Backward Melting

Imagine a wintry day with ice glazing every sidewalk and tree branch in sight. Then the temperature plummets and the ice begins to melt. Ice, of course, never behaves this way. But some materials really do turn from solid to liquid as they get colder, a phenomenon known as retrograde melting. An MIT team has now observed this bizarre, backward-seeming process in one of the most common, and useful, materials around us: silicon, the element used to make most solar cells and computer chips.

Silicon ordinarily melts at 1,414 °C. But researchers found that a silicon wafer contaminated with small amounts of copper, nickel, and iron formed a slushy mix of solid and liquid material as it cooled below 900 °C. At these temperatures, it’s possible to observe the behavior of silicon during melting, using a specialized x-ray fluorescence microprobe. The discovery has some potentially practical implications, and it may lead to useful new methods of purifying the material.

The finding was described in a paper published this year in Advanced Materials by Tonio Buonassisi, an assistant professor of mechanical engineering and manufacturing, along with Steve Hudelson, SM ‘09, and postdoctoral fellow Bonna Newman, PhD ‘08.

They first diffused copper, nickel, and iron into pure silicon and heated the mixture to 1,140 °C, well below silicon’s melting point but hot enough to produce a solution of the metals in the silicon. Rapidly cooling the mixture left the silicon supersaturated–that is, it held more dissolved metal than would normally be possible, just as soda in a bottle contains more carbon dioxide than the liquid would hold at ordinary pressure outside the bottle. But when the researchers reheated the material and allowed it to cool more slowly, liquid droplets of metal and silicon formed amid the solid material. “You hit a point where you induce precipitation, and it has no choice but to precipitate out in a liquid phase,” Buonassisi says.

The researchers found that impurities–in this case, the added copper, nickel, and iron, which are the main impurities normally found in silicon–tended to migrate to the liquid. Exploiting this phenomenon could make it possible to use cheaper grades of silicon for devices such as solar cells, because the material could be purified during the manufacturing process. “If you can create little liquid droplets inside a block of silicon,” Buonassisi says, “they serve like little vacuum cleaners to suck up impurities.” The droplets then solidify, retaining the concentrated impurities and leaving the rest of the material purer.