Dr. Moritz Oberberg and two team colleagues are preparing to set up the spin-off “House of Plasma”. The company is launching the Multipole Resonance Probe in various designs. It allows industrial plasmas to be monitored in real time to ensure that they help to consistently apply identical, faultless coatings to surfaces, for example.

Mr. Oberberg, if someone had told you five years ago that you would plan to start a company today, what would you have said?
The idea of setting up a spin-off with measurement technology is not that new. My predecessor, who completed his doctorate in 2015, was already talking about it, but it wasn’t fully developed at the time. Personally, I could always picture myself being self-employed. The timing depends, of course, on the development and the response. Still, I’ve been thinking about self-employment for a while. I have a background in this field and, in addition to my education, I also have the necessary contacts.

The company has not been established yet. How do you keep busy at the moment?
Well, I’m not alone, the three of us share all tasks that come up. We are currently planning industrial tests with a company for our latest designs – we have to travel abroad for this purpose. There’s a lot of red tape. It’s not so easy to take measuring equipment out of the country.

Other than that, I’m busy pushing the development of our products and writing project proposals. I should mention that RUB offers invaluable support with the Worldfactory Startup Center. It has a team of excellent people who help us with everything. At the moment, for example, we are working out how to acquire patents held by RUB. Articles of incorporation are still to be written before we can set up the company, the financial plan is being finalised, and the search for investors is also underway. Before we can sell something, we have to be able to purchase, for which we need capital; the same is true for marketing, premises, equipment and so on. In the current funding project at the university, tests continue in the laboratory and with industrial partners, under difficult conditions due to the pandemic, and the technology is constantly being optimised. This is all going on simultaneously.

What can prospective customers expect from you?
Our prospective clientele might include, for example, a company that coats spectacle lenses. We would check with them what their expectations are and whether our technology offers added value for their company. Can it be used? If yes, then our hardware in combination with the relevant software can provide insights that were not there before.

We compare it to a breakfast egg: I like it when the yolk is still soft but the egg white is hard. However, while it’s cooking, I can only rely on my experience, not really measure what’s going on inside the egg. At most, a peripheral measurement of the temperature on the shell would be possible, but not inside the egg. What we do now, however, is supply the data straight from the egg.

adapted from Meike Drießen (RUB)

To control plasma-produced carbon monoxide and use its anti-inflammatory effects on wound healing is the aim of a cooperation between Professor Judith Golda from Ruhr University Bochum, Germany, and Professor Claire Douat from Université d’Orléans, France. In six mutual visits their groups experimented together, financed by the German Academic Exchange Service and by Campus France.

Ms. Golda, what would you like to achieve?
Judith Golda: Today plasma is not yet a well-understood, efficient and easy tool in medicine, I would say. We hope that in the future doctors can simply use a plasma-source to cure diseases of the skin.

For which medical applications is plasma already used?
Golda: A few hospitals in Germany are utilizing it for wound healing within clinical studies. Moreover, there are surveys for cancer therapy, sterilisation…

Claire Douat: … the treatment of teeth, skin diseases like psoriasis and acne, for cosmetics and blood coagulation, to avoid or reduce a bleeding.

How exactly does plasma work in wound healing?
Golda: There are different effects. One of them is sterilisation, another blood coagulation. Plasma can also increase the blood flow in the wound. And it can work anti-inflammatory by producing reactive species that resemble natural messenger substances.

In what way do your teams complement each other?
Golda: In France, the plasma source is a little bit different from ours. It produces more charged species in the vicinity of the treated substrate and a different mixture of chemical species, for example. On the one hand, we compare the plasma sources, on the other hand we exchange complementary diagnostics: In Orléans they use optical sensors for measuring the gas density – such as the one for carbon monoxide, in Bochum we employ mass spectrometry.

Why is it difficult to prove the impact of a single plasma component?
Douat: In a plasma there are many chemical reactions happening simultaneously. We have the electric field, reactive oxygen species and the light. So, it is not only difficult to explore but also to control it. At some concentrations those reactive species are beneficial, but at higher ones they can become toxic. In sterilisation or cancer treatment we want this toxicity, but in wound healing we don’t.

What is characteristic of the cold atmospheric pressure plasma you use for the treatment of biological substrates?
Golda: Unlike other plasmas it stays cold enough not to harm the skin. The idea is to channel the energy we put in the plasma into the electrons to heat them. In this condition they are able to dissociate molecules and create reactive species while the gas remains close to room temperature. That is this non equilibrium character that everybody is talking of.

