Showing papers on "Thermoelectric effect published in 2002"

Quantum dot superlattice thermoelectric materials and devices.

Abstract: PbSeTe-based quantum dot superlattice structures grown by molecular beam epitaxy have been investigated for applications in thermoelectrics. We demonstrate improved cooling values relative to the conventional bulk (Bi,Sb) 2 (Se,Te) 3 thermoelectric materials using a n-type film in a one-leg thermoelectric device test setup, which cooled the cold junction 43.7 K below the room temperature hot junction temperature of 299.7 K. The typical device consists of a substrate-free, bulk-like (typically 0.1 millimeter in thickness, 10 millimeters in width, and 5 millimeters in length) slab of nanostructured PbSeTe/PbTe as the n-type leg and a metal wire as the p-type leg.

Lead telluride as a thermoelectric material for thermoelectric power generation

Abstract: The specialized applications of thermoelectric generators are very successful and have motivated a search for materials with an improved figure of merit Z, and also for materials which operate at elevated temperatures. Lead telluride, PbTe, is an intermediate thermoelectric power generator. Its maximum operating temperature is 900 K. PbTe has a high melting point, good chemical stability, low vapor pressure and good chemical strength in addition to high figure of merit Z. Recently, research in thermoelectricity aims to obtain new improved materials for autonomous sources of electrical power in specialized medical, terrestial and space applications and to obtain an unconventional energy source after the oil crises of 1974. Although the efficiency of thermoelectric generators is rather low, typically ∼5%, the other advantages, such as compactness, silent, reliability, long life, and long period of operation without attention, led to a wide range of applications. PbTe thermoelectric generators have been widely used by the US army, in space crafts to provide onboard power, and in pacemakers batteries. The general physical properties of lead telluride and factors affecting the figure of merit have been reviewed. Various possibilities of improving the figure of merit of the material have been given, including effect of grain size on reducing the lattice thermal conductivity λ L . Comparison of some transport properties of lead telluride with other thermoelectric materials and procedures of preparing compacts with transport properties very close to the single crystal values from PbTe powder by cold and hot-pressing techniques are discussed.

Heat Transfer in Nanostructures for Solid-State Energy Conversion

Abstract: Solid-state energy conversion technologies such as thermoelectric and thermionic refrigeration and power generation require materials with low thermal conductivity but good electrical conductivity and Seebeck coefficient, which are difficult to realize in bulk semi-conductors. Nanostructures such as superlattices, quantum wires, and quantum dots provide alternative approaches to improve the solid-state energy conversion efficiency through size and interface effects on the electron and phonon transport. In this review, we discuss recent research and progress using nanostructures for solid-state energy converxion. The emphasis is placed on fundamental issues that distinguish energy transport and conversion between nanoscale and macroscale, as well as heat transfer issues related to device development and property characterization.

Thermoelectric properties of the n-type filled skutterudite Ba0.3Co4Sb12 doped with Ni

Abstract: Synthesis and electrical and thermal transport properties are reported for several filled skutterudite compounds doped with Ni: Ba0.3NixCo4−xSb12 with 0

High figure of merit in Eu-filled CoSb3-based skutterudites

Abstract: We report measurements of electrical resistivity, thermopower, thermal conductivity, and Hall coefficient of polycrystalline Eu-doped CoSb3-based skutterudites with compositions Eu0.20Co4Sb12, Eu0.43Co4Sb11.59Ge0.31, and Eu0.42Co4Sb11.37Ge0.50. The relatively high mobility of these compounds, as compared to that of La- and Ce-filled skutterudites, may play a role in the large thermoelectric figure of merit (ZT>1 at 700 K) of Eu0.42Co4Sb11.37Ge0.50. We discuss the significant potential of these compounds for thermoelectric applications.

Bi2Sr2Co2Oy whiskers with high thermoelectric figure of merit

Abstract: Bi2Sr2Co2Oy (BC-222) whiskers were grown with an excellent Seebeck coefficient (S) and electrical resistivity (ρ) for power generation applications at high temperatures in air. The S value of these long BC-222 whiskers reached almost 300 μV K−1 at 973 K. Thermal conductivity (κ) was measured and found to be suppressed to a low value of about 2.0 W m−1 K−1. This is considered due not only to a phonon–phonon interaction but also other scattering processes. The resulting dimensionless thermoelectric figure of merit ZT(=S2T/ρκ) was more than 1.1, which corresponds to a conversion efficiency of almost 10% at 973 K in air.

