Wednesday, May 26, 2010
Compared to silicon, semiconductors like gallium arsenide can be made into solar cells that convert more sunlight into electricity and transistors that are faster than their silicon counterparts. But devices made from these materials are expensive.
Now a new method for making large-area devices from gallium arsenide promises to bring down costs by eliminating manufacturing steps and wasting less materials. Researchers have used the method to make high-performance image sensors, transistors, and solar cells. Semprius, a Durham, NC, company, is using it to make solar modules that should be on the market by the end of the year.
Gallium arsenide solar cells convert twice as much of the energy in sunlight into electricity compared to silicon cells, says John Rogers, professor of materials science and engineering at the University of Illinois at Urbana-Champaign, who led the research. Gallium arsenide is also being eyed by microchip manufacturers such as Intel as a potential replacement for silicon.
The problem with gallium arsenide, however, is its price tag. To make a gallium arsenide solar panel today, manufacturers grow a semiconductor crystal on an expensive template in a high-vacuum, high-temperature chamber. The gallium arsenide is then diced into thin pieces, assembled, and bonded. This process destroys the underlying template, which is necessary to create a high-quality crystal. And making only a single layer of gallium arsenide at a time is inefficient--it takes more time to load and unload the vacuum chamber than it does to grow the crystal.
To address the problem, Rogers developed a method for growing multiple layers of devices at one time, and a way to release them from the substrate without destroying it. "Once the substrate is in the chamber at the right temperature, we grow a multilayer stack," explains Rogers. The stack alternates a device layer with a sacrificial layer. After all the layers are put down, the stack is etched in a chemical bath that eats away at the sacrificial layer, made of aluminum arsenide, releasing thin rectangular films of gallium arsenide. As the gallium arsenide films are released, they're picked up and placed on a substrate.
Sunday, January 17, 2010
The right side of a mirror has a polymer coating that fights fogging — making the camera visible — and helps wash away oil.
IMAGINE how great it would be if, after dinner, you could stack the greasy dishes, pots, pans and utensils in the sink and let plain old water rinse away the grime — with no help from detergents, and little or no scrubbing. Bye-bye, dishpan hands.
Plastic coatings under development may someday bring that moment to pass, rendering dinnerware, bathroom mirrors and even factory equipment sparkling clean with water alone.
The new materials may be appreciated not only by dish-washing family members, but also by environmentalists concerned about all of the soap that disappears down the drain. Detergents that end up in wastewater can cause algae to bloom, among other effects.
In experiments, Dr. Youngblood and his colleagues attached the coatings chemically to the surface of glass. But he is now working on polymer additives for liquids that can be poured into a spray bottle, for example, and then used to clean mirrors and even eyeglasses or goggles.
Scientists call the coatings self-cleaning because, once they are applied to a surface, they do much of the work of scrubbing away oily residue — like that from a greasy fingerprint. “The oil beads up and then the water moves under the oil, lifting it up so it floats away,” said Kirsten Genson, a postdoctoral researcher in Dr. Youngblood’s group.
Getting the coating to do this is ingenious, said Michael F. Rubner, a professor of polymer materials science and engineering, and director of the Center for Materials Science and Engineering at the Massachusetts Institute of Technology. “Jeff figured out a way to have molecules on the surface that can rearrange themselves so they can self-clean, rejecting grease,” he said.
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