Showing papers on "Thermoelectric effect published in 2003"

Recent developments in thermoelectric materials

Abstract: Efficient solid state energy conversion based on the Peltier effect for cooling and the Seebeck effect for power generation calls for materials with high electrical conductivity σ, high Seebeck coefficient S, and low thermal conductivity k. Identifying materials with a high thermoelectric figure of merit Z(= S2σ/k) has proven to be an extremely challenging task. After 30 years of slow progress, thermoelectric materials research experienced a resurgence, inspired by the developments of new concepts and theories to engineer electron and phonon transport in both nanostructures and bulk materials. This review provides a critical summary of some recent developments of new concepts and new materials. In nanostructures, quantum and classical size effects provide opportunities to tailor the electron and phonon transport through structural engineering. Quantum wells, superlattices, quantum wires, and quantum dots have been employed to change the band structure, energy levels, and density of states of elect...

Measuring Thermal and Thermoelectric Properties of One-Dimensional Nanostructures Using a Microfabricated Device

Abstract: We have batch-fabricated a microdevice consisting of two adjacent symmetric silicon nitride membranes suspended by long silicon nitride beams for measuring thermophysical properties of one-dimensional manostructures (nanotubes, nanowires, and mmobelts) bridging the two membranes. A platinum resistance heater/thermometer is fabricated on each membrane. One membrane can be Joule heated to cause heat conduction through the sample to the other membrane. Thermal conductance, electrical conductance, and Seebeck coefficient can be measured using this microdevice in the temperature range of 4-400 K of an evacuated Helium cryostat. Measurement sensitivity, errors, and uncertainty are discussed. Measurement results of a 148 nm and a 10 nm-diameter single wall carbon nanotube bundle are presented.

Spin entropy as the likely source of enhanced thermopower in Na x Co 2 O 4

Abstract: In an electric field, the flow of electrons in a solid produces an entropy current in addition to the familiar charge current. This is the Peltier effect, and it underlies all thermoelectric refrigerators. The increased interest in thermoelectric cooling applications has led to a search for more efficient Peltier materials and to renewed theoretical investigation into how electron-electron interaction may enhance the thermopower of materials such as the transition-metal oxides. An important factor in this enhancement is the electronic spin entropy, which is predicted to dominate the entropy current. However, the crucial evidence for the spin-entropy term, namely its complete suppression in a longitudinal magnetic field, has not been reported until now. Here we report evidence for such suppression in the layered oxide Na(x)Co2O4, from thermopower and magnetization measurements in both longitudinal and transverse magnetic fields. The strong dependence of thermopower on magnetic field provides a rare, unambiguous example of how strong electron-electron interaction effects can qualitatively alter electronic behaviour in a solid. We discuss the implications of our finding--that spin-entropy dominates the enhancement of thermopower in transition-metal oxides--for the search for better Peltier materials.

Thermoelectric Efficiency and Compatibility

Abstract: The intensive reduced efficiency eta(r) is derived for thermoelectric power generation (in one dimension) from intensive fields and currents, giving eta(r)=(E x J) divided by (- inverted Delta)T x J(S). The overall efficiency is derivable from a thermodynamic state function, Phi=1 divided by u + alphaT, where we introduce u=J divided by kappa (inverted Delta)T as the relative current density. The method simplifies the computation and clarifies the physics behind thermoelectric devices by revealing a new materials property s=(sqrt[1+zT]-1) divided by (alphaT), which we call the compatibility factor. Materials with dissimilar compatibility factors cannot be combined by segmentation into an efficient thermoelectric generator because of constraints imposed on u. Thus, control of the compatibility factor s is, in addition to z, essential for efficient operation of a thermoelectric device, and thus will facilitate rational materials selection, device design, and the engineering of functionally graded materials.

Thermoelectric microdevice fabricated by a MEMS-like electrochemical process.

Abstract: Microelectromechanical systems (MEMS) are the basis of many rapidly growing technologies, because they combine miniature sensors and actuators with communications and electronics at low cost. Commercial MEMS fabrication processes are limited to silicon-based materials or two-dimensional structures. Here we show an inexpensive, electrochemical technique to build MEMS-like structures that contain several different metals and semiconductors with three-dimensional bridging structures. We demonstrate this technique by building a working microthermoelectric device. Using repeated exposure and development of multiple photoresist layers, several different metals and thermoelectric materials are fabricated in a three-dimensional structure. A device containing 126 n-type and p-type (Bi, Sb)2Te3 thermoelectric elements, 20 microm tall and 60 microm in diameter with bridging metal interconnects, was fabricated and cooling demonstrated. Such a device should be of technological importance for precise thermal control when operating as a cooler, and for portable power when operating as a micro power generator.

