Showing papers on "Seebeck coefficient 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.

Thermoelectric power of bismuth nanocomposites.

Abstract: Because of the increase in the electronic density of states in low-dimensional systems, semiconductor quantum wires constitute a most promising thermoelectric material. We report here the first experimental observation of a very large enhancement of the thermoelectric power of composites containing bismuth nanowires with diameters of 9 and 15 nm, embedded in porous alumina and porous silica. The temperature dependence of the electrical resistance shows that the samples are semiconductors with energy gaps between 0.17 and 0.4 eV, consistent with the theoretical predictions.

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

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.

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.

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.

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.

PbTe based superlattice structures with high thermoelectric efficiency

Abstract: We report on an enhanced thermoelectric figure of merit ZT=σS2T/λ (where σ is electrical conductivity, S is thermopower, T is absolute temperature, and λ is thermal conductivity) for PbTe/PbSe0.20Te0.80 superlattices (SLs) and PbTe doping SLs due to a reduction of the thermal conductivity λ parallel to the layer planes. Despite a small decrease of the power factors σS2 due to a reduction of σ in these superlattices, the figure of merit is higher as compared to the corresponding bulk materials and reaches maximum values in the temperature range between 400 and 570 K.

BaTiO3−δ: Defect Structure, Electrical Conductivity, Chemical Diffusivity, Thermoelectric Power, and Oxygen Nonstoichiometry

Abstract: Electrical conductivity, thermoelectric power, and chemical diffusivity are the most typical, charge-and-mass transport properties of a mixed ionic electronic conductor oxide which are essentially governed by its defect structure, and the oxygen nonstoichiometry is a direct measure of its overall defect concentration. For the system of BaTiO3−δ, the total electrical conductivity has been the most extensively and systematically studied as a function of oxygen partial pressure at elevated temperatures. The other properties have also been studied, but much less extensively and systematically. The electrical conductivity and thermopower were occasionally measured together on the same specimens so that mutual compatibility or consistency might be secured. But, the rest were all determined separately on the specimens of differing quality, consequently lacking in mutual consistency. It, thus, has remained hard to evaluate the canonical, defect-chemical parameters which are consistent with each and every of these defect structure-sensitive properties that were observed. Very recently the authors have determined the total conductivity, chemical diffusivity and thermoelectric power altogether on the same specimens of BaTiO3−δ, and the nonstoichiometry on the same-quality specimens at temperatures of 1073 ≤ T/K ≤ 1373 over wide enough a range of oxygen partial pressure (normally, 10−16 ≤ Po2/atm ≤ 1) that encloses an electron/hole/ion mixed regime. In this article, we will compile all the literature data on these defect-structure-sensitive properties and extract from the authors' own, without using any ad hoc assumptions regarding, e.g., the electronic carrier mobilities and effective density of states, the basic defect-chemical parameters including defect-equilibrium constants, carrier mobilities and densities, and electronic heats of transport, which are the most consistent with the properties observed. Compared to the conventional picture of the defect structure of “undoped” BaTiO3, thus, some new insights into the defect chemical nature of BaTiO3−δ are provided.

Thermoelectricity of clathrate I Si and Ge phases

Abstract: We report on investigations of type I clathrate Si and Ge compounds with Ba partially substituted by rare earth atoms. Novel compounds from framework-deficient solid solutions Ba8 Alx Si42−3/4x 4−1/4x and Ba8 Gax Si42−3/4x 4−1/4x (x = 8, 12, 16; , open square... lattice defect) have been prepared and characterized. All x-ray intensity data are consistent with the standardized clathrate I-Ba8Al16Ge30 type structure (space group Pmn). In rare earth substituted clathrates, Eu2Ba6MxSi46−x (M = Cu, Al, Ga), rare earth atoms completely occupy the 2a position and thus form a new quaternary ordered version of the Ba8Al16Ge30 structure type. From a geometrical analysis of clathrate crystal structures, a systematic scheme for all known clathrate compounds is proposed. All clathrates studied are metals with low electrical conductivity. The highest Seebeck coefficient in the present series is deduced for Ba8In16Ge30, S = −75μV K−1, indicating transport processes dominated by electrons as carriers. The Eu-based clathrates investigated exhibit long-range magnetic order as high as 32 K for Eu2Ba6Al8Si36 of presumably ferromagnetic type. Magnetic susceptibility indicates in all cases a 2+ ground state for the Eu ions, in fine agreement with LIII absorption edge spectra.

