Researchers are constantly seeking materials that are high-strength but low weight, similar to the properties of titanium. Key to achieving these is the way in which a metal’s atoms are stacked, and also maintaining that arrangement in the manufacturing process, which has been tricky.
Researchers at the University of Pennsylvania, the University of Illinois at Urbana–Champaign, and the University of Cambridge believe they’ve achieved a breakthrough in this aim with the development of a new material they’ve dubbed “metallic wood”: a sheet of nickel with nanoscale pores that make it as strong as titanium but four to five times lighter.
|A microscopic sample of the “metallic wood” developed by researchers at the University of Pennsylvania. Its porous structure is responsible for its high strength-to-weight ratio, and makes it more akin to natural materials, like wood. (Image source: University of Pennsylvania)|
The Key is Empty Space
Key to material’s behavior are the pores’ empty spaces as well as the self-assembly process in which they’re made, which make the metal similar to a natural material, such as wood, explained James Pikul, assistant professor in the Department of Mechanical Engineering and Applied Mechanics at Penn Engineering in a news release.
“The reason we call it metallic wood is not just its density, which is about that of wood, but its cellular nature,” he said. “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.”
Similarly, the empty space between the pores also can be used to support another function with the infusion of other materials, Pikul said. For example, putting inside the scaffolding an anode and cathode material could allow it to form a double function as a battery as well as whatever industrial designed part it is, he said.
The metal’s similarity to wood is no accident. The team took inspiration from the natural world to try to overcome the limitation of the atomic arrangement of metals, the best natural examples of which even have defects that limit their strength.
If materials researchers could achieve a block of titanium where every atom was perfectly aligned with its neighbors, the result would be 10 times stronger than what can currently be produced. They typically can’t, however, so to solve this problem, some materials researchers have taken 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.
For their work, Pikul and his colleagues developed a structure similar to that of wood, with areas that are thick and dense with strong metal struts, and areas that are porous with air gaps, he said.
“We’re just operating at the length scales where the strength of struts approaches the theoretical maximum,” Pikul explained.
They also knew that they had to design aspects of the material on as small a scale as possible, since “going smaller gets you stronger,” Pikul said. However, so far, this has meant there aren’t structures big enough to develop something useful out of the materials, a problem the team wanted to remedy, he said.
“Most examples made from strong materials have been about the size of a small flea, but with our approach, we can make metallic wood samples that are 400 times larger,” he said.
The Building Blocks
That approach includes struts in the metallic wood about 10 nanometers wide, or about 100 nickel atoms across. These work together with tiny plastic spheres—a few hundred nanometers in diameter, suspended in water—that are the base of the method for building the material.
“We’ve made foils of this metallic wood that are on the order of a square centimeter, or about the size of a playing die side,” Pikul said. “To give you a sense of scale, there are about 1 billion nickel struts in a piece that size.”
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. Researchers published a paper on their work in the journal Nature Scientific Reports.
While the materials involved in the process aren’t rare or expensive on their own, currently there isn’t readily available infrastructure to work with them on the nanoscale, researchers said. The next challenge researchers aim to solve is to replicate this production process at sizes that are relevant to the commercial market.
After that the team needs to conduct more macroscale tests to acquire a better understanding of the material’s tensile properties, for example, Pikul said.
“We don’t know, for example, whether our metallic wood would dent like metal or shatter like glass,” he said. “Just like the random defects in titanium limit its overall strength, we need to get a better understand of how the defects in the struts of metallic wood influence its overall properties.”
Researchers also are exploring the ways other materials can be integrated into the pores in their metallic wood’s scaffolding to expand the material’s functionality, Pikul added.
Elizabeth Montalbano is a freelance writer who has written about technology and culture for 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco and New York City. In her free time she enjoys surfing, traveling, music, yoga and cooking. She currently resides in a village on the southwest coast of Portugal.