optimum NO production. The maximum production of NO is found at 70% N2/(N2+O2) mixture ratio at 120 circle C using sandblasted glass, with a conversion rate of 0.085%.

Nanosecond plasmas ignited inside water at tungsten and platinum/iridium electrode tips are used to create very small nanoparticles with radii around 1 nm. Due to the very high power density of 1016 W m-2 at an electrode hot spot with a diameter of 10 mu m, the surface is ablated during the short plasma pulse, and the metal vapour expands in the cavitation bubble after the plasma. This creates a very large cooling rate and the formation of nanoparticles by condensation from the created metal vapour. Finally, the nanoparticles disperse in the liquid. This sequence is quantified by measuring the net tip erosion by shadowgraphy and the created nanoparticles by transmission electron microscopy and x-ray photoelectron spectroscopy. The condensation process is modelled in conjunction with cavitation theory for the expanding cavitation bubble, which shows very good agreement with experimental data.

The in-plasma-catalytic synthesis of ammonia from nitrogen and hydrogen admixed to a helium RF plasma is studied with infrared absorption spectroscopy, optical emission spectroscopy, and through chemical kinetics modeling. Sandblasted glass is used as support for iron, platinum, and copper catalysts up to a surface temperature of 150 circle C . It is shown that the optimum ammonia production occurs at very small N2/(N2+H2) ratios with an increase of concentration with plasma power. The global kinetic modelling agrees well with the data for a variation of the N2+H2 admixture and the absorbed plasma power. The introduction of the catalyst enhances ammonia production by up to a factor of 2. Based on the comparison with the modelling, this is linked to a change in the electron kinetics due to the presence of the catalyst. It is postulated that introducing the catalyst increases the reduced electric field because it reduces the secondary electron emission coefficient. As a result, the dissociation of N2 is stimulated, thereby enhancing the NH3 formation. These experiments show that the impact of the catalyst on the plasma performance in noble gas-diluted RF plasmas can be more important than the impact of the plasma on any catalytic surface process.