Electric Breakdown in Liquids
Discharges in Water and Their Applications

   
 

Breakdown development in water for a 400-μm gap. With positive biased pin, a single streamer bridges the gap in less than 10 ns.
 
Electrical breakdown in liquids has been a research topic for well over a century. While the understanding of insulating liquids often strives to avoid the phenomenon, especially water and other polar liquids are investigated with the goal to take advantage of the mechanism.

One prevalent application is the switching of high voltages. Closing switches that use water as switching medium excel through high hold-off voltages of and fast closing times. Either depends on the geometry of the switch and the hold-off time, i.e. for how long the voltage is applied. Switching voltages as high as 4 MV have been demonstrated and some configurations have shown switching times on the order of 1 ns.

Whereas electron avalanche ionization and streamer mechanisms were accepted within a few decades to explain the breakdown in gases, the nature of the breakdown in dielectric liquids is still disputed. In principle the dispute can be referred to two schools of thought. The first assumes the development of an initial gaseous phase, which then serves as the medium for avalanche development. Consequently, electrical breakdown in liquids is considered a 'shrouded gas discharge'. The alternate hypothesis states that a gaseous phase is not required and impact ionization in the liquid phase itself will result in a charge carrier multiplication, responsible for breakdown.

   
 

Time integrated image of a streamer discharge generated in a water-filled coaxial reactor of 44 mm diameter, 100 mm length, and a tungsten-wire center electrode of 75 μm diameter.
 
A major force, which is driving the interest in streamer discharges in water are the very promising applications for water purification. The potential of streamers for the chemical and bacteriological decontamination stems from multiple interaction mechanisms that attack cells viruses and chemical contaminants alike. Four main agents can be identified, which contribute, depending on target and ambient conditions, to varying degree to the efficacy of the treatment: strong electric fields in particular at the streamer head, free radicals in particular hydroxyl and other oxidative species generated in the plasma, ultraviolet radiation from the plasma, and shockwaves emanating from the discharge channel. How pronounced different mechanisms are, depends on ambient conditions, such as water-conductivity, but in particular also how discharges are generated in the liquid, i.e. operating parameters of the applied high voltage pulses, i.e. pulse duration, pulse rise time and pulse amplitude.

   
 

Removal of different pharamaceuticals dissolved in water for the application of corona discharges that were generated in water by the application of short high voltage pulses.

 
In our research we investigate conditions and mechanisms of electrical breakdown in water for the application of short pulses of nanosecond duration. This regime is of considerable interest in the design of pulse generators for a variety of applications ranging from inertial confinement fusion to the treatment of cancer. At the same time an understanding of the physical response of the medium under extreme conditions of electric field and pulse duration is extremely interesting and challenging. (For example not even the permittivity can be considered a constant any longer.)

Another direction of our research is the use of streamer discharges (i.e. the "progress" of electrical breakdown) in water for water purification and waste water treatment. As foremost physical attack agent, streamers are indiscriminate against biological and chemical agents and can be used against either of them accordingly and simultaneously. The method does further not involve any additional chemicals and is as a non-thermal process (at least theoretically) very efficient.

In particular anthropogenic micropollutants (e.g. pharmaceuticals) are found in increasing concentrations in our drinking water and are raising concern. Obviously, state-of-the-art water treatment methods are not sufficient for their removal. Plasma can provide radicals which are able to break apart even the most recalcitrant substances. Accordingly, we are exploring the respective potential of different plasma methods.

For more publications and details or information check out our publications or send me an email.
Projects

Investigation of TOC-parameters for surface- and wastewater before and after plasma treatment
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Bachelor Research Project, Niklas Barthel, September 2018 - February 2019.

The aim of the project was to evaluate the total organic carbon (TOC) as a summary parameter for the degree of organic pollution of water. The parameter allows a comprehensive evaluation of water qualities without the need of detailed information on the concentration of particular substances, such as pharmaceuticals. Regardless, or rather in particular therefore, is the respective analysis an important guide for any water treatment method. However, the measurement principle is based on an oxidation of the organic matter in the sample and for the nature of a plasma treatment, interaction mechanisms of the species that are generated by the plasma, and remaining in the water, with the measurment is possible. Accordingly, the possibilities of TOC-measurements for the plasma-treated water needed to be investigated. The results show that TOC-measurements can be used to determine degradatio efficiencies of plasma treatments. However, attention needs to be paid to possible reactions of the reactive species generated by the plasma and especially metal ions that could be introduced from the electrodes that were used for the generation of a discharge.

