Recently, there has been a great deal of interest in strong and lightweight materials for use in the aerospace industry. By replacing the aluminum alloys and polymer composites that are currently used in aircraft, fuel consumption could be dramatically reduced, or weight could be reallocated to enable innovative designs such as electric planes. An active area of research is porous, lattice structures in which the lattice geometry is designed to maximize strength while using the least amount of material. Porous structures made of metals take advantage of the high strength and high ductility that make metals invaluable as structural materials. Reducing the wall thickness of these metallic structures to the nanoscale utilizes an additional strengthening mechanism. This is because it is difficult for defects to form in this limited thickness, which leads to extremely high strengths.
Fabricating materials that combines all of these attributes has been difficult. In our Nature Communications paper on “Hardening in Au-Ag Nanoboxes from Stacking Fault-Dislocation Interactions”, we adapt colloidal synthesis techniques to make metallic nanoboxes of ~100 nm in size, in the shape of hollow cubes with the corners cut off. The two key features of the nanoboxes are their ~15 nm wall thickness (approximately 100 atoms), and their cubic structural geometry, which is a common motif in structural applications.
These nanoboxes were too small to test using conventional tools, so compression testing was performed inside of a scanning electron microscope. These measurements revealed an interesting hardening behavior, in which it takes progressively higher loads to compress the nanoboxes to larger strains. This is an attractive mechanical property to have in a structural material, but usually involves microstructural features such as precipitates. As the nanoboxes did not contain these type of microstructural features, we did not know why hardening was occurring.
To investigate this issue, we teamed up with computer simulation experts from the A*Star Institute of High Performance Computing in Singapore to look at atomistic motion within the nanobox walls under mechanical loading. This was also a great opportunity to reconnect with researchers that I have known since graduate school. The simulation team found that planes of atoms were being displaced during compression testing (stacking faults), running into each other and intersecting, and that this was leading to the hardening behavior.
To obtain complementary experimental data, one of my graduate students traveled to the Center of Integrated Nanotechnologies at Sandia National Laboratory, to use in-situ transmission electron microscope mechanical testing facilities that were not available at Stanford. He was able to obtain high-resolution videos of nanobox compression, which we then compared to our proposed deformation mechanism.
This paper represents an exciting step towards our goal of using colloidal nanomaterials in multifunctional structural materials. Now that we know how one nanobox behaves under stress, we can think about making strong and lightweight self-assembled structures formed of many nanoboxes, or other nanoscale building blocks. The use of colloidal synthesis also opens up the possibility of going beyond traditional structural materials. For instance, semiconductor materials could be used to impart catalytic or optical properties that could be controlled by applying strain.