The demand for new technologies to enhance the efficiency of our Nation's automobiles is at the forefront of new materials research towards power conversion technologies. Thermoelectric (TE) elements have been used as heat sensors for temperature measurement, heat pumps in applications with low power demand or where compression refrigerating systems cannot be used for other reasons, and in thermoelectric generators (TEG). However, unit costs are relatively high and the efficiency too low in TEGs, to allow for general commercialization of TE devices. There exists a vast array of potential applications for TE elements due to decreasing energy resources and increasing energy demand. TE power conversion from exhaust, “waste”, heat is a viable technology that can be instrumental in improving the efficiency, and thus reducing emitted pollutants, of our nation's automobiles. Because TE devices can translate heat flow into electrical current, thermoelectric generators may be used as a renewable energy source in automotive fuel economy, electricity generation. TE technology is advantageous in many respects, including reliability (no moving parts), safety and environmental friendliness. When a temperature gradient is maintained across a TE device electric potential is generated, due to the Seebeck effect, which can be used to drive a “load”, or generator.
Research into higher efficiency thermoelectric (TE) materials continues to require advanced synthesis techniques. The specific material property requirements for TE materials can be quantified by the dimensionless figure of merit, ZT=S2 σ/κ where S is the Seebeck coefficient, σ the electrical conductivity and κ the total thermal conductivity (κ=κL+κe; the lattice and electronic contributions, respectively). The power factor, S2σ or S2/ρ where ρ is the electrical resistivity, is typically optimized as a function of carrier concentration (typically ˜1019 carriers/cm3 in materials presently available in devices), through doping, to give the largest ZT. The current TE materials used in devices are rather inefficient, ZT≈1, even though they are able to address many niche applications. In order that TE devices achieve their full potential, new materials and new material synthesis approaches are needed. Materials with larger values of ZT warrant better thermoelectric devices, thus researchers have devoted much effort in finding ways to increase ZT. Since the transport properties that define ZT are interrelated, it is difficult to control them independently for a three-dimensional crystal.
The thermoelectric potential, which determines the electrical voltage that can be generated, depends on the material-specific thermoelectric characteristics based on the Seebeck coefficient and temperature differences. High Seebeck coefficients and large temperature differences lead to high thermoelectric voltages. In order to be able to draw large electrical power, a large temperature difference must be maintained between the elements of a TE device. This necessitates the use of bulk materials with high Seebeck coefficients. With the present methods for manufacturing TE elements, either it is not possible in practice to manufacture such bulk materials or the manufacture of such bulk TE elements would lead to exorbitant manufacturing costs.
Recent research on TE materials has resulted in ZT values greater than unity at different temperatures of application, due to the advanced material synthesis, characterization and modeling capabilities developed. Several classes of materials have added to these advances. These include bulk materials such as skutterudites and clathrates (G. S. Nolas, et al., “Skutterudites: A phonon-glass-electron-crystal approach to advanced thermoelectric energy conversion applications”, Ann Rev. Mat. Res. 29, 8. (1999); C. Uher, “Skutterudites: Perspective novel materials” in Semiconductors and Semimetals 69, 139 (2000); B. C. Sales, et al., “Filled skutterudite antimonides: A new class of thermoelectric materials”, Science 272, 1325 (1996); G. S. Nolas, et al., “Semiconductor Clathrates: A phonon-glass electron-crystal material with potential for thermoelectric applications”, in Semiconductors and Semimetals, Vol. 69, edited by T. M. Tritt, Academic Press, 2000, p. 255; G. S. Nolas, “Structure, transport and thermoelectric properties of clathrate compounds”, in Thermoelectrics Handbook: Macro to Nano-Structured Materials, edited by D. M. Rowe, CRC Press, Boca Raton, Fla., 2005, 33-1; A. Saramat, et al. “Large thermoelectric figure of merit at high temperature in Czochralski-grown clathrate Ba8Ga16Ge30”, J. Appl. Phys. 99, 023708 (2006), and complex chalcogenides (M. G. Kanatzidis, “The role of solid-state chemistry in the discovery of new thermoelectric materials” in Semiconductors and Semimetals 69, 51 (2000)) with unique crystal structures allowing for low κL, and therefore enhanced thermoelectric properties.