Development of a combination system for the targeted chemo-physical modification of constituents in suspension by ultrasound and cold plasma: Kombi-Max
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Project funding: "im Rahmen der Richtlinie zur Förderung von Forschung, Entwicklung und Innovation des Landes Mecklenburg-Vorpommern", since August 2018.

Partner: Power, Recycling, Energyservice GmbH (PRE), Neubrandenburg; Agrar and Process Technology, University of Rostock.

   
 
Combination of a microwave induced plasma (bottom) and ultrasound treatment (top) in a high conductivity aqueous solution.
 

The aim of the project is to increase the desintegration of biomass in biogas facilities and this way promote the better access of microorganisms (and eventually increase the yield and efficiency of biogas production) by combining ultrasound treatment and plasma exposures. Ultrasound is already successfully used to rupture in particular difficult plant material, however, the desintegration is still not complete. Plasma exposures can offer additional pathways, especially by providing potent reaction chemistries in the biomass suspension. Furthermore it is likely that ultrasound and plasma treatment can mutually enhance the efficacy of the respective individual methods, potentially resulting even in synergies. The plasma in the rather dense and viscous biomass substrate is generated by the delivery and focusing of microwaves, ideally into the ultrasound field, With comparable electrical powers for plasma and generation and ultrasound irradiation, even rather robust substrates can be comprehensivly disintegrated.

"Ultraschall und Plasma als Appetitanreger für Gärbakterien," Süddeutsche Zeitung online (SZ.de), 10. September 2018.

"Forscher wollen Wirtschaftlichkeit von Biogasanlagen verbessern," top agrar online, 27. August 2018.

"Ultraschall und Plasmaverfahren verbessern in Kombination die Aufbereitung der Einsatzstoffe in Vergärungsprozessen," LABO 09/2018, Artikel-Nr. LAB1809.

Initiation and development of sub-microsecond discharges in water and relation to chemical processes
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Doctoral Research Project, Raphael Rataj, since December 2017.

  Raphael Rataj  
 
Raphael Rataj, M.Sc. (Physics), University of Greifswald, 2017.
 

The aim of the project is a better understanding of the initiation and development of discharges that are generated in water by the application of short high voltage pulses of nanosecond durations. For the initiation of particular interest is the dependence of pertinent pulse parameters, such as pulse duration and pulse amplitude. Likewise crucial, and therefore often discussed but rarely systematically investigated, are pulse rise and fall times. Accordingly, pulse parameters and discharge geometry are tightly controlled in the experiments to determine respective inception voltages and discharge developments. Streamer-like filaments that are emerging during the application of a high voltage pulse, are subsquently re-illuminated in a way that strongly depends on pulse fall times with emissions that can be even stronger than observed during the expansion phase of the discharge channels. This phenomenon can be related to the backflow of charges into the filaments. It can be expected that the observed differences of the post-expansion phase characteristics affect the generation of reactive species.

Raphael Rataj, Hans Hoeft, Juergen F. Kolb, "Reillumination of submicrosecond pulsed corona-discharges in water," Plasma Sources Science and Technology (2019) in print. doi:10.1088/1361-6595/ab54e0

Degradation of cylindrospermopsin using advanced non-thermal plasma technologies
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Doctoral Research Project, Marcel Schneider, since May 2017.

  Marcel Schneider  
 
Marcel Schneider, M.Sc. (Pharmaceutical and Chemical Engineering), Beuth Hochschule für Technik, Berlin, 2016.
 

Cyanobacterial toxins are natural toxins that are an increasing thread to drinking water supplies due to rising insitances of cyanobacterial blooms, especially during the hotter summer seasons. The doctoral research is part of the European Training Network (ETN) NaToxAq, which is lead by the University of Copenhagen and dedicated to the study of "Natural Toxins and Drinking Water Quality - From Source to Tap" (https://natoxaq.ku.dk/). Marcel Schneider is a student at the Research Centre for Toxic Compounds in the Environment (RECETOX) at Masaryk University in Brno, Czech Republic, investigating the possibilities to degrade cylindrospermopsin (one of the most prominent represantatives for cyanotoxins) by the application of non-thermal plasmas together with the INP in Greifswald. The focus is in particular a comparison of pulsed dielectric barrier discharges that are operated in air, while contaminated water is sprayed into the plasma, and corona-like discharges that are generated directly in the water. The latter method has already been successful for the abatement of recalcitrent pharmaceuticals in water. Besides degradation efficiencies, depending on operating parameters of the plasma, Marcel has also studied in detail reaction pathways and associated chemistries of the fundamentally different approaches.

