The Nature paper describes a unique electroless etching method by which arrays of silicon nanowires are synthesized in an aqueous solution on the surfaces of wafers that can measure dozens of square inches in area. The technique involves the galvanic displacement of silicon through the reduction of silver ions on a wafers surface. Unlike other synthesis techniques, which yield smooth-surfaced nanowires, this electroless etching method produces arrays of vertically aligned silicon nanowires that feature exceptionally rough surfaces. The roughness is believed to be critical to the surprisingly high thermoelectric efficiency of the silicon nanowires.
The rough surfaces are definitely playing a role in reducing the thermal conductivity of the silicon nanowires by a hundredfold, but at this time we dont fully understand the physics, said Majumdar. While we cannot say exactly why it works, we can say that the technique does work.
Nearly all of the worlds electrical power, approximately 10 trillion Watts, is generated by heat engines, giant gas or steam-powered turbines that convert heat to mechanical energy, which is then converted to electricity. Much of this heat, however, is not converted but is instead released into the environment, approximately 15 trillion Watts. If even a small fraction of this lost heat could be converted to electricity, its impact on the energy situation would be enormous.
Thermoelectric materials, which have the ability to convert heat into electricity, potentially could be used to capture much of the low-grade waste heat now being lost and convert it into electricity, said Majumdar. This would result in massive savings on fuel and carbon dioxide emissions. The same devices can also be used as refrigerators and air conditioners, and because these devices can be miniaturized, it could make heating and cooling much more localized and efficient.<
|Contact: Lynn Yarris|
DOE/Lawrence Berkeley National Laboratory