Article from DLRmagazine 161
Barbara Milow looks intently at a glass beaker, taking in every detail. She likes the excitement of waiting to find out whether the experiment has worked, or whether something has gone askew. Only afterwards will she know whether a test has been a success or an ostensible failure from which she might nevertheless be able to learn something that proves crucial for later research. "This always fascinates me," says Milow. "I'm enthralled by chemical synthesis, how materials and their properties are related, and the development of entirely new materials." The scientist, who holds the Chair of Nanostructured Cellular Materials at the University of Cologne, and her colleagues at the DLR Institute of Materials Research have cause for excitement today. The experiment has gone according to plan: the gelation of the silica solution is clearly discernible. And Milow does not want to leave anything to chance. Nevertheless, things do not always go as expected when she and her team of technicians, engineers and scientists literally cook up new recipes for aerogels in their laboratory.
Aerogels are state-of-the-art materials. They essentially consist of air surrounded by a fine yet solid structure, not unlike a sponge. Materials such as silicates, (bio-)polymers and metal oxides are used for the solid structure, accounting for between one and 20 percent of the finished aerogel. Aerogels have a very low density, which makes them extremely light. This is one of many reasons why Milow is so enthusiastic about this material. "In principle, the manufacturing process is very simple," she says. "You combine the components, they gel, you dry the gel, and the result is a material with an incredible number of applications. It can have the electric conductivity of carbon while also offering good thermal insulation due to its nanoporous structure. It is almost unprecedented for one material to have such a range of properties.”
The scientist has led the Aerogels and Aerogel Composites Department at the DLR Institute of Materials Research since late 2018. With 32 employees, it is the largest research group dedicated to this subject area in Germany. The scientists are working together to gain a better understanding of the materials' gelling process and to open up possible areas of application. Silica aerogels are very stable at extreme temperatures and are being developed for sound and thermal insulation in aircraft cabins or fuel tanks. Thermoset aerogels, meanwhile, make outstanding insulating materials, as they are neither toxic nor flammable. Biopolymer aerogels have an inner structure that is similar to felt, and are used for the adsorption of air pollutants, humidity or materials that are susceptible to oxidation. Due to their definable nanostructures, carbon aerogels can contribute to new battery concepts or be used to optimise casting processes. When combined with other materials, their positive properties can be harnessed to form innovative aerogel composites. In conjunction with the University of Duisburg-Essen, the DLR researchers have also developed an aerogel concrete that is not only very light, but also provides excellent heat insulation. "The possibilities that we are now seeing are almost limitless, and new areas of application are emerging all the time. It is amazing, considering that research into aerogels began nearly 90 years ago," Milow says.
Good things take time
In the 1930s, scientists worked on removing fluids from wet gels – also known as hydrogels. Aerogels were considered to be any materials that were produced from wet gels and that – after drying – bore the network- and pore-like structure of a hydrogel within the material. These early researchers found it extremely difficult to control the forces that acted upon the pore walls inside the material during the drying process without destroying the structure of the gel and causing the pores to collapse. In the beginning, the drying process would take around a week, so industrial use was out of the question. It was not until the 1960s that scientists succeeded in greatly simplifying and thus accelerating the aerogel production process. They realised that the pore fluid in wet gels might predominantly consist of methanol. Researchers also discovered that material properties, such as the network-like structure of the aerogels, could be controlled via the pH value. Milow was just a child at the time.
During the period when companies like BASF, Hoechst and Henkel were deploying various solutions for wet gels in the 1980s, with a view to developing aerogels with a range of properties, she opted to study chemistry at the University of Cologne, followed by an internship at the Chemical Investigations Office in Trier. "For my doctorate, I went to the DLR Institute of Space Simulation, where I conducted experiments with liquid-liquid systems in microgravity conditions. I was even lucky enough to be able to support the ground-based research for the D2 space mission as a doctoral candidate, which was something that many of my predecessors wanted to do," Milow says. "At that time, I had very little to do with aerogels."
A few scientists at the same institute were working with this interesting material, but it would be another 10 years before Milow became intensively involved in aerogel research. That is how long she worked at what is now the DLR Institute of Solar Research, focusing on the decontamination of waste water using solar energy. "When my contract ended, I needed a reference from my old institute and contacted one of my former colleagues," she recalls. "He was the one working on aerogels with his team back when I was doing my doctorate. I wrote to him, saying, 'Lorenz, I need a reference or a job,' and he replied, 'I can give you both.' That was in 2005. Aerogels have been my focus ever since."
