Showing papers on "Thermoelectric effect published in 2011"

Convergence of electronic bands for high performance bulk thermoelectrics

Abstract: Thermoelectric generators, which directly convert heat into electricity, have long been relegated to use in space-based or other niche applications, but are now being actively considered for a variety of practical waste heat recovery systems—such as the conversion of car exhaust heat into electricity. Although these devices can be very reliable and compact, the thermoelectric materials themselves are relatively inefficient: to facilitate widespread application, it will be desirable to identify or develop materials that have an intensive thermoelectric materials figure of merit, zT, above 1.5 (ref. 1). Many different concepts have been used in the search for new materials with high thermoelectric efficiency, such as the use of nanostructuring to reduce phonon thermal conductivity, which has led to the investigation of a variety of complex material systems. In this vein, it is well known, that a high valley degeneracy (typically ≤6 for known thermoelectrics) in the electronic bands is conducive to high zT, and this in turn has stimulated attempts to engineer such degeneracy by adopting low-dimensional nanostructures. Here we demonstrate that it is possible to direct the convergence of many valleys in a bulk material by tuning the doping and composition. By this route, we achieve a convergence of at least 12 valleys in doped PbTe_(1) − _(x)Se_(x) alloys, leading to an extraordinary zT value of 1.8 at about 850 kelvin. Band engineering to converge the valence (or conduction) bands to achieve high valley degeneracy should be a general strategy in the search for and improvement of bulk thermoelectric materials, because it simultaneously leads to a high Seebeck coefficient and high electrical conductivity.

Recent Developments in Semiconductor Thermoelectric Physics and Materials

Abstract: Recent advances in semiconductor thermoelectric physics and materials are reviewed. A key requirement to improve the energy conversion efficiency is to increase the Seebeck coefficient (S) and the electrical conductivity (σ) while reducing the electronic and lattice contributions to thermal conductivity (κe + κL). Some new physical concepts and nanostructures make it possible to modify the trade-offs between the bulk material properties through changes in the density of states, scattering rates, and interface effects on electron and phonon transport. We review recent experimental and theoretical results on nanostructured materials of various dimensions: superlattices, nanowires, nanodots, and solid-state thermionic power generation devices. Most of the recent success has been in the reduction of lattice thermal conductivity with the concurrent maintenance of good electrical conductivity. Several theoretical and experimental results to improve the thermoelectric power factor (S2σ) and to reduce the Lorenz ...

Giant anharmonic phonon scattering in PbTe

Abstract: Understanding the microscopic processes affecting the bulk thermal conductivity is crucial to develop more efficient thermoelectric materials. PbTe is currently one of the leading thermoelectric materials, largely thanks to its low thermal conductivity. However, the origin of this low thermal conductivity in a simple rocksalt structure has so far been elusive. Using a combination of inelastic neutron scattering measurements and first-principles computations of the phonons, we identify a strong anharmonic coupling between the ferroelectric transverse optic mode and the longitudinal acoustic modes in PbTe. This interaction extends over a large portion of reciprocal space, and directly affects the heat-carrying longitudinal acoustic phonons. The longitudinal acoustic-transverse optic anharmonic coupling is likely to play a central role in explaining the low thermal conductivity of PbTe. The present results provide a microscopic picture of why many good thermoelectric materials are found near a lattice instability of the ferroelectric type.

Thermoelectric Property Studies on Cu-Doped n-type CuxBi2Te2.7Se0.3 Nanocomposites

