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ScienceWeek
MATERIALS SCIENCE: ON THERMOELECTRICITY IN SEMICONDUCTORS
The following points are made by Arun Majumdar (Science 2004 303:777):
1) With the widespread use of semiconductors in microelectronics and optoelectronics, it is hard to imagine that the initial excitement was due to their promise not in electronics, but in refrigeration (1). The discovery in the 1950s that semiconductors can act as efficient heat pumps led to premature expectations of environmentally benign solid-state home refrigerators and power generators containing no moving parts. Except for specialized applications, however, the vision of widespread use of thermoelectric energy-conversion devices has remained elusive. At issue are some fundamental scientific challenges, which could be overcome by deeper understanding of charge and heat transport in semiconductor nanostructures.
2) Thermoelectric materials are ranked by a figure of merit, ZT, which is defined as ZT = S^(2)sT/k, where (S) is the thermopower or Seebeck coefficient, (s) is the electrical conductivity, (k) is the thermal conductivity, and (T) is the absolute temperature. To be competitive compared with conventional refrigerators and generators, one must develop materials with ZT > 3. Yet in five decades the room-temperature ZT of bulk semiconductors has increased only marginally, from about 0.6 to 1. The challenge lies in the fact that (S), (s), and (k) are interdependent --changing one alters the others, making optimization extremely difficult. The only way to reduce (k) without affecting (S) and (s) in bulk materials is to use semiconductors of high atomic weight such as Bi2Te3 and its alloys with Sb, Sn, and Pb. High atomic weight reduces the speed of sound in the material, and thereby decreases the thermal conductivity. Although it is possible in principle (4) to develop bulk semiconductors with ZT > 3, there are no candidate materials on the horizon.(2,3,5)
References (abridged):
1. G. Mahan, B. Sales, J. Sharp, Phys. Today 50, 42 (March 1997)
2. H. K. Lyeo et al. Science 303, 816 (2004)
3. K. F. Hsu et al., Science 303, 818 (2004)
4. F. J. DiSalvo, Science 285, 703 (1999)
5. L. D. Hicks, M. S. Dresselhaus, Phys. Rev. B 47, 12727 (1993)
Science http://www.sciencemag.org
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ON THERMOELECTRIC MATERIALS
The following points are made by Brian C. Sales (Science 2002 295:1248):
1) In the early 1950s, most semiconductor research focused on using semiconductors not in integrated circuits but in thermoelectric modules for home refrigeration. This never became practical because of poor cooling efficiency. New materials and new synthesis techniques have now reawakened interest in the use of semiconductors in refrigeration and power generation, and some of the promising new thermoelectric structures contain carefully arranged films or clusters on nanometer length scales.
2) Thermoelectric devices are extremely simple, have no moving parts, and involve no greenhouse gases. The devices use two types of semiconductor "legs" that are connected in series: negatively charged electrons carry electric current in the n-type leg, whereas positively charged holes carry the current in the p-type leg.
3) Thermoelectric refrigeration with semiconductor devices is possible because electrons and holes carry heat as well as electric charge. An external battery forces the hot electrons and holes away from the cold side of the device, resulting in cooling. In some multistage thermoelectric modules, temperatures as low as 160 kelvins can be achieved. Today, spot cooling of electronics is the primary application for thermoelectric refrigerators.
4) If heat is applied to only one side of the device, a voltage develops across the (n) and (p) legs that can be used to convert part of the heat into electrical power. NASA has used this principle to provide hundreds of watts of electrical power for deep space probes such as Voyager I and II and the Cassini mission to Saturn.
5) The major problem with thermoelectric devices is poor efficiency. The efficiency of a thermoelectric module is fundamentally limited by the material properties of the n- and p-type semiconductors, regardless of how cleverly the module is engineered.
Science http://www.sciencemag.org
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MATERIALS SCIENCE: ON THERMOELECTRICS
In nature, what are called "thermoelectric effects" occur when both electric and thermal currents are present. (So-called "transverse thermoelectric effects" occur when in addition there is a magnetic field normal to these currents.) There are essentially three ordinary thermoelectric effects:
The "Seebeck effect", discovered in 1822 by Thomas Seebeck (1770-1831), relates to the electromotive force developed in a circuit consisting of different conducting elements, not all of whose contacts are at the same temperature.
