Growing ‘metallic wood’ to new heights

Natural wood remains a ubiquitous building material because of its high strength-to-density ratio; trees are strong enough to grow hundreds of feet tall but remain light enough to float down a river after being logged.

A metallic arch on a surface reflecting a rainbow prism.
This strip of metallic wood, about an inch long and one-third inch wide, is thinner than household aluminum foil but is supporting more than 50 times its own weight without buckling. If the weight was suspended from it, the same strip could support more than six pounds without breaking. (Image: Penn Engineering Today)

For the past three years, engineers at the School of Engineering and Applied Science have been developing a type of material they’ve dubbed “metallic wood.” Their material gets its useful properties and name from a key structural feature of its natural counterpart: porosity. As a lattice of nanoscale nickel struts, metallic wood is full of regularly spaced cell-sized pores that radically decrease its density without sacrificing the material’s strength.

The precise spacing of these gaps not only gives metallic wood the strength of titanium at a fraction of the weight, but unique optical properties. Because the spaces between gaps are the same size as the wavelengths of visible light, the light reflecting off of metallic wood interferes to enhance specific colors. The enhanced color changes are based on the angle that light reflects off of the surface, giving it a dazzling appearance and the potential to be used as a sensor.

Penn engineers have now solved a major problem preventing metallic wood from being manufactured at meaningful sizes: eliminating the inverted cracks that form as the material is grown from millions of nanoscale particles to metal films big enough to build with. Preventing these defects, which have plagued similar materials for decades, allows strips of metallic wood to be assembled in areas 20,000 times greater than they were before.

James Pikul, assistant professor in the Department of Mechanical Engineering and Applied Mechanics, and Zhimin Jiang, a graduate student in his lab, have published a study demonstrating this improvement in the journal Nature Materials.

Read more at Penn Engineering Today.