Abstract: Combining high energy ball-milling and hot-pressing, significant enhancements of the thermoelectric figure-of-merit (ZT) have been reported for p-type Bi0.4Sb1.6Te3 nanocomposites. However, applying the same technique to n-type Bi2Te2.7Se0.3 showed no improvement on ZT values, due to the anisotropic nature of the thermoelectric properties of n-type Bi2Te2.7Se0.3. Even though texturing was effective in improving peak ZT of Bi2Te2.7Se0.3 from 0.85 to 1.04, reproducibility from batch to batch remains unsatisfactory. Here, we show that good reproducibility can be achieved by introducing an optimal concentration of 0.01 copper (Cu) per Bi2Te2.7Se0.3 to make Cu0.01Bi2Te2.7Se0.3 samples. A peak ZT value of 0.99 was achieved in Cu0.01Bi2Te2.7Se0.3 samples without texturing. With texturing by re-pressing, the peak ZT was increased to 1.06. Aging in air for over 5 months did not deteriorate but further improved the peak ZT to 1.10. The mechanism by which copper improves the reproducibility, enhances the carrier mobility, and reduces the lattice thermal conductivity is also discussed.

High Thermoelectric Performance in PbTe Due to Large Nanoscale Ag2Te Precipitates and La Doping

Abstract: Thermoelectrics are being rapidly developed for waste heat recovery applications, particularly in automobiles, to reduce carbon emissions. PbTe-based materials with small ( 1.5 at 775 K.

Power factor enhancement by modulation doping in bulk nanocomposites.

Abstract: We introduce the concept of modulation doping in three-dimensional nanostructured bulk materials to increase the thermoelectric figure of merit. Modulation-doped samples are made of two types of nanograins (a two-phase composite), where dopants are incorporated only into one type. By band engineering, charge carriers could be separated from their parent grains and moved into undoped grains, which would result in enhanced mobility of the carriers in comparison to uniform doping due to a reduction of ionized impurity scattering. The electrical conductivity of the two-phase composite can exceed that of the individual components, leading to a higher power factor. We here demonstrate the concept via experiment using composites made of doped silicon nanograins and intrinsic silicon germanium grains.

Heavily Doped p-Type PbSe with High Thermoelectric Performance: An Alternative for PbTe

Abstract: PbSe was expected to have a smaller bandgap and higher thermalconductivity than PbTe. Instead, these values are about the same at high temperature leading to comparable thermoelectric figure of merit, with zT> 1 achieved in heavily doped p-type PbSe.

Al-doped zinc oxide nanocomposites with enhanced thermoelectric properties.

Abstract: ZnO is a promising high figure-of-merit (ZT) thermoelectric material for power harvesting from heat due to its high melting point, high electrical conductivity σ, and Seebeck coefficient α, but its practical use is limited by a high lattice thermal conductivity κL. Here, we report Al-containing ZnO nanocomposites with up to a factor of 20 lower κL than non-nanostructured ZnO, while retaining bulklike α and σ. We show that enhanced phonon scattering promoted by Al-induced grain refinement and ZnAl2O4 nanoprecipitates presages ultralow κ ∼ 2 Wm −1 K–1 at 1000 K. The high α∼ −300 μV K–1 and high σ ∼ 1–104 Ω–1 m–1 result from an offsetting of the nanostructuring-induced mobility decrease by high, and nondegenerate, carrier concentrations obtained via excitation from shallow Al donor states. The resultant ZT ∼ 0.44 at 1000 K is 50% higher than that for the best non-nanostructured counterpart material at the same temperature and holds promise for engineering advanced oxide-based high-ZT thermoelectrics for appl...

Light-Weight Flexible Carbon Nanotube Based Organic Composites with Large Thermoelectric Power Factors

Abstract: Typical organic materials have low thermal conductivities that are best suited to thermoelectrics, but their poor electrical properties with strong adverse correlations have prevented them from being feasible candidates. Our composites, containing single-wall carbon nanotubes, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and/or polyvinyl acetate, show thermopowers weakly correlated with electrical conductivities, resulting in large thermoelectric power factors in the in-plane direction of the composites, ∼160 μW/m·K2 at room temperature, which are orders of magnitude larger than those of typical polymer composites. Furthermore, their high electrical conductivities, ∼105 S/m at room temperature, make our composites very promising for various electronic applications. The optimum nanotube concentrations for better power factors were identified to be 60 wt % with 40 wt % polymers. It was noticed that high nanotube concentrations above 60 wt % decreased the electrical conductivity of the composites ...