Thermoelectric properties of the Bi- and Na-substituted Ca3Co4O9 system

Abstract: Bi- and Na-substituted Ca3Co4O9 polycrystalline samples have been prepared using a hot-pressing technique and their thermoelectric properties were carefully studied in air from room temperature to 1000 K. The substitutions of Bi3+ and Na+ for Ca2+, as well as Bi3+ and Na+ double substitution, cause both the electrical conductivity (σ) and thermoelectric power (S) to increase simultaneously. The double substitution has also been demonstrated to be effective to decrease the thermal conductivity (κ). The dimensionless figure of merit ZT (=S2σT/κ) reaches 0.32 at 1000 K in the double-substituted sample.

Semimetal–semiconductor transition in Bi1−xSbx alloy nanowires and their thermoelectric properties

Abstract: The resistivity of Bi1−xSbx nanowire arrays exhibits complex variations as a function of Sb content x and temperature T due to the unique semimetal-to-semiconductor (SM–SC) transition experienced by the nanowires. Seebeck coefficient measurements show enhanced thermopower due to Sb alloying and the reduction in wire diameter. The theoretical model not only explains these transport measurements, but also suggests a useful technique to experimentally determine (i) whether the wire is semimetallic or semiconducting, (ii) the carrier concentration, and (iii) the conditions for the SM–SC transition.

Preparation of sintered degenerate n-type PbTe with a small grain size and its thermoelectric properties

Abstract: Sintered degenerate n-type PbTe samples with small grain sizes ranging from 07 to 4 μm were prepared and the effects of grain size on their thermoelectric properties were then investigated The Seebeck coefficient of the sintered samples increased almost double when the grain size decreased from 4 to 07 μm On the other hand, their electrical and thermal conductivity decreased with decreasing grain size Accordingly, decreasing their grain size increased their thermoelectric figure-of-merit A maximum value of the figure-of-merit of the obtained small grain-size samples was significantly higher than that of large grain-size samples with the same carrier concentration reported This favorable result was caused mainly by the increase in the Seebeck coefficient The influences of grain boundaries on the increase in the Seebeck coefficient were discussed It is concluded that the Seebeck coefficient was increased by potential barrier scattering, which occurred at the grain boundaries in the sintered samples

Thermal transporting properties of electrically conductive polyaniline films as organic thermoelectric materials

Abstract: Thermal transporting properties of electrically conductive polyaniline films were first investigated in wide range of temperatures above room temperature as organic thermoelectric materials. Thermal conductivities of various protonic acid-doped polyaniline films were measured by combination of a laser flash method and a differential scanning calorimeter in relation with electrical conductivity and a kind of dopant. The thermal conductivities thus measured are in the range of conventional organic polymers, indicating that the doped polyaniline films have extremely low thermal conductivities among electrically conductive materials, and have correlation with neither electrical conductivity, nor a kind of dopant. Consequently the polyaniline film, which shows very high electrical conductivity, has comparable thermoelectric figure-of-merit (ZT) with feasible inorganic thermoelectric materials such as iron silicide.

Thermoelectric properties of the semimetallic Heusler compounds Fe 2 − x V 1 + x M ( M = Al , Ga)

Abstract: We have studied the thermoelectric properties of the stoichiometric and off-stoichiometric Heusler compounds ${\mathrm{Fe}}_{2\ensuremath{-}x}{\mathrm{V}}_{1+x}M$ $(M=\mathrm{Al},$ Ga) between 10 and 300 K. Seebeck coefficient (S) measurements indicate that stoichiometric ${\mathrm{Fe}}_{2}\mathrm{VAl}$ and ${\mathrm{Fe}}_{2}\mathrm{VGa}$ are p-type materials with moderate S values of about $30--40\ensuremath{\mu}\mathrm{V}/\mathrm{K}$ at room temperature. Upon substituting the Fe site with V, a significant enhancement on Seebeck coefficients accompanied by a sign change are observed. On the other hand, the low-temperature resistivity $(\ensuremath{\rho})$ grows drastically with V substitution for Fe, while the thermal conductivity $(\ensuremath{\kappa})$ remains almost unaffected in the substitution levels we investigated. These features are consistent with other experimental results and are related to issues raised by band-structure calculations. The thermoelectric performance in these semimetallic Heusler compounds is estimated to be an order of magnitude lower than conventional thermoelectric materials.

