Scientists at the California Institute of Technology have found a way to make sophisticated nanoscale metal structures using a 3D printer. If effectively scaled, the technology could have a range of commercial applications.

Objects made via 3D printing are constructed using additive manufacturing, the deposition of material layer by layer. The technology can yield unique substructures -- nanostructures that would be impossible to produce using more common manufacturing methods like etching or milling.

Scientists have succeeded at creating unique nanostructures using polymers, ceramics and other materials, but 3D printing metals has proven difficult.

When 3D printing polymer structures, extremely precise lasers use just a pair of photons to harden the liquified polymer. To fuse metals, however, more energy is required.

"Metals don't respond to light in the same way as the polymer resins that we use to manufacture structures at the nanoscale," Caltech materials scientist Julia Greer said in a news release. "There's a chemical reaction that gets triggered when light interacts with a polymer that enables it to harden and then form into a particular shape. In a metal, this process is fundamentally impossible."

Andrey Vyatskikh, a grad student and researcher in Greer's lab, developed a workaround using organic ligands to create a metal-enriched resin. Organic ligands naturally bind with metal. The resin functions like a polymer, but can carry along tiny fragments of metal.

In the lab, Vyatskikh and Greer created a resin of metal and organic ligands. The resin is used just like a normal polymer, zapped with the laser to harden into a preprogrammed design. With each zap, the chemical bonds between the organic molecules are strengthened, and because the organic ligands are already bonded with the metal, the nickel becomes incorporated into the newly hardened material.

Once printed, scientists put the metal-enriched polymer scaffolding into vacuum oven and heated it to 1,000 degrees Celsius, roughly 1,800 degrees Fahrenheit -- just hot enough to vaporize the organic molecules but not hot enough to melt the nickel.

Through a process known as pyrolysis, the heat strengthens the bonds between the metal molecules. Though the material shrinks, the organization of the scaffolding's nanoscale structure is maintained.

"That final shrinkage is a big part of why we're able to get structures to be so small," said Vyatskikh. "In the structure we built for the paper, the diameter of the metal beams in the printed part is roughly 1/1000th the size of the tip of a sewing needle."

Vyatskikh and Greer -- who described the work this week in the journal Nature Communications -- believe their technology could be used to build parts of computer chips or lightweight aerospace components.

The technology could also be used to create new kinds of metal-organic frameworks, or MOFs, a unique material with an expansive internal surface area. In a recent study, another group of scientists used MOFs to remove salt and ions from water.

In future studies, Vyatskikh and Greer hope to experiment with other metals and measure the impurities caused by pyrolysis.

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Changing the color of 3-D printed objects
Boston MA (SPX) Feb 05, 2018
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