The "Peltier effect", discovered in 1834 by Jean Peltier (1785-1845), involves the reversible heating or cooling which occurs at a contact between two dissimilar conductors when electric current flows from one conductor to another.
The "Thomson effect" (Kelvin effect) discovered by Lord Kelvin (William Thomson) (1824-1907) in 1856, refers to the reversible heat absorption which occurs when an electric current flows in a homogeneous conductor in which there is a temperature gradient. All three effects are related by thermodynamics: if one effect is known, the other two can be derived.
The so-called Seebeck and Peltier coefficients are measures of the respective effects: the larger the coefficient, the greater the effect. In the Seebeck effect, for example, the generated voltage difference is proportional to the temperature difference, with the Seebeck coefficient another name for the proportionality factor.
The term "clathrates" refers to substances in which molecules of one compound (the "host") form a crystalline lattice that contains open spaces ("cages") that may be occupied by molecules of a second compound or element (the "guest"). Ordinarily, no chemical bonds are formed between the host and the guests, with the stability of a clathrate dependent on the closeness with which the guest entities fit into the holes as well as on the polarizabilities of the guest entities.
Devices based on the thermoelectric effects are widely used, but the applications of such devices are limited by a low efficiency. This has led to a search for new thermoelectric materials, and various clathrate compounds have been a recent focus of research.
The following points are made by G.S. Nolas and G.A. Slack (American Scientist 2001 89:136):
1) The authors point out that having large Peltier and Seebeck coefficients is not the only requirement for a useful thermoelectric material. The material must be both a good electrical conductor and an equally good thermal insulator. A thermoelectric cooler with poor electrical conductivity, for example, would increase in temperature through resistance heating when power was applied. And if the material did not act as a good thermal insulator, the heat transported by the flow of electric current would tend to leak backward from the hot side to the cold side.
2) Semiconductor material can be made to conduct electricity well by supplying mobile electrons or holes derived from the doping of the material with "impurities" that slightly disorder the crystal lattice. In these same semiconductors, heat is carried mostly by vibrational waves (phonons) moving through the atomic lattice. Most semiconductors have high thermal conductivity. But G.A. Slack pointed out more than 20 years ago that if the propagation of phonons in a semiconductor can be impeded by lattice collisions (scattering) due to disorder in the crystal structure, the normally high value of thermal conductivity will drop: the more disorderly the internal atomic arrangement, the more poorly the material will conduct heat. An ideal thermoelectric material, therefore, would be a well-ordered atomic structure allowing electric charge to move freely, but an atomic structure that at the same time scatters phonons. Various amorphous solids (glasses) transmit heat poorly because their internal atomic arrangements are so disorderly. An ideal thermoelectric material should be a "phonon glass" and an "electron crystal".
3) Investigations more than two decades ago suggested that semiconducting clathrates might have low values of thermal conductivity while retaining relatively high electrical conductivity. The elements commonly used to construct semiconductors do, in fact, form clathrates. The general idea is that as long as the voids in a semiconductor clathrate structure remain empty, the material will transmit heat too well to serve as a thermoelectric device. But if the cages are filled with small heavy atoms, the thermal conductivity of the composite material can be made extremely low as a result of scattering of phonons by the heavy atoms. At the same time, the surrounding crystal structure is still highly ordered and allows a high mobility of electrons and holes. Since 1997, the authors have been engaged in research on a fabricated germanium clathrate. This clathrate is a strontium-gallium-germanium compound, with the heavy strontium atoms acting as phonon scatterers. The authors report they have achieved thermal conductivity close to the theoretical minimum, with all heat-carrying phonons apparently scattered. "So far we have definitely achieved our aim of synthesizing a crystal that acts as a phonon glass. Still, we are a long way from complete success: The best semiconducting clathrates we have fabricated so far... are only about half as good as the materials now being used commercially for cooling."
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