Hello,

We noticed you're browsing in private or incognito mode.

To continue reading this article, please exit incognito mode or log in.

Not an Insider? Subscribe now for unlimited access to online articles.

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.

This story is part of the November/December 2010 Issue of the MIT News Magazine
See the rest of the issue
Subscribe

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.

Tech Obsessive?
Become an Insider to get the story behind the story — and before anyone else.

Subscribe today

Uh oh–you've read all of your free articles for this month.

Insider Premium
$179.95/yr US PRICE

Want more award-winning journalism? Subscribe and become an Insider.
  • Insider Plus {! insider.prices.plus !}* Best Value

    {! insider.display.menuOptionsLabel !}

    Everything included in Insider Basic, plus the digital magazine, extensive archive, ad-free web experience, and discounts to partner offerings and MIT Technology Review events.

    See details+

    What's Included

    Unlimited 24/7 access to MIT Technology Review’s website

    The Download: our daily newsletter of what's important in technology and innovation

    Bimonthly print magazine (6 issues per year)

    Bimonthly digital/PDF edition

    Access to the magazine PDF archive—thousands of articles going back to 1899 at your fingertips

    Special interest publications

    Discount to MIT Technology Review events

    Special discounts to select partner offerings

    Ad-free web experience

  • Insider Basic {! insider.prices.basic !}*

    {! insider.display.menuOptionsLabel !}

    Six issues of our award winning print magazine, unlimited online access plus The Download with the top tech stories delivered daily to your inbox.

    See details+

    What's Included

    Unlimited 24/7 access to MIT Technology Review’s website

    The Download: our daily newsletter of what's important in technology and innovation

    Bimonthly print magazine (6 issues per year)

  • Insider Online Only {! insider.prices.online !}*

    {! insider.display.menuOptionsLabel !}

    Unlimited online access including articles and video, plus The Download with the top tech stories delivered daily to your inbox.

    See details+

    What's Included

    Unlimited 24/7 access to MIT Technology Review’s website

    The Download: our daily newsletter of what's important in technology and innovation

/
You've read all of your free articles this month. This is your last free article this month. You've read of free articles this month. or  for unlimited online access.