Research into the unknown
At that point, the group was facing an existential dilemma: although funding opportunities were opening up for their projects, the researchers learned that the working group was to be disbanded after a change in the institute's management. "We could not just bury our heads in the sand, so we began to actively raise the profile of this area." The team visited countless foundries across Germany in order to research and develop their production process. In doing so, they gradually opened up a wide range of applications for their materials. For instance, the group developed aerogel structures as model systems for bone implants. Today, their working group at the University of Cologne is making implants out of 'foam-like' biopolymer structures and have reached a stage where these aerogel structures are not rejected after being inserted into the tissue.
Milow and her team consider it important to replace syntheses that include toxic substances and to use other processes instead. "Apart from formaldehyde, the chemicals that we use are all environmentally friendly. We are always looking for materials that we can work with in a sustainable way." The group has found some unorthodox ways of achieving this feat: "On one occasion, we used cinnamaldehyde to make aerogels. It made the whole laboratory smell like Christmas. Unfortunately, it proved to be more detrimental in the required concentrations."
An ambitious vision
"At first, I was sceptical as to whether the team would stick together, everything would work out and the funding would come through. I wrote numerous proposals. But it turned out even better than I could have hoped, and today we are bursting at the seams." Since then, Milow's attention has turned to the next generation. Today, she and her department are working on components for all areas of research at DLR. They are currently developing a material to prevent aircraft fuselages from icing up on long flights. Hydrophobic – meaning water-repellent – aerogels should stop water from accumulating and forming heavy layers of ice.
"My dream is to have a separate centre of excellence for aerogels here at DLR, where we can produce prototypes and very small series of new raw materials and components," the researcher says. The staff at the University of Cologne, where Milow recently became a professor, would also be able to work there. She is proud of that, and her delight at working in this field is apparent when she talks about future projects and new areas of application. Of course, it all takes a lot of work, and perhaps she could have restricted herself to one or two specific areas. But Milow shakes her head: "Keeping things small just isn't my kind of thing." Frank Seidler is responsible for marketing and communications at the DLR Institute of Materials Research.
How do aerogels dry?
Drying aerogels without damaging their inner structure is essential but difficult. The solution lies in what is known as supercritical drying. In this process, the pressure and temperature are increased until the density of the liquid and gaseous phase – two of the three classic phases of aggregation – is balanced. This homogeneous phase turns the fluid into a supercritical fluid. The compression of the molecules creates a fluid without capillary stresses, so that there are no forces acting upon the pore structure. The gel does not shrink during drying, but rather retains the shape and structure of the wet gel, while becoming an aerogel.
Aerogels and aerogel composites
Biopolymer aerogels consist of naturally occurring macromolecules such as cellulose, chitin or carrageenan. Chemically bound nanometre-thin fibres give the material its stability. This felt-like structure makes them particularly suitable as filters, for example, for regulating humidity in aircraft cabins, as well as for the adsorption of carbon dioxide.
Thermoset aerogels are lightweight materials that insulate heat and sound. Accordingly, they are of particular interest for the construction of vehicles, trains and aircraft, or as an alternative to polystyrene. They can range from brittle to super flexible, depending on the composition and drying process. They are based on an aqueous resorcinol-formaldehyde solution. When combined with other aerogels, such as silica-aerogel granulates, they form composites with better thermal and mechanical properties.
Carbon- and silicon-oxycarbide aerogels are formed during the thermal treatment of thermoset, biopolymer-based or hybrid aerogels. They are very stable in high temperatures and are used in the sand casting process in foundries. Their large internal surfaces allow them to absorb the gases that occur during the casting process, thus preventing casting defects such as gas bubbles, non-metallic inclusions or sand adhesion. DLR is working to transfer its manufacturing process from the laboratory to a pilot-plant scale.
Silica-based aerogels, also known as frozen smoke, are the most studied type of aerogels. Due to their low thermal conductivity, they are used as an insulating material, but other applications include aerogel particles in cosmetics. In recent years, researchers have developed a new variant: soft, flexible aerogels that remain stable at temperatures of up to 500 degrees Celsius. DLR scientists are working on new formulations to further improve the material.
The article is taken from DLRmagazine 161. You can receive the DLRmagazine free of charge when you subscribe on the DLRmagazine subscription page. Here you can also find all issues. The author Frank Seidler is responsible for marketing and communications at the DLR Institute of Materials Research.