‘Metallic wood’ with titanium strength can be grown at scale

In spite of technological advances in construction, wood remains a ubiquitous building material thanks to its high strength-to-density ratio.

For the past three years, University of Pennsylvania engineers have been developing a novel metallic material with the attractive qualities of wood. It gets its nickname - “metallic wood” - due to mimicking a key structural feature of natural wood: porosity. Metallic wood is a lattice of nanoscale nickel struts with regularly spaced cell-sized pores. This radically decreases its density without sacrificing strength; the material has the strength of titanium at a fraction of the weight.

The precise spacing of these pores also gives the material some unique optical properties. The spaces between gaps are the same size as wavelengths of visible light meaning that light reflected from the wood interferes, with the result that specific colours – depending on the angle of reflection – are enhanced. This gives the material an attractive and bright rainbow appearance, with the potential to be incorporated into sensing devices.

While the material has been under development for several years, the engineers have now solved a serious problem which prevented them manufacturing metallic wood at useful sizes: eliminating the inverted cracks which form as the material is grown from nanoparticles into metal films. Preventing these defects allows strips of the material to be grown in areas 20,000 times larger than before. The solution has been detailed in a Nature Materials paper.

When a crack forms in a conventional material, bonds between atoms break, eventually causing the material to split. An inverted crack, however, is an excess of atoms. In the case of metallic wood, these are extra nickel atoms filling the nanopores which give it its unique properties.

“Inverted cracks have been a problem since the first synthesis of similar materials in the late 1990s,” said graduate student Zhimin Jiang, who worked on the project. “Figuring out a simple way of eliminating them has been a long-standing hurdle in the field.”

Inverted cracks emerge from the way metallic wood is grown. It starts as a 'template' of stacked nanospheres. When nickel is deposited through the template, it forms a lattice around the spheres, which are subsequently dissolved to leave behind the nickel pore structure. However, the researchers found that if there are any places where the nanospheres’ regular stacking pattern is disrupted, nickel will fill those gaps and produce an inverted crack when the template is dissolved.

“The standard way to build these materials is to start with a nanoparticle solution and evaporate the water until the particles are dry and regularly stacked. The challenge is that the surface forces of water are so strong that they rip the particles apart and form cracks, just like cracks that form in drying sand,” explained Professor James Pikul. “These cracks are very 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 template wet.

“This prevents the films from cracking, but because the particles are wet we have to lock them in place using electrostatic forces so that we can fill them with metal.”

Now that it is possible to create larger, more consistent strips of metallic wood, Pikul and his colleagues are particularly interested in using it to build new devices. He said: “Our new manufacturing approach allows us to make porous metals that are three times stronger than previous porous metals at similar relative density and 1,000 times larger than other nanolattices.

“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.”