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Making metal with the lightness of air

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Posted June 4, 2019

Gold, silver and copper are heavy metals, but LLNL scientists can now make them nearly as light as air — in a form so tiny it can ride on a mosquito’s back.

The groundbreaking science, part of a joint NIF/Physical and Life Sciences (PLS) project supported by the Laboratory Directed Research and Development (LDRD) Program, created these ultra-low density metal foams to give physicists better X-ray sources to employ in experiments that support NIF’s Stockpile Stewardship mission.

The foam is the product of a nearly decade-long research effort by members of the Lab’s NIF and PLS directorates for use on inertial confinement fusion (ICF) experiments at NIF, the world’s most energetic laser system.

A mosquito standing on cotton fibers carries a sample of ultra-low density gold aerogel. Photos by Joshua DeOtte.

A mosquito standing on cotton fibers carries a sample of ultra-low density gold aerogel. Photos by Joshua DeOtte/LLNL

“We are looking primarily at fundamental science questions that govern how to synthesize, assemble and shape metal nanowire-based aerogels,” said materials scientist Michael Bagge-Hansen, the LDRD project’s principal investigator.

The material is called foam because that’s historically what these types of materials were named, but it’s not a material made by foaming. It’s a spaghetti-like web of randomly connected nanometer-sized wires, formed into the shape of a miniature marshmallow and containing the same or fewer number of atoms as air.

Physicist Sergei Kucheyev calls it a “porous metal monolith. There’s a lot going on here in terms of both chemistry and physics.”

X-ray sources

An ultra-low density metal foam sample dangles from a single strand of a spider’s web.

Scientists sought different ultra-low density metals that can be used as targets for laser-driven X-ray sources for experiments further probing the properties of various materials placed under the extreme conditions possible when NIF’s 192 high-powered lasers are directed inside the target chamber, said Tyler Fears, a staff scientist with the LLNL’s Materials Science Division (MSD).

Each element emits a characteristic set of X-rays when heated by lasers into a plasma, Fears explained. Metal foams can mimic gas even though they are made from materials that are not gas at room temperature.

The underlying physics of laser-driven X-ray sources, however, sets the bar high with rigorous specifications for the types, densities, shapes and sizes of metal foams needed for experiments.

“We need heavy metal targets to be around the density of air and a few millimeters in size within well-defined dimensions,” he said. “Our challenge is to try to meet all those goals at the same time.”

The team also had to make sure the techniques they developed could be repeated to consistently produce the foams, even if the size, shape and composition are changed to meet future experimental needs.

Source: LLNL

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