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Posted: Apr 25, 2012

Thermoelectric nanocomposites switch between heat and electricity in a unique and clean solid-state approach

(Nanowerk Spotlight) In the past couple of decades, thermoelectrics have been drawing more and more research interest due to the limited availability and the negative environmental impact of conventional energy strategies.
In the past, as a measuring stick of the conversion efficiency, the term "dimensionless figure-of-merit," also referred to as ZT, has been widely used. A high ZT value usually promises high thermoelectric performance. Typically, good thermoelectric materials should simultaneously display low thermal conductivity and good electrical conductivity.
Striving to enhance the performance of thermoelectric materials, researchers from Boston College and MIT have recently reported a novel materials design to achieve a 30 to 40% enhancement in the peak ZT value for n-type SiGe semiconducting alloys.
Bo Yu, lead author of a paper in Nano Letters ("Enhancement of Thermoelectric Properties by Modulation-Doping in Silicon Germanium Alloy Nanocomposites") describing the recent work, says that SiGe has been almost the exclusive choice for high temperature thermoelectric applications. The material has been used in the radioisotope thermoelectric generators (RTGs) employed by US NASA ever since 1976.
Nevertheless, the broader application of SiGe has been limited by the fact that germanium, which is used to reduce the thermal conductivity in such alloys, is extremely expensive and the cost has to justify the performance.
Bo Yu, is a graduate researcher in the Department of Physics at Boston College working in Zhifeng Ren's group. He worked on this project with MIT collaborators, Mona Zebarjadi, Gang Chen, and Mildred S. Dresselhaus.
The scientists reported that the modulation-doping strategy, conventionally used in the thin-film semiconductor industry, could also be utilised in the 3D bulk thermoelectric materials to enhance their carrier mobility and therefore the electrical conductivity, by over 50% in this case.
By improvising materials design, the team also achieved a simultaneous reduction in the thermal conductivity which combines to provide a high ZT value of about 1.3 at 900°C.
Schematics of 3D modulation doping
Schematics of 3D modulation doping (top, courtesy of Boston College), band alignment of Si/Ge heterostructure (bottom left, Nano Lett., 2012, 12 (4), pp 20772082) and figure-of-merit of modulation-doped sample (bottom right, Nano Lett., 2012, 12 (4), pp 20772082).
"To improve materials ZT is extremely challenging because all the internal parameters are closely related to each other. Once you change one of them, the others may most likely change accordingly to the other extreme, leading to no net improvement. As a result, a more popular trend in this field of study is to look into new opportunities, or say new material system. However, our study proved that opportunities are still there for the existing materials, if one could work smartly enough to find some alternative material designs," explains Bo Yu.
Zhifeng Ren also points out that this reported ZT peak value competes well with the state-of-art n-type SiGe alloy materials while the new material design requires over 30% less of germanium. "That is a significant advantage to cut down the fabrication cost as we want all the materials we studied in the group be really used by people in reality and that is always the goal for our everyday research," adds Ren.
By using a similar strategy, researchers are also looking into other traditional materials systems trying for more breakthroughs. Actually, this Boston College and MIT team, led by Ren and Chen, has been a pioneer in the clean energy research community for years especially for their contribution in understanding and controlling the phonon and electron transport in bulk thermoelectric composite materials (see for instance a previous Nanowerk Spotlight: "Nanocomposite approach enhances the performance of thermoelectric materials").
Currently, their research is funded by the S3TEC (Solid state solar thermal energy conversion) Centre which is part of the US DOE Energy Frontier Research Centre program, aiming at advancing fundamental science and developing materials to harness heat from the sun and convert this heat into electricity via solid-state thermoelectric and thermophotovoltaic technologies.
Source: A Nanowerk exclusive provided by Boston College

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