Marcel Schneider, Raphael Rataj, Juergen F. Kolb, Ludek Blaha, "Degradation of Cylindrospermopsin using Advanced Non-Thermal Plasma Technologies," Conf. Rec. 16th International Conference on Environmental Science and Technology (CEST 2019), September 4-7, 2019, Rhodes, Greece.

Economic synthesis of contamination-free graphene: PlasGraph
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Project funding: im Rahmen des Förderprogramms 'Zentrales Innovationsprogramm Mittelstand" des Bundesministeriums für Wirtschaft und Energie (BMWi)'", April 2017 - July 2019.

   
 
Plasma generated by nanosecond high voltage pulses in an organic liquid.
 

The aim of the project was the investigation of possibilities for the generation of graphene from liquid precursors. This competitive new approach offers the general possibility for a cleaner and more efficient production of mono- and few layer graphene. Therefore the most suitable organic liquids need to be determined, together with the an appropriate plasma process for the conversion of the precusors. Especially aromatic compounds seem most promising. Depending on the molecular properties of these molecules and the general properties of the respective liquids, a plasma can either be applied at the surface of the solvent or in the liquid directly. Relevant plasma-chemical reactions require further to control oxidation and reduction processes, which can be achieved by a dedicated operating gas atmosphere, i.e. noble gases with and without the admixture of oxygen (or other oxidizing agents), ambient conditions, such as pressure and temperature, and - of course - plasma operating parameters. In particular decidedly non-equilibrium plasmas, instigated by short high voltage pulses, offer possibilities to manage plasma and process parameters.

Optimization of the impulse-method for the application for drinking water treatment: OxIDANT
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Project funding: EFRE Europäischen Fonds für regionale Entwicklung über die Sächsische Aufbaubank, April 2016 - September 2018.

Partner: PICON GmbH, Dresden; Process Engineering in Hydrosystems, Technical University Dresden.

   
 
Treatment of water sprayed into plasma configuration.
 

The aim of the project was to optimize a plasma discharge for drinking water treatment. A non-thermal, i.e. 'cold', plasma is generated by the repetitive application of short high voltage pulses to a dedicated electrode geometry in an air-water mixtures. Therefore the water is sprayed by a nebulizer from the top into the plasma configuration. Highly reactive species that are generated in the plasma can readily mix with the water and attack and degrad undesired organic compounds. Especially hydroxyl radical are known to be relatively non-selective, i.e. preferential, with respect to different pollutants, and react with all of them with high kinetics. As a result they are especially suitable for the remediation of recalcitrant organic substances. However, hydroxyl radicals are rather short lived and hence strategies for their efficient delivery need to be devised. Experiments with respect to water treatment, reactive species that are generated and possible by-products were conducted at the Technical University of Dresden. The INP has developed and build the plasma system for these experiments as a subcontracter.

A. Weigert, S. Schönekerl, S. Uhlig, C. Pfefferkorn, A. Lerch, "Optimierung des Impulsverfahrens für die Anwendung in der Trinkwasseraufbereitung (OxIDANT)," IFAT 2016, Munich, Germany, May 30 - June 3, 2016.

A. Weigert, S. Schönekerl, S. Uhlig, R. Pörschke, K. Wellner, C. Pfefferkorn, A. Lerch, "Untersuchung der Leistungsfähigkeit des Hochspannungsimpulsverfahrens gegenüber der konventionellen Ozonung," IFAT 2018, Munich, Germany, May 14-18, 2018.

Extraction of Bioactive Primary and secondary metabolites from microalgae by atmospheric pressure plasmas and pulsed discharges in water
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Doctoral Research Project, Katja Zocher, August 2015 - December 2019.

  Katja Zocher  
 
Katja Zocher, Dipl.-Pharm. and licensed pharmacist, University of Greifswald, 2015.
 

The aim of the project was the investigation of possibilities for 'gentle' methods for compounds from microalgae. Besides a source of lipids for biofuels, other components are in particular interesting for the pharmaceutical industry and as a nutritional supplement. The problem of microalgae is their robust cell wall and the rather high energy demand of available extraction methods. Therefore alternative approaches were investigated, such as pulsed electric field exposures but especially different plasma methods. Among the latter have spark discharges, that are generated directly in the algae suspension by the application of short high voltage pulses of 100 ns, demonstrated to effectively damage cell walls and promote the release of cell contents. Arguably more important is also the possibility to control the temperature increase in the suspension with this method and prevent the degradation of heat sensitive compounds. The responsible mechanism that was identfied are strong shockwaves, that result in mechanical damage and allows to optimize the method accordingly. Nevertheless, the plasma treatment will also generate different reactive species, which might react with the extracted compounds. Therefore also a careful study was conducted on a possible oxidation, nitrification or any other presumable change of different proteins of interest.

