Showing papers on "Thermoelectric effect published in 2001"

Thin-film thermoelectric devices with high room-temperature figures of merit

Abstract: Thermoelectric materials are of interest for applications as heat pumps and power generators. The performance of thermoelectric devices is quantified by a figure of merit, ZT, where Z is a measure of a material's thermoelectric properties and T is the absolute temperature. A material with a figure of merit of around unity was first reported over four decades ago, but since then-despite investigation of various approaches-there has been only modest progress in finding materials with enhanced ZT values at room temperature. Here we report thin-film thermoelectric materials that demonstrate a significant enhancement in ZT at 300 K, compared to state-of-the-art bulk Bi2Te3 alloys. This amounts to a maximum observed factor of approximately 2.4 for our p-type Bi2Te3/Sb2Te3 superlattice devices. The enhancement is achieved by controlling the transport of phonons and electrons in the superlattices. Preliminary devices exhibit significant cooling (32 K at around room temperature) and the potential to pump a heat flux of up to 700 W cm-2; the localized cooling and heating occurs some 23,000 times faster than in bulk devices. We anticipate that the combination of performance, power density and speed achieved in these materials will lead to diverse technological applications: for example, in thermochemistry-on-a-chip, DNA microarrays, fibre-optic switches and microelectrothermal systems.

Anomalous barium filling fraction and n-type thermoelectric performance of BayCo4Sb12

Abstract: Barium-filled skutterudites BayCo4Sb12 with an anomalously large filling fraction of up to y=0.44 have been synthesized. The lattice parameters increase linearly with Ba content. Magnetic susceptibility data show that Ba0.44Co4Sb12 is paramagnetic, which implies that some of the Co atoms in BayCo4Sb12 have acquired a magnetic moment. The presence of the two different valence states of Co (Co3+ and Co2+) leads to the anomalously large barium filling fraction even without extra charge compensation. All samples show n-type conduction. The electrical conductivity increases with increasing the Ba filling fraction. The lattice thermal conductivity of BayCo4Sb12 is significantly depressed as compared to unfilled Co4Sb12. The dimensionless thermoelectric figure of merit, ZT, increases with increasing temperature reaching a maximum value of 1.1 for Ba0.24Co4Sb12 at 850 K.

Effects of partial substitution of Ni by Pd on the thermoelectric properties of ZrNiSn-based half-Heusler compounds

Abstract: We report on the effects of partial substitution of nickel by palladium on the thermoelectric properties of ZrNiSn-based half-Heusler compounds. It is shown that the substitution of palladium for nickel results in a significant, beneficial reduction of the thermal conductivity. The Seebeck coefficient also decreases, but only by a small amount. In the Hf0.5Zr0.5Ni0.8Pd0.2Sn0.99Sb0.01 compound, a power factor of 22.1 μW K−2 cm−1 and a thermal conductivity as low as 4.5 W/m K are measured at room temperature. The dimensionless figure of merit ZT increases with increasing temperature and reaches a maximum value of 0.7 at about 800 K.

High-Temperature Thermoelectric Properties of NaxCoO2-δ Single Crystals

Abstract: We measure the high-temperature thermoelectric properties of the NaxCoO2-? single crystal for the first time. The NaxCoO2-? single crystals are prepared by a flux technique, and the resulting flaky single crystals are very thin on the c-axis. The in-plane electrical resistivity (?), the thermoelectric power (S) and the in-plane thermal conductivity (?) are measured in the range of 300 K to 800 K. The estimated power factor (PF=S2??-1) and the figure-of-merit (Z=S2??-1?-1) are about 2.4 mW m-1 K-2 and 0.12 mK-1 at 300 K, respectively. These factors increase with temperature, and reach the value of PF=7.7 mW m-1 K-2 and Z=1.5 mK-1 at 800 K, and the dimensionless value of ZT exceeds the criterion of 1. The PF and Z exceed the values of typical ?high-temperature thermoelectric materials?, such as p-type FeSi2 and Si0.7Ge0.3. These results suggest that NaxCoO2-? could be a very promising material for use in high-temperature thermoelectric power generation devices.

