‘Metallic wood’ throws down a challenge to titanium

Titanium is pretty impressive (as strong as steel yet half the weight), but scientists reckon they can do better.

In a paper published in the journal Nature Scientific Reports, a team from the University of Pennsylvania and the University of Illinois in the US, Middle East Technical University, Turkey, and the University of Cambridge in the UK describe building a sheet of nickel with nanoscale pores that make it as strong as titanium but four to five times lighter again.

The empty space of the pores, and the self-assembly process in which they’re made, make the porous metal akin to a natural material, such as wood.

And just as the porosity of wood grain serves the biological function of transporting energy, the empty space in this “metallic wood” could be infused with other materials, the researchers say. Infusing the scaffolding with anode and cathode materials, for example, would allow it to be used for a plane wing or prosthetic leg that’s also a battery.

“The reason we call it metallic wood is not just its density, which is about that of wood, but its cellular nature,” says Pennsylvania’s James Pikul, who led the research. 

“Cellular materials are porous; if you look at wood grain, that’s what you’re seeing. Parts that are thick and dense and made to hold the structure, and parts that are porous and made to support biological functions, like transport to and from cells.”

Their material’s structure is similar, he adds, but they’re operating at the length scales where the strength of struts approaches the theoretical maximum.

And that’s the basis of the story. Even the best natural metals have defects in their atomic arrangement that limit their strength. Fix or avoid the defects, and you limit the limits.

A block of titanium where every atom was perfectly aligned with its neighbours would be 10 times stronger, the researchers say, than what can currently be produced. 

Materials scientists have been trying to exploit this phenomenon by taking an architectural approach, designing structures with the geometric control necessary to unlock the mechanical properties that arise at the nanoscale, where defects have reduced impact.

“We’ve known that going smaller gets you stronger for some time,” Pikul says, “but people haven’t been able to make these structures with strong materials that are big enough that you’d be able to do something useful.{%recommended 7284%} 

“Most examples made from strong materials have been about the size of a small flea, but with our approach we can make metallic wood samples that are 400 times larger.”

That approach starts with tiny plastic spheres, a few hundred nanometres in diameter, suspended in water. When the water is slowly evaporated, the spheres settle and stack like cannonballs, providing an orderly, crystalline framework. 

Using electroplating, the same technique that adds a thin layer of chrome to a hubcap, the researchers then infiltrate the plastic spheres with nickel. Once the nickel is in place, the plastic spheres are dissolved with a solvent, leaving an open network of metallic struts.

Because roughly 70% of the resulting material is empty space, this nickel-based metallic wood’s density is extremely low in relation to its strength. With a density on par with water’s, a brick of the material would float.

Replicating this production process at commercially relevant sizes is the next challenge. Unlike titanium, none of the materials involved are particularly rare or expensive on their own, but the infrastructure necessary for working with them on the nanoscale is currently limited. 

Once the researchers can produce samples of their metallic wood in larger sizes, they can begin subjecting it to more macroscale tests. A better understanding of its tensile properties, for example, is critical.

“We don’t know, for example, whether our metallic wood would dent like metal or shatter like glass.” Pikul says. “Just like the random defects in titanium limit its overall strength, we need to get a better understand of how the defects in the struts of metallic wood influence its overall properties.”