Which substrates do you use for your experiments?
Douat: Right now, we are utilizing bacteria. The next step will be on cells and afterwards on animals. But before, we need to make sure that the treatment is safe, for example that the electric current through the biological target is not too high.

What could you find out until now?
Douat: We performed electrical measurement and determined the concentration of some species in the different plasma-sources. For that we have one from Bochum in Orléans and they have one from us in Bochum. In the end, we treated bacteria to see the effects of the two plasma-sources: the bacteria didn’t behave in the same way when treated with diverse plasmas. Which is nice, because now we have a lot of science to discuss and are writing a paper about it.

Ms. Douat, what is your impression of Ruhr University?
Douat: I really like being in Bochum. Every time I come here, it’s easy. There is all this infrastructure with the hotel and everything I need in walking distance to university – including supermarkets and even a swimming pool.

From my point of view, when we are every day in the same place, we see things not the way we see them as a visitor. In Bochum they are well organised and there is always room for ease. Also internationally, Ruhr University is a very well-known place for plasma research, with leading plasma and modelling experimental physicists.

It’s fine to be here and to discuss with them. In addition, they have a big pack of experimental setup. I would not say everything, but a lot. When we want to test something, there is always a way to make it possible.

Golda: Also, the mensa has vegan and vegetarian options. One of the French students is vegetarian …

Douat: … and the view is great!

What are the advantages of your cooperation?
Golda: It’s a lot of fun. I like co-operating with Claire and her group. When you are working on a topic on your own, from time to time you get stuck on your ideas. In a cooperation we can discuss the issues and develop new ideas together. Sometimes it’s really inspiring to be in a new environment like another lab or university to get new ideas. In addition, Claire and her team have some expertise we don’t have.

Douat: It’s always fine to discuss and see another point of view. The students also work together and learn from each other. That’s a very good experience for them.

Golda: You really see how they grow. The first time the students go to France they are rather shy and seem kind of lost. The second time it’s like they feel at home there. The relationships between them change a lot and it’s awesome to watch them meeting at the weekend and spending free time together.

Douat: When we come to Bochum or our colleagues visit us in Orléans, it’s a bit like holidays, at least for Judith and me. Not in the sense that we wouldn’t work. We work a lot when we travel. But in that time, we have only one task to do, so we can focus on it. At home, we are busy with so many different things every day. Right now, we enjoy just to be at the laboratory, the whole day working on one project.

adapted from Carina Huber (RUB)

In ten years, researchers will have understood the interactions between catalysts, which determine the speed of chemical reactions, and plasmas. This will facilitate the excitation of the plasma at atmospheric pressure in such a way that its properties accelerate the reactions on the catalyst surface in a controlled manner. As a result, chemical engineers will not only increase the turnover of the starting materials, but also the percentage of these materials that will be converted into the required product.

Therefore, the vision is that plasma generators will control catalytic processes. New compact plasma catalyst modules will be created, through which large gas flows can pass with little pressure drop. This will enable exhaust gas streams to be purified and other important industrial reactions to be carried out. In order for the modules to work in a resource-saving way, researchers still have to boost their energy efficiency. In future, catalysis, plasma and reaction engineering experts will work hand in hand to develop plasma catalyst modules. Computer-aided plasma, velocity and flow simulations will help to optimise them.

by Martin Muhler

When you add energy to gases or gas mixtures, a plasma can be created, and inside it things go haywire: atoms turn into ions, free electrons whiz through space and collide with everything, some ingredients decay, other substances form anew. Depending on what is added to the starting material, plasmas can therefore be used to produce larger compounds. Hydrocarbons and silicon hydrogens are turned into long chains of molecules called polymers.

“If you want to etch with plasmas that tend to form polymers, it’s bad because the nanoparticles that form are a hindrance,” explains Professor Peter Awakowicz, holder of the Chair of General Electrical Engineering and Plasma Technology at RUB. But his team has taken advantage of the situation. If polymers are specifically made to form and deposit on the surfaces surrounding the plasma, they can be coated in a targeted manner. Thanks to this so-called Plasma Enhanced Chemical Vapor Deposition, or PECVD for short, it is possible, for example, to apply ultra-thin, gas-tight coatings to the inside of PET bottles, ensuring that the contents last longer, or to protect organic light-emitting diodes (OLEDs) from moisture so that the TV screens work for a long time. This and much more is only possible because the plasmas are cold and thus do not damage the PET bottle or other surfaces to be coated with heat. Only the fast electrons in the plasma are hot, and they do not damage the surfaces.

