Julia Greer, professor of materials science and mechanics, has made more progess with the fractal nanotruss—nano because the structures are made up of members that are as thin as five nanometers (five billionths of a meter); truss because they are carefully architected structures that might one day be used in structural engineering materials.
Greer’s group has developed a three-step process for building such complex structures very precisely. They first use a direct laser writing method called two-photon lithography to “write” a three-dimensional pattern in a polymer, allowing a laser beam to crosslink and harden the polymer wherever it is focused. At the end of the patterning step, the parts of the polymer that were exposed to the laser remain intact while the rest is dissolved away, revealing a three-dimensional scaffold. Next, the scientists coat the polymer scaffold with a continuous, very thin layer of a material—it can be a ceramic, metal, metallic glass, semiconductor, “just about anything,” Greer says. In this case, they used alumina, or aluminum oxide, which is a brittle ceramic, to coat the scaffold. In the final step they etch out the polymer from within the structure, leaving a hollow architecture.
“Having full control over the architecture gives us the ability to tune material properties to what was previously unattainable with conventional monolithic materials or with foams,” says Greer. “For example, we can decouple strength from density and make materials that are both strong (and tough) as well as extremely lightweight. These structures can contain nearly 99 percent air yet can also be as strong as steel. Designing them into fractals allows us to incorporate hierarchical design into material architecture, which promises to have further beneficial properties.”
Strong and stiff and materials comprised of mostly air are well on their way to commercialization. In this World Economic Forum Discussion, Caltech materials scientist Julia Greer talks about their use in “hierarchal design,” and the impact it will have on increased efficiency and the prominence of solar cells.
The catalytic properties of materials depend strongly on their microscopic structure, with the atomic-level chemistry and structure directly influencing the activity and durability of the catalyst. However, these microscopic properties can be difficult to understand and control. Furthermore, most efficient catalysts contain substantial amounts of precious metals, rendering them prohibitively expensive. The search for efficient, inexpensive catalysts has, therefore, been challenging. On page 1339 of this issue, Chen et al. report the synthesis of a new class of electrocatalysts built from platinum-nickel nanocrystals. Their Pt3Ni nanoframes have more than 22 times the catalytic activity of conventional platinum/carbon catalysts at 0.9 V, yet contain about 85% less precious metal.
Nanometer thick walls that are made into hollow trusses to enable far lighter and stronger material.
In the analysis of complex, hierarchical structural meta-materials, it is critical to understand the mechanical behavior at each level of hierarchy in order to understand the bulk material response. We report the fabrication and mechanical deformation of hierarchical hollow tube lattice structures with features ranging from 10 nm to 100 μm, hereby referred to as nanolattices. Titanium nitride (TiN) nanolattices were fabricated using a combination of two-photon lithography, direct laser writing, and atomic layer deposition. The structure was composed of a series of tessellated regular octahedra attached at their vertices. In situ uniaxial compression experiments performed in combination with finite element analysis on individual unit cells revealed that the TiN was able to withstand tensile stresses of 1.75 GPa under monotonic loading and of up to 1.7 GPa under cyclic loading without failure. During the compression of the unit cell, the beams bifurcated via lateral-torsional buckling, which gave rise to a hyperelastic behavior in the load–displacement data. During the compression of the full nanolattice, the structure collapsed catastrophically at a high strength and modulus that agreed well with classical cellular solid scaling laws given the low relative density of 1.36 %. We discuss the compressive behavior and mechanical analysis of the unit cell of these hollow TiN nanolattices in the context of finite element analysis in combination with classical buckling laws, and the behavior of the full structure in the context of classical scaling laws of cellular solids coupled with enhanced nanoscale material properties.
This paper presents the design and fabrication of 3-dimensional hollow metallic nanolattices using 2-photon lithography. The ability to fabricate structures of any geometry, with resolution down to 150 nm, provides opportunities to engineer structures spanning multiple length scales with potential to capitalize on combined structural and material size effects for use in many technological applications.