Metallic 'wood' is light as water but as strong as titanium

Scientists are constantly researching ways to make materials stronger and lighter.

In doing so they could help improve everything from aeroplanes to sports equipment, making them more efficient, faster and increasing performance.

And while much time and money are poured into materials such as carbon fibre and titanium, for example, new technologies are also being developed.

Researchers at the University of Pennsylvania, the University of Illinois at Urbana-Champaign, the University of Cambridge and the Middle East Technical University in Ankara, Turkey have developed a sheet of nickel with nanoscale pores that make it as strong as titanium but up to five times lighter.

Crystalline framework

It’s constructed by suspending tiny plastic spheres in water. The water is slowly evaporated, leaving the spheres to settle and stack, providing a crystalline framework. This structure is then electroplated with nickel and the plastic spheres are dissolved with a solvent, leaving a porous network of metallic struts.

The pores make the metal similar to materials found in nature such as wood, which is why the researchers have dubbed the material metallic wood. The research was led by James Pikul, assistant professor in the department of mechanical engineering and applied mechanics at the University of Pennsylvania.

“The goal was firstly to understand the mechanical properties of the nanoporous material we made, and secondly to see if we can engineer the material to have very high strength and light weight, beyond what classical materials can achieve today. We succeeded in both,” he said.

The research project took a roundabout way to developing the material, initially looking at how to improve battery technology, rather than simply engineering a stronger and lighter substance.

“We made a highly engineered material to make battery electrodes that have very high power density. This was our earlier work, and we made batteries that were 100-1,000 times more powerful than commercial batteries. We then were reading literature on high-strength materials and predicted that our battery materials might have the right properties to achieve ultra-high strength,” said Pikul.

With four universities involved, the workload was split; fabrication and characterisation were conducted at Penn, Illinois and Middle East, while modelling was done at Cambridge. 

The advantages the metallic wood brings are obvious and could be incredibly useful in any number of applications.

Pikul said: “It is as light as water but as strong as titanium, which has many advantages, but I am most excited by the fact that we can achieve these properties with a material that is 70-90% porous. That means that, in addition to being strong, there is room to fill active functional materials inside. In a way similar to bone, which is a strong scaffold filled with active materials that allow it to heal, we are interested in using the pores to make multifunctional and strong materials.”

The team thinks it could replace materials in high-value applications where strength and weight are important, such as in quadcopters and phones. Pikul and his team also hope that, when they add functional materials into the pores, it will be useful in robotics too.

Fabrication challenge

But although the material offers a host of benefits there are challenges that will need to be overcome if it is ever to enter the market. Fabrication is perhaps the most challenging aspect. “In a square centimetre of material, we are precisely aligning billions of tiny particles, which we then make into metallic wood,” said Pikul.

Which brings the project’s next challenge. Pikul and his team will develop the scaleability and manufacturing processes of the material, from its current cm² areas to m² areas. 

“There is significant technology development that needs to be done to realise this. We are also working on understanding the fracture of these materials and how to make composite materials from our metallic wood,” said Pikul.

If industrialised, metallic wood could help any number of applications to become more efficient and faster and offer increased performance, so Pikul and his team’s next development steps will be critical to its future success.

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