Advances in thermoelectrics: From single phases to hierarchical nanostructures and back
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uction Thermoelectricity is defined by the “Seebeck effect,” after the German physicist Thomas Seebeck. In 1821, Seebeck discovered that, by using wires of different metals together and heating one end, he could induce a small electric current flow from the hot end to the cold end, demonstrating that heat could be converted to electricity. Seebeck’s ingenious experiment is illustrated in Figure 1; it shows a piece of metal with a magnet underneath it and a hot source. The magnet moves as the current flows. This phenomenon is the basis of thermolectrics.1–5 Thermoelectricity can also operate in reverse and create temperature differences using electric currents, so it can be used for solid-state refrigerators. The Seebeck effect is based on a property of a material called thermopower. This has a simple explanation: A piece of metal or semiconductor
contains charge carriers, and if it is subjected to a temperature difference, the charge carriers will move to create a current from the region of higher temperature to that of lower temperature in order to lower their energy. If the temperature difference ΔT induces a voltage difference ΔV, then the thermopower is defined as ΔV/ΔT, so the higher the value of ΔV for a given ΔT, the higher the thermopower.4,5 The best thermoelectric devices uses an n-type and p-type material placed in parallel, bounded at each end by a heat source and a heat sink, as shown in Figure 2. The charge carriers then move from the heat source to the heat sink, creating an electric current. This process can also be driven in reverse to cool the material. The dream of the thermoelectrics research community is to harness this energy-conversion technology for widespread power generation and energy management.
Mercouri G. Kanatzidis, Northwestern University, USA; [email protected] DOI: 10.1557/mrs.2015.173
© 2015 Materials Research Society
MRS BULLETIN • VOLUME 40 • AUGUST 2015 • www.mrs.org/bulletin
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ADVANCES IN THERMOELECTRICS: FROM SINGLE PHASES TO HIERARCHICAL NANOSTRUCTURES AND BACK
Figure 1. Seebeck’s experiment to demonstrate thermoelectricity. The apparatus consists of a piece of metal attached to a magnet, and under the magnet is a hot source. Heating the apparatus induces current flow, which then causes the magnet to move.
There are some obvious examples: exhaust heat from vehicles, constituting approximately three-quarters of the total fuel energy; as well as engines on ships, utilities, chemical plants, the brick industry, glass industries, industries producing a large amount of heat, and space power. The National Aeronautics and Space Administration (NASA) must be credited for selecting thermoelectrics for deep space missions early on beginning in the 1960s because of reliability. Thermoelectric devices were tested and proven successful for all thermoelectric applications at NASA. For example, the hot source for the NASA applications is PuO2 and the semiconductor material is SiGe. The Voyager spacecraft left the earth in the late 1970s. Voyager 1 has now escaped the solar
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