The conversion of heat to electricity by thermoelectric devices may play a key role in the future for energy production and utilization. However, in order to meet that role, more efficient thermoelectric materials are needed that are suitable for high-temperature applications. We show that the material system AgPb m SbTe 2+ m may be suitable for this purpose. With m = 10 and 18 and doped appropriately, n -type semiconductors can be produced that exhibit a high thermoelectric figure of merit material ZT max of ∼2.2 at 800 kelvin. In the temperature range 600 to 900 kelvin, the AgPb m SbTe 2+ m material is expected to outperform all reported bulk thermoelectrics, thereby earmarking it as a material system for potential use in efficient thermoelectric power generation from heat sources.

http://science.sciencemag.org/content/sci/303/5659/818.full.pdf

Cubic AgPbmSbTe2+m: Bulk Thermoelectric Materials with High Figure of Merit

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

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?†

Magnetocaloric effect: From materials research to refrigeration devices

Transport properties of polycrystalline Ge clathrates with general composition Sr8Ga16Ge30 are reported in the temperature range 5 K⩽T⩽300 K. These compounds exhibit N-type semiconducting behavior with relatively high Seebeck coefficients and electrical conductivity, and room temperature carrier concentrations in the range of 1017–1018 cm−3. The thermal conductivity is more than an order of magnitude smaller than that of crystalline germanium and has a glasslike temperature dependence. The resulting thermoelectric figure of merit, ZT, at room temperature for the present samples is 14 that of Bi2Te3 alloys currently used in devices for thermoelectric cooling. Extrapolating our measurements to above room temperature, we estimate that ZT>1 at T>700 K, thus exceeding that of most known materials.

/pdf/semiconducting-ge-clathrates-promising-candidates-for-wehk84knp6.pdf

Semiconducting Ge clathrates: Promising candidates for thermoelectric applications

Publisher Summary This chapter describes semiconductor clathrates. The word “clathrate” derives from the Latin “clathratus” meaning “furnished with a lattice.” It is currently used in chemistry to describe a particular type of compound, usually a polyatomic compound, in which one component forms a cage structure imprisoning the other. The crystalline complexes of water, H 2 O, with simple molecules such as chlorine, Cl 2 , have been known to form clathrate compounds for more than a century. The chapter discusses the two common forms of ice clathrates, which are formed when the water is cooled and agitated in the presence of a sufficient concentration of the guest atoms. Type-I silicon (Si) and germanium (Ge) clathrates are metallic, whereas type-II clathrates maintain the semiconducting properties of Si and Ge for low alkaline metal concentrations. The semiconductivity diminishes as the metal concentration increases. A clathrate material is one in which the voids in the host lattice are present in the absence of a guest atom or molecule. These are called “true clathrates.” One good example in the field of thermoelectrics are the semiconducting skutterudites based on CoAs 3 . The chapter outlines several methods employed in forming Si, Ge, and tin (Sn) clathrates and in mixed-crystal clathrates containing two or more of these elements.

Chapter 6 Semiconductor clathrates: A phonon glass electron crystal material with potential for thermoelectric applications

Current-injection ultraviolet lasers are demonstrated on low-dislocation-density bulk AlN substrates. The AlGaInN heterostructures were grown by metalorganic chemical vapor deposition. Requisite smooth surface morphologies were obtained by growing on near-c-plane AlN substrates, with a nominal off-axis orientation of less than 0.5°. Lasing was obtained from gain-guided laser diodes with uncoated facets and cavity lengths ranging from 200 to 1500 μm. Threshold current densities as low as 13 kA/cm2 were achieved for laser emission wavelengths as short as 368 nm, under pulsed operation. The maximum light output power was near 300 mW with a differential quantum efficiency of 6.7%. This (first) demonstration of nitride laser diodes on bulk AlN substrates suggests the feasibility of using such substrates to realize nitride laser diodes emitting from the near to deep ultraviolet spectral regions.

Ultraviolet semiconductor laser diodes on bulk AlN

Large-area AlN substrates for electronic applications: An industrial perspective

The surface acoustic wave velocity has been measured on a-plane (c-propagation) and c-plane oriented bulk aluminum nitride (AlN) single crystals using the S/sub 11/-parameter method in the frequency range 160-360 MHz. The SAW velocity is 5760 m/s for both orientations. From comparison of this value with the simulations using various elastic constants of AlN available in literature, we estimated the elastic constant C/sub 44/ to be 122 /spl plusmn/ 1 GPa.

S. B. Schujman

Papers

Semiconducting Ge clathrates: Promising candidates for thermoelectric applications

Chapter 6 Semiconductor clathrates: A phonon glass electron crystal material with potential for thermoelectric applications

Ultraviolet semiconductor laser diodes on bulk AlN

Large-area AlN substrates for electronic applications: An industrial perspective

Surface acoustic wave velocity in single-crystal AlN substrates