Researchers at Boston College and MIT have achieved a major increase in the thermoelectric efficiency of bismuth antimony telluride—a semiconductor alloy that has been commonly used in commercial devices since the 1950s—in bulk form. Specifically, the team realized a 40% increase in the alloy’s figure of merit—a dimensionless term calculated to indicate a material’s relative performance—from 1 to a peak of 1.4.
The team’s low-cost approach, details of which are published in the online version of the journal Science, involves building tiny alloy nanostructures. The discovery sets the stage for broader use of this new nanocomposite approach in developing high-performance, low-cost bulk thermoelectric materials.
Among their other potential applications, thermoelectric materials could enable more efficient recovery of waste heat from combustion engines, using the resulting electric power to improve the overall fuel economy of the vehicle. (Earlier post.)
The achievement marks the first such gain in a half-century using the cost-effective material that functions at room temperatures and up to 250° C. The success using the relatively inexpensive and environmentally friendly alloy in bulk form means the discovery can quickly be applied to a range of uses, leading to higher cooling and power generation efficiency.
By using nanotechnology, we have found a way to improve an old material by breaking it up and then rebuilding it in a composite of nanostructures in bulk form. This method is low cost and can be scaled for mass production. This represents an exciting opportunity to improve the performance of thermoelectric materials in a cost-effective manner.—Zhifeng Ren, Boston College
The researchers crushed bismuth antimony telluride into a nanoscopic dust and then reconstituted it in bulk form, albeit with nanoscale constituents. The grains and irregularities of the reconstituted alloy increased phonon scattering, radically transforming the thermoelectric performance by blocking heat flow while allowing the electrical flow. Phonons, a quantum mode of vibration, play a key role because they are the primary means by which heat conduction takes place in insulating solids.
In addition to Ren and six researchers at his BC lab, the international team involved MIT researchers, including Chen and Institute Professor Mildred S. Dresselhaus; research scientist Bed Poudel at GMZ Energy, Inc, a Newton, Mass.-based company formed by Ren, Chen, and CEO Mike Clary; as well as BC visiting Professor Junming Liu, a physicist from Nanjing University in China.
The research was supported by the Department of Energy and by the National Science Foundation.
Bed Poudel, et. al, High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys, Science (20 March 2008) DOI: 10.1126/science.1156446
Vehicular Thermoelectrics Applications Overview (John Fairbanks, DOE, DEER 2007)