ThermoelectricsThermoelectrics are a class of materials that can convert heat energy directly into electrical energy, by exploiting the thermal energy carried by charges (electrons and holes). This energy conversion process is also highly reversible; thermoelectric materials can be used for power generation as well as solid-state refrigeration. Thermoelectrics are currently used in applications ranging from thermocouple sensors to solar power generators.
The efficiency of thermoelectric power generators is determined by the nondimensional figure-of-merit, ZT=S2σT/k, where S is the Seebeck coefficient, σ the electrical conductivity, k the thermal conductivity, and T the absolute temperature. Efforts to increase thermoelectric performance center around increasing the electrical properties S2σ, or decreasing the thermal conductivity.
Electron TransportTo characterize electron transport in nanostructured thermoelectric materials, we measured four transport coefficients (electrical conductivity, Seebeck, Hall, and Nernst coefficients) and deduced four important transport parameters such as Fermi level, density-of-states effective mass, mobility and scattering exponent. These transport parameters give insight into how to engineer a material with improved Seebeck coefficient. Bandgap measurements are being pursued using transmission measurements and photoacoustic measurements. The photoacoustic effect uses light to produce an acoustic signal from the measured sample. From this, the spectral absorption coefficient can be deduced, leading to knowledge of the bandgap. For electrical property calculations, it is important to know the bandgap of the material. Also, understanding the bandgap gives insight into what effect doping has on the material.
Thermal ConductivityOn the thermal conductivity side, it is important to study phonons, to understand how to impede and scatter them. We have developed tools to calculate, from first principles density-functional theory, the phonon mean free paths due to 3-phonon scattering processes. We apply these tools to lead selenide (PbSe), lead telluride (PbTe), and bismuth antimonide (BiSb). Furthermore, the transmission across grain interfaces has also been calculated using accurate Green's functions methods. These two major steps provide the necessary ingredients to perform multi-scale simulations of phonon transport in complex nanostructured materials using the Monte Carlo method. We have recently applied the concept of modulation doping to make thermoelectrics (SiGe) of higher mobility. The alloy will limit the mean free path of optical and high-energy phonons of shorter wavelength. Long wavelength phonons will however be scattered by nanoparticles of similar size. So we introduce B-doped Si nanoparticles in SiGe alloys to reduce further their thermal conductivity, and to provide charge carriers. Our recent made samples reach a high-temperature ZT of 1.3 with a minimal usage of Ge to reduce the fabrication cost.
ApplicationsTraditionally, thermoelectrics have been used as cooling and heating elements. They can also be used in power generation applications to scavenge waste heat. Recently, we have developed devices that convert the sun's heat to electricity via thermoelectrics. See our solar energy conversion page for more about solar thermoelectric generators.
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