Katja Zocher, Jan-Wilm Lackmann, Johann Volzke, Leif Steif, Michael Lalk, Klaus-Dieter Weltmann, Kristian Wende, Juergen F. Kolb, "Profiling microalgal protein extraction by microwave burst heating in comparison to spark plasma exposures," Algal Research 39 (2019) 101416. doi:10.1016/j.algal.2019.101416

Katja Zocher, Robert Banaschik, Christian Schulze, Tilo Schulz, Jana Kredl, Camelia Miron, Michael Schmidt, Sabine Mundt, Wolfgang Frey, Juergen F. Kolb, "Comparison of extraction of valuable compounds from microalgae by atmospheric pressure plasma and pulsed electric fields," Plasma Medicine 6 (2016) 273-302. doi:10.1615/PlasmaMed.2017019104

Pulsed plasma in water for the remediation of pharmaceutical residues: PlasWas
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Project funding: BMBF als "Wissenschaftliches Vorprojekt", April 2015 - March 2017.

   
 
Corona-like discharges generated in water by the application of rectangular pulsed high voltages.
 

The aim of the project was to optimize a plasma discharge for drinking water treatment. A non-thermal, i.e. 'cold', plasma is generated by the repetitive application of short high voltage pulses to a dedicated electrode geometry in an air-water mixtures. Therefore the water is sprayed by a nebulizer from the top into the plasma configuration. Highly reactive species that are generated in the plasma can readily mix with the water and attack and degrad undesired organic compounds. Especially hydroxyl radical are known to be relatively non-selective, i.e. preferential, with respect to different pollutants, and react with all of them with high kinetics. As a result they are especially suitable for the remediation of recalcitrant organic substances. However, hydroxyl radicals are rather short lived and hence strategies for their efficient delivery need to be devised. Experiments with respect to water treatment, reactive species that are generated and possible by-products were conducted at the Technical University of Dresden. The INP has developed and build the plasma system for these experiments as a subcontracter.

A. Weigert, S. Schönekerl, S. Uhlig, C. Pfefferkorn, A. Lerch, "Optimierung des Impulsverfahrens für die Anwendung in der Trinkwasseraufbereitung (OxIDANT)," IFAT 2016, Munich, Germany, May 30 - June 3, 2016.

A. Weigert, S. Schönekerl, S. Uhlig, R. Pörschke, K. Wellner, C. Pfefferkorn, A. Lerch, "Untersuchung der Leistungsfähigkeit des Hochspannungsimpulsverfahrens gegenüber der konventionellen Ozonung," IFAT 2018, Munich, Germany, May 14-18, 2018.

Nanosecond high voltage discharges in liquids for polymer modifications
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Postdoctoral Research Project, Dr. Camelia Miron, July 2013 - May 2015.

  Dr. Camelia Miron  
 
Dr. Miron at her experimental setup for a newspaper interview.
 

The aim of the project was to invetigate the possiblities of nanosecond discharges in liquids, primarily aqueous solutions, for the modification and synthesis of new materials. Experiments were focusing in particular on the application of very short high voltagepulses with durations of either 10 ns or 1 ns, and amplitudes of up to of 150 kV or 200 kV, respectively. Novel mechanisms and interactions are associated with this extreme conditions. In organic liquds, few-layer graphene structures could be synthesized and in aqueous electrolytes polymers could be effectively modified, i.e. functionalized. For polyimide films, treated in the vicinity of the discharge, an increase in the degree of inter-molecular ordering of the polymer was observed that could held responsible for a decrease of the relative permittivity. Concurrently, did the hydrophoicity of the polyimide films increase. The mechanisms of surface modification were notable different from previousl described interactions with plasmas that wer generated with microsecond high voltage pulses.

