AVS2014 Session PS+SE-ThA: Atmospheric Pressure Plasma Processing; Fundamental and Applications

Over the last decade there have been considerable advances in the utilization of atmospheric pressure cold plasmas for thin film deposition. Intense research efforts have been made to develop a large variety of processes which exploit different types of atmospheric pressure discharges for the direct and remote deposition of thin films from monomers in gas, vapor and aerosol form [1]. However diagnostic studies of the plasma chemistry and mechanistic investigations of thin film growth are still scarce, while being crucial prerequisite for further process optimization. The research should be addressed to detect the film precursors, to identify the main reaction pathways (both homogeneous and heterogeneous processes) and to clarify the plasma-surface interaction.

In this contribution, our studies on thin film deposition by dielectric barrier discharges (DBDs) will be presented to provide insights into the chemistry of atmospheric pressure plasma processes. Examples discussed will include the investigation of the role of the monomer chemical structure in the plasma-enhanced chemical vapor deposition of organosilicon coatings from different methyldisiloxanes [2], and the study of the influence of air and water vapor feed gas impurities in the deposition of fluorocarbon coatings from argon-hexafluoropropene fed DBDs [3]. Our recent work on the deposition and characterization of organic-inorganic hydrocarbon polymer/ZnO nanoparticles nanocomposite coatings by a DBD fed with helium and the aerosol of a dispersion of oleate-capped ZnO nanoparticles in hydrocarbon solvents will be presented [4]. Results from the plasma jet co-deposition of acrylic acid and ethylene towards water-stable coatings containing carboxylic functionalities will be also shown.

The overall deposition mechanisms will be outlined on the basis of the results from the chemical and morphological characterization of the coatings, the optical emission spectroscopy investigation of the plasma phase, the analysis of the exhaust gas by means of gaschromatography-mass spectrometry. The latter is a powerful indirect diagnostic technique of the gas phase which allows the assessment of the monomer depletion and the quali-quantitative determination of stable byproducts formed by plasma activation.

[1] F. Massines, C. Sarra-Bournet, F. Fanelli, N. Naudé, N. Gherardi, Plasma Process. Polym., 9, 1041 (2012).

[2] F. Fanelli, S. Lovascio, R. d’Agostino, F. Fracassi, Plasma Process. Polym., 9, 1132 (2012).

[3] F. Fanelli, R. d’Agostino, F. Fracassi, Plasma Process. Polym., 8, 557 (2011).

[4] F. Fanelli, A. M. Mastrangelo, F. Fracassi, Langmuir, 30, 857 (2014).

The interaction of atmospheric-pressure plasma jets with liquids is becoming increasingly important for medical and materials applications. Despite empirical evidence of reactions occurring both in the bulk plasma and liquid phases, a basic understanding of the chemistry, particularly at the interface of the plasma and liquid, remains poorly understood. Previous studies have shown that species produced in the plasma phase such as nitric oxide (NO) and hydroxyl radicals (OH) can solvate in the liquid, yielding products such as nitrous acid (HNO₂) and hydrogen peroxide (H₂O₂) in the bulk solution [1]. In addition, we have recently shown that charge can be transferred from a DC microplasma jet into an aqueous solution to promote electrolytic reduction reactions [2]. However, it remains unclear how these charge transfer reactions occur and, in particular, if plasma electrons solvate in the liquid before subsequently reducing solution species.

To clarify the role of electrons solvating at a plasma-liquid interface, we have designed and built an experiment to detect solvated electrons using optical absorption spectroscopy. Electrons solvated in aqueous solutions are well known to absorb strongly in the red. Initial models suggest that because of the short lifetime of solvated electrons in aqueous solutions (~1 μ s) and anticipated solvation depths on the order 10 – 100 nm, optical absorption will be on the order of one part in 10⁶, making this an inherently challenging measurement. In this presentation, we will give an overview of our experimental method and present preliminary findings on direct measurements of electron solvation.

[1] P. Rumbach, M. Witzke, R. M. Sankaran, and D. B. Go, J. Am. Chem. Soc. 135 16264-16267 (2013).

[2] M. Witzke,P. Rumbach, D. B. Go, and R. M. Sankaran, J. Phys. D: Appl. Phys. 45 442001 (2012).