Thermoelectric effect in molecular electronics

Abstract: We provide a theoretical estimate of the thermoelectric current and voltage over a Phenyldithiol molecule. We also show that the thermoelectric voltage is (1) easy to analyze, (2) insensitive to the detailed coupling to the contacts, (3) large enough to be measured, and (4) give valuable information, which is not readily accessible through other experiments, on the location of the Fermi energy relative to the molecular levels. The location of the Fermi-energy is poorly understood and controversial even though it is a central factor in determining the nature of conduction $(n$ or p type). We also note that the thermoelectric voltage measured over Guanine molecules with a scanning tunneling microscope by Poler et al., indicate conduction through the highest occupied molecular orbital level, i.e., p-type conduction.

Functionally graded materials for sensor and energy applications

Abstract: Principles, preparation, characterisation, and application of functional materials containing a gradient of their functional properties are surveyed, with main emphasis on thermoelectric (TE) materials for application in sensors and thermogenerators. Further examples of the implementation of functionally graded materials (FGM) presented are dielectric thin-film stacks for capacitors with low temperature coefficient, microwave-processed structural gradients in fuel cell electrodes, and zone-melted graded (Bi1 − xSbx)2Te3 materials for Peltier coolers. Preparation and properties of compositional gradients in TE solid solutions (FeSi2 doped by alloying, (Bi1 − xSbx)2Te3, Mg2(Si,Ge,Sn), PbTe) are analysed, as well as composites joining thermoelectrics of dissimilar chemistry and joints to metallic contacts and interlayers. Thermal spraying of doping-graded FeSi2 was developed as a preparation technique of TE silicide-based FGM. Design, preparation and test of a layered heat-flux sensor based on FeSi2 are described. A calibration test evidenced the feasibility of linearisation of thermal sensor characteristics. A theoretical design tool for functionally graded and segmented thermoelectric structures was based on a local selection criterion to identify the optimal spatial compositional distribution.

Thermoelectric properties of superlattice nanowires

Abstract: We report here on a theoretical model for the electronic structure and transport properties of superlattice nanowires, considering their cylindrical wire boundary and multiple anisotropic carrier pockets. The thermoelectric properties of superlattice nanowires made of various lead salts (PbS, PbSe, and PbTe) are investigated as a function of the segment length, wire diameter, crystal orientation along the wire axis, and the length ratio of the constituent nanodots of the superlattice, based on the Kronig-Penney potential for each one-dimensional (1D) subband and on the 1D Boltzmann transport equations. A potential barrier--well inversion induced by quantum confinement, which is a unique phenomenon in superlattice nanowires, is predicted as the wire diameter decreases. $\mathrm{ZT}$ values higher than 4 and 6 are predicted for 5-nm-diameter PbSe/PbS and PbTe/PbSe superlattice nanowires at 77 K, respectively. These ZT values are significantly larger than those of their corresponding alloy nanowires, indicating that superlattice nanowires are promising systems for thermoelectric applications.

Bismuth telluride compounds with high thermoelectric figures of merit

Abstract: The thermoelectric properties of the p-type (Bi0.25Sb0.75)2Te3 doped with 8 wt. % excess Te and the n-type Bi2(Te0.94Se0.06)3 doped substantially with 0.07 wt. % I, 0.02 wt. % Te, and 0.03 wt. % CuBr which were grown by the Bridgman method at a rate of 6 cm/h were measured before and after annealing, where annealing was done in the temperature range from 473 up to 673 K for 2–5 h in a vacuum and a hydrogen stream. No annealing effect on the power factor was observed for the p-type specimen, but the as-grown p-type specimen exhibited a large power factor of 5.53×10−3 W/mK2 at 308 K and a low thermal conductivity of 1.21 W/mK. When the n-type specimen was annealed at 473 K for 2 h in a hydrogen stream, however, the power factor at 308 K increased significantly from 3.26×10−3 to 4.73×10−3 K−1 but its thermal conductivity then increased by about 3% from 1.26 to 1.30 W/mK. As a result, the maximum thermoelectric figure of merits Z at 308 K for the as-grown p- and annealed n-type specimens reached surprisingly ...

