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)

Plasmas play a vital role in many industrial applications. The energetically excited gases can be used, for example, to deposit coatings on surfaces, such as scratch-resistant protective layers on plastic spectacle lenses, or high-precision optical filters on quartz glass. In the process, the plasma is used to bombard the coatings that grow on the substrate by vapour deposition with ions, thus effectively hammering them into place.

But such processes must meet many requirements. For example, they must take place at low temperatures to prevent damage to the coated surfaces. “In modern production processes, more and more emphasis is also being placed on precision,” stresses Professor Ralf Peter Brinkmann, who holds the Chair for Theoretical Electrical Engineering at RUB. All resulting products must be exactly the same, and the coating must be defect-free. To achieve this level of precision, it is necessary to constantly monitor the plasma. The electron density is particularly important in coating processes. If it were to fluctuate too much, this would negatively affect the quality of the finished coating. “Ideally, the electron density should be constantly measured and automatically readjusted if necessary, so that no human has to interfere in the process,” explains Brinkmann.

Small, reliable, maintenance-free

A measuring instrument that can do all this has to meet a variety of requirements: It should be as small as possible, reliable, maintenance-free, and it must neither interfere with the coating process nor be damaged in the plasma. Experts refer to this as “process-compatible plasma diagnostics”.

Researchers have been pursuing one idea for a long time: The electrons, which move freely in the plasma, can be made to oscillate by applying a small external voltage. If the right frequency is hit, a detectable resonance occurs. Since the resonance frequency depends on the electron density, this can then be theoretically calculated.

However, earlier attempts to put this idea into practice encountered some difficulties. A Japanese research group, for example, proposed a very simple construction: The team used a coaxial cable, similar to an analogue antenna cable, allowed the inner conductor to protrude a little, and inserted the end of the cable into the plasma. If a voltage was applied, resonances of the plasma could be measured. However, resonances of equal value occurred at several different frequencies – a veritable zoo. “The problem was: which one should be used for diagnostics,” explains Ralf Peter Brinkmann.

Analyses by the Bochum-based researchers provided an answer to the question of where the different resonances came from: As simple as the design of the measuring equipment was, different oscillations with different resonance frequencies arose at different sections of the apparatus. “Picture this like driving an old car,” as Ralf Peter Brinkmann illustrates the principle. “At a certain speed, the exhaust rattles, at another, it’s the wing mirror.”

The more symmetrical, the better

To remedy the situation, the team devised a concept that aimed for the simplest possible oscillations. Their aim was: the more symmetrical, the better. “The spherical shape is the simplest configuration imaginable,” says Brinkmann. “A floating marble would’ve been our first choice.” However, it was not quite that simple, of course. Electricity always needs a forward and a return conductor. Accordingly, two metallic hemispheres were chosen. The construction is rotationally and mirror-symmetrical, both electrodes are the same size. “Here, too, we measure resonances at different frequencies,” explains Ralf Peter Brinkmann. “But they can be clearly sorted. The strongest resonance is the dipole resonance; the other, weaker ones, represent the overtones to this keynote, so to speak.” The name “multipole resonance probe” (MRP) was derived from the mathematical method of “multipole analysis” used for this purpose.

The reason why researchers opt for the MRP is that the relationship between plasma density and resonance frequency is given by a simple mathematical formula. The plasma density is the only unknown in this equation. Once the equation is solved, it can be calculated from the measured values. “This means that it is not necessary to calibrate the measuring probe before use, i.e. to compare it with other measured values,” elaborates Ralf Peter Brinkmann.

Plasma was kept consistent through constant monitoring

So much for the underlying theory. For practical implementation, the researchers teamed up with three other chairs at the faculty. Professor Ilona Rolfes’ High Frequency Engineering Institute carried out a high-frequency optimisation of the entire system consisting of probe head and holder. For example, it was possible to make the holder practically invisible to the plasma. Professor Thomas Musch’s Institute of Electronic Circuits designed control electronics based on radar technology. And finally, Professor Peter Awakowicz’ Institute for Electrical Engineering and Plasma Technology tested the finished probe in a number of plasma processes.

The Federal Ministry of Education and Research funded the joint projects Pluto and Pluto plus to develop the MRP to the point where it could be used in practice. This also provided the opportunity to test the probe with industrial partners. And it turned out that if the electron density in the plasma was kept consistent through constant monitoring by means of MRP and automatic adjustment of the control, the fluctuations in the process results were significantly reduced.

Today, a spin-off is about to get off the ground.

adapted from Meike Drießen (RUB)

Plasma physics is the study of the behaviour of ionised gases. Statistical physics, fluid dynamics, electrodynamics as well as atomic and molecular physics come together to form a discrete discipline. Plasmas determine both stellar evolution on astronomical scales and etching of nanostructures in the semiconductor industry. Plasma-based engines are already powering satellites in space, and very hot magnetised plasmas may provide clean energy through controlled nuclear fusion in the future. Tiny cold plasmas at atmospheric pressure offer a wide range of applications, from CO₂ conversion to medicine and biology. Major advances in measuring the internal parameters of plasmas and in their simulation have recently contributed to a much better understanding of these complex systems. Wherever the journey into the future takes us, it will not be without plasmas.

by Uwe Czarnetzki (RUB)