High-performance golf clubs and aircraft wings are made of titanium, which is as strong as steel but weighs half the weight of steel. These characteristics depend on how the metal atoms are stacked, but the random defects that occur during manufacturing mean that these materials are only a small part of the theory. An architect working on a single atomic scale can design and manufacture new materials with better strength-to-weight ratios.

In a new study published in the Natural Science Report , researchers at the University of Pennsylvania School of Engineering and Applied Sciences, the University of Illinois at Urbana-Champaign and Cambridge University did just that. They made a piece of nickel with nano-scale pores that was as strong as titanium, but weighed four to five times less.

The voids of the pores and their self-assembly process make the porous metal similar to natural materials such as wood .

Just as the graininess of wood grain acts as a biological function that transfers energy, the blank space in the researchers' "metal wood" can also be injected into other materials . Injecting the scaffold together with the anode and cathode materials will enable this metal wood to serve a dual purpose: the planar wing or prosthesis is also a battery.

The study was led by James Pikul, assistant professor of the Department of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania. Bill King and Paul Braun of the University of Illinois at Urbana-Champaign and Vikram Deshpande of the University of Cambridge contributed to the research.

Even the best natural metals have atomic alignment defects that limit their strength. The titanium block, in which each atom is perfectly aligned with its neighbors, is ten times stronger than the blocks produced today. Materials researchers have been trying to exploit this phenomenon by using building methods to design the necessary geometric control structures to unlock the mechanical properties produced at the nanoscale, where defects reduce the effects.

The success of Pikul and his colleagues is due to the inspiration of nature.

"The reason we call metal wood is not just its density, but the density of the wood, but its cellular properties," Pikul said. "The cell material is porous; if you look at the wood grain, is that what you see? - Thick and dense parts for the fixed structure, some of which are porous, used to support biological functions, such as transport to cells .

“Our structure is similar,” he said. “Our area is thick and dense, with strong metal pillars, and the porous area has air gaps. We only operate on the length scale, and the strength of the pillars is close to the theoretical maximum.”

The researchers' pillars in metal wood are about 10 nanometers wide, or about 100 nickel atoms. Other methods involve the use of techniques similar to 3D printing to fabricate nanoscale scaffolding with hundred nanometer accuracy, but slow and arduous processes are difficult to scale to useful sizes.

“We already know that getting smaller will make you stronger in a while,” Pikul said. “But people can't make these structures with powerful materials that are big enough that you can do something useful. The size of the material is similar to the size of a small flea, but by our method we can produce 400 times larger metal wood samples."

Pikul's method starts with a tiny plastic ball, a few hundred nanometers in diameter, suspended in water. As the water evaporates slowly, the sphere settles and accumulates like a projectile, forming an ordered crystal frame. Using a plating technique, a thin layer of chromium was added to the hub cap and the researchers then immersed the nickel in the plastic sphere. Once the nickel is in place, the plastic ball dissolves in the solvent and forms an open network of metal pillars.

“The foil we made from this metal wood is about a square centimeter, or about the size of a game mold,” says Pikul. "In order to give you a sense of scale, there are about 1 billion nickel pillars."

Since about 70% of the resulting material is empty, the density of such nickel-based metal wood is extremely low compared to its strength. The density is the same as water, and the bricks of the material will float.

Replicating this production process on a business-related scale is the team's next challenge. Unlike titanium, the materials involved are not particularly rare or expensive, but the infrastructure required to use them at the nanoscale is currently limited. Once the infrastructure is developed, economies of scale should produce meaningful quantities of metal wood faster and cheaper.

Once researchers can produce larger sizes of metal wood samples, they can begin more macro testing. For example, a better understanding of its tensile properties is critical.

"For example, we don't know if our metal wood will be sunken like metal, or it will break like glass," Picur said. “Just like the random defect of titanium limits its overall strength, we need to better understand how the defects in the metal wood pillar affect its overall performance.”

At the same time, Pikul and his colleagues are exploring ways in which other materials can be incorporated into the pores of metal wood scaffolding.

“The long-term interesting thing about this job is that we are able to produce materials with the same strength properties as other ultra-high-strength materials, but now we have 70% space,” says Pikul. “One day, you can fill this space with something else, like a living organism or a material that stores energy.”

Further exploration: The team developed a series of bio-inspired artificial wood from traditional resins

For more information: James H. Pikul et al., High-strength metal wood from nanostructured nickel inverse opal materials, Scientific Reports (2019). DOI: 10.1038 / s41598-018-36901-3