Ceramic aerogel

C&I Issue 3, 2019

Researchers have created a new extremely light and durable ceramic aerogel. The material could be useful for applications like insulating spacecraft because it can withstand intense heat and severe temperature changes.

The aerogel comprises a network of tiny air pockets, with each pocket separated by two atomically thin layers of hexagonal boron nitride. It’s at least 99% space. To build the aerogel, Duan’s team used a graphene template coated with borazine, which forms crystalline boron nitride when heated (Science, doi: 10.1126/science.aav7304). When the graphene template oxidises, this leaves a ‘double-pane’ boron nitride structure.

‘The key to the durability of our new ceramic aerogel is its unique architecture,’ says study co-author Xiangfeng Duan of the University of California, Los Angeles. ‘The “double-pane” ceramic barrier makes it difficult for heat to transfer from one air bubble to another, or to spread through the material by traveling along the hexagonal boron nitride layers themselves, because that would require following long, circuitous routes.’

Unlike other ceramic aerogels, the material doesn’t become brittle under extreme conditions. The new aerogel withstood 500 cycles of rapid heating and cooling from –198°C to 900°C, as well as 1400°C for one week. A piece of the insulator shielded a flower held over a 500°C flame.

When heated, the material contracts rather than expanding like other ceramics. When compressed, it contracts perpendicularly to the direction of compression, the opposite of how most materials react. As a result, the material is far more flexible and less brittle than current state-of-the-art ceramic aerogels. It can be compressed to 5% of its original volume and recovers; existing aerogels manage about 20%.

Duan believes the aerogels could be made from many materials, including carbon or metal oxides. ‘They could be useful for thermal insulation in spacecraft and automobiles, thermal energy storage, catalysis or filtration.’

Aerogels made out of 2D materials, for example, graphene aerogels, have been made before, notes Julia Greer of the California Institute of Technology. ‘Hollow shell-based architectures, as well as many different periodic hollow beam-based architectures, have also been made.

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