Supercooling of Peltier cooler using a current pulse

Abstract: The operation of a Peltier cooler can be temporarily enhanced by utilizing the transient response of a current pulse. The performance of such a device, using (Bi,Sb)2Te3-based thermoelectric elements, was examined from −70 to 55 °C. We establish both theoretically and experimentally the essential parameters that describe the pulse cooling effect, such as the minimum temperature achieved, maximum temperature overshoot, time to reach minimum temperature, time while cooled, and time between pulses. Using simple theoretical and semiempirical relationships the dependence of these parameters on the current pulse amplitude, temperature, thermoelectric element length, thermoelectric figure of merit and thermal diffusivity is established. At large pulse amplitudes the amount of pulse supercooling is proportional to the maximum steady-state difference in temperature. This proportionality factor is about half that expected theoretically. This suggests that the thermoelectric figure of merit is the key materials para...

Measurements of anisotropic thermoelectric properties in superlattices

Abstract: Thermoelectric properties, i.e., thermal conductivity, electrical conductivity, and the Seebeck coefficient, have been measured in the directions parallel (in-plane) and perpendicular to the interface of an n-type Si(80 A)/Ge(20 A) superlattice. A two-wire 3ω method is employed to measure the in-plane and cross-plane thermal conductivities. The cross-plane Seebeck coefficient is deduced by using a differential measurement between the superlattice and reference samples and the cross-plane electrical conductivity is determined through a modified transmission-line method. The in-plane thermal conductivity of the Si/Ge superlattice is 5–6 times higher than the cross-plane one, and the electrical conductivity shows a similar anisotropy. The anisotropy of the Seebeck coefficients is smaller in comparison to electrical and thermal conductivities in the temperature range from 150 to 300 K. However, the cross-plane Seebeck coefficient rises faster with increasing temperature than that of the in-plane direction.

Molecular dynamics calculation of the thermal conductivity of superlattices

Abstract: We report on molecular dynamics studies of heat flow in superlattices. The computer simulations are performed using classical mechanics with periodic boundary conditions. The heat flow is in the direction normal to the layers. We have studied the variation of the conductivity with the repeat distance and the effect of interfacial roughness. We discuss the relation of these results to experimental data in the literature. Superlattices are structures composed of alternating layers of two materials that have nearly the same lattice parameter. Semiconductor superlattices have optical, electronic, and thermal properties that vary significantly from those of the bulk constituent materials. These novel properties have led to the use of superlattice structures in a number of applications, including semiconductor lasers 1 and thermoelectric devices. 2,3 The operation of these devices can be greatly af- fected by the thermal conductivity of the superlattice. For instance, the efficiency of a semiconductor laser is reduced when the active region of the device is at high temperature, and so a high thermal conductivity superlattice is preferred. On the other hand, the efficiency of a thermoelectric device is inversely proportional to the thermal conductivity, and so low thermal conductivity materials are preferable. The study of heat flow in a superlattice is also of interest from a fun- damental perspective. The periodicity of the superlattice modifies the phonon dispersion relation. The effects of this modification on the lattice thermal conductivity have been studied by several authors, 4-6 but discrepancies between the- oretical calculations and experimental values have not yet been resolved. Measurements on Si/Ge ~Ref. 7! and GaAs/AlAs ~Ref. 8! superlattices have shown that the thermal conductivity in the direction perpendicular to the layers ~growth direction! is reduced by as much as an order of magnitude compared to the conductivity of the bulk constituents. Part of the decrease in the thermal conductivity can be attributed to the reduction in the group velocity of phonons due to zone folding. 5 How- ever, quantitative calculations show that this effect should lead to a thermal conductivity that decreases as the thickness of the layers making up the superlattice is increased within the range from one to ten monolayers. Experimental results for samples with layer thickness in this range have shown the opposite effect, as can be seen from the data of Capinski et al. 8 shown in Fig. 1, which show a monotonic increase in the thermal conductivity with increasing superlattice period. The disagreement between the zone-folding theory and the data may be due to interface effects, 9 but the extent to which interfacial roughness and other superlattice defects affect the experimentally measured thermal conductivity is not yet known. In order to investigate the effects of the different superlattice parameters on the thermal conductivity in the growth direction, we have performed molecular dynamics simulations on a simple, classical model of a superlattice and present the results here.

