Skip to Content
MIT News magazine

Backward Melting

Getting silicon to melt as it cools

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.

A different kind of melting A tiny silicon chip (the glowing orange square at the center of this special heating device), having been supersaturated with metal contaminants, undergoes a retrograde melting process after being heated and then very slowly cooled.

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.

Keep Reading

Most Popular

Russian servicemen take part in a military drills
Russian servicemen take part in a military drills

How a Russian cyberwar in Ukraine could ripple out globally

Soldiers and tanks may care about national borders. Cyber doesn't.

Death and Jeff Bezos
Death and Jeff Bezos

Meet Altos Labs, Silicon Valley’s latest wild bet on living forever

Funders of a deep-pocketed new "rejuvenation" startup are said to include Jeff Bezos and Yuri Milner.

conceptual illustration showing various women's faces being scanned
conceptual illustration showing various women's faces being scanned

A horrifying new AI app swaps women into porn videos with a click

Deepfake researchers have long feared the day this would arrive.

ai learning to multitask concept
ai learning to multitask concept

Meta’s new learning algorithm can teach AI to multi-task

The single technique for teaching neural networks multiple skills is a step towards general-purpose AI.

Stay connected

Illustration by Rose WongIllustration by Rose Wong

Get the latest updates from
MIT Technology Review

Discover special offers, top stories, upcoming events, and more.

Thank you for submitting your email!

Explore more newsletters

It looks like something went wrong.

We’re having trouble saving your preferences. Try refreshing this page and updating them one more time. If you continue to get this message, reach out to us at with a list of newsletters you’d like to receive.