Electronic structure and transport in type-I and type-VIII clathrates containing strontium, barium, and europium

Abstract: The thermoelectric properties of filled gallium-germanium clathrates are analyzed from a band-structure point of view. Using the virtual crystal approximation the undoped clathrates are calculated to be semiconductors with band gaps around 0.6--0.9 eV. The conduction bands hybridize with the unoccupied d states of the guest atoms. This means that the thermoelectric properties of n-doped type-I clathrates depend strongly on the guest atom while p doped clathrates are relatively unaffected. The type-VIII structures have disperse bands centered around the voids in the framework structure. This explains the low effective masses observed for n-doped type-VIII structures. It also means that the electronic structure and thermoelectric properties are relatively independent of the guest atom and that p-doped should have favorable thermoelectric properties while n-doped type-VIII structures have poor properties. We estimate a figure of merit of 1.2 at 400 K for an optimally p-doped europium-VIII clathrate. In addition we predict that the strontium-VIII clathrate should be stable.

Preparation and thermoelectric properties of Al-doped ZnO ceramics

Abstract: Al 2 O 3 added ZnO powders were prepared via sol–gel processing, using zinc acetate, ammonia and Al 2 O 3 powders as starting materials. Scanning electron microscopy (SEM) observations indicated that the Al 2 O 3 added ZnO powders consisted of very fine particles (0.1–2 μm). The thermal conductivity, electrical conductivity and Seebeck coefficient of hot-pressed Al 2 O 3 added ZnO ceramic samples were measured in dependence of temperature up to 600 °C. The influence of Al 2 O 3 addition on the thermoelectric (TE) properties of ZnO ceramics is discussed.

Mechanism for thermoelectric figure-of-merit enhancement in regimented quantum dot superlattices

Abstract: We propose a mechanism for enhancement of the thermoelectric figure-of-merit in regimented quantum dot superlattices. A proof-of-concept calculation has been carried out for p-type regimented superlattice of Ge dots on Si. It is shown that when conditions for miniband formations are satisfied, carrier transport in such structures can be tuned in a favorable way leading to large carrier mobility, Seebeck coefficient, and, as a result, to the thermoelectric figure-of-merit enhancement. To maximize the improvement, one has to tune the parameters of quantum dot superlattice in such a way that electrical current is mostly through the well-separated minibands of relatively large width (at least several kBT, where kB is Boltzmann’s constant and T is temperature).

Thermoelectric performance of Bi- and Na-substituted Ca3Co4O9 improved through ceramic texturing

Abstract: Plate-like particles of Ca3Co4O9 have been prepared in which Bi and Na were partially substituted for Ca to improve the thermoelectric performance. Successfully fabricated dense and highly textured ceramics have been obtained by combining the templated grain growth technique (TGG) with hot-pressing (HP). Ca3Co4O9 single crystals, which have alternating layers of Co–O and Ca–Co–O in the direction of the c-axis, show high electrical conductivity along the layer compared with that across the layer. Hence, a highly oriented TGG + HP specimen showed high electrical conductivity compared with the specimen sintered under uniaxial pressure (UP + PLS). The improved electrical conductivity with high Seebeck coefficient of the highly oriented specimen (TGG + HP) gave a high thermoelectric power factor of 5.9 × 10−4 W m−1 K−2 at 1073 K. The figures-of-merit at 773 K and 1073 K were calculated to be 8.54 × 10−5 K−1 (ZT = 0.066 at 773 K) and 1.69 × 10−4 K−1 (ZT = 0.18 at 1073 K), respectively. These values are quite high among Ca–Co–O polycrystalline systems reported so far.

Effect of partial void filling on the transport properties of NdxCo4Sb12 skutterudites

Abstract: Polycrystalline samples of the partially filled skutterudites NdxCo4Sb12 have been prepared and characterized by x-ray powder diffraction and differential thermal analysis. The saturation limit of the Nd void filling in CoSb3 was found to be around 13%. All samples decompose incongruently at a temperature of 1149 ± 6 K. Room temperature Hall measurements show that each Nd atom donates approximately 0.8 electrons, which is significantly less than the Nd oxidation state (3+). The temperature dependence of the electrical and thermal transport properties has been measured over the range of 11–700 K. The electrical resistivity and absolute value of the Seebeck coefficient decrease with increasing Nd content and for samples with x > 0.02 the temperature dependence is typical of heavily doped semiconductors. Filling CoSb3 with Nd causes a rapid initial decrease in the lattice thermal conductivity with a minimum at the composition Nd0.1Co4Sb12. Nd-filled skutterudites exhibit the lowest value of the lattice thermal conductivity in comparison with other partially filled skutterudites at x < 0.1, which could be attributed to a smaller radius of Nd than that of other filling elements. At high temperature the ZT value of the Nd-filled skutterudites is limited due to intrinsic conduction caused by the relatively low carrier concentration. The effect of the partial Nd filling on the transport properties of the filled skutterudite compounds is discussed in the context of potential thermoelectric materials.

