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01/10/2008 08:00:00

Energy-Efficient Refrigeration from Ultranarrow Silicon Wires

PASADENA, Calif.-- Supernarrow silicon wires, or silicon nanowires, are laying the foundation for a new type of cheap yet energy-efficient microscopic refrigeration, with no moving parts, report researchers from the California Institute of Technology in a study published today in the journal Nature.

The researchers found that making silicon into nanowires could create highly efficient thermoelectric materials. Thermoelectric materials create a voltage--a difference in electric potential--when there is a difference in temperature across the surface of the material. The thermoelectric effect has been known for more than 200 years, and the materials have had niche applications, such as power generation in satellites. However, the efficiency with which thermoelectric materials heat at one end and cool at the other in response to electric current has been too poor to be of general use. To improve performance, other researchers have experimented with increasingly complex compositions and arrangements of rare elements. Although they have found newer materials with improved efficiency, those materials are expensive and difficult to miniaturize.

The Caltech researchers, led by James Heath, the Elizabeth W. Gilloon Professor and professor of chemistry, took a completely different tack by using silicon, the most abundant element on earth. Using a method developed in Heath's labs, they constructed nanowires that were from 10 to 100 times narrower than the wires used in current computer microchips and found that the nanowires became extremely efficient at converting between thermal energy and electrical energy, exhibiting a 100-fold increase in performance. Near-term applications may involve recovering waste heat from microprocessor chips to make those chips more energy efficient. Longer-term applications include their use in efficient cooling units for refrigeration, or in thermal to electrical energy conversion for large-scale applications.

"At these tiny dimensions, nature is doing things that were previously not thought possible," says Heath, whose research group carried out the experiments described in the study. "Optimizing materials for cooling or heat recovery applications involves a tricky trade-off of several different parameters, including the electrical conductivity and the thermal conductivity." It is often the case that an improvement in one of these parameters will adversely affect the performance of the others, Heath says, but "we find that we can greatly drop the thermal conductivity in these nanowires without affecting the other parameters, and this leads to dramatic improvements in the thermoelectric efficiency."

An additional parameter that the researchers were surprised to see improved in the nanowires is the thermopower, which is the amount of voltage generated in a material for a given thermal gradient. The improvement likely arises from a phenomenon known as "phonon drag," which comes when the sound-carrying vibrations in the atomic lattice of the nanowires are not in thermal equilibrium with the current carrying electrons. "We find that for ultrathin nanowires the electrons drag certain sound waves along with them as they move down the nanowire. This extra heat from the sound is enhancing the thermoelectric efficiency," says Jamil Tahir-Kheli, a theoretician with Caltech's Materials and Process Simulation Center and a contributing author to the study.

Although silicon nanowires are still about a factor of two less efficient than the most efficient known thermoelectric materials, researchers are optimistic that further improvements in the materials will soon be made. "Our theoretical models indicate that a number of exciting avenues are available to significantly improve the efficiency," says William A Goddard, the Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics at Caltech, the director of the Materials and Process Simulation Center, and a contributing author to the study. "However, even at their current efficiencies, these nanowires already outperform many commercially available systems, and so could potentially find near-term applications. This is one more example of the surprising properties of in the world of nanomaterials, an area stimulated by the pioneering work of Richard Feynman, Tolman Professor of Theoretical Physics at Caltech, in 1959, just as I was arriving at Caltech," says Goddard.

Other authors on the study were Caltech chemistry graduate students Akram Boukai, Yuri Bunimovich, and Jen Kan Yu.

Written by Kathy Svitil