Most materials melt when heated, on other words, they change from a solid to a liquid state. But a few nonconformist materials do the opposite, they melt as they get cooler. A team of MIT researchers has now discovered that silicon, which is the most widely used material in computer chips and solar cells, can display this bizarre property of retrograde melting if it contains high concentrations of certain metals dissolved in it. The finding could lead to applications in solar cells and other devices
The material, a compound of silicon, copper, nickel and iron, “melts” – essentially turning from a solid to a slushlike blend of solid and liquid material- as it cools below 900 degrees Celsius. Silicon ordinarily melts at 1414 degrees C. The significantly lower temperatures make it feasible to observe the behavior of the material during melting. This is based on specialized X-ray fluorescence microprobe technology using a synchrotron particle accelerator as a source.
In the photo here, a minuscule silicon chip is seen; it is the glowing orange square at the center of a special heating device. Itis heated to a temperature well below silicon’s melting point, and then very slowly cooled down. The chip inside this heating device was placed in the path of a synchrotron beam to analyse its changes at a molecular level as it went through the retrograde melting process.
“If you can create little liquid droplets inside a block of silicon, they serve like little vacuum cleaners to suck up impurities,” team leader Tonio Buonassisi says.
The research, described in a paper just published online in the journal Advanced Materials could also lead to new methods for making arrays of silicon nanowires — tiny tubes that are highly conductive to heat and electricity.
Retrograde Melting and Internal Liquid Gettering in Silicon