High-efficiency thermoelectric (TE) materials are important for power-generation devices that are designed to convert waste heat into electrical energy.They can also be used in solid-state refrigeration devices.The conversion of waste heat into electrical energy may play an important role in our current challenge to develop alternative energy technologies to reduce our dependence on fossil fuels and reduce greenhouse gas emissions.

An overview of various TE phenomena and materials is provided in this issue of MRS Bulletin. Several of the current applications and key parameters are defined and discussed.Novel applications of TE materials include biothermal batteries to power heart pacemakers, enhanced performance of optoelectronics coupled with solid-state TE cooling, and power generation for deep-space probes via radioisotope TE generators.A number of different systems of potential TE materials are currently under investigation by various research groups around the world, and many of these materials are reviewed in the articles in this issue.These range from thin-film superlattice materials to large single-crystal or polycrystalline bulk materials, and from semiconductors and semimetals to ceramic oxides.The phonon-glass/electron-crystal approach to new TE materials is presented, along with the role of solid-state crystal chemistry.Research criteria for developing new materials are highlighted.

Good thermoelectric materials possess low thermal conductivity while maximizing electric carrier transport. This article looks at various classes of materials to understand their behavior and determine methods to modify or “tune” them to optimize their thermoelectric properties. Whether it is the use of “rattlers” in cage structures such as skutterudites, or mixed-lattice atoms such as the complex half-Heusler alloys, the ability to manipulate the thermal conductivity of a material is essential in optimizing its properties for thermoelectric applications.

Layered CoO2 materials are excellent candidates for potential thermoelectric applications. Their single crystals show good p-type thermoelectric properties at temperatures higher than 800K in air.Recently, the mechanism of thermoelectric properties was clarified through a discussion of electronic and crystallographic structure. In order to fabricate thermoelectric modules possessing good power-generation properties, thermoelectric materials and metallic electrodes must be connected with low contact resistance and high mechanical strength.It has been found that good junctions can be formed using Ag paste including p- and n-type oxide powders.The role of spin entropy contributions to thermopower will be presented, in connection with strong electron correlation and triangular lattices.

Superlattices consist of alternating thin layers of different materials stacked periodically.The lattice mismatch and electronic potential differences at the interfaces and resulting phononand electron interface scattering and band structure modifications can be exploited to reduce phonon heat conduction while maintaining or enhancing the electron transport.This article focuses on a range of materials used in superlattice form to improve the thermoelectric figure of merit.

Over a decade ago, Dresselhaus predicted that low-dimensional systems would one day serve as a route to enhanced thermoelectric performance.In this article, recent results in the thermoelectric properties of nanowires and nanotubes are discussed. Various synthesis techniques will be presented, including chemical vapor deposition for the growth of thermoelectric nanostructures in templated alumina.Electrical transport measurements of carbon nanostructures, such as resistivity and thermopower, have revealed some very interesting thermoelectric properties.Challenges still remain concerning the measurement of individual nanostructures such as nanowires.Much work has been performed on the thermoelectric properties of carbon nanotubes, and these results will be highlighted.In addition, routes for enhanced thermoelectric materials have focused on incorporating nanostructures within the bulk materials.The role of these “hybrid composite structures” based on nanomaterials incorporated into the bulk matrix and the potential for enhanced performance are discussed.

Historically, thermoelectric technology has only occupied niche areas, such as the radioisotope thermoelectric generators for NASA's spacecrafts, where the low cooling coefficient of performance (COP) and energy-conversion efficiency are outweighed by the application requirements.Recent materials advances and an increasing awareness of energy and environmental conservation issues have rekindled prospects for automotive and other applications of thermoelectric materials.This article reviews thermoelectric energy-conversion technology for radioisotope space power systems and several proposed applications of thermoelectric waste-heat recovery devices in the automotive industry.