Transformable smart materials move closer to realisation

The promise of smart materials whose mechanical properties can be changed on demand is a step closer to realisation, following work by German and Chinese scientists.

The team has developed an alloy that can increase its breaking strength upon application of an electrical charge by an unprecedented factor of two.

‘We were pretty happy to see this, I wouldn’t have expected it,’ said one of the collaborators, Dr Jörg Weissmüller from Hamburg University of Technology. ‘In terms of mechanical properties, this has never been done — this is the first time and there is no comparison.’

The material was made using a silver-gold alloy that underwent a corrosion process to remove the silver and form a gold structure containing a network of nanopores 10–20nm in size. This gave the gold a sponge-like structure, massively increasing its surface area. 

By filling the structure with a perchloric-acid electrolyte, an electrochemical double layer was formed between the liquid and metal. The team then applied an electrical charge to alter the absorption and desorption of oxygen on the gold’s surface. Standard mechanical tests revealed that the electric charge reversibly altered the material’s breaking strength and ductility by a factor of two.

The resulting material can currently be produced on the scale of cubic millimetres to a strength of around 50MPa (in ‘activated’ form), which Weissmüller concedes is around 10 times lower than what would be needed for a ‘good structural alloy’.

And so the next step will be to improve the activated strength — and to this end Weissmüller believes that the nanoscale fabrication method could be applied to other alloys that are more suitable for such structural applications.

Another area that needs improvement is the time it takes for the material to complete its mechanical transformation, which is currently on the order of ’many seconds’ due to the limiting nature of the underlying chemical processes.

Nevertheless, the potential of instantaneous property shifting on a larger scale is clear, Weissmüller noted.

‘Imagine you can build your car strong and light but when it crashes you can switch it to ductile and it becomes energy absorbing — that would be a big incentive.’

In the interim, smaller sections of smart materials could be used for individual components for various industrial applications. In addition, the same principles might be used for different properties such as actuation or optical transmission, which would be useful in the emerging field of optoelectronics.