The microstructural material of the future

Airgel is an excellent thermal insulator. Until now, however, it has been used mainly on a large scale, for example in environmental technology, in physical experiments or in industrial catalysis. Empa researchers have now succeeded in making aerogels accessible to microelectronics and precision engineering: An article in the latest issue of the scientific journal Nature shows how 3-D-printed parts are made of silica airgel and high-precision silica composite materials can be produced. This opens up several new application possibilities in the high tech industry, for example in microelectronics, robotics, biotechnology and sensor technology.

Behind the simple headline “Additive manufacturing of silica airgel” – the article was published on July 20 in the famous scientific journal Nature– a groundbreaking development is hidden. Silica aerogels are light, porous foams that provide excellent thermal insulation. In practice, they are also known for their brittle behavior, which is why they are usually reinforced with fibers rather than with organic or biopolymers for large-scale applications. Due to their brittle fracture behavior, it is also not possible to saw or grind small pieces from a larger airgel block. Solidifying the gel directly in solidified molds is also unreliable – resulting in high scraping speeds. This is the reason why aerogels were hardly useful for small scale applications.

Stable, well-formed microstructures

The Empa team led by Shanyu Zhao, Gilberto Siqueira, Wim Malfait and Matthias Koebel have now succeeded in producing stable, well-formed microstructures of silica airgel using a 3-D printer. The print structures can be as thin as a tenth of a millimeter. The thermal conductivity of the silica air gel is just under 16 mW / (m * K) – but half that of polystyrene and even significantly less than that of a non-moving layer of air, 26 mW / (m * K). At the same time, the novel printed silica airgel has even better mechanical properties and can even be drilled and milled. This opens up completely new possibilities for the post-processing of 3-D printed airgel lists.

With the method for which a patent application has now been filed, it is possible to precisely adjust the flow and solidifying properties of the silica ink, from which the airgel is later produced, so that both self-supporting structures and wafer-thin membranes can be printed. As an example of overhanging structures, the researchers printed leaves and blooms of a lotus flower. The test object floats on the water surface due to the hydrophobic properties and the low density of the silica airgel – just like its natural model. The new technology also makes it possible for the first time to print complex 3-D multi-material microstructure.

Insulation material for microtechnology and medicine

With such structures, it is now relatively trivial to thermally insulate even the smallest electronic components. The researchers were able to demonstrate in an impressive way the thermal protection of a temperature-sensitive component and the thermal management of a local “hot spot”. Another possible application is the protection of heat sources in medical implants, which no longer need a surface temperature of 37 degrees to protect body tissue.

A functional airgel membrane

Through 3-D printing, multilayer / multi-material combinations can be produced much more reliably and reproducibly. Fine structures of new airgel become possible and open up new technical solutions, as a second example of application shows: Using a printed airgel membrane, the researchers constructed a “thermosmolecular” gas pump. This permeation pump manages without moving parts at all and is also known to the technical community as a Knudsen pump, named after the Danish physicist Martin Knudsen. The principle of operation is based on the limited gas transport in a network of nanoscale pores as one-dimensional channels whose walls are hot at one end and cold at the other. The team built such an airgel pump, which is doped on one side with black manganese oxide nanoparticles. When this pump is placed under a light source, it gets hot on the dark side and starts pumping gases as solvent vapors.

Air cleaning without moving parts

These applications demonstrate the capabilities of 3-D printing in an impressive way: 3-D printing turns the high-performance material airgel into a building material for functional membranes that can be quickly adapted for a wide range of applications. The Knudsen pump, which is powered solely by sunlight, can do more than just pump: If the air is contaminated with a pollutant or an environmental toxin such as the solvent toluene, the air can circulate through the membrane several times and it pollutant is chemically degraded by a reaction catalyzed by the nanoparticles of manganese oxide. Such solar-powered, autocatalytic solutions are particularly appealing in the field of air analysis and purification on a very small scale because of their simplicity and durability.

The Empa researchers are now looking for industry partners who want to integrate 3-D-printed airgel structures into new high-tech applications.