Enhanced Thermoelectric Figure of Merit of p-Type Half-Heuslers

Abstract: Half-Heuslers would be important thermoelectric materials due to their high temperature stability and abundance if their dimensionless thermoelectric figure of merit (ZT) could be made high enough. The highest peak ZT of a p-type half-Heusler has been so far reported about 0.5 due to the high thermal conductivity. Through a nanocomposite approach using ball milling and hot pressing, we have achieved a peak ZT of 0.8 at 700 °C, which is about 60% higher than the best reported 0.5 and might be good enough for consideration for waste heat recovery in car exhaust systems. The improvement comes from a simultaneous increase in Seebeck coefficient and a significant decrease in thermal conductivity due to nanostructures. The samples were made by first forming alloyed ingots using arc melting and then creating nanopowders by ball milling the ingots and finally obtaining dense bulk by hot pressing. Further improvement in ZT is expected when average grain sizes are made smaller than 100 nm.

Low Effective Mass Leading to High Thermoelectric Performance

Abstract: High Seebeck coefficient by creating large density of state (DOS) around the Fermi level through either electronic structure modification or manipulating nanostructures, is commonly considered as a route to advanced thermoelectrics. However, large density of state due to flat bands leads to large effective mass, which results in a simultaneous decrease of mobility. In fact, the net effect of high effective mass is a lower thermoelectric figure of merit when the carriers are predominantly scattered by acoustic phonons according to the deformation potential theory of Bardeen-Shockley. We demonstrate the beneficial effect of light effective mass leading to high power factor in n-type thermoelectric PbTe, where doping and temperature can be used to tune the effective mass. This clear demonstration of the deformation potential theory to thermoelectrics shows that the guiding principle for band structure engineering should be low effective mass along the transport direction.

Enhancement of the thermoelectric properties in nanoscale and nanostructured materials

Abstract: Thermoelectric materials can be used for solid state power generation and heating/cooling applications. The figure of merit of thermoelectric materials, ZT, which determines their efficiency in a thermoelectric device, remains low for most conventional bulk materials. Nanoscale and nanostructured thermoelectric materials are promising for increasing ZT relative to the bulk. This review introduces the theory behind thermoelectric materials and details the predicted and demonstrated enhancements of ZT in nanoscale and nanostructured thermoelectric materials. We discuss thin films and superlattices, nanowires and nanotubes, and nanocomposites, providing a ZT comparison among various families of nanocomposite materials. We provide some perspectives regarding the origin of enhanced ZT in nanoscale and nanostructured materials and suggest some promising and fruitful research directions for achieving high ZT materials for practical applications.

High performance Na-doped PbTe-PbS thermoelectric materials: electronic density of states modification and shape-controlled nanostructures.

Abstract: Thermoelectric heat-to-power generation is an attractive option for robust and environmentally friendly renewable energy production. Historically, the performance of thermoelectric materials has been limited by low efficiencies, related to the thermoelectric figure-of-merit ZT. Nanostructuring thermoelectric materials have shown to enhance ZT primarily via increasing phonon scattering, beneficially reducing lattice thermal conductivity. Conversely, density-of-states (DOS) engineering has also enhanced electronic transport properties. However, successfully joining the two approaches has proved elusive. Herein, we report a thermoelectric materials system whereby we can control both nanostructure formations to effectively reduce thermal conductivity, while concurrently modifying the electronic structure to significantly enhance thermoelectric power factor. We report that the thermoelectric system PbTe-PbS 12% doped with 2% Na produces shape-controlled cubic PbS nanostructures, which help reduce lattice thermal conductivity, while altering the solubility of PbS within the PbTe matrix beneficially modifies the DOS that allow for enhancements in thermoelectric power factor. These concomitant and synergistic effects result in a maximum ZT for 2% Na-doped PbTe-PbS 12% of 1.8 at 800 K.

