Cheap, Efficient Thermoelectrics

Thursday, March 20, 2008

Nanomaterials could be used for lower-emission cars and solar panels.

By Katherine Bourzac

Efficient crystals: Researchers increased the efficiency of a commonly used thermoelectric material, bismuth antimony telluride, by grinding it into a fine powder and pressing it back together. This technique creates random crystal lattices (lines in this tunneling-electron microscope image of the material) that interrupt the flow of heat. Credit: Zhifeng Ren, Boston College

Thermoelectric materials promise everything from clean power for cars to clean power from the sun, but making these materials widely useful has been a challenge. Now researchers at MIT and Boston College have developed an inexpensive, simple technique for achieving a 40 percent increase in the efficiency of a common thermoelectric material. Thermoelectric materials, which can convert heat into electricity and electricity into heat, hold promise for turning waste heat into power. But thermoelectric materials have not been efficient enough to move beyond niche applications. The new jump in efficiency, achieved with a relatively inexpensive material, may finally make possible such applications as solar panels that turn the sun's heat into electricity, and car exhaust pipes that use waste heat to power the radio and air conditioner.

The researchers started with bismuth antimony telluride, a thermoelectric material used in niche products such as picnic coolers and cooling car seats. Then Gang Chen, a professor of mechanical engineering at MIT; Institute Professor Mildred Dresselhaus; and Boston College physics professor Zhifeng Ren crushed it into a powder with a grain size averaging about 20 nanometers, and pressed it into discs and bars at high heat. The resulting material has a much finer crystalline lattice structure than the original material, which is made up of millimeter-scale grains. Chen and Ren's nanocomposite formulation of the material is 40 percent more efficient than the conventional form of the material at 100 °C, and it works at temperatures ranging from room temperature to 250 °C.

"Power-generation applications [for thermoelectrics] are not big now because the materials aren't good enough," says Chen. He believes that his group's more efficient version of the material will finally make such applications commercially viable.

Thermoelectric materials must be able to maintain a heat gradient, which means that they must be good conductors of electrons and good thermal insulators. When one end of a bar of thermoelectric material is heated, electrons move from the hot side to the cold, creating an electrical current. If a material conducts heat well, this current-generating temperature gradient will dissipate. Unfortunately, in most bulk materials, electrical conductivity and thermal conductivity "go hand in hand," says John Fairbanks, who heads thermoelectrics efforts in the Department of Energy's Vehicle Technologies Program.

One approach to making better thermoelectric materials has been to build nanostructured materials from the bottom up. Interfaces in these materials reflect the flow of heat without impeding electrical current. Researchers who have grown arrays of silicon nanowires, pressed silicon and germanium nanowires into millimeter-scale bars, and tested single organic molecules have had success on a small scale, but making such materials in bulk is a major hurdle.

The researchers' nanocomposite technique creates many interfaces in the material that reflect thermal vibrations, says Chen. Peidong Yang, a professor of chemistry at the University of California, Berkeley, says that the work is "a great example of how defect engineering can significantly impact on the [vibration] transfer in solids."

Ren says that it's easy to make large amounts of the nanocomposite material: "We're not talking grams; we're not talking kilograms. We can make metric tons." Because bismuth antimony telluride is already used in commercial products, Ren and Chen predict that their technique will be integrated into commercial manufacturing in several months.

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Thermoelectric Breakthrough in Silicon Nanowires

