A new way of making crystalline silicon could make the crucial ingredient of computers and solar cells much cheaper and greener. It may also be useful in the topic of the post January 23rd, Nano Silicon Powder Produces Hydrogen Without Energy.
Silicon dioxide, that we see as typical sand, makes up about 40 percent of the earth’s crust. The problem is the industrial method for converting sand into crystalline silicon is expensive and has a major environmental impact due to the extreme processing conditions. Growing silicon crystals is not so simple as making glass objects.
Stephen Maldonado, professor of chemistry and applied physics at the University of Michigan (UM) provides the background, “The crystalline silicon in modern electronics is currently made through a series of energy-intensive chemical reactions with temperatures in excess of 2,000 degrees Fahrenheit that produces a lot of carbon dioxide.”
Recently, Maldonado and chemistry graduate students Junsi Gu and Eli Fahrenkrug discovered a way to make silicon crystals directly at just 180º F (80 °C), the internal temperature of a cooked turkey. And they did it by taking advantage of a phenomenon you can see right in your kitchen.
Maldonado explains that when water is super-saturated with sugar, that sugar can spontaneously form crystals, popularly known as rock candy, “Instead of water, we’re using liquid metal, and instead of sugar, we’re using silicon.”
Maldonado’s team 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 gallium electrodes. So far, the diamond cubic crystal structures are very small faceted nanocrystals with diameters in excess of 500 nm, about 1/2000th of a millimeter in diameter, but Maldonado hopes to improve the technique and make larger silicon crystals, 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.
One wonders if the UM and University at Buffalo teams know about one another’s work.
If the process pathway proves viable and can scale up, the implications would 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 alternatives to semiconductors. The price differential has created an industry based on alternate technologies that are catching up on efficiency.
The UM team published their results “Direct Electrodeposition of Crystalline Silicon at Low Temperatures” in the Journal of the American Chemical Society. The American Chemical Society Petroleum Research Fund funded the lab’s work.
Maldonado said, “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. 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.”
Clean, well, very pure silicon chips are the bulk of end use of silicon production for now. There is a very large worldwide market. The basic problem is the clean silicon is cut into tiny little pieces, etched for circuits and buried in plastics and ceramics. Recycling such an inert material in those circumstances, no matter the value, is a problematic system.
Last week we saw the University at Buffalo working to release hydrogen using nano crystals on the 10 nanometer range. It would be grand if the UM team sent a sample over to the Buffalo group to see what the cost might be to get from 500 down to 10 nanometers.
The UM work looks to be very high potential. Electronics, solar, perhaps hydrogen – UM is pursuing patent protection for the intellectual property and is seeking commercialization partners to help bring the technology to market.
2000º to under 200º is going to make a big difference.