retrograde melting in silicon at MITThe physical world is full of surprises and head-scratching peculiarities, and a process called retrograde melting ranks right up there with the most counterintuitive phenomena.

Retrograde melting is the process of a solid substance turning to liquid as its temperature decreases. (Imagine sticking a popsicle in the freezer to turn it into juice, and you're in the ballpark.) Several metallic assemblages have been shown to exhibit retrograde melting, and a study published online in Advanced Materials July 29 demonstrates that silicon impregnated with metals can also turn to liquid as it cools.

Silicon's capacity to retain metal dopants in its crystalline structure increases with temperature, explain the study's authors, many of whom work in Tonio Buonassisi's Laboratory for Photovoltaic Research at the Massachusetts Institute of Technology. Buonassisi and his colleagues heated a silicon wafer along with copper, nickel and iron to 1,140 degrees Celsius—hot enough to increase the solubility of the metals in silicon but not hot enough to melt the silicon itself—and then cooled it quickly. The process left the silicon wafer supersaturated with metals; that is, it retained more metal atoms than it would be able to under normal conditions.

By reheating the material and then allowing it to cool past the point of solubility, the researchers witnessed the formation of droplets of metal and silicon—the supersaturated wafer precipitated out its metals in liquid form as it cooled.

The liquid droplets that form from the cooling silicon draw out dissolved impurities, Buonassisi's group reports. The researchers note that the melting process could find use as a way to control or remove impurities in silicon-based semiconductors such as solar cells and microchips.

Counterintuitive as retrograde melting may be, at least this trick of the physical world goes both ways. The opposite phenomenon was recently demonstrated in water, which a group of researchers coaxed into freezing as its temperature increased.

Photo of glowing silicon chip (orange square inside heating device): Patrick Gillooly