Engineers develop strong, light "metallic wood"

A nickel structure researchers are calling "metallic wood" because of the open spaces that lend biological materials like wood lightness and an ability to transport liquids through the solid form
© James Pikal, Penn Engineering

This nickel structure as strong as titanium but four to five times lighter could do double duty as a battery

Metallic wood has got it all: a clever name, inspiring potential applications, and a promising method for manufacturing the material at larger scales. And Mother Nature is at least in part to thank.

The team calls their material "metallic wood" not just because it has the density of wood, but because it mimics the structure of trees. Lead researcher James Pikul of Penn Engineering notes:

"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.”
Of course, it won't hurt that "metallic wood" could catch on with engineers while "nanostructured nickel inverse opal materials" would seem destined to remain hidden in the corners of a lab.

The potential applications are exciting. The material could be used in place of titanium in airplane wings and other high performance parts. But while just as strong as titanium, the metallic wood's porous structure could allow the open spaces to be filled, for example with an electrolyte that could turn the part into a battery. Imagine a prosthetic leg which can store energy to produce power while in use!

Perhaps best of all, Pikul - and his collaborators Bill King and Paul Braun from the University of Illinois at Urbana-Champaign, and Vikram Deshpande from the University of Cambridge - have developed a process for manufacturing the material that looks like it could be scaled up and fairly cost effective.

Metallic wood construction begins with a template of nano-balls arrayed like a pile of canon balls. The pile is sintered and then filled with electroplated nickel and then the template is dissolved away so that the porous metallic structure remains, at which point additional materials can be applied. The resulting light metal material consists of about 70% open space.

The researchers report that the infrastructure for working with the nanoscale materials is currently limited, but since the materials used are not rare or expensive and the processes are reasonably simple - evaporating water in which the nanoballs are suspended allows them to settle into the template array - it is only a matter of time before larger samples of metallic wood can be manufactured.

Larger samples will be subject to further testing. Although the compressive properties like strength can be measured on the small samples currently existing, the tensile properties are not fully explored. Pikul says “We don’t know, for example, whether our metallic wood would dent like metal or shatter like glass.”

Small anomalies in the regularity of the template could also affect the properties of the engineered metal, which needs to be understood in order to control the manufacturing process adequately. So while metallic wood might not be coming to a DIY store near you anytime soon, this is one to keep our eyes on.

Read the published report on metallic wood in Scientific Reports (2019): High strength metallic wood from nanostructured nickel inverse opal materials DOI: 10.1038/s41598-018-36901-3
Other co-authors include Sezer Özerinç (now at the Department of Mechanical Engineering at Middle East Technical University, Ankara, Turkey) and Runyu Zhang of the University of Illinois at Urbana-Champaign, and Burigede Liu of the University of Cambridge.

Engineers develop strong, light "metallic wood"
This nickel structure as strong as titanium but four to five times lighter could do double duty as a battery

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