Nanocardboard made out of an aluminum oxide film
Nanocardboard made out of an aluminum oxide film

Corrugated paper cardboard is commonplace in manufacturing and packaging as it so light yet stiff. Now, researchers at the University of Pennsylvania have shown how make 'nanocardboard’ that is similar to corrugated cardboard but is ultrathin and ultralight, and acts as an excellent thermal insulator. A square centimeter of the material weighs less than a thousandth of a gram and it can spring back into shape after being bent very sharply – as opposed to standard cardboard, which becomes creased and permanently weakened – offering potential for applications in aerospace and microrobotics.

As described in Nature Communications [Lin et al. Nat. Commun. (2018) DOI: 10.1038/s41467-018-06818-6], the material is made from an aluminum oxide film with a thickness of tens of nanometers that forms a hollow plate, making a sandwich structure that is over ten thousand times as stiff as a solid plate of the same mass. To make these optimal structures at the nanoscale they cannot glue the face sheets to the webbing as with ordinary corrugated cardboard, so a monolithic structure was designed that is slightly less stiff than a standard sandwich plate and which offers some unique mechanical properties.

“That's the surprising thing about our nanocardboard; when you bend it, it recovers as if nothing happened. That has no precedent at the macroscale.”Igor Bargatin

The team used a solid silicon template with channels running through it. Aluminum oxide was then chemically deposited in a nanometer-thick layer on top of the silicon; once the template is encased, the nanocardboard can be cut to size. When the sides are exposed, the silicon on the inside can be etched away, leaving a hollow shell of aluminum oxide with a network of tubes connecting the top and bottom faces.

A basket-weave pattern with close-set, slit-shaped channels arranged in alternating directions was used, as this limits the potential for wrinkles to form but is tough under extreme bending. As co-leader Igor Bargatin said “That's the surprising thing about our nanocardboard; when you bend it, it recovers as if nothing happened. That has no precedent at the macroscale.”

Nanocardboard is fairly expensive to make as it requires machines typically used to make electronic chips, which constrains applications to quite small devices with high value. However, with its mechanical and thermal properties being crucial to potential uses, the researchers are exploring a propulsion mechanism that allows the material to levitate when illuminated by light, which could lead to new types of small-scale flying robots, called microrobotic flyers, which can operate at both atmospheric pressure and very high altitudes. They are also investigating thermal insulators in microfabricated energy converters, since the material can recover its shape regardless of what deformations or temperatures it experiences, as well as MEMS sensor applications that would benefit from the very low thickness of the nanocardboard structure.