Ultralight ceramic material withstands extreme temperatures

US scientists have developed a ceramic airgel that is extremely light, durable and heat resistant. It could be used, for example, for the insulating tiles of spacecraft.

Sample of the new airgel

99% of the ceramic airgel consists of air. It is very flexible and heat resistant.

Photo: Oszie Tarula / UCLA

When a Space Shuttle enters the Earth’s atmosphere from a space flight, extreme heat develops on the outer shell. Super insulators serve as a barrier in such situations to protect the technology from the effects of heat. Ceramic aerogels have great potential in this respect, but the materials used to date have shown signs of fatigue after repeated variations in temperature and have been breaking relatively rapidly. University of California scientists have now developed a new ceramic material that is more resilient and flexible than traditional aerogels. At the same time, it is much easier.

Aerogels are a phenomenon. These materials are obtained from aqueous solutions, by gelation and suitable drying methods. This results in solids which, among other things, have a very low density. The bottom line is that they often consist of more than 99% air. Added to this is a filigree network of thin layers, which are suitable for ceramics, carbon or metal oxides, among other things. Ceramic aerogels are very strong in structure, but also very porous. Although they are extremely resistant to heat, they are generally unable to withstand large temperature fluctuations or long-lasting heat – they break after several changes in their ambient temperature. The new material, on the other hand, is said to be exceptionally elastic, as proven by tests conducted under extreme conditions.

Long shelf life of ceramic aerogels thanks to great flexibility

The new ceramic airgel is based on boron nitride. The atoms of this ceramic are arranged in hexagonal patterns, visually similar in plan view so honeycombs. From this structure, the researchers have formed a three-dimensional net-like structure, which has very special properties: with increasing warming, it does not expand, but contracts. If it is stretched mechanically, it does not become thinner in the middle, but thicker. Physically, these properties can be described with a negative coefficient of thermal expansion and with a negative Poisson number, ie an expansion of the material transverse to the tensile direction. This behavior is comparable with a tennis ball that narrows in the middle when you press it on a table. As a result, the new ceramic airgel is significantly more flexible and less brittle than its predecessors. It can be compressed to 5% of its original volume and fully recover. Other aerogels can only be compressed to a maximum of 20%.

Airgel on pistil

The ceramic airgel is so light it holds on a pistil.

Photo: Xiangfeng Duan and Xiang Xu / UCLA

This flexibility is the key to the durability of the ceramic airgel. Ordinary ceramic materials expand in the heat. Repeated temperature changes therefore burden their structure and lead to fractures relatively quickly. As the new airgel contracts in the heat, it holds it better: the engineers put samples in a container, which they heat within a few seconds from minus 198 degrees Celsius to over 900 degrees Celsius. Even after the scientists repeated this experiment hundreds of times, the material showed no damage. A second test survived just as well. The researchers stored the ceramic airgel for one week at 1,400 degrees Celsius. It lost less than 1% of its mechanical strength after that time.

Possible for use in aerospace or the automotive industry

The new ceramic airgel would, among other things, be suitable for heat in spacecraft or to insulate in cars. It could also be used for the storage of heat energy or for catalysts. Before that, however, the scientists still have to solve a practical problem. The production of the airgel is extremely complicated. For industrial use, therefore, researchers must first succeed in transferring their process to mass production. But the team has bigger goals: they are already planning to develop a ceramic airgel that outperforms these positive qualities and is even more heat-resistant and lighter.

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