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The Incredible Lightness of Being

December 5, 2011

Lighter is usually better. That clunky laptop computer that you may have owned years ago has been replaced by something smaller and lighter, possibly a tablet. If you have the function, why do you need the mass? On a fundamental mechanics level, this is what carbon nanotubes offer, strength without the bloat. Another low density functional material is the aerogel, which I wrote about in a previous article. (Carbon Nano-Aerogel, March 14, 2011)

A regular gel, is a colloid of interconnected particles "floating" in a liquid. For example, a gelatin dessert is a gel in which protein molecules are contained in water. Gels are interesting materials, since they have mechanical properties like solids, but they have a density similar to that of a liquid.

In an aerogel, the liquid is replaced by a gas, so aerogels have very low density. They are commonly used as a high-tech thermal insulators in aerospace applications for which weight is an important parameter. It's not surprising that aerogels were used as a thermal insulation material on the Mars Rovers. They are useful, also, for vibration damping.

A common aerogel is composed of silica glass and air, and it has just 0.01% the density of the glass itself. Carbon is another popular solid phase for aerogel materials, and carbon aerogels are excellent electrodes for electric double-layer capacitors (supercapacitors), since they have a high surface area.

A research team of scientists from HRL Laboratories (Malibu, CA), the University of California at Irvine, and the California Institute of Technology have just published a paper on a new, lightweight material in the November 18, 2011, issue of Science.[2-5] The density of this material, which they term an ultralight metallic microlattice, is just 0.9 mg/cc. This density just breaks the apparent record of 1.0 mg/cc for a silica gel, as quoted in Wikipedia.

This new material is a lattice array of hollow tubes produced by electrolessly nickel plating a template formed from a photopolymer.[1] The fabrication process was developed by HRL senior scientist, Alan Jacobsen,[5] and it's adaptable to metals other than nickel.[3] The interconnected hollow tubes have a wall thickness of just 100 nanometers.[5]

One important property of this material is that its Young's modulus scales with density (ρ) as ρ2. Ultralight aerogels and carbon nanotube foams have a Young's modulus that scales a ρ3. The ultralight metallic microlattice material shows complete recovery from 50% compressive strain, and it has an energy absorption like that of elastomers.

Copyright notice A picture is worth a thousand words, but my repeated requests to HRL Laboratories for permission to use a photograph of their microlattice material were ignored.

Click here for the image at their web site.

The unique mechanical properties of this material are a consequence of its hierarchical architecture that has features at the nanometer, micrometer and millimeter scales.[2] Bill Carter, manager of the Architected Materials Group at HRL, makes the analogy to larger structures.
"Modern buildings, exemplified by the Eiffel Tower or the Golden Gate Bridge, are incredibly light and weight-efficient by virtue of their architectures. We are revolutionizing lightweight materials by bringing this concept to the materials level and designing their architectures at the nano and micro scales."[5]

Says Lorenzo Valdevit, coauthor of the paper, Assistant Professor of Mechanical and Aerospace Engineering, and also Chemical Engineering and Materials Science, at the University of California at Irvine,
"Materials actually get stronger as the dimensions are reduced to the nanoscale... Combine this with the possibility of tailoring the architecture of the micro-lattice and you have a unique cellular material."[2]

As reported in the Los Angeles Times, the material will float in the air for more than ten seconds when dropped from shoulder height.[3] The research was funded by the DARPA, the Defense Advanced Research Projects Agency.[2,5] HRL Laboratories is a corporate research-and-development laboratory owned by The Boeing Company and General Motors.

The Unbearable Lightness of Being is a 1984 novel by Milan Kundera about Czechoslovakia in 1968. It was made into a 1988 movie.[6]

References:

  1. T. A. Schaedler, A. J. Jacobsen, A. Torrents, A. E. Sorensen, J. Lian, J. R. Greer, L. Valdevit and W. B. Carter, "Ultralight Metallic Microlattices," Science, vol. 334 no. 6058 (November 18, 2011), pp. 962-965.
  2. Multidisciplinary team of researchers develop world's lightest material, University of California at Irvine Press Release, November 17, 2011.
  3. Deborah Netburn, "Scientists invent lightest material on Earth. What now?" LA Times Blog, November 17, 2011.
  4. PR Web, HRL Researchers Develop World's Lightest Material, HRL Laboratories Press Release, November 18, 2011.
  5. HRL Researchers Develop World's Lightest Material, HRL Laboratories Press Release, November 18, 2011.
  6. The Unbearable Lightness of Being (1988, Philip Kaufman, Director) on the Internet Movie Database.