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 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...

Impurity-induced transition and impurity-enhanced thermopower in the thermoelectric oxide NaCo 2 − x Cu x O 4

Abstract: Various physical quantities were measured and analyzed for the Cu-substituted thermoelectric oxide ${\mathrm{NaCo}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{O}}_{4}.$ As was previously known, the substituted Cu enhances the thermoelectric power, while it does not increase the resistivity significantly. The susceptibility and the electron specific heat are substantially decreased with increasing x, which implies that the substituted Cu decreases the effective-mass enhancement. Through a quantitative comparison with the heavy-fermion compounds and the valence-fluctuation systems, we have found that the Cu substitution effectively increases the coupling between the conduction-electron and the magnetic fluctuations. The Cu substitution induces a phase transition at 22 K that is very similar to a spin-density-wave transition.

Particle size and magnetic field dependent resistivity and thermoelectric power of La0.5Pb0.5MnO3 above and below metal–insulator transition

Abstract: The effect of particle size on the transport properties (resistivity and thermopower) of La0.5Pb0.5MnO3 has been investigated both in the presence and in the absence of magnetic field B=0.0–1.5 T (maximum). Grain size, dc conductivity; and the metal–insulator transition temperature Tp of the sample increase with increasing annealing time. Grain size has, however, comparatively little effect on the Seebeck coefficient S. Magnetoresistance is higher for the samples with smaller grain sizes. dc magnetic susceptibility also increases with increasing grain size. High temperature (T>θD/2) resistivity data well fit the small polaron hopping model. Polaron hopping energy WH decreases but polaron radius rp increases with the increase of grain size. In the metallic regime (for T

Ferromagnetism in new diluted magnetic semiconductor Bi2−xFexTe3

Abstract: Single crystals of the new diluted magnetic semiconductors of p-Bi2−xFexTe3 (0⩽x⩽008) and n-Bi2−xFexSe3 (0⩽x⩽004) have been produced Magnetization and magnetic susceptibility have been measured in the temperature interval 2–300 K The ferromagnetic phase was found in p-Bi2−xFexTe3 at a temperature Tc which increases with Fe content x up to Tc=12 K for x=008, while hole concentration decreases, ie, iron atoms exhibit donor properties The easy-axis for magnetization is parallel to the C3 crystallographic axis In n-type samples, Bi2−xFexSe3 ferromagnetic transition was not found The de Haas–van Alphen effect was clearly observed with the frequency of oscillation decreasing with Fe doping We observed also the jump of the resistivity at T=Tc The Seebeck coefficient α also has been measured at room temperature With an increase in the content x of Fe atoms, the value of α increases from α=215 μV/K in the host material p-Bi2Te3 to 260 μV/K in Bi192Fe008Te3 with maximal Fe content

Effect of Ni on the transport and magnetic properties of (formula presented)

Abstract: The filled skutterudite compounds based on the binary skutterudite CoSb 3 are currently being investigated for their potential applications as thermoelectric materials. One route to optimization of these compounds is by doping on the Co site. An obvious candidate for an n-type dopant is Ni, since it has one more electron in its valence shell than Co. Up to now, however, only high concentrations of Ni in CoSb 3 have been studied; and the valence of Ni in this compound and its influence on the transport and magnetic properties has been an open question. We present electrical resistivity, thermopower, Hall effect, magnetoresistance, and magnetic susceptibility measurements on polycrystalline, n-type Co 1 - x Ni x Sb 3 with x = 0, 0.001, 0.003, 0.005, 0.0075, and 0.01, Our results show that in these low concentrations Ni has a dramatic effect on the transport properties. A two-band model is proposed that takes into account transport in both the conduction band and within donor impurity states formed by the Ni impurities. This model provides a consistent explanation of both the anomalous low-temperature transport properties as well as the Curie-Weiss behavior of the magnetic susceptibility. We conclude that Ni takes the tetravalent Ni 4 + state, assumes the d 6 electronic configuration for the lower energy nonbonding orbitals, and gives an electron to the conduction band.