Herein we cover the key concepts in the field of thermoelectric materials research, present the current understanding, and show the latest developments. Current research is aimed at increasing the thermoelectric figure of merit (ZT) by maximizing the power factor and/or minimizing the thermal conductivity. Attempts at maximizing the power factor include the development of new materials, optimization of existing materials by doping, and the exploration of nanoscale materials. The minimization of the thermal conductivity can come through solid-solution alloying, use of materials with intrinsically low thermal conductivity, and nanostructuring. Herein we describe the most promising bulk materials with emphasis on results from the last decade. Single-phase bulk materials are discussed in terms of chemistry, crystal structure, physical properties, and optimization of thermoelectric performance. The new opportunities for enhanced performance bulk nanostructured composite materials are examined and a look into the not so distant future is attempted.

New and Old Concepts in Thermoelectric Materials

There has been a renaissance of interest in exploring highly efficient thermoelectric materials as a possible route to address the worldwide energy generation, utilization, and management. This review describes the recent advances in designing high-performance bulk thermoelectric materials. We begin with the fundamental stratagem of achieving the greatest thermoelectric figure of merit ZT of a given material by carrier concentration engineering, including Fermi level regulation and optimum carrier density stabilization. We proceed to discuss ways of maximizing ZT at a constant doping level, such as increase of band degeneracy (crystal structure symmetry, band convergence), enhancement of band effective mass (resonant levels, band flattening), improvement of carrier mobility (modulation doping, texturing), and decrease of lattice thermal conductivity (synergistic alloying, second-phase nanostructuring, mesostructuring, and all-length-scale hierarchical architectures). We then highlight the decoupling of th...

Rationally Designing High-Performance Bulk Thermoelectric Materials

The field of thermoelectrics has progressed enormously and is now growing steadily because of recently demonstrated advances and strong global demand for cost-effective, pollution-free forms of energy conversion. Rapid growth and exciting innovative breakthroughs in the field over the last 10-15 years have occurred in large part due to a new fundamental focus on nanostructured materials. As a result of the greatly increased research activity in this field, a substantial amount of new data--especially related to materials--have been generated. Although this has led to stronger insight and understanding of thermoelectric principles, it has also resulted in misconceptions and misunderstanding about some fundamental issues. This article sets out to summarize and clarify the current understanding in this field; explain the underpinnings of breakthroughs reported in the past decade; and provide a critical review of various concepts and experimental results related to nanostructured thermoelectrics. We believe recent achievements in the field augur great possibilities for thermoelectric power generation and cooling, and discuss future paths forward that build on these exciting nanostructuring concepts.

Nanostructured Thermoelectrics: Big Efficiency Gains from Small Features

This review discusses recent developments and current research in bulk thermoelectric materials in which nanostructuring is a key aspect affecting thermoelectric performance. Systems based on PbTe, AgPbmSbTe2+m, NaPbmSbTe2+m, Bi2Te3, and Si are given particular emphasis. To date the dramatic enhancements in figure of merit in bulk nanostructured materials come from very large reductions in lattice thermal conductivity rather than improvement in power factors. A discussion of future possible strategies is aimed at enhancing the thermoelectric figure of merit of these materials.

Nanostructured Thermoelectrics: The New Paradigm?†

Health, infrastructure, and environmental monitoring as well as networking and defense technologies are only some of the potential areas of application of micro-/nanosystems (MNSs). It is highly desirable that these MNSs operate without an external electricity source and instead draw the energy they require from the environment in which they are used. This Review covers various approaches for energy harvesting to meet the future demand for self-powered MNSs.

Nanotechnology-Enabled Energy Harvesting for Self-Powered Micro-/Nanosystems

The Czochralski method was used to grow a 46-mm-long crystal of the Ba8Ga16Ge30 clathrate, which was cut into disks that were evaluated for thermoelectric performance. The Seebeck coefficient and electrical and thermal conductivities all showed evidence of a transition from extrinsic to intrinsic behavior in the range of 600–900K. The corresponding figure of merit (ZT) was found to be a record high of 1.35 at 900K and with an extrapolated maximum of 1.63 at 1100K. This makes the Ba8Ga16Ge30 clathrate an exceptionally strong candidate for medium and high-temperature thermoelectric applications.

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Large thermoelectric figure of merit at high temperature in Czochralski-grown clathrate Ba8Ga16Ge30

Catalytic oxidation of CO over ordered mesoporous platinum

Conventional Pt/Al2O3 catalysts prepared by wet-impregnation are composed of Pt nanoparticles exposing convex and facetted surfaces deposited on high-surface area gamma-Al2O3 supports. A hexagonal phase mesoporous Pt material (denoted H-1-Pt) prepared by chemical reduction in the aqueous domains of a lyotropic liquid crystalline template exposes however mainly a concave surface with expected different catalytic properties. A series of Pt/Al2O3 catalysts were prepared using H-1-Pt, Pt-black or wet-impregnated Pt, and the samples were characterized by SEM-EDX and TEM, and finally evaluated for CO oxidation. The H-1-Pt/Al2O3 catalyst showed an ignition profile for CO oxidation at lower temperatures and thus appeared less sensitive to CO poisoning than the two other types of samples. This difference may be related to the differences in surface curvature.

Differences in catalytic properties between mesoporous and nanoparticulate platinum

CoSb/sub 3/ skutterudites have been prepared by a new chemical method of co-precipitating the constituents from an aqueous solution and subsequent heat treatment at comparably low temperature, resulting in nano-grained powder and thus in a very high concentration of grain boundaries. Under certain conditions grain boundary scattering is expected to be more effective on heat carrying phonons than on charge carriers, causing an increased ratio of electrical to thermal conductivity (/spl sigma///spl kappa/ ratio). Furthermore to overcome the expected reduction of the electrical conductivity by the grain boundary scattering skutterudites with various substitution of Co by Ni and of Sb by Te as well as combined substitutions have been prepared. Their structural and thermoelectric properties have been investigated, i.e. electrical and thermal conductivity as well as Seebeck coefficient and figure of merit. ZT values up to 0.67 at 680 K have been found for these samples.

Thermoelectric properties of nano-grained CoSb/sub 3/ skutterudites doped with Ni and Te

A method for growing single crystals of the clathrate Ba,Ga,,Ge,, has been developed and a suitable specimen prepared. The crystal with a diameter of 7 mm and a length of 46 mm was cut into discs, which were analyzed with respect to chemical composition, crystal structure and thermoelectric properties. Typical disc samples were found to be n-doped with the Seebeck coefficient increasing from -SO pVK at 300 K to -185 pVK at 900 K. The thermal conductivity was found to be close to 2 W/mK and only slightly decreasing with increasing temperature. The resistivity of the sample increased from 0.65 mQcm at 300 K to 1.8 mQcm at 900 K. Combining these results the dimensionless thermoelectric figure-of-merit, ZT (= TS'/RA), was found to increase from ZT= 0.08 at room temperature to ZT > 1.0 at 900 K. Preliminary data indicates that ZT appears to further increase with temperature above 900 K. These results emphasize the position of the Ba,Ga,,Ge,, clathrate as being in the same range of thermoelectric performance as the current state of the art materials at the medium to high temperature range.

Ali Saramat

Papers

Large thermoelectric figure of merit at high temperature in Czochralski-grown clathrate Ba8Ga16Ge30

Catalytic oxidation of CO over ordered mesoporous platinum

Differences in catalytic properties between mesoporous and nanoparticulate platinum

Thermoelectric properties of nano-grained CoSb/sub 3/ skutterudites doped with Ni and Te

Thermoelectric performance of large single crystal clathrate Ba8Ga16Ge30