Conductive polymers as a new type of thermoelectric material

Abstract: Thermoelectric properties were investigated for the films of electrically conductive doped polyanilines. The thermoelectric performance, evaluated by thermoelectric figure-of-merit (ZT = T (S 2 σ) / κ), of various protonic acid-doped polyaniline bulk films was found to depend on the electrical conductivity a of the film. Thus, the higher the electrical conductivity, the higher the figure-of-merit is, because the thermal conductivity κ of polyaniline films does not depend on the electrical conductivity. Among the conductive bulk films of polyaniline, the highest figure-of-merit (ZT = 1 X 10 -4 ) was observed for (±)-10-camphorsulfonic acid (CSA)-doped polyaniline in an emeraldine form ( σ = 188 S cm -1 ) at room temperature. The multilayered film, composed of electrically insulating emeraldine base layers and electrically conducting CSA-doped emeraldine salt layers, exhibited 6 times higher ZT at 300 K than that of a bulk film of CAS-doped polyaniline, showing the highest ZT value of 1.1 X 10 -2 at 423 K. Stretching of the CAS-doped polyaniline film also increased the figure-of-merit of doped polyaniline films along the direction of the stretching.

Thermoelectric module with thin film substrates

Abstract: A flexible thermoelectric module having a pair of flexible substrates, a plurality of electrically conductive contacts on one side of each of the flexible substrates, and a plurality of P-type and N-type thermoelectric elements electrically connected between opposing sides of the pair of flexible substrates having the plurality of conductive contacts where the plurality of conductive contacts connect adjacdnt P-type and N-type elements to each other in series and where each of the P-type and N-type elements has a first end connected to one of the plurality of conductive contacts of one of the substrates and a second end connected to one of the plurality of electrical contacts of the other of the substrates.

Temperature dependence of the Seebeck coefficient and the potential barrier scattering of n-type PbTe films prepared on heated glass substrates by rf sputtering

Abstract: Thermoelectric properties of polycrystalline degenerate n-type PbTe films have been investigated in order to understand potential barrier scattering. The Seebeck coefficients of the PbTe films obtained in this study were larger than those of bulk samples having the same carrier concentrations in the temperature range from 100 to 450 K. Some of their power factors were larger than those of bulk samples having the same carrier concentrations in the temperature range from 200 to 450 K, while their electrical conductivities were smaller than those of bulk samples. From a comparison of these results with those previously reported, we concluded that potential barrier scattering occurred at grain boundaries in our films, resulting in the above favorable changes in thermoelectric properties. By analyzing their properties using the energy filtering model, we estimated the height of grain-boundary potential barriers, which probably influenced the increases in the Seebeck coefficient. We also examined the origin of the potential barriers accordingly. Consequently, we consider that the origin of the potential barriers was mainly due to point defects, probably Te vacancies, and that the barrier height may be controlled, for example, by the preparation conditions.

Grain structure effects on the lattice thermal conductivity of Ti-based half-Heusler alloys

Abstract: Half-Heusler alloys with the general formula TiNiSn1−xSbx are currently being investigated for their potential as thermoelectric (TE) materials. A systematic investigation of the effect of Sb doping on the Sn site and Zr doping on the Ti site on the electrical and thermal transport of the TiNiSn system has been performed. Unexpectedly, lattice thermal conductivity κL appears to increase somewhat randomly with small amounts (x<5%) of Sb doping. Subsequently, an investigation of grain structure in these Sb-doped materials has been found to correlate with the anomalous behavior of κL. Furthermore, effects of submicron grain sizes on κL in ball milled and shock compressed samples are also presented.

Simultaneous measurements of Seebeck coefficient and thermal conductivity across superlattice

Abstract: A method is developed to simultaneously measure the Seebeck coefficient and thermal conductivity in the cross-plane direction of thin films and applied to an n-type Si/Ge quantum-dot superlattice. In this method, an Au/Cr pattern serves as both a heater and a thermometer, and a microprobe is prepared between the heater and the thin film to extract the Seebeck voltage. Using a differential measurement between the thin films with different thickness, the temperature and voltage drops across the thin film are determined to deduce its cross-plane thermal conductivity and Seebeck coefficient. At room temperature, the cross-plane Seebeck coefficient and thermal conductivity are 312 μV/K and 2.92 W/mK, respectively, for the n-type Si(75 A)/Ge(15 A) quantum-dot superlattice doped to 8.7×1019 cm−3.