Bipolarity in electrical conduction of transparent oxide semiconductor CuInO2 with delafossite structure

Abstract: A transparent oxide semiconductor with delafossite structure, CuInO2, was found to exhibit both p-type and n-type conduction by doping of an appropriate impurity and tuning of proper film-deposition conditions. Thin films of Ca-doped or Sn-doped CuInO2 (optical band gap=∼3.9 eV) were prepared on α-Al2O3(001) single crystal substrates by pulsed laser deposition method. The films were deposited at 723 K in O2 atmosphere of 1.0 Pa for the Ca-doped films or 1.5 Pa for the Sn-doped films. The positive sign of the Seebeck coefficient demonstrated p-type conduction in the Ca-doped films, while the Seebeck coefficient of the Sn-doped films was negative indicating n-type conductivity. The electrical conductivities of Ca-doped and Sn-doped CuInO2 thin films were 2.8×10−3 S cm−1 and 3.8×10−3 S cm−1, respectively, at 300 K.

Anomalously high thermoelectric figure of merit in Bi1−xSbx nanowires by carrier pocket alignment

Abstract: Electronic transport calculations were carried out for Bi1−xSbx nanowires (0⩽x⩽0.30) of diameters 10 nm⩽dW⩽100 nm at 77 K. A band structure phase diagram was generated, showing the dependence of the relative band edge positions on diameter and composition. Calculations of the thermoelectric figure-of-merit (ZT) predict that the performance of Bi1−xSbx nanowires is superior to that of Bi nanowires and to that of the bulk alloy. An exceptionally high value of ZT for p-type nanowires at 77 K was found for dW∼40 nm and x∼0.13, which is explained by the coalescence in energy of up to ten valence subband edges to maximize the density-of-states at the Fermi energy.

High Performance Thermoelectric Tl 9 BiTe 6 with an Extremely Low Thermal Conductivity

Abstract: $\mathrm{Tl}{}_{9}\mathrm{BiTe}{}_{6}$ exhibits a thermoelectric figure of merit of $\mathrm{ZT}\ensuremath{\sim}1.2$ around 500 K, which significantly exceeds the state-of-the-art materials in this temperature range. The extraordinary thermoelectric performance is mainly due to the extremely low thermal conductivity of $\mathrm{Tl}{}_{9}\mathrm{BiTe}{}_{6}$ [ $0.39\mathrm{W}/(\mathrm{m}\ifmmode \dot{}\else \.{}\fi{}\mathrm{K})$ at 300 K]. In fact, the minimum lifetime of the phonons has to be taken into account to describe the thermal conductivity data.

Large Improvement in Thermoelectric Properties in Pressure-Tuned p-Type Sb1.5Bi0.5Te3

Abstract: There is widespread interest in the search for materials that would allow the fabrication of more efficient thermoelectric devices for cooling and power generation applications. Here, we report a large increase in the thermoelectric power of p-doped antimony bismuth telluride alloys upon pressure tuning under nonhydrostatic compression conditions. Together with measurements of the electrical conductivity and an upper bound estimated for the thermal conductivity under pressure, these results indicate that values of the dimensionless thermoelectric figure of merit, ZT, in excess of 2 have been achieved, substantially larger than the best observed values in bulk materials to date. We suggest an explanation for the observed behavior and strategies for attempting to reproduce it at ambient pressure.

Fabrication of an all-oxide thermoelectric power generator

Abstract: An oxide thermoelectric device was fabricated using Gd-doped Ca3Co4O9 p-type legs and La-doped CaMnO3 n-type legs on a fin. The power factors of p legs and n legs were 4.8×10−4 Wm−1 K−2 and 2.2×10−4 Wm−1 K−2 at 700 °C in air, respectively. With eight p–n couples the device generated an output power of 63.5 mW under the thermal condition of hot side temperature Th=773 °C and a temperature difference ΔT=390 °C. This device proved to be operable for more than two weeks in air showing high durability.

