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D APPENDIX D: TECHNOLOGY SOLUTIONS FOR ENERGY RECOVERY in Ag-Sb and are epitaxially embedded in a matrix that is depleted of Ag and Sb. Presumably, the electronic band structure and vibrational properties of these nano-regions are different from those of the surrounding material, suggesting quantum confinement. Several questions remain: Are there quantum effects in these nanostructures and, if so, do they play any role in raising the ZT of the material? Is the acoustic impedance of the nanodots very different from that of the matrix and, if so, do they scatter acoustic phonons and thereby reduce thermal conductivity? How does the structure and size depend on m, and are there ways to maximize ZT? What are the thermoelectric effects and efficiencies of other metal-Group V-VI elemental compounds? Seen historically, this is a huge jump over an extremely short period (see Figure D.4). What is the underlying science? In semiconductors, electrons and holes carry charge, whereas lattice vibrations or phonons dominate heat transport. Electrons (or holes) and phononshavetwolengthscalesassociatedwiththeirtransport–wavelength, ,andmean free path, . By nanostructuring semiconductors with sizes comparable to sharp edges, peaks in their electronic density of states are produced whose location in energy space depends on size. By matching the peak locations and shape with respect to the Fermi energy, one can tailor the thermopower S. Furthermore, such quantum confinement increases electronic mobility, which could lead to high values of . Hence, quantum confinement allows manipulation of S2 that is otherwise difficult to achieve in bulk materials. It is entirely possible, though, that the increase of ZT may be less dependent on quantum confinement of electrons and holes, and more on phonon dynamics and transport. For example, if the size of a semiconductor is smaller than the mean free path of phonons and larger than that of electrons or holes, one can reduce thermal conductivity by boundary scattering without affecting electrical transport. Although charge transport in thermo- electricity is almost monoenergetic (energy levels within a few kT around the Fermi energy), heat transport by phonons is broadband over the Brillouin zone. Many bulk thermoelectric materials are alloys because alloy scattering of the short-wavelength acoustic phonons suppresses thermal conductivity without substantially altering S2 . However, the mid- to long-wavelength phonons remain largely unaffected and conduct heat in alloys. Can researchers beat the alloy limit? It is possible that embedded nanostructures in semiconductor alloys could scatter the mid- to long-wavelength phonons as well because of the similarity in sizes, and thereby reduce the thermal conductivity below the alloy limit. Given all these possibilities, it is unclear which effect can be exploited most readily to increase ZT. What is clear, though, is that if researchers must increase ZT beyond 3, it is imperative that they understand how charge and heat transport occur in semiconductor nanostructures. 105PDF Image | Analysis for Recovering Energy from Industrial Waste Heat
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