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When it comes to metallurgy, it is common knowledge that smaller grains make for harder metals. But how exactly do you achieve these grains?

A group of Brown University researchers has found a method for smashing individual metal nanoclusters that leads to metals that are up to four times harder than naturally occurring structures. This new method is quite different from conventional hardening techniques.

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“Hammering and other hardening methods are all top-down ways of altering grain structure, and it’s very hard to control the grain size you end up with,” said in a press release Ou Chen, an assistant professor of chemistry at Brown and corresponding author of the new research.

“What we’ve done is create nanoparticle building blocks that fuse together when you squeeze them. This way we can have uniform grain sizes that can be precisely tuned for enhanced properties.”

For this research, the team used nanoparticles of gold, silver, palladium, and other metals and chemically stripped them of the organic molecules called ligands, which generally prevent the formation of metal-metal bonds between particles. The clusters were then able to fuse together with just a bit of pressure.

The new metal coins made with the technique were found to have electrical conduction and light reflectance virtually identical to standard metals but their optical properties were dramatically changed.

“Because of what’s known as the plasmonic effect, gold nanoparticles are actually purplish-black in color,” Chen said. “But when we applied pressure, we see these purplish clusters suddenly turn to a bright gold color. That’s one of the ways we knew we had actually formed bulk gold.”

The researchers are now looking to apply the technique to commercial products as the chemical treatment is relatively simple to execute. Chen has currently patented the technique and sees great potential for it” both for industry and for the scientific research community.”

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