Light-as-air ceramics could be used for cooling spacecraft

Materials World magazine
25 Mar 2019

A superinsulating nitride aerogel with an ability to withstand extreme conditions could help spacecraft stay cool. Shardell Joseph reports. 

A light-as-air ceramic aerogel has been created that can withstand temperatures up to 1,400°C without losing its mechanical strength. The superinsulating material, made of boron nitride, is only 1% as dense as water, but can protect spacecraft equipment from extreme temperatures.

Other ceramic aerogels tend to become brittle under adverse conditions, as they have solid dispersions containing more than 99% air. This material, however, can be heated up to 1,400°C for one week and undergo 500 temperature shocks that cool it from 900°C to -198°C in seconds, with its mechanical stability still intact.

‘Due to the high crystallinity, mechanically negative Poisson’s ratio and thermally negative thermal expansion, our aerogels could efficiently dissipate the thermal stress under thermal shocks or high temperature condition,’ says University of California, Los Angeles (UCLA), Associate Professor, Yu Huang.

Huang, Associate Professor at UCLA, Dr Xiangfeng Duan, and their team designed and synthesised the ceramic aerogel with nanolayered double-pane walls with a negative Poisson’s ratio – demonstrating compressive deformation – and a negative linear thermal expansion – contracting rather than expanding when heated. The material showed near-zero strength loss after sharp thermal shocks or intense thermal stress.

The aerogel’s hyperbolic design gives it unusual properties as it becomes thinner when squeezed and contracts when heated. In addition, where most ceramic aerogels can only recover compressions of up to 80%, its superelasticity means that after being compressed it recovers up to 95% of its volume.

Ceramic aerogels

Despite being more than 99% air, aerogels are solid with very strong structures for their weight. Ceramic aerogels have been popular for insulating industrial equipment since the 1990s and have been used by NASA for insulating scientific equipment for Mars rover missions.

Conventional ceramic aerogels possess traits desired for demanding environments, yet most are brittle and susceptible to degradation due to exposure to high temperatures and rapid temperature changes. According to the researchers, these have largely limited the use of ceramic aerogels as superinsulators.

Designing strength

In the paper, Double-negative-index ceramic aerogels for thermal superinsulation, published in ScienceMag, the researchers reported on the design of the unique aerogel and its demonstrated capabilities.

Using a hexagonal boron nitride, a resistant chemical that does not oxidise at high temperatures, the team created a template with hyperbolic surfaces – saddle shapes with negative curvature. This template is then coated with borazine – a boron-nitrogen version
of benzene. The borazine polymerises into polyborazylene when heated up to 1,500°C, which then loses its hydrogen atoms to
form crystalline boron nitride. Finally, the template is oxidised and burned away.

After this process, a double-pane boron nitride structure is created, which suppresses air condition and convention, making it superinsulating. The researchers tested the material’s mechanical and thermal capabilities by heating the aerogel at 900°C then rapidly cool it to -198°C repeatedly, at a rate of 275°C per second.

They demonstrated its properties using a flower, placing it on top of a 2cm-thick aerogel sitting on top of a 500°C flame for 15 minutes. The result was that the flower showed only slight withering.

Another test, evaluating the effect of long-term temperature stress, exposed the material to temperatures approaching 1,500°C in a vacuum. The aerogel remained largely unchanged with near-zero strength loss throughout the vigorous tests.

The aerogel has been tested and proven functional under extreme temperatures – the research team is looking to how its properties can be used. ‘Due to the low thermal conductivity both in vacuum and air, and high thermal stability, our ceramic aerogels exhibit thermal superinsulation to keep the spacecraft cool,’ Huang says.

Although this material hasn’t been used in any real-world applications yet, this will be the next step, transitioning their research into practical use. ‘We plan to continue on this project to make ceramic aerogels more flexible and able to withstand higher working temperatures,’ Huang says. ‘The current cost to make the material is still high. We plan to develop low-cost, large-scale fabrication techniques in the future.’