Scientists have developed micro-scale, liquid-metal particles that can be used for heat-free soldering and fabricating, repairing and processing of metals - all 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. This is called 'undercooling' and it has been widely studied for insights into metal structure and metal processing. However, it had been a challenge to produce large and stable quantities of undercooled metals. Researchers from Iowa State University in the US thought if tiny droplets of liquid metal could be covered with a thin, uniform coating, they could form stable particles of undercooled liquid metal. They experimented with a new technique that uses a high-speed rotary tool to sheer liquid metal into droplets within an acidic liquid. The particles were exposed to oxygen and then an oxidation layer was allowed to cover the particles, essentially creating a capsule containing the liquid metal.
The layer was then polished until it was thin and smooth. The researchers 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 micrometres in diameter, about the size of a red blood cell. "We wanted to make sure the metals don't turn into solids," said Martin Thuo, an assistant professor at Iowa State University. "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," said Thuo, who is also an associate of the US Department of Energy's Ames Laboratory. Those liquid metal particles could have significant implications for manufacturing, researchers said. "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," they said. The research was published in the journal Scientific Reports.