Nanoscale Joule heating, Peltier cooling and current crowding at graphene–metal contacts

Abstract: The temperatures of the graphene–metal contacts in working transistors have been measured with a resolution of ∼10 nm, revealing the presence of both heating and cooling effects.

Conducting polymers: Efficient thermoelectric materials

Abstract: This review highlights the recent progress made in the area of thermoelectric (TE) applications of conducting polymers and related composites. Several examples of such materials and their TE properties are discussed. TE properties of new poly(2,7-carbazole) derivatives are highlighted. References are also made to carbon nanotube/polymer composites and their improved electrical and TE performance. Studies on polymer/inorganic materials composites have also taken a step forward and have shown very promising TE properties. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Enhancement in Thermoelectric Figure‐Of‐Merit of an N‐Type Half‐Heusler Compound by the Nanocomposite Approach

Abstract: An enhancement in the dimensionless thermoelectric fi gure-of-merit ( ZT ) of an n-type half-Heusler material is reported using a nanocomposite approach. A peak ZT value of 1.0 was achieved at 600 ° C‐700 ° C, which is about 25% higher than the previously reported highest value. The samples were made by ball-milling ingots of composition Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 into nanopowders and hot-pressing the powders into dense bulk samples. The ingots were formed by arc-melting the elements. The ZT enhancement mainly comes from reduction of thermal conductivity due to increased phonon scattering at grain boundaries and crystal defects, and optimization of antimony doping.

Nanostructures Boost the Thermoelectric Performance of PbS

Abstract: In situ nanostructuring in bulk thermoelectric materials through thermo-dynamic phase segregation has established itself as an effective paradigm for optimizing the performance of thermoelectric materials. In bulk PbTe small compositional variations create coherent and semicoherent nanometer sized precipitates embedded in a PbTe matrix, where they can impede phonon propagation at little or no expense to the electronic properties. In this paper the nanostructuring paradigm is for the first time extended to a bulk PbS based system, which despite obvious advantages of price and abundancy, so far has been largely disregarded in thermoelectric research due to inferior room temperature thermoelectric properties relative to the pristine fellow chalcogenides, PbSe and PbTe. Herein we report on the synthesis, microstructural morphology and thermoelectric properties of two phase (PbS)(1-x)(PbTe)(x)x = 0-0.16 samples. We have found that the addition of only a few percent PbTe to PbS results in a highly nanostructured material, where PbTe precipitates are coherently and semicoherently embedded in a PbS matrix. The present (PbS)(1-x)(PbTe)(x) nanostructured samples show substantial decreases in lattice thermal conductivity relative to pristine PbS, while the electronic properties are left largely unaltered. This in turn leads to a marked increase in the thermoelectric figure of merit. This study underlines the efficiency of the nanostructuring approach and strongly supports its generality and applicability to other material systems. We demonstrate that these PbS-based materials, which are made primarily from abundant Pb and S, outperform optimally n-type doped pristine PbTe above 770 K.

A high temperature apparatus for measurement of the Seebeck coefficient

Abstract: A high temperature Seebeck coefficient measurement apparatus with various features to minimize typical sources of error is designed and built. Common sources of temperature and voltage measurement error are described and principles to overcome these are proposed. With these guiding principles, a high temperature Seebeck measurement apparatus with a uniaxial 4-point contact geometry is designed to operate from room temperature to over 1200 K. This instrument design is simple to operate, and suitable for bulk samples with a broad range of physical types and shapes.

Analysis of the thermoelectric properties of n-type ZnO

Abstract: We report an investigation of the temperature- and doping-dependent thermoelectric behavior of n-type ZnO. The results are based on a combination of experimental data from the literature and calculated transport functions obtained from Boltzmann transport theory applied to the first-principles electronic structure. From this we obtain the dependence of the figure of merit ZT on doping and temperature. We find that improvement of the lattice thermal conductivity is essential for obtaining high ZT in n-type ZnO.

