Research

The strength of titanium, the density of water: engineers have created a "metallic wood"

High performance golf clubs and wingsAirplanes are made of titanium, which is stronger than steel, but twice as light. These properties depend on the way in which the metal atoms are laid, but random defects that arise during the production process mean that these materials can be much stronger, but they will not. An architect collecting metals from individual atoms could design and build new materials that would have the best balance of strength and weight.

Is a metal tree possible?

In a new study published in NatureScientific Reports, researchers from the School of Engineering and Applied Sciences at the University of Pennsylvania, the University of Illinois and the University of Cambridge, did just that. They collected a nickel sheet with nanoscale pores that make it as durable as titanium, but four to five times lighter.

The empty space of the pores and the process of self-assembly make the porous metal look like a natural material, such as wood.

And just like the porosity of a tree trunkperforms the biological function of transporting energy, the empty space in the "metallic wood" can be filled with other materials. Filling forests with anode and cathode materials will allow the metal tree to serve a dual purpose: to be an airplane wing or a prosthetic leg with a battery.

The research was led by James Pikul, an assistant professor of mechanical engineering and applied mechanics at the University of Pennsylvania.

Even the best natural metals are defective.in the arrangement of atoms that limit their strength. A block of titanium, where every atom would be perfectly aligned with its neighbors, would be ten times stronger than what can be produced now. Material scientists tried to use this phenomenon by applying an architectural approach, designing structures with geometric controls necessary to unlock the mechanical properties that arise on a nanoscale scale, where defects have a reduced impact.

Pikul and his colleagues owe their success to nature.

"The reason we call itmetal tree is not only in its density, which is equal to the density of wood, but also in its cellular nature, ”says Pikul. “Cellular materials are porous; If you look at wood grain (typical wood laminate pattern), what will you see? Thicker and denser parts hold the structure, and more porous parts are necessary to maintain biological functions, such as transport into and out of the cell. ”

“Our structure is similar,” he says. “We have areas that are thick and dense, with strong metal struts, and areas that are porous, with air gaps. We simply work on a scale of length, where the strength of the struts is close to the theoretical maximum. ”

Spacers in metallic wood are wideabout 10 nanometers, or 100 nickel atoms across. Other approaches include the use of technologies such as three-dimensional printing, to create nanoscale forests with an accuracy of 100 nanometers, but the slow and painstaking process is difficult to scale to usable sizes.

"We knew that reducing sizes would make youstronger for a while, but people could not make large enough structures of these durable materials so that they could do something useful. Most of the examples made from durable materials were about the size of a small flea, but with our approach we can make samples of metallic wood that are 400 times larger. ”

Pikul’s method starts with tiny plasticspheres with a diameter of several hundred nanometers suspended in water. When the water evaporates slowly, the spheres settle and fold up like cannonballs, forming an ordered, crystalline skeleton. Using electroplating, with the help of which they usually add a thin layer of chromium to the cap, the scientists then fill the plastic spheres with nickel. As soon as the nickel is in place, the plastic spheres dissolve, leaving an open net of metal struts.

"We made a foil of this metal treethe size of the order of a square centimeter is the verge of the dice, ”says Pikul. “To give you an idea of ​​the scale, I’ll say that in one piece of this size there are about 1 billion nickel struts.”

Since the resulting material is 70% composed ofempty space, the density of nickel-based metallic wood is extremely low relative to its strength. At a density equal to the density of water, the brick of such a material will float.

The next task is to playThis production process is on a commercial scale. Unlike titanium, none of the materials involved is particularly rare or expensive in itself, but the infrastructure required for working at nanoscale is currently limited. Once it is developed, economies of scale will make the production of significant quantities of metallic wood faster and cheaper.

As soon as researchers can producesamples of their metallic wood in large sizes, they will be able to subject them to more extensive tests. For example, it is very important to better understand their tensile properties.

"We do not know, for example, whether ourmetal tree bend like metal or break like glass. Just as random defects in titanium limit its overall strength, we need to better understand how defects in metal struts affect its overall properties. ”

What applications could appear in such an iron tree? Offer in our channel in Telegram.