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'Nanotruss' structures combine high strength with extreme light weight

25 May 2014

Professor Julia Greer and her group at Caltech have developed a three-step process for building complex 'nanotruss' structures with great precision.

A fractal nanotruss created by Professor Julia Greer and her team at Caltech
A fractal nanotruss created by Professor Julia Greer and her team at Caltech

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 material — a ceramic, metal, semiconductor, or whatever. In this case, they used the brittle ceramic, alumina to coat the scaffold.

In the final step they etch out the polymer from within the structure, leaving a hollow architecture.

Taking advantage of some of the size effects that many materials display at the nanoscale, these nanotrusses can have unusual, desirable qualities. For example, intrinsically brittle materials, like ceramics, including the alumina used here, can be made deformable so that they can be crushed and still rebound to their original state without global failure.

"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."

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