Thermodynamics of Thermoelectric Phenomena and Applications

Abstract: Fifty years ago, the optimization of thermoelectric devices was analyzed by considering the relation between optimal performances and local entropy production. Entropy is produced by the irreversible processes in thermoelectric devices. If these processes could be eliminated, entropy production would be reduced to zero, and the limiting Carnot efficiency or coefficient of performance would be obtained. In the present review, we start with some fundamental thermodynamic considerations relevant for thermoelectrics. Based on a historical overview, we reconsider the interrelation between optimal performances and local entropy production by using the compatibility approach together with the thermodynamic arguments. Using the relative current density and the thermoelectric potential, we show that minimum entropy production can be obtained when the thermoelectric potential is a specific, optimal value.

Role of Lone-Pair Electrons in Producing Minimum Thermal Conductivity in Nitrogen-Group Chalcogenide Compounds

Abstract: Fully dense crystalline solids with extremely low lattice thermal conductivity (κ(L)) are of practical importance for applications including thermoelectric energy conversion and thermal barrier coatings. Here we show that lone-pair electrons can give rise to minimum κ(L) in chalcogenide compounds that contain a nominally trivalent group VA element. Electrostatic repulsion between the lone-pair electrons and neighboring chalcogen ions creates anharmonicity in the lattice, the strength of which is determined by the morphology of the lone-pair orbital and the coordination number of the group VA atom.

Remarkable Enhancement in Thermoelectric Performance of BiCuSeO by Cu Deficiencies

Abstract: A significant enhancement of thermoelectric performance in layered oxyselenides BiCuSeO was achieved. The electrical conductivity and Seebeck coefficient of BiCu(1-x)SeO (x = 0-0.1) indicate that the carriers were introduced in the (Cu(2)Se(2))(2-) layer by Cu deficiencies. The maximum of electrical conductivity is 3 × 10(3) S m(-1) for Bicu(0.975)Seo at 650 °C, much larger than 470 S m(-1) for pristine BiCuSeO. Featured with very low thermal conductivity (∼0.5 W m(-1) K(-1)) and a large Seebeck coefficient (+273 μV K(-1)), ZT at 650 °C is significantly increased from 0.50 for pristine BiCuSeO to 0.81 for BiCu(0.975)SeO by introducing Cu deficiencies, which makes it a promising candidate for medium temperature thermoelectric applications.

Seebeck effect in magnetic tunnel junctions

Abstract: Creating temperature gradients in magnetic nanostructures has resulted in a new research direction, that is, the combination of magneto- and thermoelectric effects. Here, we demonstrate the observation of one important effect of this class: the magneto-Seebeck effect. It is observed when a magnetic configuration changes the charge-based Seebeck coefficient. In particular, the Seebeck coefficient changes during the transition from a parallel to an antiparallel magnetic configuration in a tunnel junction. In this respect, it is the analogue to the tunnelling magnetoresistance. The Seebeck coefficients in parallel and antiparallel configurations are of the order of the voltages known from the charge-Seebeck effect. The size and sign of the effect can be controlled by the composition of the electrodes' atomic layers adjacent to the barrier and the temperature. The geometric centre of the electronic density of states relative to the Fermi level determines the size of the Seebeck effect. Experimentally, we realized 8.8% magneto-Seebeck effect, which results from a voltage change of about -8.7 μV K⁻¹ from the antiparallel to the parallel direction close to the predicted value of -12.1 μV K⁻¹. In contrast to the spin-Seebeck effect, it can be measured as a voltage change directly without conversion of a spin current.