Our solution uses a getter material to readily sequester oxygen as surface oxides. A non-thermal, atmospheric pressure plasma then combines hydrocarbons in the tank with the getter to reduce the oxide and reverse the process to form water vapor. No consumable inerting product is used. Experimental data shows the getter absorbs 20 times its volume in oxygen before saturation. Scaling up this reaction would inert a 500 gallon fuel tank in 11.4 minutes with one liter getter at standard temperature and pressure conditions. Time-to-inert decreases at higher altitudes due to lower pressure. Time-to-inert curves were calculated for the typical aircraft flight envelope based on this model. We have successfully demonstrated the feasibility of our inerting system in a laboratory environment. Our inerting system is highly selective to oxygen, and eliminates the membranes and high power pumps used in existing systems. The next step will be to evaluate a prototype system through demonstration testing on the replica of a military aircraft fuel tank system. A customized plasma source will be developed optimized in size and weight competitive with aircraft applications.

Optical emission spectroscopy, (infrared) absorption spectroscopy along with rotational Raman scattering were employed to study the CO₂ conversion and deduce gas temperatures in mid-frequency (kHz) driven DBDs at elevated pressures. To study the kinetics of CO formation the DBD was additionally operated in pulsed mode. Absolute densities of CO, O₂ and O₃ were established downstream the plasma reactor. The CO yield was typically below 5% for gas flow rates that would allow reasonable throughput. The generally weak emission of electronically excited species (CO_2⁺, CO) was monitored during individual AC cycles. Additionally, phase- and time-resolved signals of ro-vibrational absorption lines of CO and CO₂ in their (electronic) ground state were detected by quantum cascade laser absorption spectroscopy. In contrast to emission, the CO absorption remained constant throughout individual AC cycles. The modulation of CO absorption signals during pulsed operation can be successfully modelled by considering the power density of the DBD, the reactor wall and gas temperatures (up to 550 K), and the residence time of the gas along with the reactor geometry.

The results suggest electron-impact CO₂ excitation and ionisation followed by potentially surface enhanced recombination. The stoichiometric CO:O₂ ratio is described by a uniform trend as function of the number of charges transferred during the residence time of CO₂ in the active plasma zone.

In atmospheric plasma processes, the use of a main gas, usually considered as inert, to sustain the plasma is common. Nitrogen, argon and helium are the most regularly used gases. If the nature of the gas has a strong influence in the breakdown discharge voltage (due to Paschen Law), it also leads to plasmas which are optically and electrically different. This has drastic consequences on the chemistry and structure of plasma deposited coatings and on the chemistry of plasma treated gases. In this study, we compare, and try to explain, the results obtained on different topics using either argon or helium as main plasma gas.

All experiments have been conducted in dielectric barrier discharge reactors, powered with sinusoidal AFS generators, operating at frequencies lower than 40 kHz. For all the studies the reactors have been pumped down to evacuate contaminations and then backfilled with the plasma gas (Ar or He). The precursor is then injected in the DBD through the appropriate flow of the carrier gas.

Internal chemistry of coatings :

Through a combined XPS, FTIR and SIMS study, it is shown that the degree of cross-linking, the branching and the number of insaturations of plasma-polymerized polystyrene or sulfonated polystyrene (for fuel cell applications) is higher with argon than with helium, whereas the density of aromatics exhibits the opposite behavior.

Surface roughness of coatings

Similarly, for all the organic coatings synthesized by a dielectric barrier discharge (PS, CFx from C₆F₁₂, PA), the roughness, determined by AFM is always higher with argon than with helium.

Conversion of CO₂ by DBD

Finally, the conversion rate of CO₂ into CO in atmospheric plasma conditions (DBD), is higher when argon is the main plasma gas than with helium.

It is suggested that all these effects are strongly related to the plasma operating mode and to the energy distribution inside the plasma.

This work is supported by PSI-IAP 7 (plasma surface interactions) from the Belgian Federal Government BELSPO agency

The maximum rotational temperature was approximately 3000 K, which was observed in the plasma column in contact with the electrolyte surface. The frequency of collisional quenching was also high in the contacting region to the electrolyte surface, suggesting that the dominant quencher of the excited state of OH was water vapor. The absolute density of water vapor was evaluated from the quenching frequency and the rate coefficient of collisional quenching. It was found that the partial pressure of water vapor was more than five times higher than the saturated vapor pressure of water at room temperature.

The OH radical density was high in the neighboring region to the electrolyte surface, suggesting that water vapor produced from the electrolyte is the parent of OH radical. However, the peak of the OH radical density was separated from the electrolyte surface, and also the OH radical density on the electrolyte surface was not zero. It is noted that, if OH radicals are not reactive on the electrolyte surface, the decrease in the OH density toward the electrolyte surface would not be observed. On the other hand, if OH radicals are too reactive in the gas phase, the OH radical density on the electrolyte surface would become zero (Note that the production of OH in the cathode sheath is negligible), and in this case we cannot expect the interaction of OH radicals with the electrolyte. The axial distribution of the OH density observed in this work indicates that the electrolyte surface interacts with OH radicals as their sink. In other words, OH radicals are lost on the electrolyte surface. A possibility of the loss process is recombination to form hydrogen peroxide and water. In addition, we can also expect the transport of OH radicals into the inside of the electrolyte.