Contact: Lynn Yarris (510) 386-5375, lcyarris@lbl.gov

Jan. 9, 2008

BERKELEY, CA — Energy now lost as heat during the production of electricity could be harnessed through the use of silicon nanowires synthesized via a technique developed by researchers with the U.S. Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) at Berkeley. The far-ranging potential applications of this technology include DOE’s hydrogen fuel cell-powered “Freedom CAR,” and personal power-jackets that could use heat from the human body to recharge cell-phones and other electronic devices. Rough silicon nanowires synthesized by Berkeley Lab researchers demonstrated high performance thermoelectric properties even at room temperature when connected between two suspended heating pads. In this illustration, one pad serves as the heat source (pink), the other as the sensor. “This is the first demonstration of high performance thermoelectric capability in silicon, an abundant semiconductor for which there already exists a multibillion dollar infrastructure for low-cost and high-yield processing and packaging,” said Arun Majumdar, a mechanical engineer and materials scientist with joint appointments at Berkeley Lab and UC Berkeley, who was one of the principal investigators behind this research. “We’ve shown that it’s possible to achieve a large enhancement of thermoelectric energy efficiency at room temperature in rough silicon nanowires that have been processed by wafer-scale electrochemical synthesis,” said chemist Peidong Yang, the other principal investigator behind this research, who also holds a joint Berkeley Lab and UC Berkeley appointment. Majumdar, who was recently appointed director of Berkeley Lab's Environmental Energy Technologies Division (EETD) and is a member of the Materials Sciences Division, is an expert on energy conversion and nanoscale science and engineering. Yang is a leading nanoscience authority with Berkeley Lab's Materials Sciences Division and with the UC Berkeley Chemistry Department. Majumdar and Yang are the co-authors of a paper appearing in the January 10, 2008 edition of the journal Nature, entitled “Enhanced Thermoelectric Performance of Rough Silicon Nanowires.” Also co-authoring this paper were Allon Hochbaum, Renkun Chen, Raul Diaz Delgado, Wenjie Liang, Erik Garnett and Mark Najarian. 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 wafer’s 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 don’t 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 world’s 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. From left, Renkun Chen, Arun Majumdar, Peidong Yang and Allon Hochbaum were co-authors of a Nature paper that described a wafer-scale electrochemical synthesis technique for producing rough silicon nanowires that can convert heat into electricity with surprisingly high efficiency. “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.” However the on-going challenge for scientists and engineers has been to make thermoelectric materials that are efficient enough to be practical. The goal is a value of 1.0 or more for a performance measurement called the “thermoelectric figure of merit” or ZT, which combines the electric and thermal conductivities of a material with its capacity to generate electricity from heat. Because these parameters are generally interdependent, attaining this goal has proven extremely difficult. In recent years, ZT values of one or more have been achieved in thin films and nanostructures made from the semiconductor bismuth telluride and its alloys, but such materials are expensive, difficult to work with, and do not lend themselves to large-scale energy conversions. “Bulk silicon is a poor thermoelectric material at room temperature, but by substantially reducing the thermal conductivity of our silicon nanowires without significantly reducing electrical conductivity, we have obtained ZT values of 0.60 at room temperatures in wires that were approximately 50 nanometers in diameter,” said Yang. “By reducing the diameter of the wires in combination with optimized doping and roughness control, we should be able to obtain ZT values of 1.0 or higher at room temperature.” The ability to dip a wafer into solution and grow on its surface a forest of vertically aligned nanowires that are consistent in size opens the door to the creation of thermoelectric modules which could be used in a wide variety of situations. For example, such modules could convert the heat from automotive exhaust into supplemental power for a Freedom CAR-type vehicle, or provide the electricity a conventional vehicle needs to run its radio, air conditioner, power windows, etc. When scaled up, thermoelectric modules could eventually be used in co-generating power with gas or steam turbines. Figure (a) is a cross-sectional scanning electron microscope image of an array of rough silicon nanowires with an inset showing a typical wafer chip of these wires. Figure (b) is a transmission electron microscope image of a segment of one of these wires in which the surface roughness can be clearly seen. The inset shows that the wire is single crystalline all along its length. “You can siphon electrical power from just about any situation in which heat is being given off, heat that is currently being wasted,” said Majumdar. “For example, if it is cold outside and you are wearing a jacket made of material embedded with thermoelectric modules, you could recharge mobile electronic devices off the heat of your body. In fact, thermoelectric generators have already been used to convert body heat to power wrist watches.” The Berkeley Lab researchers will be studying the physics behind this phenomenon to better understand and possibly manipulate it for even further improvements. They will also concentrate on the design and fabrication of thermoelectric modules based on silicon nanowire arrays. Berkeley Lab’s Technology Transfer Department is now seeking industrial partners to further develop and commercialize this technology. This research was funded by the U.S. Department of Energy's Office of Basic Energy Science, through the Division of Materials Sciences and Engineering. Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California. Visit our Website at www.lbl.gov. http://www.lbl.gov/Science-Articles/Archive/MSD-silicon-nanowires.html Additional Information Arun Majumdar can be reached via e-mail at majumdar@me.berkeley.edu Peidong Yang can be reached via e-mail at p_yang@uclink.berkeley.edu For more information on the research of Arun Majumdar, visit his Website at http://www.me.berkeley.edu/faculty/majumdar/ For more information on the research of Peidong Yang, visit his Website at http://www.cchem.berkeley.edu/pdygrp/main.html If interested in licensing the silicon nanowire technology for commercial development, please contact Berkeley Lab’s Technology Transfer Department at TTD@lbl.gov