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 could be theoretically.
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. Just 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 such as anode and cathode materials to enable the metallic wood to serve double duty; for example, a plane wing or prosthetic leg that is also a battery.
A block of titanium in which every atom was perfectly aligned with every other atom would be ten times stronger than what can currently be produced. Cellular materials are porous; in wood grain, parts are thick and dense to hold the structure, and other parts are porous to support biological functions like transport to and from cells. Similarly, the metallic wood contains areas that are thick and dense with strong metal struts, and areas that are porous with air gaps. The struts in the metallic wood are about 10 nanometers wide, or about 100 nickel atoms across.
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, the 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.
Unlike 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 samples of the metallic wood can be produced in larger sizes, it can be subject to more macroscale tests. A better understanding of its tensile properties, for example, is critical.