Do You Know “Thermoelectric Materials” ?
Thermoelectric materials show the thermoelectric effect in a strong and/or convenient form. The thermoelectric effect refers to phenomena in which a temperature difference creates an electric potential or electric potential creates a temperature difference: Specifically, the Seebeck effect (temperature->current), Peltier effect (current->temperature), and Thomson effect (conductor heating/cooling). While all materials have a nonzero thermoelectric effect, in most materials it is too small to be useful. However, low cost materials that have a sufficiently strong thermoelectric effect (and other required properties) could be used for applications including power generation, refrigeration and a variety of other applications.
Production methods for these materials can be divided into powder and crystal growth based techniques. Powder based techniques offer excellent ability to control and maintain desired carrier distribution. In crystal growth techniques dopants are often mixed with melt, but diffusion from gaseous phase can also be used. In the zone melting techniques disks of different materials are stacked on top of others and then materials are mixed with each other when a travelling heater causes melting. In powder techniques, either different powders are mixed with a varying ratio before melting or they are in different layers as a stack before pressing and melting.
Approximately 90% of the world’s electricity is generated by heat energy, typically operating at 30-40% efficiency, losing roughly 15 terawatts of power in the form of heat to the environment. Thermoelectric devices could convert some of this waste heat into useful electricity. Thermoelectric efficiency depends on the figure of merit, ZT. There is no theoretical upper limit to ZT, and as ZT approaches infinity, the thermoelectric efficiency approaches the Carnot limit. However, no known thermoelectrics have a ZT>3. As of 2010, thermoelectric generators serve application niches where efficiency and cost are less important than reliability, light weight, and small size.
Internal combustion engines capture 20-25% of the energy released during fuel combustion. Increasing the conversion rate can increase mileage and provide more electricity for on-board controls and creature comforts (stability controls, telematics, navigation systems, electronic braking, etc.). 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.
Thermoelectric materials can be used as refrigerators, called “thermoelectric coolers”, or “Peltier coolers” after the Peltier effect that controls their operation. As a refrigeration technology, Peltier cooling is far less common than vapor-compression refrigeration. The main advantages of a Peltier cooler (compared to a vapor-compression refrigerator) are its lack of moving parts or circulating fluid, and its small size and flexible shape (form factor). Another advantage is that Peltier coolers do not require refrigerant fluids, such as chlorofluorocarbons (CFCs) and related chemicals, which can have harmful environmental effects.
Strategies to improve thermoelectrics include both advanced bulk materials and the use of low-dimensional systems. Such approaches to reduce lattice thermal conductivity fall under three general material types :
- Alloys : create point defects, vacancies, or rattling structures (heavy-ion species with large vibrational amplitudes contained within partially filled structural sites) to scatter phonons within the unit cell crystal.
- Complex crystals : separate the phonon-glass from the electron crystal using approaches similar to those for superconductors. The region responsible for electron transport would be an electron-crystal of a high-mobility semiconductor, while the phonon-glass would be ideal to house disordered structures and dopants without disrupting the electron-crystal (analogous to the charge reservoir in high-Tc superconductors.
- Multiphase nanocomposites : scatter phonons at the interfaces of nanostructured materials, be they mixed composites or thin film superlattices.
Materials under consideration for thermoelectric device applications include :
Materials such as Bi2Te3 and Bi2Se3 comprise some of the best performing room temperature thermoelectrics with a temperature-independent thermoelectric effect, ZT, between 0.8 and 1.0. Nanostructuring 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). Note that this high value has not entirely been independently confirmed.
Bismuth telluride and its solid solutions are good thermoelectric materials at room temperature and therefore suitable for refrigeration applications around 300 K. The Czochralski method has been used to grow single crystalline bismuth telluride compounds. These compounds are usually obtained with directional solidification from melt or powder metallurgy processes. Materials produced with these methods have lower efficiency than single crystalline ones due to the random orientation of crystal grains, but their mechanical properties are superior and the sensitivity to structural defects and impurities is lower due to high optimal carrier concentration.
Jeffrey Snyder and his colleagues have shown in 2008 that with thallium doped lead telluride alloy (PbTe) it is possible to achieve zT of 1.5 at 773 K (Heremans et al., Science, 321(5888): 554-557). In an article published in January 2011, they showed that replacing thallium with Sodium zT~1.4 at 750 K is possible (Y. Pei et al., Energy Environ. Sci., 2011). In May 2011 they reported in Nature in collaboration with Chinese research group that PbTe1-xSex alloy doped with sodium gives zT~1.8±0.1 at 850 K (Y. Pei et al., Nature, 473 (5 May, 2011)). Snyder’s group has determined that both thallium and sodium alter the electronic structure of the crystal increasing electric conductivity. The Snyder group also claims that selenium increases further electric conductivity and also reduces thermal conductivity. These works show that other bulk alloys have also potential for improvement, which could open many new applications for thermoelectrics.
Higher silicides seem promising materials for thermoelectric energy conversion, because their figure of merit is at the level with materials currently in use and they are mechanically and chemically strong and therefore can often be used in harsh environments without any protection. More detailed studies are needed to assess their potential in thermoelectrics and possibly to find a way to increase their figure of merit. Some of possible fabrication methods are Czochralski and floating zone for single crystals and hot pressing and sintering for polycrystalline.
Magnesium group IV compounds
Mg2BIV (BIV=Si, Ge, Sn) compounds and their solid solutions are good thermoelectric materials and their figure of merit values are at the level with established materials. Due to the lack of the systematic studies about their thermoelectric properties suitability of these materials, and in particular their quasi-ternary solutions, for thermoelectric energy conversion remains in question. The appropriated production methods are based on direct comelting but mechanical alloying has also been used. During synthesis, magnesium losses due to evaporation and segregation of components (especially for Mg2Sn) need special attention.
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