'Metallic wood' has titanium's strength, but cellular structure

February 06, 2019 // By Rich Pell
Researchers at the University of Pennsylvania’s School of Engineering and Applied Science (Philadelphia, PA), the University of Illinois at Urbana–Champaign, and the University of Cambridge have built a sheet of nanostructured nickel that is as strong as titanium, but four to five times lighter.

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

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