Thermoelectric power, electrical and thermal resistance, and magnetoresistance of nanowire composites
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Thermoelectric power, electrical and thermal resistance, and magnetoresistance of nanowire composites. Joseph P. Heremans Delphi Research Laboratories Shelby Township, MI 48315, U. S. A. ABSTRACT The thermoelectric power of bismuth nanowires is theoretically calculated to be greatly enhanced over that of bulk Bi. This is a result of the size-quantization of the electron wavefunction in nanowires with diameters below 50 nm. The effect is expected to lead to the development of high figure of merit thermoelectric materials. We review here the experimental observation of such enhancement in composites containing nanowires with diameters down to 9 nm. When the wire diameter is further decreased, localization effects take over and limit the thermopower. The theory further predicts the appearance of an energy gap in bismuth nanowires with diameters below 50 nm. We observe such a gap in the temperature dependence of the resistivity. The dependence of the gap on nanowire diameter is consistent with theory. Comparisons of the transport properties of Bi nanowires with those in other nanowire systems show the influence of localization effects. INTRODUCTION
Coefficient of Performance
Recent advances in nanoscale thermoelectric (TE) materials pave the way to applications that were hitherto unconceivable. For instance, the value of ZT=2 reported [1] for a quantum-dot PbTe superlattice at room temperature is the threshold value above which solid-state cooling and air 5
Quantum Dot Commercial Superlattice [1] TE
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New adiabatic design [2]
R134a systems
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conventional isothermal module
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∆T=40oC
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ZT Figure 1. Maximum coefficient of performance of a TE cooler at 300 K, as a function of the figure of merit ZT of the material.
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conditioning can be considered. We show, in Fig. 1, the maximum coefficient of performance (COP) of a thermoelectric cooler calculated for a temperature gradient of 40oC, as a function of the thermoelectric figure of merit, ZT, of the material. The COP is defined as the flow of heat pumped at the cold plate divided by the electrical power used. Two cases are considered. The first case, the “isothermal module”, is that of a conventional module in which all pairs of Peltier couples are thermally connected to a single hot plate on one end, and to a cold plate on the other. The second case is proposed in Ref [2]: each Peltier couple is thermally isolated from each other couple (“adiabatic design”) and the heat is carried from one couple to the next by the fluid to be heated on one end of the Peltier elements, and to be cooled on the other end. We also show the range of values of COP that are reached by a conventional vapor-compression based air automotive air conditioner using R134a as medium. Clearly, conventional TE materials used in a conventional module reach at best a COP of about 0.5, and cannot compete with vapor-compression refrigeration. The use of quantum-dot superlattices, combined with the “adiabatic” Peltier modules does offer the promise to reach COPs on
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