The lightest solid on Earth

It is so light, it’s said that this material can balance on a Dandelion without even deforming the individual seed heads. Engineering Materials finds out more about this modern day marvel.

This is a piece of graphene aerogel – officially the lightest solid material in the world.

It was developed in a lab at Zhejiang University in China and it is so light it can be placed on a delicate flower and not even cause any deformation. The sponge-like material weighs just 160g/m3, around 7.5 times less than air, slightly less than helium and it’s just twice the density of hydrogen.

Aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and low thermal conductivity. Aerogel was discovered in 1931 by US scientist Steven Kistler, who used silicon dioxide to produce it and nicknamed it ‘frozen smoke’. However, in 2011 returned to the material and developed nickel aerogel with a density of 900g/m3.

Gao Chao from the Zhejiang University had been developing macroscopic graphene materials, such as one-dimensional graphene fibres and two-dimensional graphene films. This time he decided to make a 3D porous material out of graphene to break the record.

“It’s somewhat like large space structures such as big stadiums, with steel bars as supports and high strength film as walls to achieve both lightness and strength,” said fellow researcher Sun Haiyan. “Here, carbon nanotubes are supports and graphene is the wall.”

The basic principle of developing aerogel is to remove solvent in the gel and retain the integrity of the solidifying material. In the past, scientists used the sol-gel method and template-oriented method. The former can synthesise aerogel on a large scale, but has poor controllability, while the latter generated ordered structures.

However, Professor Chao’s team explored a new method known as freeze-drying. Here, they basically freeze-dried solutions of carbon nanotubes and combined it with a large amounts of graphene oxide. Residual oxygen was then removed chemically.

“With no need for templates, its size only depends on that of the container,” he explained. “Bigger container can help produce the aerogel in bigger size, even to thousands of cubic centimetres or larger.”

The properties of the graphene aerogel are thought to be much more elastic than previous attempts. Traditionally the material is known for being incredibly delicate, with no elasticity making it easy to damage. However, the graphene aerogel is said to bounce back when compressed.

As well as being light, the material is said to be particularly good at oil-absorption, reportedly 900 times its own weight in oil.

“Maybe one day when oil spill occurs, we can scatter them on the sea to absorb the oil quickly,” said Chao. “Due to its elasticity, both the oil absorbed and the aerogel can be recycled.”

Apart from that, the aerogel may also be made into ideal phase change energy storage insulation material, catalytic carrier or efficient composite.

The question that might puzzle some is that if the material is lighter than air, and helium, why doesn’t it float?

Justin Cunningham

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This was widely reported on in April 2013.

The reason it does not float is that it is untrue that it is seven times lighter than air. If you go to the original paper, its density was measured in air, not vacuum, so the weight measured is in excess of the weight of air (the tare, zero, of the scale includes the weight of air).

Comment Peter Andrews, 19/06/2016

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