Thermoelectric catalytic power generator with preheat

Abstract: An automobile catalytic converter that utilizes the energy of the exothermic reactions that take place in the catalysis substrate to produce electrical energy with a thermoelectric generator On vehicle cold start, the thermoelectric generator is used as a heat pump to heat the catalyst substrate to reduce the time to catalyst light-off In this way, the catalytic converter comes up to operating temperature more rapidly, reducing the amount of pollutant emissions at vehicle start-up

High performance functionally graded and segmented Bi2Te3-based materials for thermoelectric power generation

Abstract: Bi2Te3-based materials possess a figure of merit maximum over a narrow temperature range. When used in a generating mode over a large temperature difference the material operates at a substantially lower overall figure of merit than its maximum value. The conversion efficiency of a thermoelectric generator for low temperature waste heat recovery can be increased by employing functionally graded or segmented materials. In this work functionally graded p-type Bi2Te3-based thermoelectric materials have been prepared from melt by the Bridgman method using double doping technique. Segmented n-type thermoelement has been fabricated by joining two Bi2Te3-based materials with figure of merit maximum at 270 K and 380 K. The thermoelectric properties of the materials and a thermocouple comprised of p-type functionally graded and n-type segmented materials have been measured over a temperature range 200 K–450 K. The material efficiency of the thermocouple over the temperature gradient 223 K–423 K is estimated to be 10% compared with 8.8% for a standard Bi2Te3-based materials.

On thermoelectric power conversion from heat recirculating combustion systems

Abstract: The thermodynamic theory governing the absolute maximum efficiency of energy conversion by thermoelectric devices that operate as part of the heat recycle in regenerative burners is examined. Comparison with a series of elementary Carnot cycles helps to address the question of whether higher system efficiencies are realizable by rejecting the unconverted heat to the cold surroundings or to the incoming reactants as part of the recycle. While for the second law (Carnot) heat engine cycles the maximum power that can be extracted is independent of the layout, in the case of irreversible thermoelectric assemblies a particular combination of both in a novel configuration is shown to be most advantageous. This heat exchanger/thermoelectric converter configuration consists of a coaxial assembly of many annular thermoelectric elements in series and a section in which heat is rejected to the incoming reactants that is followed by a second section, which discards unconverted heat to the cold surroundings. It is shown that the efficiencies of such devices could substantially exceed the maximum efficiencies of the best present-day thermolectric of such devices could substantially exceed the maximum efficiencies of the best present-day thermoelectric conversion systems, and the theory suggests practical designs for small, combustion-driven, power supplies.

High thermoelectric figure of merit ZT in PbTe and Bi2Te3-based superlattices by a reduction of the thermal conductivity

Abstract: Experimental evidence is presented for an enhanced thermoelectric figure of merit ZT=σS2T/λ (where σ is the electrical conductivity, S the thermopower, T the temperature, and λ is the thermal conductivity) by a reduction of the thermal conductivity λ in PbTe- and Bi2Te3-superlattices (SLs) parallel to the layer planes. Data on thermoelectric properties of Bi2Te3/Bi2(SexTe1−x)3-SLs, of PbTe-based doping SLs, and PbTe/PbSe0.2Te0.8-SLs are presented. In these structures, a decrease of λ was measured compared to the corresponding bulk compounds or homogeneous alloys. Despite a drop of the total power factor σS2 in the SLs in total an enhancement of the figure of merit ZT is found in these highly-doped PbTe and Bi2Te3-based SLs.

Thermoelectric transient cooling and heating systems

Abstract: An improved efficiency thermoelectric system operates the thermoelectric elements (208) in the system in a non-steady state manner. The thermoelectric elements (208) are powered for predefined periods of time to obtain increased efficiency. This benefit can be improved by also altering the resistance of the thermoelectric elements (208) during the power-on period such that resistive heating is minimized.

Thermoelectric efficiency in graded indium-doped PbTe crystals

Abstract: High efficiency thermoelectric conversion is achieved by using materials with a maximum figure of merit Z=S2σ/k, where S is the Seebeck coefficient, σ and k, the electrical and thermal conductivities, respectively. High quality homogeneous thermoelectric materials, based on PbTe crystals, usually display an elevated value of Z over a narrow temperature range. A maximal value of figure of merit Z, as a function of electron density, is attained only for one specific location of the Fermi level, EF, with respect to the conduction band edge, EC. In order to maintain this optimal Z value, namely, maintain a constant location of the Fermi level, the electron density, which is determined by the dopant concentration, must increase with increasing temperature. We present a method for the generation of a dopant (indium) concentration profile in n-type PbTe crystals that gives rise to a constant location of the Fermi level, and hence, to an optimal value of Z over a wide temperature range. The resulting functionally...