Natural wood is still a popular building material 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, University of Pennsylvania engineers have been working on a new type of material called "metallic wood". The key structural characteristic of the material's natural counterpart, porosity, is what gives it its useful properties and name. Metallic wood, which is a lattice made of nanoscale nickel struts, has regular cell-sized pores. These pores dramatically decrease the material's density without compromising 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 gaps are exactly the same size as visible light wavelengths, so metallic wood's light reflections can cause certain colors to be enhanced. The angle at which light reflects off the surface gives it enhanced color variations. This can give it a stunning appearance and the potential for being 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), published a study demonstrating the improvement in 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 atoms. For example, metallic wood has inverted cracks that contain extra nickel, which fills in the critical nanopores necessary for its unique properties.Jiang says that inverted cracks have been a problem ever since the early synthesis of similar materials in late 1990s. They have been a problem for a long time.This is how metallic wood is made. It begins as a template made up of nanoscale spheres stacked on top one another. The template forms a lattice structure of metallic wood around the spheres when nickel is deposited through it. This can be then dissolved to reveal its signature pores.If the spheres' normal stacking pattern is disturbed, the nickel will fill in the gaps and produce an inverted crack when it is 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 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 the particles must be locked in place by electrostatic forces.Researchers are now able to use larger, more consistent pieces of metallic wood in order to make better devices.Pikul states that "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 are already using them as membranes to separate biomaterials for cancer diagnostics, protective coatings, and flexible sensors.
