lecture20 - 2.57 Nano-to-Macro Transport Processes Fall...

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2.57 Fall 2004 – Lecture 20 1 2.57 Nano-to-Macro Transport Processes Fall 2004 Lecture 20 Guest lecture by Prof. Mildred S. Dresselhaus 1. Outline -Overview of low dimensional thermoelectricity -New physics to yield enhanced performance in 1D nanowires -Quantum dot superlattice nanowires – model calculations -Newly emerging research directions -New methods for synthesis and assembly of nanowires Self assembled composite nanostructures New 3D crystalline materials with quantum dots New thermoelectric tools 2. Introduction to Thermoelectricity Physically, when one side of the conductor (or semiconductor) is hot, electrons have higher thermal energy and will diffuse to the cold side. The higher charge concentration in the cold side builds an internal electric field that resists the diffusion. The Seebeck voltage is the steady-state voltage accumulated under the open circuit condition. We can express this as 21 V S TT =− (V/K), where S > 0 for p-type semiconductors, and S < 0 for n-type materials. In the following figure, we demonstrate the idea of Peltier Effects. + V T 1 T 2 I Heat Q Heat Q I Q = π
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2.57 Fall 2004 – Lecture 20 2 In this case, heat is carried by carriers (absorbed at the left and rejected at the right end). The Peltier coefficient is defined as / QI Π = . 3. Applications In the above figure, we show two major applications of thermoelectricity: refrigerator and power generator. The efficiency of the refrigerator can be evaluated by 2 S Z TT k σ = , in which S is seebeck coefficient, is electrical conductivity, and k denotes thermal conductivity. Compared with mechanical refrigeration, thermoelectric cooling offers the following advantages: -No moving parts -Environmentally friendly -No loss of efficiency with size reduction -Can be integrated with electronic circuits (e.g. CPU) -Localized cooling with rapid response However, the current ZT value (~1) for TE cooling still lags behind that of mechanical refrigeration (ZT~3). 4. Thermoelectric Properties of Conventional Materials In the following figure, we present the variations of three parameters against the carrier concentration. The ZT value reaches the maximum in the middle of the figure. However, we should not use semimetals because they have both holes and electrons as carriers, which have different signs of S and the effects will cancel out in the cooling process.
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lecture20 - 2.57 Nano-to-Macro Transport Processes Fall...

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