Pulsed laser deposition of Bi2Te3-based thermoelectric thin films

Abstract: Thin films of p-type Bi05Sb15Te3, n-type Bi2Te27Se03, and n-type (Bi2Te3)90(Sb2Te3)5(Sb2Se3)5 (with 013 wt % SbI3) were deposited on substrates of mica and aluminum nitride (on silicon) using pulsed laser ablation at substrate temperatures between 300 °C to 500 °C The films were characterized using x-ray diffraction and transmission electron microscopy for crystalline quality and epitaxial growth on the substrates The surface morphology and microstructure were examined using scanning electron microscopy X-ray mapping and energy-dispersive spectroscopy were performed to determine nonstoichiometry in the composition and homogeneity The quality of the films, in terms of stoichiometric composition and crystal perfection, was studied as a function of growth temperature and laser fluence The values of the Seebeck coefficient, electrical resistivity, and Hall mobility in the thin films were measured and compared with those in the bulk Thermoelectric figure of merit of the films was evaluated from the

Chemistry, physics, and materials science of thermoelectric materials : beyond bismuth telluride

Abstract: Overview of Bismuth Nanowires for Thermoelectric Applications.- The Synthetic Search for Better Thermoelectrics.- New Bulk Materials for Thermoelectric Applications: Synthetic Strategies Based on Phase Homologies.- Boron Carbides: Unconventional High-Temperature Thermoelectrics.- Block-Layer Concept for the Layered Cobalt Oxide: A Design for Thermoelectric Oxides.- Thermoelectric Properties of Early Transition Metal Antimonides.- Clathrate Thermoelectrics.- Structure-Property Relations in Skutterudites.- Phonon Heat Conduction in Superlattices.- Thermal Conduction in CoSb3-Based Skutterudites.- Thermoelectric Transport in Bismuth Nanowires: Experimental Results.- Pressure Tuning of Thermoelectric Materials.- Thermoelectrics with Thermionic Boundary Conditions.- Electronic Structure of Complex Bismuth Chaleogenide Systems.- Toward a First-Principles Determination of Transport Coefficients.- Can Theory Help in the Search for Better Thermoelectric Materials?.- Participants.

Compact, high-efficiency thermoelectric systems

Abstract: A number of compact, high-efficiency thermoelectric system utilizing the advantages of thermal isolation in the direction of a working medium flow or movement, in manufacturable systems, are described. Such configurations exhibit high system efficiency and power density. Several different embodiments and applications are disclosed utilizing a plurality of thermoelectric modules or thermoelectric elements sandwiched between heat exchangers.

Thermal conductivity of elemental crystalline silicon clathrate Si136

Abstract: The thermal conductivity and heat capacity of a guest-free polycrystalline silicon clathrate with the type-II hydrate crystal structure is reported. The magnitude of the thermal conductivity at room temperature is only slightly larger than that of vitreous silica, and is thirty times lower than that of diamond structured Si. The temperature dependence of the thermal conductivity of Si136 follows the well-known Debye form, and is dissimilar to that of clathrates with “guest” atoms inside their polyhedra. The Debye temperature of Si136, estimated from low temperature heat capacity measurements, is 470 K. The potential of guest-free clathrates for thermoelectric applications is discussed.

Nanostructured Co1-xNixSb3 skutterudites : Synthesis, thermoelectric properties, and theoretical modeling

Abstract: Nanostructured skutterudite Co1-xNixSb3 has been synthesized by chemical alloying with Ni substitution for Co up to 27.5 at. %. High concentration of grain boundaries provided by nanostructuring is ...

Hydrogen-selective thermoelectric gas sensor

Abstract: A thermoelectric (TE) hydrogen gas sensor was fabricated by depositing a platinum catalyst thin film on the half surface of nickel oxide thick film. When it was exposed to combustible gas diluted by synthetic air, the catalyst layer converts hydrogen and oxygen effectively to water vapor, and give out heat energy, resulting temperature difference across the sensor, and consequently voltage signal. The voltage signal for the 3% H2/air mixture gas at temperature 100 °C, was 4.65 mV while that for 3% CH3OH/air mixture gas was 0.31 mV. The lower detection limit (LDL) of the sensor at 100 °C was lower than 500 ppm, with an excellent linearity of signal voltage to the hydrogen gas concentration.