Despite the extraordinary mechanical, energy conversion and optical properties reported for three-dimensional (3D) architectures at the micro- and nanoscale, we have yet to capitalize on this to create billets of material at substantial sizes. If made accessible at the bulk scale, these architected materials could have widespread applications, ranging from photonic devices, to energy storage and conversion systems, and biomedical and electronic devices. However, the applicability of these types of 3D metamaterials is significantly limited by their achievable length scales and architectures within the available structural bandwidth.

—Zheng et al.

The Duke team created scalable, metallic, mechanical metamaterials that simultaneously achieve high strength and ultralow density, as well as high compressive and superelastic tensile behavior at a bulk scale 107-times larger than their smallest nanoscale feature sizes within the structure.

The scalable metamaterials contain hierarchical 3D topologies the feature size of which spans seven orders of magnitude in length scale—from tens of nanometers to tens of centimeters.

Creation of the metamaterial with this unprecedented scalability was enabled by a new type of additive manufacturing technique that is capable of miniaturized architectures over large areas, combined with nanoscale post-processing.

With these 3D features we’ve been fabricating on a nanoscale, you can get some really interesting properties, but people have never been able to scale them up and see how they behave. We’ve figured out a strategy of hierarchically building them to take advantage of the nanoscale features but use them at a large scale. It turned out better than we could have imagined.

—lead author Xiaoyu “Rayne” Zheng

An optical image of a bulk-scale hierarchical nickel phosphorous lattice material with a network of hierarchical stretch-dominated octet unit cells. Photo courtesy of Xiaoyu Zheng and William Smith. Click to enlarge.

The lattices were initially printed out of polymer, using a one-of-a-kind Large Area Projection Micro-Stereolithography (LAPuSL) printer invented by LLNL engineer Bryan Moran, who won an R&D 100 award for the design. The lattice structure was then coated with a nickel-phosphorus alloy and put through post-processing to remove the polymer core, leaving extremely lightweight, hollow tube structures.

Taking inspiration from the hierarchical structure of materials found in nature, such as wood and bone, the finished material incorporates multiple levels of feature size, from nanoscale films to the macroscale unit cells and lattices, within a single piece roughly five centimeters long. While testing the nickel-plated material, researchers found that structures fashioned with walls 700 nanometers thick broke under a 5% strain, while those with 60-nanometer walls stretched about 20% before failing.

Besides flexibility, researchers said the concept could overcome the current limitations of 3D-printed micro and nano-architectures by addressing the usual tradeoff between high resolution and build size and extend to a variety of large-scale applications, including aircraft parts, batteries or stretchable armor for the military.

We’re seeing some of the benefits you only get from the nanoscale at the macroscale. Using the structural concept of unit cells and lattices, combined with nanoscale features, you can achieve high strength at a very light weight, as well as a ductile-like behavior in materials that are normally brittle.

—Chris Spadaccini, director of the Lab’s Center for Engineered Materials and Manufacturing

The researchers said they could begin to scale up further to tens of centimeters and beyond, explore photonic and electronic properties, as well as incorporating other materials, including ceramics.

The Laboratory Directed Research & Development (LDRD) program, the Defense Advanced Research Projects Agency (DARPA) and Virginia Tech startup provided funding for study.

Resources

• Xiaoyu Zheng, William Smith, Julie Jackson, Bryan Moran, Huachen Cui, Da Chen, Jianchao Ye, Nicholas Fang, Nicholas Rodriguez, Todd Weisgraber & Christopher M. Spadaccini (2016) “Multiscale metallic metamaterials” Nature Materials doi: 10.1038/nmat4694