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‘Metallic wood’ has titanium’s strength, but cellular structure

‘Metallic wood’ has titanium’s strength, but cellular structure

Technology News |
By Rich Pell



The material has a porous structure that is responsible for its high strength-to-weight ratio. The researchers have dubbed the material “metallic wood,” because, they say, it has the high mechanical strength and chemical stability of metal, as well as a density close to that of natural materials such as wood.

“The reason we call it metallic wood is not just its density, which is about that of wood, but its cellular nature,” says James Pikul, Assistant Professor in the Department of Mechanical Engineering and Applied Mechanics at Penn Engineering, who led the study. “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.”

“Our structure is similar,” says Pikul. “We have areas that are thick and dense with strong metal struts, and areas that are porous with air gaps. We’re just operating at the length scales where the strength of struts approaches the theoretical maximum.”

Just as the porosity of wood grain serves the biological function of transporting energy, say the researchers, the empty space in the porous structure of metallic wood could be infused with other materials. For example, infusing the scaffolding with anode and cathode materials would enable it to serve double duty, such as a plane wing or prosthetic leg that’s also a battery.

Such engineered materials, designed on the scale of individual atoms, could present superior alternatives to existing high-performance natural materials like titanium – even the best of which have defects in their atomic arrangement that limit their strength. A block of titanium where every atom was perfectly aligned with its neighbors, say the researchers, would be ten times stronger than what can currently be produced.

Materials researchers 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.

The struts in the researcher’s metallic wood are around 10 nanometers wide, or about 100 nickel atoms across. Other approaches involve using 3D-printing-like techniques to make nanoscale scaffoldings with hundred-nanometer precision, but the process – which is slow and painstaking – is hard to scale to useful sizes.

“We’ve known that going smaller gets you stronger for some time,” says Pikul, “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. 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.”

To create their material, the researchers started with tiny plastic spheres, a few hundred nanometers 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.

“We’ve made foils of this metallic wood that are on the order of a square centimeter, or about the size of a playing die side,” says Pikul. “To give you a sense of scale, there are about one billion nickel struts in a piece that size.”

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

The researchers say their next challenge is to replicate their production process at commercially relevant sizes. Unlike with 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 that infrastructure is developed, say the researchers, economies of scale should make producing meaningful quantities of metallic wood faster and less expensive. Then, they say, they can begin subjecting it to more macroscale tests.

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

In the meantime, the researchers are exploring the ways other materials can be integrated into the pores in their metallic wood’s scaffolding.

“The long-term interesting thing about this work is that we enable a material that has the same strength properties of other super high-strength materials but now it’s 70 percent empty space,” says Pikul. “And you could one day fill that space with other things, like living organisms or materials that store energy.”

For more, see “High strength metallic wood from nanostructured nickel inverse opal materials.”

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