Liquid metal makes silicon crystals at record low temperatures

Jan 24, 2013

A new way of making crystalline silicon, developed by U-M researchers, could make this crucial ingredient of computers and solar cells much cheaper and greener.

, or sand, makes up about 40 percent of the earth's crust, but the industrial method for converting sand into is expensive and has a major environmental impact due to the extreme processing conditions.

"The crystalline silicon in is currently made through a series of energy-intensive with temperatures in excess of 2,000 degrees Fahrenheit that produces a lot of carbon dioxide," said Stephen Maldonado, professor of chemistry and applied physics.

Recently, Maldonado and chemistry graduate students Junsi Gu and Eli Fahrenkrug discovered a way to make silicon directly at just 180 F, the internal temperature of a cooked turkey. And they did it by taking advantage of a phenomenon you can see right in your kitchen.

When water is super-saturated with sugar, that sugar can spontaneously form crystals, popularly known as rock candy.

"Instead of water, we're using , and instead of sugar, we're using silicon," Maldonado said.

Maldonado and colleagues made a solution containing silicon tetrachloride and layered it over a liquid gallium electrode. Electrons from the metal converted the silicon tetrachloride into raw silicon, which then dissolved into the liquid metal.

"The liquid metal is the key aspect of our process," Maldonado said. "Many solid metals can also deliver electrons that transform silicon tetrachloride into disordered silicon, but only metals like gallium can additionally serve as liquids for silicon crystallization without additional heat."

The researchers reported dark films of silicon crystals accumulating on the surfaces of their liquid electrodes. So far, the crystals are very small, about 1/2000th of a millimeter in diameter, but Maldonado hopes to improve the technique and make larger , tailored for applications such as converting light energy to electricity or storing energy. The team is exploring several variations on the process, including the use of other low-melting-point metal alloys.

If the approach proves viable, the implications could be huge, especially for the solar energy industry. Crystalline silicon is presently the most-used solar energy material, but the cost of silicon has driven many researchers to actively seek alternative semiconductors.

"It's too premature to estimate precisely how much the process could lower the price of silicon, but the potential for a scalable, dramatically less expensive and more environmentally benign process is there," Maldonado said. "The dream ultimately is to go from sand to crystalline silicon in one step. There's no fundamental law that says this can't be done."

The study, which appears in the Journal of the American Chemical Society, was funded by the American Chemical Society Petroleum Research Fund.

The university is pursuing patent protection for the intellectual property and is seeking commercialization partners to help bring the technology to market.

Explore further: New startup will develop non-stick surfaces for broad range of industrial applications

More information: The study is titled "Direct Electrodeposition of Crystalline Silicon at Low Temperatures" DOI: 10.1021/ja310897r

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Caliban
1 / 5 (1) Jan 24, 2013
"...The team is exploring several variations on the process, including the use of other low-melting-point metal alloys."


Let's hope so, as this process, as it stands, will go nowhere fast, due to its dependence upon a very expensive rare earth in short supply, that --in an of itself-- presents some very high environmental costs, which I doubt have been factored into the calculation of the "greenness" of this process.

Perhaps some serendipitous discovery of a cheap, abundant and less environmentally damaging "liquid metal" can be found, but until then, this discovery is only an interesting experimental demonstration.
DonaldJLucas
1 / 5 (1) Jan 25, 2013
The article didn't say if the liquid gallium was consumed in any way (including evaporation) by this process. If not, and if evaporation of the liquid metal could be entirely avoided, then perhaps the liquid metal could be mercury. Not at all environmentally friendly, but if it is more like a catalyst than a consumed ingredient, then perhaps some type of reactor vessel could be used to contain it properly. While very environmentally dangerous, it is not nearly as dangerous as many of the chemicals used to create various semiconductors and that industry seems to be able to prevent the release of these very noxious chemicals.
RealScience
5 / 5 (2) Jan 27, 2013

Let's hope so, as this process, as it stands, will go nowhere fast, due to its dependence upon a very expensive rare earth in short supply


Gallium is not a "rare earth".
Gallium is not even particularly rare (which is different from being a "rare earth") - it is more plentiful in earth's crust than lead or tin, for example.
Furthermore gallium is not consumed in this process.

The small crystal size is more likely to be a problem than the gallium is.
Caliban
1 / 5 (1) Jan 27, 2013

Let's hope so, as this process, as it stands, will go nowhere fast, due to its dependence upon a very expensive rare earth in short supply


Gallium is not a "rare earth".
Gallium is not even particularly rare (which is different from being a "rare earth") - it is more plentiful in earth's crust than lead or tin, for example.
Furthermore gallium is not consumed in this process.

The small crystal size is more likely to be a problem than the gallium is.


You are correct, it isn't a rare earth. I was thinking of Indium, for some reason, when I wrote that.

Still, its relative abundance is not so vast, nor is it so readily available, that this might prove an impediment to this process on an industrial scale.

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