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The vial contains liquid-metal particles suspended in ethanol. The particles were used to demonstrate heat-free soldering. (Credit: Christopher Gannon/Iowa State University)

liquids

This liquid can solder metals without heat

Researchers have created new liquid particles that can solder metals at room temperature.

The project started as a search for a way to stop liquid metal from returning to a solid—even below the metal’s melting point. That’s something called undercooling and it has been widely studied for insights into metal structure and metal processing. But it had been a challenge to produce large and stable quantities of undercooled metals.

Researchers at Iowa State University thought if tiny droplets of liquid metal could be covered with a thin, uniform coating, they could form stable particles of undercooled liquid metal. The engineers experimented with a new technique that uses a high-speed rotary tool to sheer liquid metal into droplets within an acidic liquid.

And then nature lends a hand: The particles are exposed to oxygen and then an oxidation layer is allowed to cover the particles, essentially creating a capsule containing the liquid metal. That layer is then polished until it is thin and smooth.

They proved the concept by creating liquid-metal particles containing Field’s metal (an alloy of bismuth, indium, and tin) and particles containing an alloy of bismuth and tin. The particles are 10 micrometers in diameter, about the size of a red blood cell. A paper published in Scientific Reports describes the process.

“We wanted to make sure the metals don’t turn into solids,” says Martin Thuo, an assistant professor of materials science and engineering and an associate of the US Department of Energy’s Ames Laboratory. “And so we engineered the surface of the particles so there is no pathway for liquid metal to turn to a solid. We’ve trapped it in a state it doesn’t want to be in.”

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Those liquid-metal particles could have significant implications for manufacturing.

“We demonstrated healing of damaged surfaces and soldering/joining of metals at room temperature without requiring high-tech instrumentation, complex material preparation, or a high-temperature process,” the engineers wrote in their paper.

Thuo and the Iowa State Research Foundation Inc. have filed for a patent on the technology. Thuo supported the project with faculty startup funds from Iowa State and funds from a Black and Veatch faculty fellowship. The project also included imaging work at the Center for Nanoscale Systems at Harvard University.

Source: Iowa State University

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