Additionally, research and development in nanotechnology has shown promising improvements in Seebeck coefficient with new TE materials. The controlled fabrication of nanoscale semiconductors with enhanced physical properties is a current goal of technical as well as fundamental interest. Thin-film coatings and nanotube technology allows production of two-dimensional or one-dimensional thermoelectric structures with improved TE characteristics compared to traditional “bulk” materials, but still do not solve the problem of manufacturing bulk materials that are inexpensive. Nevertheless, nanostructured materials have become of great interest because they offer the opportunity of independently varying the transport properties that define ZT (G. S. Nolas, J. Sharp, and H. J. Goldsmid, Thermoelectrics: Basic Principles and New Materials Developments (Springer, New York, 2001); G. Chen, M. S. Dresselhaus, G. Dresselhaus, J. P. Fleurial, and T. Caillat, International Materials Review, Vol. 48 (London, 2003), p. 45-66).
Semiconductor grain boundaries on carrier transport becomes increasingly important in nanoscale polycrystalline systems, where surface, point defect, dislocation, and interfacial energy barrier scatterings can dominate the transport (G. Blatter and F. Greuter, Phys. Rev. B 34, 8555 (1986); H. Marom, M. Ritterband, M. Eizenberg, Thin Solid Films 510, 62 (2006); C. H. Seager, J. Appl. Phys. 52, 3960 (1981)). Recent identification of several higher efficiency thermoelectric (TE) materials can be attributed to nanoscale enhancement (M. S. Dresselhaus, G. Chen, M. Y. Tang, R. Yang, H. Lee, D. Wang, Z. Ren, J. P. Fleurial, and P. Gogna, Adv. Mater. 19, 1043 (2007); L. D. Hicks, M. S. Dresselhaus, Phys. Rev. B 47, 16631 (1993); J. P. Heremans, C. M. Thrush and D. T. Morelli, J. Appl. Phys. 98, 063703 (2005); K. F. Hsu, S. Loo, F. Guo, W. Chen, J. S. Dyck, C. Uher, T. Hogan, E. K. Polychroniadis, M. G. Kanatzidis, Science 303, 818 (2004); M. S. Dresselhaus, G. Chen, M. Y. Tang, R. G. Yang, H. Lee, D. Z. Wang, Z. F. Ren, J. P. Fleurial and P. Gogna, Proc. Mater. Res. Soc. 886 3 (2006)). These materials demonstrate increased Seebeck coefficient and decreased thermal conductivity due to the phenomenological properties of nanometer length scales, including enhanced interfacial phonon scattering and charge carrier filtering.
Nanometer-scale materials have been at the core of one of the most substantial advances in thermoelectric technology since the development of semiconductor technology (Materials and Technologies for Direct Thermal-to-electrical Energy Conversion, Mater. Res. Soc. Proc. Vol. 886, edited by J. Yang. T. P. Hogan, R. Funahachi and G. S. Nolas, 2006)). The two mechanisms that led to this success were a strong reduction in κL and an increase in S2σ. The reduction in κL can be understood as a function of phonon mean-free-path (J. W. Sharp, et al., “Boundary Scattering of Phonons and thermoelectric figure of merit”, Physica Status Solidi (a) 187, 507 (2001); G. S. Nolas and H. J. Goldsmid, “The Figure of Merit in Amorphous Thermoelectrics”, Physica B 194, 271 (2002); S. Bhattacharya, et al., “Grain Structure Effects on the Lattice Thermal Conductivity of Ti-Based Half-Heusler Alloys”, Appl. Phys. Lett. 81, 43 (2002)). Quantum confinement results in a strongly increased Seebeck coefficient due to the increase in the density-of-states of the carriers (L. D. Hicks and M. S. Dresselhaus, “Effect of quantum-well structures on the thermoelectric figure of merit”, Phys. Rev. B 47, 12727 (1993)). This observation was at the origin of the interest in nanoscale thermoelectricity and experimentally observed on bismuth nanowires (J. P. Heremans, et al., “Thermoelectric Power of Bismuth Nanocomposites”, Phys. Rev. Lett. 88, 216801 (2002)).