Flexible thermoelectric circuit

Abstract: A flexible thermoelectric circuit is disclosed. Thermoelectric circuits have traditionally been of the rigid or substantially rigid form. Several different embodiments of thermoelectric circuits are disclosed which permit flexion in one or more directions to permit applications where flexible thermoelectric circuits are advantageous.

Microcombustor and combustion-based thermoelectric microgenerator

Abstract: A generally toroidal counterflow heat exchanger is the main element of a combustor that operates at a micro scale. The combustor includes a central combustion region with openings to a reactant gas channel and an exhaust gas channel. The reactant channel and exhaust channels are coiled around each other in a spiral configuration that reduces heat loss. An electric current microgenerator is similar and also includes a thermoelectric active wall composed of n-type and p-type thermoelectric elements as part of a channel wall of the microcombustor. The thermoelectric active wall includes fins configured to increase the temperature differential across the thermoelectric elements relative to the temperature difference between the thermoelectric elements and the reactant and exhaust gases. A method of monolithically fabricating such microdevices by electrodepositing multiple layers of material is also provided.

Thermal characterization of Bi2Te3/Sb2Te3 superlattices

Abstract: Superlattices offer the potential to enhance the figure of merit for thermoelectric cooling by increasing the Seebeck coefficient while decreasing the thermal conductivity compared to bulk samples. The large bulk value of ZT makes superlattices containing Bi2Te3 attractive for demonstrating benefits of using low-dimensional materials in thermoelectric applications. The present work describes measurements of the effective thermal conductivity normal to Bi2Te3/Sb2Te3 superlattices deposited on GaAs using noncontact pulsed laser heating and thermoreflectance thermometry. The data show a strong reduction in the effective thermal conductivity of the Bi2Te3/Sb2Te3 superlattices compared to bulk Bi2Te3, which can further increase thermoelectric figure of merit. The dependence of thermal conductivity on superlattice period is found to be weak, particularly at periods above 60 A. This indicates that disorder in Bi2Te3/Sb2Te3 superlattices may limit the heat conduction process at shorter periods than in Si/Ge super...

Chapter 3 The role of solid-state chemistry in the discovery of new thermoelectric materials

Abstract: Publisher Summary This chapter discusses the role of solid-state chemistry in the discovery of new thermoelectric materials. The challenge in any effort to discover new thermoelectric (TE) materials lies in achieving simultaneously high electronic conductivity, high thermoelectric power, and low thermal conductivity in the same solid. The challenge in TE research is to incorporate all the desirable features associated with the charge and thermal transport in a single solid-state material. The chapter presents a review on the criteria and the types of compounds that might be suitable. Materials suitable for TE applications fall mainly into two categories: semiconductors and mixed-valent compounds, although some semimetals may also be viable. Research groups are also investigating whether multiple quantum wells in semiconductors will improve thermoelectric behavior. The improvements for the electronic properties so far seem small, but quantum wells may have potentially a much smaller thermal conductivity than bulk counterparts, which may eventually make them useful for devices. The chapter discusses the properties for best materials of semiconductors. A highly successful approach has been performing reactions using molten salts as solvents. The future of TE relies heavily on new materials and, consequently, solid state chemistry has an important role to play in the field of TE.

Efficiency thermoelectrics utilizing thermal isolation

Abstract: An improved efficiency thermoelectric system and method of making such a thermoelectric system are disclosed. Significant thermal isolation between thermoelectric elements in at least one direction across a thermoelectric system provides increased efficiency over conventional thermoelectric arrays. Significant thermal isolation is also provided for at least one heat exchanger coupled to the thermoelectric elements. In one embodiment, the properties, such as resistance or current flow, of the thermoelectric elements may also be varied in at least one direction across a thermoelectric array. In addition, the mechanical configuration of the thermoelectric elements may be varied, in one embodiment, according to dynamic adjustment criteria.

