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  • Silicon Exhibits Retrograde Melting
Aug 2010
CAMBRIDGE, Mass., Aug. 10, 2010 — A new silicon material has been produced that actually melts as it cools off, which could lead to applications in solar cells and other devices.

A tiny silicon chip — the glowing orange square at the center of this special heating device — is 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 probe its changes at a molecular level as it went through the retrograde melting process. (Images: Patrick Gillooly)

According to the research team from MIT, the material, a compound of silicon, copper, nickel and iron, “melts” (actually turning from a solid to a slush-like mix of solid and liquid material) as it cools below 900 degrees Celsius, whereas silicon ordinarily melts at 1414 °C. The much lower temperatures make it possible to observe the behavior of the material during melting, based on specialized x-ray fluorescence microprobe technology using a synchrotron — a type of particle accelerator — as a source.

A wafer of silicon, which is the most widely used material for computer chips and solar cells.

The material and its properties are described in a paper just published online in the journal Advanced Materials. Team leader Tonio Buonassisi, the SMA Assistant Professor of Mechanical Engineering and Manufacturing, is the senior author and the lead authors are Steve Hudelson and postdoctoral fellow Bonna Newman.

The findings could be useful in lowering the cost of manufacturing some silicon-based devices, especially those in which tiny amounts of impurities can significantly reduce performance. In the material that Buonassisi and his researchers studied, impurities tend to migrate to the liquid portion, leaving regions of purer silicon behind. This could make it possible to produce some silicon-based devices, such as solar cells, using a less pure, and therefore less expensive, grade of silicon that would be purified during the manufacturing process.

“If you can create little liquid droplets inside a block of silicon, they serve like little vacuum cleaners to suck up impurities,” Buonassisi said. This research could also lead to new methods for making arrays of silicon nanowires — tiny tubes that are highly conductive to heat and electricity.

Postdoctoral fellow Bonna Newman and Tonio Buonassisi, the SMA assistant professor of Mechanical Engineering and Manufacturing, discuss their work studying the retrograde melting property of silicon.

Buonassisi predicted in a 2007 paper that it should be possible to induce retrograde melting in silicon, but the conditions needed to produce such a state, and to study it at a microscopic level, are highly specialized and have only recently become available. To create the right conditions, Buonassisi and his team had to adapt a microscope “hot-stage” device that allowed the researchers to precisely control the rate of heating and cooling. And to actually observe what was happening as the material was heated and cooled, they drew upon high-power synchrotron-based X-ray sources at Lawrence Berkeley National Laboratory in California and at Argonne National Laboratory in Illinois (researchers from both national labs are co-authors of the paper).

The material for the tests consisted of a kind of sandwich made from two thin layers of silicon, with a filling of copper, nickel and iron between them. This was first heated enough to cause the metals to dissolve into the silicon, but below silicon’s melting point. The amount of metal was such that the silicon became supersaturated — that is, more of the metal was dissolved in the silicon than would normally be possible under stable conditions. For example, when a liquid is heated, it can dissolve more of another material, but then when cooled down it can become supersaturated, until the excess material precipitates out.

In this case, where the metals were dissolved into the solid silicon, “if you begin cooling it down, you hit a point where you induce precipitation, and it has no choice but to precipitate out in a liquid phase,” Buonassisi said. It is at that point that the material melts.

The study was funded by the US Department of Energy, the National Science Foundation, the Clare Booth Luce Foundation, Doug Spreng and the Chesonis Family Foundation, and some equipment was provided by McCrone Scientific.

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