Metallic microlattice

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A block of metallic microlattice being supported by a dandelion seed head.

A metallic microlattice is a synthetic porous metallic material, consisting of an ultra-light metal foam. With a density as low as 0.9 kg/m3 (0.00561 lb/ft3), it is one of the lightest structural materials known to science.[1] It was developed by a team of scientists from HRL Laboratories, in collaboration with researchers at University of California, Irvine and Caltech, and was first announced in November 2011. The prototype samples were made from a nickel-phosphorus alloy.[2] In 2012, the microlattice prototype was declared one of 10 World-Changing Innovations by Popular Mechanics.[3]

Synthesis[edit]

To produce their metallic microlattice, the HRL/UCI/Caltech team first prepared a polymer template using a technique based on self-propagating waveguide formation,[4][5] though it was noted that other methods can be used to fabricate the template.[6] The process passed UV light through a perforated mask into a reservoir of UV-curable resin. Fiber-optic-like "self-trapping" of the light occurred as the resin cured under each hole in the mask, forming a thin polymer fiber along the path of the light. By using multiple light beams, multiple fibers could then interconnect to form a lattice.

The process was similar to photolithography in that it used a two-dimensional mask to define the starting template structure, but differed in the rate of formation: where stereolithography might take hours to make a full structure, the self-forming waveguide process allowed templates to be formed in 10–100 seconds. In this way, the process enables large free-standing 3D lattice materials to be formed quickly and scalably. The template was then coated with a thin layer of metal by electroless nickel plating, and the template is etched away, leaving a free-standing, periodic porous metallic structure. Nickel was used as the microlattice metal in the original report. Owing to the electrodeposition process, 7% of the material consisted of dissolved phosphorus atoms, and it contained no precipitates.[6]

Properties[edit]

A metallic microlattice is composed of a network of interconnecting hollow struts. In the least-dense microlattice sample reported, each strut is about 100 micrometres in diameter, with a wall 100 nanometres thick. The completed structure is about 99.99% air by volume,[2] and by convention, the mass of air is excluded when the microlattice density is calculated.[6] Allowing for the mass of the interstitial air, the true density of the structure is approximately 2.1 mg/cm3 (2.1 kg/m3), which is only about 1.76 times the density of air itself at 25 °C. The material is described as being 100 times lighter than Styrofoam.[7]

Metallic microlattices are characterized by very low densities, with the 2011 record of 0.9 mg/cm3 being among the lowest values of any known solid. The previous record of 1.0 mg/cm3 was held by silica aerogels, and aerographite is claimed to have a density of 0.2 mg/cm3.[8] Mechanically, these microlattices are behaviorally similar to elastomers and almost completely recover their shape after significant compression.[9] This gives them a significant advantage over current aerogels, which are brittle, glass-like substances. This elastomeric property in metallic microlattices furthermore results in efficient shock absorption. Their Young's modulus E exhibits different scaling, with the density ρ, E ~ ρ2, compared to E ~ ρ3 in aerogels and carbon nanotube foams.[6]

Applications[edit]

Metallic microlattices may find potential applications as thermal and vibration insulators such as shock absorbers, and may also prove useful as battery electrodes and catalyst supports.[6] Additionally, the microlattices' ability to return to their original state after being compressed may make them suitable for use in spring-like energy storage devices.[2]

Similar materials[edit]

A similar but denser material, consisting of an electrodeposited nanocrystalline nickel layer over a polymeric rapid-prototyped truss, was created by researchers at the University of Toronto in 2008.[10] In 2012, German researchers created a carbon foam known as aerographite, with an even lower density than a metallic microlattice.[11] In 2013, Chinese scientists developed a carbon-based aerogel which was claimed to be lighter still.[1]

References[edit]

  1. ^ a b "In pictures: Ultra-light material". BBC. 9 April 2013. Retrieved 1 July 2013. 
  2. ^ a b c "Metallic microlattice 'lightest structure ever'". Chemistry World. 17 November 2011. Archived from the original on 21 November 2011. Retrieved 21 November 2011. 
  3. ^ Sterling, Robert (29 October 2012). "The world's lightest material". Boeing. Archived from the original on 2 November 2012. Retrieved 2 November 2012. 
  4. ^ Jacobsen, A.J.; Barvosa-Carter, W.B.; Nutt, S. (2007). "Micro-scale Truss Structures formed from Self-Propagating Photopolymer Waveguides". Advanced Materials 19 (22): 3892–3896. doi:10.1002/adma.200700797. 
  5. ^ US patent 7382959, Alan J. Jacobsen, "Optically oriented three-dimensional polymer microstructures", assigned to HRL Laboratories, LLC. 
  6. ^ a b c d e Schaedler, T. A.; Jacobsen, A. J.; Torrents, A.; Sorensen, A. E.; Lian, J.; Greer, J. R.; Valdevit, L.; Carter, W. B. (Received 25 July 2011, published 12 October 2011). "Ultralight Metallic Microlattices". Science 334 (6058): 962–5. Bibcode:2011Sci...334..962S. doi:10.1126/science.1211649. PMID 22096194.  Check date values in: |date= (help)
  7. ^ "World's 'lightest material' unveiled by US engineers". BBC News. 18 November 2011. Retrieved 25 November 2011.
  8. ^ New carbon nanotube struructure aerographite is lightest material champ. Phys.org. 13 July 2012. Retrieved 14 July 2012.
  9. ^ Stephen Shankland (18 November 2011). "Breakthrough material is barely more than air". CNET. Retrieved 26 April 2013.
  10. ^ Gordon, L.M.; Bouwhuis, B.A.; Suralvo, M.; McCrea, J.L.; Palumbo, G.; Hibbard, G.D. (2009). "Micro-truss nanocrystalline Ni hybrids". Acta Materialia 57: 932–939. doi:10.1016/j.actamat.2008.10.038. 
  11. ^ "Aerographit: Forscher entwickeln leichtestes Leichtgewicht". Der Spiegel (in German). 11 July 2012. Retrieved 1 July 2013. 

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