Electron Crystals and Phonon Glasses: A New Path to Improved Thermoelectric Materials
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MRS BULLETIN/JANUARY 1998
erant, and last indefinitely. The material requirements for thermoelectric devices depend upon whether sensing, cooling, or power generation is required. Thin wires of various metal al-
loys are usually used in the construction of thermocouples. For refrigeration or power-generation applications however, metal alloys are too inefficient. In a refrigerator for example, (Figure 1) heat has to be pumped from the cold end of the device to the warm end. The amount of heat that can be pumped using the Peltier effect is just STI, so clearly we want a material with a large S. The electrical resistivity p of the material should be small. Otherwise irreversible heat losses will occur via Joule heating (I2R). Finally the thermal conductivity of the material should be small to reduce the natural flow of heat from the hot to the cold end. If all these effects are taken into account, it can be shown that the parameter determining how device efficiency depends on material properties is the dimensionless figure of merit ZT where ZT=TS2/KP,
(1)
p is the electrical resistivity, and K is the thermal conductivity of the material at temperature T (K). In an electrically conducting solid, heat is transported by both the charge carriers and phonons. Hence K, the total thermal conductivity, is composed of two parts: K = Kek,ctronk + K|at,ict, where KL,iectronic describes the heat conducted by the electrons (or holes) and K\micl. describes the heat conducted by the crystalline lattice. For both refrigeration
Active Cooling
Refrigeration
Power Generation
Figure 1. A single thermoelectric couple that has been configured for refrigeration or power generation. The labels "P" (positive) and "N" (negative) refer to the sign of the charge carriers in each leg. (Open circles correspond to holes and filled circles to electrons.) Refrigeration is possible in these devices because electrons (or holes) carry heat and hot electrons (holes) can be forced away from the cold end of the device by the battery. If a temperature difference is externally imposed on the device, useful power can be extracted.
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Electron Crystals and Phonon Glasses: A New Path to Improved Thermoelectric Materials
Figure 2. This multistage thermoelectric-cooling module is capable of reaching temperatures as low as 160 K at the small copper pad on the final (highest) stage. The large copper pad at the bottom of the first stage is normally attached to a heatsink. (Photograph courtesy of Marlow Industries, Dallas, Texas.) The cooling module is composed of over a hundred thermoelectric couples similar to those shown in Figure 1.
and power-generation (Figure 1) applications, materials with larger values of ZT result in devices that will operate at a higher efficiency. Doped semiconductors are best suited to meet these requirements. Most metals have small values of S and hence very small values of ZT. The major problem with these devices is poor efficiency. Current state-of-the art Peltier refrigerators use semiconducting Bi2Te.-,-Sb2Te3 alloys (Z
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