Engineers from Iowa State University (ISU) have developed a method to make micro-scale, liquid-metal particles that can be used for heat-free soldering, fabricating, repairing and processing of metals at room temperature.

The process, developed by ISU Assistant Professor of Materials Science and Engineering Martin Thuo and colleagues, started as a search for a way to stop liquid metal from returning to a solid—even at temperatures below the metal’s melting point. Known as "undercooling," 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

Thuo’s research team thought that 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 shear liquid metal into droplets within an acidic liquid.

ISU Assistant Professor Martin Thuo holds a vial of liquid-metal particles. Working behind him are (l-r) Simge Cinar, Jiahao Chen and Ian Tevis. Image credit: Christopher Gannon.The particles were then exposed to oxygen, and an oxidation layer was allowed to cover the particles, essentially creating a capsule containing the liquid metal. That layer was then polished until it was thin and smooth.

Thuo’s research group 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 were 10 micrometers in diameter, about the size of a red blood cell.

“We wanted to make sure the metals don’t turn into solids,” Thuo says. “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.”

Those liquid metal particles could have significant implications for manufacturing, as the team 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 Iowa State Research Foundation has filed for a patent on the innovation. Thuo is working with a local startup, SAFI-Tech, to develop the technology for product demonstrati