Stretched polyaniline films doped by (±)-10-camphorsulfonic acid : Anisotropy and improvement of thermoelectric properties

Abstract: The stretched polyaniline films doped by (±)-10-camphorsulfonic acid exhibited significant anisotropy of the electrical and thermoelectric properties. The thermoelectric figure-of-merit (ZT) of the stretched films in the direction parallel to the stretching direction is much higher than that of the unstretched counterpart, and moreover, the ZT increased with increasing drawing ratio. According to the experimental results of X-ray diffraction and UV-Vis-near IR spectroscopy the improvement of the ZT by the stretching process is revealed to be attributable to the increment of carrier mobility, which is proposed to be induced by the extended coil-like conformation of polyaniline.

Thermoelectric properties of Pb- and Ca-doped (Bi2Sr2O4)xCoO2 whiskers

Abstract: An oxide single crystalline whisker has been prepared which displays superior thermoelectric properties at high temperature in air. The whisker has an average composition of (Bi, Pb)2.2(Sr, Ca)2.8Co2Oy (abbreviated to BC-232 whisker) and a layered structure in which two different sublattices, CoO2 and rocksalt (Bi, Pb)2(Sr, Ca)2O4, alternate in the c-axis direction. Edge-sharing CoO2 layers act as conducting and thermoelectric units. Although a plateau is observed at around 500 K, the Seebeck coefficient of the whiskers is about 100 μV K−1 at 100 K, basically increases with temperature up to 773 K, and is saturated at temperatures higher than 773 K. Temperature dependence of electric resistivity shows semiconducting-like behavior but its value is very low compared with general semiconductors. These electrical phenomena would indicate pseudogap formation with its width of a few meV at the Fermi level. The power factor of the BC-232 whiskers is estimated at over 0.5 mWm−1 K−2 at temperatures higher than 650...

Band structures and thermoelectric properties of the clathrates Ba8Ga16Ge30,Sr8Ga16Ge30,Ba8Ga16Si30, and Ba8In16Sn30

Abstract: Density functional calculations in the generalized gradient approximation are used to study the transport properties of the clathrates Ba8Ga16Ge30, Sr8Ga16Ge30, Ba8Ga16Si30, and Ba8In16Sn30. The band structures of these clathrates indicate that they are all semiconductors. Seebeck coefficients, conductivities and Hall coefficients are calculated, to assess the effects of carrier concentration on the quantity S2σ/τ (where S is the Seebeck coefficient, σ is the conductivity, and τ the electron relaxation time) which is proportional to the thermoelectric power factor. In each compound we find that both p- and n-doping will significantly enhance the thermoelectric capabilities of these compounds. For p-doping, the power factors of all four clathrates are of comparable magnitude and have similar temperature dependence, while for n-doping we see significant variations from compound to compound. We estimate that room-temperature ZT values of 0.5 may be possible for optimally n-doped Sr8Ga16Ge30 or Ba8In16Sn30; a...

Chapter 2 Electronic and thermoelectric properties of Half-Heusler alloys

Abstract: Summary Recent studies of ternary and multinary half-Heusler phases have revealed many interesting electrical and magnetic properties in this class of bandgap intermetallic compounds. Most of the half-Heusler alloys that exhibit semiconducting behavior at elevated temperatures are found to be semimetallic at low temperatures, which is characteristic of the metal-based narrow-gap compounds studied to date. Since the chemistry of half-Heusler compounds can be conveniently modified, selective doping of the three sublattices in the crystal structure can be carried out to allow tuning of the electronic structure and lattice vibration properties. Impurity band states, previously not reported for metal-based systems, are observed in some of the doped alloys investigated. In ferromagnetic half-Heusler phases, the crossover from semiconductor to semimetal is found in the region where magnetism emerges. According to band-structure calculations, the ferromagnetism is of the itinerant and highly spin-polarized type. Large thermopower and moderate resistivity, with the former attributable to the existence of heavy carrier mass, are measured at and above ambient temperature. Upon doping, a high power factor comparable to those reported for the state-of-the-art thermoelectric alloys is measured. The thermoelectric dimensionless figure of merit ZT, which is found to reach ∼0.6 at 800 K, underscores the potential of half-Heusler alloys as a new class of prospective thermoelectric materials above ambient temperature. The ZT value is to be further enhanced by reducing the lattice contribution to thermal conductivity. Unlike other thermoelectric materials, half-Heusler alloys exhibit low carrier mobility. Thus, attempts made to reduce the lattice thermal conductivity are seen to have relatively less effect on the power factor. The measured properties are found to be sensitive to annealing conditions, with the latter presumably determining the underlying crystallographic order.