Ion Sava, Asandulesa Mihai, Katja Zocher, Angela Kruth, Juergen F. Kolb, Wiktor Bodnar, Kerstin Witte, Takahiro Ishizaki, Camelia Miron, "Electrical and mechanical properties of polyimide films treated by plasma formed in water and isopropanol," React. Funct. Polym. 134 (2019) 22-30. doi:10.1016/j.reactfunctpolym.2018.11.005

Ion Sava, Angela Kruth, Juergen F.Kolb, Camelia Miron, "Optical properties of polyimides films treated by nanosecond pulsed electrical discharges in water," Japan. J. Appl. Phys. 57 (2018) 0102BF. doi:10.7567/JJAP.57.0102BF

Liviu Sacarescua, Mihaela Simionescua, Gabriela Sacarescua, Antje Quade, Juergen F.Kolb, Camelia Miron, "Nanostructuration of polysilane-SiQDs composite by pulsed electrical discharges in water," React. Funct. Polym. 120 (2017) 38-45. doi:10.1016/j.reactfunctpolym.2017.09.005

Camelia Miron, Jie Zhuang, Ion Sava, Angela Kruth, Klaus-Dieter Weltmann, Juergen Kolb, "Dielectric Spectroscopy of Polyiamide Films Treated by Nanosecond High Voltage Pulsed Driven Electrical Discharges in Water," Plasma Processes and Polymers 13 (2015) 253-257. doi:10.1002/ppap.201400170

Camelia Miron, Camelia Hulubei, Ion Sava, Antje Quade, Anna Steuer, Klaus-Dieter Weltmann, Juergen Kolb, "Polyimide Film Surface Modification by Nanosecond High Voltage Pulse Driven Electrical Discharges in Water," Plasma Processes and Polymers 12 (2015) 734-745. doi:10.1002/ppap.201500037

Degradation of pharmaceutical residues by pulsed corona discharges generated directly in water
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Doctoral Research Project, Robert Banaschik, February 2013 - November 2017.

  Robert Banaschik  
 
Robert Banaschik, Dipl.-Pharm. and licensed pharmacist, University of Greifswald, 2017. (Robert doesn't like his picture shown.)
 

The aim of the project was the study of degradation mechanisms of pharmaceutical residues in water for the treatment with corona-like discharges that were directly generated in the liquid. Therefore, an extended rather 'cold' plasma was generated by the application of high voltage pulses of sub-microsecond duration to a thin wire at the center of a coaxial geometry. Depending on the operating parameters, in particular water conductivity and pulse amplitude, the entire volume can be penetrated by a densly distributed plasma filaments. The potential of the method for the abatment of pharmaceuticals that are often found in water and was shown for carbamazepine (anti-epilepsy drug), diatrizoate (X-ray contrast agent), diazepam (anxiety medication), diclofenac (pain medication), ibuprofen (pain medication), ethinylestradiol (contraceptive) and trimethoprim (antibiotic). With the plasma that is generated from the water, especially high concentrations of hydroxyl radicals were produced. These were found to be responsible for an efficient oxidation of even persistent compounds, such as X-ray contrast agents. This was shown in detail for the degradation of diclofenac by investigating degradation products and their subsequent reactions with hydroxyl radicals. Accordingly, the entire degradation cascade, i.e. all intermediates until the complete mineralization of the initial compound could be determined together with the respective reaction pathways. Notable, reactions with ozone are not relevant, which was actually not observed for the submerged results. However, in addition to the reaction with directly formed hydroxyl radicals also post-discharge chemistries, i.e. Fento reactions, which could split up hydrogen peroxide again into hydroxyl radicals, were recognized as an important - so far neglected - mechanism for the plasma treatment. Accordingly, the choice of electrod materials is a crucial factor for the optimization of the method in addition to operating parameters.

Robert Banaschik, Helena Jablonowski, Patrick J. Bednarski, Juergen F. Kolb, "Degradation and intermediates of diclofenac as instructive example for decompostion of recalcitrant pharmaceuticals by hydroxyl radicals generated with pulsed corona plasma in water," J. Hazardous Materials 342 (2018) 651-660. doi:10.1016/j.jhazmat.2017.08.058

Robert Banaschik, Petr Lukes, Camelia Miron, Richard Banaschik, Andrei V. Pipa, Katja Fricke, Patrick J. Bednarski, Juergen F. Kolb, "Fenton chemistry promoted by sub-microsecond pulsed corona plasmas for organic micropollutant degradation in water," Electrochimica Acta 245 (2017) 539-548. doi:10.1016/j.electacta.2017.05.121

Robert Banaschik, Petr Lukes, Helena Jablonowski, Malte U. Hammer, Klaus-Dieter Weltmann, Juergen Kolb, "Potential of pulsed corona discharges generated in water for the degradation of persistent pharmaceutical residues," Water Research 84 (2015) 127-135. doi:10.1016/j.watres.2015.07.018