When choosing materials to make something, tradeoffs need to be made among properties such as thickness, stiffness, and weight. A new material called nanocardboard was developed that is made out of an aluminum oxide film with a thickness of tens of nanometers, forming a hollow plate with a height of tens of microns. Its sandwich structure, similar to that of corrugated cardboard, makes it more than 10,000 times as stiff as a solid plate of the same mass.
Nanocardboard's stiffness-to-weight ratio makes it suited for aerospace and microrobotic applications. In addition to mechanical properties, nanocardboard is an excellent thermal insulator since it mostly consists of empty space. Also, shining a light on a piece of nanocardboard allows it to levitate. Heat from the light creates a difference in temperatures between the two sides of the plate, which pushes a current of air molecules out through the bottom.
Sandwich structures such as corrugated cardboard are attractive because they reduce the overall weight of a material without sacrificing much in the way of its overall strength. Sandwich composites like the corrugated paper cardboard are known to provide the best possible combination of low weight and high stiffness. The difficulty of scaling this concept down to the nano realm has to do with the way the sandwich layers are connected to its interior.
To be made at all, nanocardboard would need to be monolithic — composed out of a single contiguous piece of material — but how to give such a material the necessary sandwich layers was yet unknown. The new process involves making a solid silicon template with channels running through it. Aluminum oxide can then be chemically deposited in a nanometer-thick layer over the silicon. After the template is encased, the nanocardboard can be cut to size. Once 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.
The basketweave pattern features close-set, slit-shaped channels arranged in alternating directions. The pattern is key to nanocardboard's toughness under extreme bending. With enough force, corrugated cardboard can be bent sharply, but it will snap, creating a crease where it is permanently weakened. With the nanocardboard, when it is bent, it recovers fully.
The unique mechanical and thermal properties are critical for nanocardboard's potential uses from microrobotic flyers to thermal insulators in microfabricated energy converters, as the material would need to recover its shape regardless of what deformations or temperatures it goes through.