Thermoelectric effects in Kondo-correlated quantum dots

Abstract: In this letter we study thermoelectric effects in ultra-small quantum dots We study the behaviour of the thermopower, Peltier coefficient and thermal conductance both in the sequential tunneling regime and in the regime where Kondo correlations develop Both cases of linear response and non-equilibrium induced by strong temperature gradients are considered The thermopower is a very sensitive tool to detect Kondo correlations It changes sign as a function of both temperature and temperature gradient We also discuss violations of the Wiedemann-Franz law

Chapter 1 Quantum wells and quantum wires for potential thermoelectric applications

Abstract: In this chapter, the predictions of an enhancement in the thermoelectric figure of merit of low-dimensional material systems relative to their corresponding bulk counterparts, as well as the present state of experimental confirmation of these predictions, are reviewed. Progress with specific quantum well, quantum wire, and quantum dot materials systems, such as the lead salts, Si-Ge, and bismuth is discussed. To date most of the effort has gone into proof-of-principle studies, though actual demonstration of the highest thermoelectric figure of merit ( Z 3D T ) of any material to date has been seen in the low-dimensional system PbSe 0.98 Te 0.02 -PbTe, where the enhancement is attributed to quantum dot formation associated with the interface between the PbTe and PbSe 0.98 Te 0.02 . In this chapter, particular attention is given to a discussion of the structure and properties of bismuth quantum wires, which are still at an early state of research. Bismuth nanowires, however, offer significant promise for practical applications, because they can be self-assembled and are predicted to have desirable thermoelectric properties when they have wire diameters in the 5- to 10-nm range. Though temperature-dependent resistance measurements have been carried out for Bi nanowires in this diameter range, reliable thermoelectric measurements have not yet been reported.

Thermoelectric power generation utilizing convective heat flow

Abstract: An improved efficiency thermoelectric power generation system is disclosed wherein convection is actively facilitated through a thermoelectric array, and the thermoelectric array is used to generate electrical power. Thermal power is convected through the thermoelectric array or arrays toward at least one side of the thermoelectric array, which leads to increased efficiency. Thermal power is applied to the array, creating a temperature gradient across the array. The thermoelectric system may also be combined with other power generation systems, forming a co-generation system.

Design and characterization of thin film microcoolers

Abstract: Thin film coolers can provide large cooling power densities compared to bulk thermoelectrics due to the close spacing of hot and cold junctions. Important parameters in the design of such coolers are investigated theoretically and experimentally. A three-dimensional (3D) finite element simulator (ANSYS) is used to model self-consistently thermal and electrical properties of a complete device structure. The dominant three-dimensional thermal and electrical spreading resistances acquired from the 3D simulation are also used in a one-dimensional model (MATLAB) to obtain faster, less rigorous results. Heat conduction, Joule heating, thermoelectric and thermionic cooling are included in these models as well as nonideal effects such as contact resistance, finite thermal resistance of the substrate and the heat sink, and heat generation in the wire bonds. Simulations exhibit good agreement with experimental results from InGaAsP-based thin film thermionic emission coolers which have demonstrated maximum cooling of 1.15 °C at room temperature. With the nonideal effects minimized, simulations predict that single stage thin film coolers can provide up to 20–30 °C degrees centigrade cooling with cooling power densities of several 1000 W/cm2.

Efficiency thermoelectrics utilizing convective heat flow

Abstract: An improved efficiency thermoelectric system is disclosed wherein convection is actively facilitated through a thermoelectric array. Thermoelectrics are commonly used for cooling and heating applications. Thermal power is convected through a thermoelectric array toward at least one side of the thermoelectric array, which leads to increased efficiency. Several different configurations are disclosed to provide convective thermal power transport, using a convective medium. In addition, a control system is disclosed which responds to one or more inputs to make adjustments to the thermoelectric system.