Making milk and medicines last longer

The glass-like coating of the plastic, which is only 20 to 30 nanometres thin, ensures that 10 to 100 times less gas escapes through the bottle. This extends the shelf life of a soda pop from the previous four weeks to about a year. The method is also of interest for the packaging of milk and other foods, as well as medicines and even microelectronic components.

“This type of coating is also environmentally friendly, because the tiny amount of material can simply be neglected during recycling,” explains Dr. Marc Böke from the Experimental Physics II department at RUB. Composite materials made of plastic and aluminum, such as Tetrapaks, are far more difficult to recycle because it is very difficult to separate the components.

Other applications of the PECVD method can be, for example, the coating of implants that grow into the bone better than conventional ones. There are also many microelectronic applications. For example, transistors can be deposited with ultrathin silicon dioxide films in plasma.

Oxygen tips the scales

The challenge lies in controlling the formation of the layers. “The layers should not only be ultra-thin, but also absolutely dense, gap-free and uniform,” explains Marc Böke. The adjusting screws for this are manifold. For one thing, it depends on the gas mixture. Atomic Oxygen is a particularly important player. Its proportion can be used to control, among other things, whether other additives evaporated into the plasma form inorganic layers, such as the glass-like silicon dioxide, or organic layers that have other interesting properties, such as giving surfaces greater biocompatibility or enabling gas separation.

The pressure at which the plasma is operated is also significant. Higher pressures and corresponding gases result in the coating of surfaces, while lower pressures are more likely to result in etching processes, which are central to all microelectronics (from cell phones to modern cars). Similarly, the geometry of the reactor and the choice of energy source influence what happens in the plasma and how it affects the surrounding surfaces. For example, an appropriate plasma can be ignited by microwaves, but also by inductively or capacitively coupled radio frequency. “In general, different sizes of plasma reactor are possible, up to the huge dimensions needed to coat entire window panes for high-rise buildings,” says Peter Awakowicz. These coatings serve to reflect infrared radiation that would otherwise cause it to get as hot behind them as in a greenhouse when the sun shines. But you can still see through it. With the sputtering of thin metal layers on foils used for this purpose, it is also possible to work in a feed-through process and thus coat many square metres.

Measurement techniques had to be developed

Only after the basic mechanisms of high-power pulsed sputtering (HiPIMS) and PECVD had been measured and understood in the first phase of the Collaborative Research Center SFB/TR 87 the research teams could get down to the business of implementing such large-area coatings. “We had to develop the appropriate measurement techniques in some cases,” Awakowicz recounts. “If you simply hold a measuring probe in the plasma, it may become coated itself and may lose its function,” he gives an example.

The researchers have gradually been able to fathom and perfect many aspects of the possible processes. For example, PET bottles are cleaned and activated before coating, also by means of plasma. But here, too, the surface of the bottle changes, which in turn influences the subsequent coating. Measurements of the particle flows during cleaning revealed what happens in the process: is wettability increased? And if so, how? Does the surface energy change? At what point in the treatment does the surface become roughened? “If the surface is too rough, you can no longer cover it evenly with an ultra-thin coating,” explains Marc Böke. If all these aspects are taken into account during cleaning and the process is run optimally, this has a considerable influence on the success of the subsequent coating: “We were able to increase the impermeability, which was initially a factor of 100 (depending on the substrate material), to a factor of 500 through the correct setting of the previous cleaning,” says Peter Awakowicz.

Keeping plasticizers away from food

Detailed knowledge of the processes in the plasma and the resulting coatings now also make it possible to coat stretchable films with gas-tight thin films. Thanks to an intervening buffer layer, Marc Böke’s team was able to increase the tolerance of the layer to the stretching of the film from originally about three to about six percent. This application is also of interest to the food industry, for example, as the dense coating prevents ingredients from the film, such as the dreaded plasticizers, from penetrating the food.

The latest application, which is currently being worked on, makes a virtue out of necessity: if one actually wishes for layers that are as dense and defect-free as possible, defects such as tiny pores in the coating are almost impossible to avoid. They allow the research teams to use plasma coating to develop non-swelling filter membranes that exhibit previously unknown properties. They can desalinate water or separate gases from each other, such as oxygen from CO₂. “Normally, the more selective a membrane is, the lower its transmission, i.e. the more inefficient the process,” explains Marc Böke. “With plasma coating, however, we can control pore formation so that selectivity no longer comes at the expense of transmission or efficiency.” The researchers of the SFB/TR87 can simulate and tailor the polar properties of the membrane. This makes it easier for certain molecules to pass through the membrane. “Water molecules, for example, are made to give up their actual angle, practically flattening out and thus sliding through the pore,” Peter Awakowicz describes. “You couldn’t target something like that before.”

adapted from Meike Drießen (RUB)

Reactive oxygen and nitrogen species (RONS) have different functions in biological contexts: At low concentrations they act as signalling substances, for example in wound healing. At high concentrations they destroy biomolecules, an effect immune cells use to kill pathogens. Non-equilibrium plasmas, in which the electrons have high temperatures but the gas temperature remains low, can produce such RONS without heating the treated samples. These non-thermal plasmas are already being used for sterilisation purposes. Their therapeutic use in wound treatment and cancer therapy is currently being explored.

