April 20, 2024

Natural wood is still popular due to its high strength-to-density ratio. Trees can grow to hundreds of feet in height but are light enough to float downstream after being logged.

Three years ago, the University of Pennsylvania engineers developed a new type of material called “metallic wood.” This material derives its usefulness and name from a fundamental structural characteristic of its natural counterpart, porosity. Metallic wood, a lattice made of nanoscale nickel streets, is filled with regularly spaced cells-sized pores, dramatically reducing its density but not affecting its strength.

These gaps are precisely spaced to give metallic wood the strength and optical properties of titanium while weighing only a fraction. The spaces between holes are the same size as visible light wavelengths, so metallic wood’s light reflections can cause specific colors to be enhanced. The angle at which light reflects off the surface enhances color variations. This can give it a stunning appearance and potentially be used as a sensor.

Penn Engineers has solved a major problem that prevented metallic wood from being made at meaningful sizes. They have eliminated the inverted cracks caused by the material growing from millions of tiny particles to large enough metal films to use for building. These defects have plagued similar materials for decades and prevent strips of metallic wood from being assembled in areas that are 20,000 times larger than before.

James Pikul, an assistant professor in the Department of Mechanical Engineering and Applied Mechanics, and Zhimin Jiang (a graduate student in his laboratory) have published a study demonstrating the improvement in the journalĀ Nature Materials.

Cracks can form in everyday materials when the bonds between their atoms are broken, eventually causing them to separate. Inverted cracks, on the other hand, are a surplus of scraps. For example, metallic wood has inverted cracks containing extra nickel, filling the critical nanopores necessary for its unique properties.

Jiang says that inverted cracks have been a problem since the early synthesis of similar materials in the late 1990s. They have been a problem for a long time.

This is how metallic wood is made. It begins as a template of nanoscale spheres stacked on one another. When nickel is deposited, the template forms a lattice structure in metallic timber around the spheres. This can then be dissolved to reveal its signature pores.

If the spheres’ regular stacking pattern is disturbed, the nickel will fill in the gaps and produce an inverted crack when removed.

The standard method to make these materials is to use a nanoparticle solution. Once the solution has evaporated, the particles can be dried and stacked regularly. Pikul says that water’s surface forces can cause the particles to break apart and crack, much like cracks in dry sand. These cracks are difficult to prevent from the structures we are trying to build. So we devised a new strategy to allow us to self-assemble particles while keeping the template moist. The films will not crack, but electrostatic forces must lock the particles in place.

Researchers can now use more significant, consistent pieces of metallic wood to make better devices.

Pikul states, “our new manufacturing process allows us to create porous metals three times stronger than previously porous metals with similar relative densities and 1,000 times more than other nanolattices.” These materials will be used to create a variety of previously impossible devices. We use them as membranes to separate biomaterials for cancer diagnostics, protective coatings, and flexible sensors.

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