Enhancement of the Thermoelectric Performance of p‐Type Layered Oxide Ca3Co4O9+δ Through Heavy Doping and Metallic Nanoinclusions

Abstract: By converting heat directly into electricity, thermoelectric (TE) generation offers a promising technology to recover waste heat emitted from industrial sectors and energy consumption processes. [ 1 ] The key to realize an effi cient TE generator lies, however, in fi nding good materials with high TE performance, a good durability at high temperature, and preferably robustness to operating in air. The performance of a TE material is evaluated by the dimensionless fi gure-of-merit ZT ( = S 2 T / ρ κ , where S , T , ρ , and κ are the Seebeck coeffi cient, absolute temperature, electrical resistivity, and thermal conductivity, respectively). By far the most widely used TE materials are alloys of Bi 2 Te 3 , PbTe, and SiGe, which often suffer from poor durability at high temperature, are harmful or scarce, and have costly constituting elements. Metal oxides have been considered as an alternative to overcome these problems. Metal oxide-based materials have been attracting continuous interest as TE materials over the years since the discovery of large TE power in p-type NaCo 2 O 4 single crystals by Terasaki et al. in 1997. [ 2 ] However, practical application of this oxide for power generation from waste heat has never been realized because of the volatility of Na and the instability of the compound against humidity. Another Co-based oxide p type material Ca 3 Co 4 O 9 + δ has also been intensively investigated because of its good TE performance ( ZT = 0.83 at 973 K for the single crystal) [ 3 ] and its high thermal and chemical stabilities even up to 1200 K in air. [ 4–7 ] An incommensurate character in the crystal structure of this compound is explicitly described as [Ca 2 CoO 3 ] b 1/ b 2 [CoO 2 ], where b 1 and b 2 are two different periodicities along the b axis for the rock salt-type Ca 2 CoO 3 subsystem and the CdI 2 -type CoO 2 subsystem, respectively. [ 4 ] Single crystals are less likely to be applied for fabricating practical TE devices, because they will be too expensive. It is hence highly desirable to achieve suffi cient TE properties in a polycrystalline form of these oxides. Although the diffi culty of discovering novel high performance

Linear-response theory of spin Seebeck effect in ferromagnetic insulators

Abstract: We formulate a linear response theory of the spin Seebeck effect, i.e., a spin voltage generation from heat current flowing in a ferromagnet. Our approach focuses on the collective magnetic excitation of spins, i.e., magnons. We show that the linear-response formulation provides us with a qualitative as well as quantitative understanding of the spin Seebeck effect observed in a prototypical magnet, yttrium iron garnet.

Self-Tuning the Carrier Concentration of PbTe/Ag2Te Composites with Excess Ag for High Thermoelectric Performance

Abstract: Thermoelectric materials can be optimized by tuning the carrier concentration with chemical doping. However, because the optimum dopant concentration typically increases with temperature, the optimum efficiency can not normally be achieved for a uniform material in a temperature gradient. Here, we show Ag-doped PbTe/Ag_2Te composites exhibit high thermoelectric performance (∼50% greater than La-doped composites) because of a temperature induced gradient in the doping concentration caused by the temperature-dependent solubility of Ag in the PbTe matrix. This demonstrates a new mechanism to achieve a higher thermoelectric efficiency afforded by a given material system, and should be applicable to other thermoelectric materials.

Thermoelectric Nanostructures: From Physical Model Systems towards Nanograined Composites

Abstract: Thermoelectric materials could play an increasing role for the efficient use of energy resources and waste heat recovery in the future. The thermoelectric efficiency of materials is described by the figure of merit ZT = (S2σT)/κ (S Seebeck coefficient, σ electrical conductivity, κ thermal conductivity, and T absolute temperature). In recent years, several groups worldwide have been able to experimentally prove the enhancement of the thermoelectric efficiency by reduction of the thermal conductivity due to phonon blocking at nanostructured interfaces. This review addresses recent developments from thermoelectric model systems, e.g. nanowires, nanoscale meshes, and thermionic superlattices, up to nanograined bulk-materials. In particular, the progress of nanostructured silicon and related alloys as an emerging material in thermoelectrics is emphasized. Scalable synthesis approaches of high-performance thermoelectrics for high-temperature applications is discussed at the end.