In ten years, we will understand in more detail the mechanisms underlying the generation of the different reactive particles in plasma as well as their biological effects. Based on this knowledge, plasma reactors can then be designed that provide RONS at concentrations and with mixing ratios required for specific applications – such as reactions catalysed by enzymes that utilise plasma-generated species or the degradation of pollutants through the joint action of plasmas and microbes.

by Julia Bandow (RUB)

Plasmas have been used in industry for decades with great success. However, in many cases their application is based on trial and error. A deeper understanding of the processes that take place in the excited gases, sometimes in a very short time and on tiny length scales, has been largely lacking until now. Researchers from Bochum and Ulm are venturing into this new territory together in the frame of the Collaborative Research Centre 1316 “Transient Atmospheric Pressure Plasmas: From Plasmas to Liquids to Solids”.

Their fields of work are complementary: Professor Thomas Mussenbrock, who holds the Chair of Applied Electrodynamics and Plasma Technology at RUB, focuses on the plasma as a whole, whereas Professor Timo Jacob, head of the Institute of Electrochemistry at Ulm University, specialises in atomic processes. “We calculate processes at different ends of the time and length scales and try to fill the gap in between,” as Mussenbrock recaps.

Complicated molecules

Timo Jacob is the expert for interfaces between plasmas and their environment. This interface can be between plasma and solid, but also between plasma and liquid. “If we want to use plasmas to coat a surface or to change its structure, we have to understand the interaction between the plasma species and the surface on an atomic scale,” he explains. This process seems simple and predictable. But his team goes into much more detail, because complicated molecules can also form in plasmas, which are present in specific excited states. They can then change both the properties and the structure of catalytic surfaces or even actively impact reactions at the interface directly. Until now, such details had been little understood. “We had to develop the tools for this first,” says Jacob.

He and his team established methods that can describe how the plasma changes the molecules during a reaction. The influence of the molecules, for example, can result in an effect on the interface that becomes meta-stable but of particular interest for catalytic reactions.

The effort usually increases cubically

The processes calculated in this manner take place on the Ångström or nanometre scale within a tiny fraction of a second. The fewer effects the researchers have to calculate simultaneously, the more accurate the simulation can become. “Our highly accurate calculations involve a maximum of 300 to 400 atoms; using semi-empirical dynamic simulations even several 100,000 atoms are possible,” explains the researcher. “We sometimes use specifically acquired computing clusters or mainframes at high-performance computing centres for this purpose.” The effort usually increases cubically: i.e. if twice as many atoms are to be included in the calculation, the computational effort increases eightfold.

The fundamental insights thus gained should help, for example, to make plasmas usable for new applications – most importantly, catalytic processes. The researchers concentrate on both the reduction of carbon dioxide (CO₂) and the decomposition of relatively simple hydrocarbons as a model, using n-butane as an indicator.

Data is extremely scarce

It is of course not enough to consider only a few hundred atoms. This is where the research of Thomas Mussenbrock’s team completes the overall picture. His length scale is a billion times larger than Jacob’s. Quantum mechanical effects that are important at the atomic level no longer need to be explicitly considered in this case. The idea is to map plasma dynamics and plasma chemistry. “We look at the plasma based on classical physics.” The methods for this are known and well understood, but the needed data is extremely scarce.

A major challenge is that industrial processes with plasmas should take place at atmospheric pressure, if possible, and preferably at air pressure. “At this relatively high pressure, many interactions come into play,” explains Mussenbrock. What reacts how strongly with which molecules and atoms? Based on the results of Timo Jacob’s calculations, his team scales up the simulation. The results can be matched with experiments – deviations must be understood and explained.

“Bringing the two ends of the time and length scales together is uncharted scientific territory,” points out Thomas Mussenbrock. Consequently, the teams are breaking new ground. Perhaps at some point, thanks to their findings, it will be possible to make large-scale industrial processes such as ammonia production more energy-efficient.

adapted from Meike Drießen (RUB)