Thermoelectric Devices and Materials

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Contents

[edit] Motivation for Research

Currently there are two primary arenas in which thermoelectric devices can lend themselves to increase energy efficiency and/or decrease pollutants: conversion of waste heat into usable energy and refrigeration.

[edit] Power Generation

In the transportation sector, although very common as a means of powering vehicles, internal combustion engines are highly inefficient in energy use (utilizing only 20-25% of the energy generated during fuel combustion)[1]. Furthermore, the electricity requirement in vehicles is increasing due to the demands of enhanced performance, on-board controls and creature comforts[2] (stability controls, telematics, navigation systems, electronic braking, etc.). In order to gain fuel efficiency, it may be possible to shift energy draw from the engine (in certain cases) to the electrical load in the car, e.g. electrical power steering or electrical coolant pump operation.[3] Thermoelectric devices are thus being investigated to convert waste-heat into usable energy utilizing the Seebeck Effect.

Currently, some power plants utilize a method known as cogeneration in which in addition to the electrical energy generated, the heat produced during the process is converted to useful heat. Thermoelectrics may find applications in such systems or in solar thermal energy generation.[4]

[edit] Refrigeration

Thermoelectric devices applied to refrigeration (utilizing the Peltier effect could reduce pollutants into the atmosphere. Hydrochlorofluorocarbons (HCFCs) and chlorofluorocarbons (CFCs) are known ozone depleting substances (ODSs); however, they have long been at the heart of refrigeration technology. Recently, there has been legislation regulating the use of such chemicals for refrigeration; current international legislation mandates caps on HCFC production and will prohibit their production after 2020 in developed countries and 2030 in developing countries.[5] These mandates as well as the environmental mindedness of consumers is leading to an increased effort in developing effective thermoelectric refrigeration units. Such units could reduce the use of such harmful chemicals and might run more quietly (since they do not require noisy compressors).

[edit] Materials Selection Criteria

[edit] Figure of Merit

The primary criterion for thermoelectric device viability is the Figure of Merit given by:

Z = {\sigma S^2 \over \lambda},

which depends on the Seebeck Coefficient, S, Thermal Conductivity, λ, and Electrical Conductivity, σ.

[edit] Slack's Proposal: Phonon-Glass, Electron-Crystal (PGEC) Behavior

Notably, in the above equation, Thermal Conductivity and Electrical Conductivity are typically intertwined. G. A. Slack[6] proposed that in order to optimize the Figure of Merit, phonons which are responsible for thermal conductivity must experience the material as they would in a glass (experiencing a high degree of phonon scattering--lowering the thermal conductivity) while electrons must experience it as a crystal (experiencing very little scattering--maintaining the electrical conductivity). It is through the adjustment of each these properties independently of the other that the Figure of Merit can be improved.

[edit] Materials of Interest

There are a number of materials being researched for thermoelectric device applications and temperature ranges. Some such materials include:

[edit] Bismuth Chalcogenides

These materials involve Bi2Te3 and Bi2Se3 and comprise some of the best performing thermoelectrics at room temperature with a temperature-independent figure of merit, ZT, between 0.8 and 1.0.[7] Nanostructuring of these materials to produce a layered superlattice structure of alternating Bi2Te3 and Bi2Se3 layers produces a device within which there is good electrical conductivity but perpendicular to which thermal conductivity is poor. The result is an enhanced ZT (approximately 2.4 at room temperature for p-type).[8]

[edit] Skutterudite Thermoelectrics

Recently, skutterudite materials have sparked the interest of researchers in search of new thermoelectrics[9] These structures are of the form (Co,Ni,Fe)(P,Sb,As)3 and are cubic with space group Im3. Unfilled, these materials contain voids into which low-coordination ions (usually rare earth elements) can be inserted in order to alter thermal conductivity by producing sources for lattice phonon scattering and decrease thermal conductivity due to the lattice without reducing electrical conductivity.[10] Such qualities make these materials behave with PGEC behavior.

[edit] Oxide Thermoelectrics

Due to the natural superlattice formed by the layered structure in homologous compounds (such as those of the form (SrTiO3)n(SrO)m--the Ruddleson-Popper phase), oxides are also being considered for high-temperature thermoelectric devices.[11] These materials exhibit low thermal conductivity perpendicular to these layers while maintaining electrical conductivity within the layers providing relatively high figure of merit of ~0.34 at 1000K.[12]

[edit] References

  1. ^ Yang, International Conference on Thermoelectrics: 2005, pp. 155.
  2. ^ Fairbanks, J., Thermoelectric Developments for Vehicular Applications, U.S. Department of Energy: Energy Efficiency and Renewable Energy. Presented on: August 24, 2006.
  3. ^ Yang, International Conference on Thermoelectrics: 2005, pp. 155.
  4. ^ Tritt et al., "Thermoelectrics: Direct Solar Thermal Energy Conversion," MRS Bulletin: April 2008, Vol. 33, pp. 366-8
  5. ^ NOAA: Earth System Research Laboratory, Hydrochlorofluorocarbon measurements in the Chlorofuorocarbon Alternatives Measurement Project, http://www.esrl.noaa.gov/gmd/hats/flask/hcfc.html.
  6. ^ Slack GA., CRC Handbook of Thermoelectrics, ed. DM Rowe, Boca Raton, FL: CRC Press (1995)
  7. ^ D.Y. Chung et al., Complex Bismuth Chalcogenides as Thermoelectrics, 16th International Conference on Thermoelectrics (1997), pp. 459-462
  8. ^ Venkatasubramanian et al., Nature, 413, 597 (2001)
  9. ^ Caillat, T., Borshchevsky, A., and Fleurial, J.-P., In Proceedings of 7th International Conference TEs, K. Rao, ed., pp. 98 – 101. University of Texas, Arlington, 1993.
  10. ^ Nolas et al., J. Appl. Phys ., 79 (1996), 4002-8
  11. ^ K. Koumoto,I. Terasaki, T. Kajitani, M. Ohtaki, R. Funahashi “Oxide Thermoelectrics”; Section 35: pp.1-14 in Thermoelectrics Handbook: Macro to Nano, Edited by D.M. Rowe, CRC Press: New York (2006).
  12. ^ W. Wunderlich, S. Ohta, and K. Koumoto, 24th International Conference on Thermoelectrics, 2005, pp. 252-255