Nanostructured TE enhancement aims to ‘split’ the interdependence of the electrical and thermal transport, allowing for better optimization of the TE figure of merit (G. S. Nolas, et al., Thermoelectrics: Basic Principles and New Materials Developments (Springer, New York, N.Y., 2001)). The reduction of κL through the interface scattering of phonons in nanostructured materials has been viewed as the primary way to increase ZT (M. S. Dresselhaus, et al., “New Directions for Low-Dimensional Thermoelectric Materials”, Adv. Mater. 19, 1043 (2007); R. Venkatasubramanian, et al., “Thin-film thermoelectric devices with high room-temperature figures of merit”, Nature 413, 597 (2001); T. C. Harman, et al., “Quantum Dot Superlattice Thermoelectric Materials and Devices”, Science 297, 2229 (2002); T. Koga, et al., “Experimental proof-of-principle investigation of enhanced Z3DT in (001) oriented Si/Ge superlattices”, Applied Physics Letters 77, 1490 (2000)). The particular mechanism is the effective scattering of phonons due to the presence of interfaces.
Superior TE performance requires enhancement of the power factor (S2/ρ). Carrier filtering, where the presence of interfacial energy barriers filters low energy charge carriers traversing the interface, has been theoretically predicted (Y. I. Ravich, In CRC Handbook of Thermoelectrics, edited by D. M. Rowe, pages 67-73, CRC Press, New York, (1995); B. Moyzhes and V. Nemchinsky, Appl. Phy. Lett. 73, 1895 (1998)). This increases ISI, as its value depends on the mean carrier energy relative to the Fermi level (G. S. Nolas, J. Sharp, and H. J. Goldsmid, Thermoelectrics: Basic Principles and New Materials Developments (Springer, New York, N.Y., 2001)).
Carrier scattering at the boundaries that inhibits the conduction of low-energy charge carriers thus causing an overall increase in S was first proposed theoretically, (L. W. Whitlow and T. Hirano, “Superlattice applications to thermoelectricity”, J. Appl. Phys. 78, 5460 (1995); Moyzhes and V. Nemchinsky, “Thermoelectric figure of merit of metal-semiconductor barrier structure based on energy relaxation length”, Appl. Phys. Lett. 73, 1895 (1998)) and has been experimentally investigated in III-V heterostructures (D. Vashaee and A. Shakouri, “Improved thermoelectric power factor in metal-based superlattices”, Phys. Rev. Lett. 92, 106103 (2004)). Very little experimental research effort has been undertaken into the investigation of S2σ enhancement of nano-scale inclusions in bulk materials. Research on PbTe, (K. Kishimoto and T. Koyanagi, “Preparation of sintered degenerate n-type PbTe with a small grain size and its thermoelectric properties”, J. Appl. Phys. 92, 2544 (2002); J. P. Heremans, et al., “Thermopower enhancement in lead telluride nanostructures”, Phys. Rev. B 70, 115334 (2004)) Si—Ge alloys (M. S. Dresselhaus, et al., “New directions for nanoscale thermoelectric materials research”, Proc. Mater. Res. Soc. 886, 3 (2006)) Bi2Te3 (B. Poudel et al, “High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys”, Science 320, 634 (2008)) and Ce-filled skutterudites with “unfilled” skutterudite nano-grains (P. C. Znai, et al., “Nanostructures and enhanced thermoelectric properties in Ce-filled skutterudite bulk materials”, Appl. Phys. Lett. 89, 052111 (2006)) have shown an improvement in S as compared to their bulk counterparts. These studies, however, have not fully developed the physics behind the underlying phenomena affecting the transport properties as a function of grain size. In all five of these studies the ball-milling technique that was used to synthesize nanoscale powders of the respective materials causes strain in these materials that surely affected their transport properties as compared to hot pressed (HP) sintered bulk polycrystalline specimens. This was specifically noted in the work of Heremans et al. (“Thermopower enhancement in lead telluride nanostructures”, Phys. Rev. B 70, 115334 (2004)). The affect on nano-feature size is therefore not well established from these studies. More recently a study by Heremans et al. (“Thermoelectric enhancement in PbTe with Pb precipitates”, J. Appl. Phys. 98, 063703 (2005)) showed an increase in S due to 3 and 6% nano-grain Pb precipitates. Most recently, we have synthesized and measured the transport properties of polycrystalline PbTe with 100 nm PbTe inclusions and showed enhanced S with minimal degradation in a as compared to bulk polycrystalline PbTe (A. J. Crocker and L. M. Rogers, Brit. “Interpretation of the Hall coefficient, electrical resistivity and Seebeck coefficient of p-type lead telluride”, J. Appl. Phys. 18, 563 (1967)).