The development of “metal wood” to new heights

This strip of metallic wood, about an inch long and a third of an inch wide, is thinner than household aluminum foil but supports more than 50 times its weight without warping. If the weight is suspended from it, the bar itself can support more than six pounds without breaking. Credit: University of Pennsylvania

Natural wood remains a ubiquitous building material due to its high strength-to-density ratio; The trees are strong enough to grow hundreds of feet tall but still light enough to float downstream after the trees are cut down.

Over the past three years, engineers in the College of Engineering and Applied Sciences have developed a type of material they call “metallic wood.” Their material gets its useful properties and its name from the main structural feature of its natural counterpart: porosity. As a network of nano-nickel supports, metallic wood is filled with regularly spaced cell-size pores reducing its density drastically without sacrificing the strength of the material.

The precise spacing between these gaps gives metallic wood not only the strength of titanium at a fraction of the weight, but also unique optical properties. Because the gaps between the gaps are the same size as the wavelengths of visible light, the light reflected off the metallic wood is interfered with to enhance certain colors. Enhanced color changes depend on the angle at which the light is reflected off the surface, giving it a great look and potential for use as a sensor.

Pennsylvania engineers have now solved a big problem that prevents metallic wood from being made to meaningful sizes: eliminating the inverted cracks that form as the material grows from millions of nanoparticles into metal films large enough to build with. Preventing these defects, which have plagued similar materials for decades, It is allowed to collect strips of metal wood in areas 20 thousand times more than before.


Credit: University of Pennsylvania

James Bicol, assistant professor in the Department of Mechanical Engineering and Applied Mechanics, and Ximin Jiang, a graduate student in his lab, published a study demonstrating this improvement in the journal. nature materials.

When a crack forms within an everyday material, the bonds between its atoms break, eventually causing the material to crack. By contrast, the inverted slit is an excess of atoms; In the case of metallic wood, the inverted slits consist of additional nickel that fills nanopores important for its unique properties.

“Inverted notches have been a problem since the first synthesis of similar materials in the late 1990s,” says Jiang. “Finding out a simple way to eliminate it has been a long-standing obstacle in the field.”

These inverted notches stem from the way metallic wood is made. It starts as a template of nanodomains, stacked on top of each other. When nickel is deposited through the mold, it forms a lattice structure of metallic wood around the spheres, which can then be melted away to leave their characteristic pores.


The nanopores are key to the properties of metallic wood, but if there was a crack in the mold before the nickel was added, it would become an “inverted crack” – a crack of solid nickel – when the mold was removed. The researchers’ technique allows for crack-free areas that are 20,000 times larger than previously possible. Credit: University of Pennsylvania

However, if there are any places where the usual stacking pattern of the balls gets disrupted, nickel will fill in those gaps, producing an inverted crack when the die is removed.

“The standard way to build these materials is to start with a solution of nanoparticles and evaporate the water until the particles dry and stack uniformly. The challenge is that the water’s surface forces are so strong that they rupture the particles and form cracks, just like cracks that form in drying sand,” Becol says. These cracks are difficult to prevent in the structures we are trying to build, so we developed a new strategy that allows us to self-assemble the particles while keeping the mold moist. This prevents the films from cracking, but because the particles are wet, we have to lock them in place using electrostatic forces until we can fill them with metal.”

With larger and more uniform strips of metallic wood, researchers are becoming particularly interested in using this material to build better devices.

“Our new fabrication approach allows us to make porous metals that are three times stronger than previous porous metals with a similar relative density and 1,000 times larger than other nanogrids,” says Bicol. “We plan to use these materials to make a number of previously impossible devices, which we are already using as membranes to separate biomaterials in cancer diagnostics, protective coatings, and flexible sensors.”

The engineer’s “metallic wood” has the strength of titanium and the density of water

more information:
Zhimin Jiang et al, Centimeter crack-free self-assembly for ultra-high tensile strength metallic nanogrids, nature materials (2021). DOI: 10.1038 / s41563-021-01039-7

Provided by University of Pennsylvania

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