Low temperature plasmas are emerging as an exciting development for therapeutics. The unique properties of cold non-equilibrium plasmas have enormous potential in disease therapeutics and plasma pharmacology as drug alternatives. Applications of these plasmas include surface sterilization and bacterial decontamination, biofilm inactivation, antimicrobial treatment in food preservation, wound healing, to cancer treatment.

Non-equilibrium plasmas, operated at ambient atmospheric pressure and temperature, are very efficient sources for highly reactive neutral particles e.g. reactive oxygen and nitrogen species (RONS) (such as atomic oxygen, atomic nitrogen, hydroxyl radical, superoxide, singlet delta oxygen, nitrogen oxides), charged particles, UV-radiation, and electro-magnetic fields. Individually many of these components have been implicated in therapeutics. RONS are known to play a crucial role in biological systems, such as signalling, and generating oxidative damage to a variety of cellular components, which can ultimately lead to cell death. Plasmas have the advantage of delivering these simultaneously providing potentially superior processes.

Transport of the plasma components to the target is complex. In the core plasma production region a large, but defined, number of species can be created (including for example O, N, NO, O₂^-). As the plasma interacts with ambient air new reactions and components are formed. Upon interaction with either humidity or liquid layers on biological samples new species of varying lifetimes can be created (e.g. OH, H, H₂O₂, ONOO^-). Energy dissipation at these interfaces is important and to date unclear.

In order to understand the chemical kinetics and plasma-liquid-biological interaction mechanisms measurements of the relevant RONS are key. Measurements and simulations under this atmospheric pressure environment are challenging, primarily due to the multi-phase (solid, liquid, gas and plasma), strongly non-equilibrium with large gradients (e.g. in electric field), high collisionality, thus requiring extremely high temporal (picosecond to nanosecond) and spatial (microns) resolution.

Two advanced optical diagnostic techniques are applied, and will be presented, to measure absolute radical densities: Pico-second two-photon absorption laser induced fluorescence (ps-TALIF) and high-resolution synchrotron VUV absorption spectroscopy will be presented. Radicals measured in an atmospheric pressure plasma operated in helium with varying admixtures of oxygen, nitrogen and water vapour will be presented.

The increase in global emissions of carbon dioxide (CO₂) due to fossil-fuel combustion and other energy-related human activities is strongly related with the global warming issue. Due to this effect, the study of CO₂ dissociation process is a highly demanding topic, which requires attention and efficient solutions. Among the proposed strategies, one possible way to limit CO₂ emission is to use plasma technology to break the CO₂ molecule into oxygen (O) and carbon monoxide (CO), which can be later used for production of valuable chemicals, e.g. for the fuel synthesis (syngas).

Recently, energy-efficient powerful plasma-chemical systems have been developed based on microwave plasmas. These discharges are often generated using electromagnetic waves in the range of 300 MHz to 10 GHz, and can operate over a wide range of conditions of pressure and power. In particular, microwave surfaguide discharges (MSGD) where the gas flow in the discharge tube is perpendicular to the waves propagation, are proven to be good solution for an efficient molecular dissociation of di- or multi-atomic species [1].

In this work, the recent results on CO₂-containing MSGD operating at 0.9 GHz and 2.45 GHz in pulsed and continuous regimes are presented. The plasma is sustained in CO₂ and CO₂+N₂ flowing gas mixtures in a quartz tube crossing a copper waveguide. The microwave discharges are characterized in terms of various plasma parameters using various optical emission spectroscopy (OES) methods. In particular, the characteristic plasma temperatures, such as the gas temperature (via CO rotational spectra), and vibrational temperature (via N₂ vibrational bands) are determined as a function of time at different axial positions along the gas flow in the discharge tube. The CO₂ conversion rate in the discharge volume, along with the measurements of plasma energetic efficiency of such conversion is performed by means of optical emission actinometry. The results of the time- and space- resolved OES measurements demonstrate a non-uniform dissociation rate of CO₂ along the gas propagation direction in the discharge. As a result, the dissociation degree can be substantially modified by varying the power balance and the composition of the gas mixture [2]. In addition, in order to get a complete picture of the process, gas chromatography measurements were performed in the post discharge region. The obtained integrated results allow better understanding the µwave plasma-based dissociation of CO₂.

[1] T. Godfroid, J. P. Dauchot and M. Hecq, Surf. Coating Technol 174-175 1276-1281 (2003)

[2] T. Silva, N. Britun, T. Godfroid, R. Snyders Plasma Sources Sci. Technol 23 025009 (2014)