Researchers at Iowa State University have produced novel micro-sized particles filled with liquid metal that retains its liquid state at room temperature and are able to meld with each other. The new form of liquid/solid hybrid has practical applications in heat-free soldering and electronic circuit damage repair.

The discovery of this new metallic form was made when researchers looked for a way to prevent liquid metals from returning to their solid state, even at temperatures below the point at which they normally solidify. Known as "undercooling,", the technique has often been used to analyze the inner workings of metal structures and to look at alternative ways of processing metals. However, the greatest hurdle in this area is that it is difficult to create significant amounts of stable quantities of these undercooled metals.

Looking at this problem in a different way, the Iowa State team hit on the idea of shrouding minute droplets of liquid metal with a thin, uniform coating that held the liquid inside a sort of particle capsule, to form a balanced batch of particles containing undercooled liquid metal.

To create these particles, the researchers used a high-velocity rotary cutting tool to slice the liquid metal into minute droplets and then suspend them in an acetic acid/diethylene glycol mixture. When the particles were exposed to oxygen, the surface of the material oxidized and created a bubble with the liquid metal trapped inside. Once the oxidization was complete, the team then polished this layer until it was smooth.

"We wanted to make sure the metals don't turn into solids," said assistant professor Martin Thuo. "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."

Unlike metal glues used as solder which join two disparate substances together, such as Boston Northeastern University's MesoGlue, the Iowa State material is created using liquid-metal particles of the same metals. In this case, Field's metal (an alloy of bismuth, indium and tin) and other particles containing an alloy of just bismuth and tin were used to produce the liquid metal bubbles.

Only about the size of red blood cells at round 10 micrometers in diameter, these bubbles were tested by the team to repair microscopic defects, to perform micro-soldering, and to join macroscopic pieces of metal together in heat-free soldering.

"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," said the team in their paper.

Thuo has filed a patent on the technology and formed a start-up company to help bring this new product to market, and he is currently working on testing and improving the electrical conductivity and mechanical reliability of the new liquid-metal material.

The team's paper appears in the journal Scientific Reports.

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