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Physics Special Colloquium, May 2, 2005Low-dimensional Materials for Energy ConversionJoseph P. HeremansDelphi Research LabsWhen the physical size of nanostructures is reduced to below the length scales that govern the transport properties, namely the phonon and electron mean free paths, the electron diffusion length, the de Broglie wavelength, and the phase coherence length, new phenomena appear that can lead to new applications in energy conversion problems. This talk will outline the physical mechanisms that result in the superior thermoelectric performance of low-dimensional solids, quantum wires and quantum dots, compared to bulk thermoelectric semiconductors. The energy efficiency of thermoelectric energy converters is a function of the "Thermoelectric Figure of Merit", ZT = T S2σ/κ, where S is the Seebeck coefficient, σ is the electrical conductivity and κ the thermal conductivity of the solid. While in bulk conventional semiconductors, ZT<1 at room temperature, the last 4 years have seen superlattice structures and quantum-dot superlattices reach ZT>2. Two mechanisms are at work: a reduction of the lattice thermal conductivity, and an increase in the Seebeck coefficient at a given carrier density. The Seebeck coefficient is particularly sensitive to the energy dependence of the electrical conductivity. As a result, it is strongly affected by variations in the density of states, for instance induced by size-quantization effects. It can also be increased by increasing the sensitivity of the scattering mechanisms to the electron energy, which can be seen as an electron-energy filtering mechanism. We will review the mechanisms that lead to enhanced thermoelectric properties in the light of thermoelectric transport data. The effects of size-quantization are illustrated on nanowires of bismuth with diameters from 4 to 200 nm. Localization effects are in evidence in antimony and zinc nanowires of the same diameters. The results of recent galvanomagnetic and thermomagnetic measurements on bulk PbTe containing 30 nm inclusions of metallic Pb also show a strong increase in Seebeck coefficient, this time due to an increased energy dependence of the scattering mechanisms. A few applications of nanoscale materials to energy conversion problems will also be reviewed. 10:30 a.m., Room 4138 Physics Research BuildingRefreshments served at 10:00 a.m. |
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