Titanium is as strong as steel but about twice as light. These properties depend on the way a metal's atoms are stacked, but random defects that arise in the manufacturing process mean that these materials are only a fraction as strong as they theoretically could be. An architect, working on the scale of individual atoms, could design and build new materials that have even better strength-to-weight ratios.
Researchers have built a sheet of nickel with nanoscale pores that make it as strong as titanium but four to five times lighter. 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. As the porosity of wood grain serves the biological function of transporting energy, the empty space in the “metallic wood” could be infused with other materials — infusing the scaffolding with anode and cathode materials would enable the metallic wood to serve double duty as a plane wing or prosthetic leg that's also a battery.
Even the best natural metals have defects in their atomic arrangement that limit their strength. A block of titanium where every atom was perfectly aligned with its neighbors would be 10 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 metallic wood are about 10 nanometers wide, or about 100 nickel atoms across. Other approaches involve using 3D-printing-like techniques to make nanoscale scaffoldings with 100-nanometer precision, but the slow and painstaking process is hard to scale to useful sizes.
The new method starts 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 plastic spheres are infiltrated 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 percent 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 is particularly rare or expensive on its own, but the infrastructure necessary for working with them on the nanoscale is currently limited. Once that infrastructure is developed, economies of scale should make producing meaningful quantities of metallic wood faster and less expensive.
For more information, contact James Pikul, Assistant Professor of Mechanical Engineering and Applied Mechanics, at