Mesh can rest on dandelion without disturbing the seeds, is lighter than a feather

Mankind has long been trailing nature in quality of materials.  A feather's intricate structural design lends it a density of 0.0025 g/cm3 [source].  By contrast, man has been forced to rely on much denser materials like Styrofoam, which has a density of ~0.02 g/cm3 [source].  

But mankind has at last one-upped nature, producing a metal thin-film mesh, which has a density of 0.0009 g/cm3 -- about a third the density of feathers.  It can rest gently atop a bed of dandelion fluff without damaging the bloom.

Metal mesh on dandelion

The material was invented by researchers at the University of California, Irvine; HRL Laboratories (a commercial partner); and the California Institute of Technology.  

William Carter, manager of the architected materials group at HRL and the senior author of the paper on the work, says the material draws inspiration from human macroscopic structural engineering triumphs, stating, "Modern buildings, exemplified by the Eiffel Tower or the Golden Gate Bridge, are incredibly light and weight-efficient by virtue of their architecture. We are revolutionizing lightweight materials by bringing this concept to the nano and micro scales."

To build the incredible nanomesh, the researchers first made a polymer mesh using a self-propagating photopolymer waveguide technique.  Thiol-ene was the selected class of photopolymers (thiol-enes are four-branched hyrocarbon molecules with a central junction of silicon and a sulfur connector midway on each branch).

An electroless nickel plating technique was then applied.  When you want to coat a solid object in metal, one common way is to use electricity to force metal atoms to stick to the surface.  Another method relies on a chemical reaction to plate.  In this case the reaction is between hydrated phosphates and nickel, which is auto-catalyzing.  

The end result is a 100 nm thick layer of NiP, that's 7% phosphorous and 93% nickel by weight.  The layer is solid, and is a (supersaturated) solution of phosphorous.

The photoplastic is then eaten away using etching techniques.  What is left behind is essential tubes made out of smaller tube "beams".  This tubes out of tubes approach yields a substance that's surprisingly strong, but is also 99.99 percent air.

Metal nanomesh making

Making of the metal nanomesh [Image Source: HRL Laboratories/Science]

Dr. Tobias Schaedler, a HRL researcher who was the paper's first author, summarizes this slightly complicated production method, commenting, "The trick is to fabricate a lattice of interconnected hollow tubes with a wall thickness 1,000 times thinner than a human hair."

Individual metal nanotubes have a "hardness of 6 GPa and a modulus of 210 GPa".  By contrast the greater lattice -- microtubes formed of nanotube "beams" -- has a "compressive modulus of 529 kPa, with deviations from linear elastic behavior starting at ~10 kPa".  The material also was able to recover 98 percent of its shape after being unloaded from "50% compression".  

Anyone who's played with tin foil knows that most metals don't recover 98 percent of their shape after 50% compression.  The strength and resiliency of the metal mesh is impressive.  Multiple compression cycles diminish the stress and strength slightly, but after the third cycle the pseudohardened material's characteristics more or less are constant.

That's just a taste of the extensive strength tests the researchers did.  To get the full results, you'll have to check out "Ultralight Metallic Microlattices", published in a November issue of the ultra-prestigious peer-reviewed journal Science.  

UCI mechanical and aerospace engineer Lorenzo Valdevit, his school's principle investigator on the project, cheers, "Materials [nanomeshes] 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."

The collaborators see the material as promising for applications such as:
thermal insulation; battery electrodes; catalyst supports; and acoustic, vibration, or shock energy damping

Of course a big issue is the cost.  Electroless nickel plating is already an expensive process due to its complexity, although it can be somewhat diminished if multiple objects are coated in a single very large bath.  When you add in the additional cost of manufacturing the wave-guide photopolymer mesh, you have an intricate process that will likely take a long time.  The net result will be a very expensive material.

Still in applications like space travel and combat aircraft, where cost is less of issue compared to material performance, the expense could become a moot point given the fantastically low density and relatively good strength.

Coincidentally, the project was entirely funded by a Defense Advanced Research Projects Agency (DARPA) grant from the U.S. Department of Defense.

The researchers are patenting the manufacturing process for the material, other than the polymer waveguard, which they say is covered under U.S. Patents 7,382,959, 7,653,279, and 8,017,193.  Conveniently, HRL Laboratories owns all three of those waveguide patents already.

Sources: UCI, Science

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