Temperature Thermoelectric Research Articles

Production of Single Crystal Thermoelectric Bismuth Telluride Alloys

Thermoelectric (TE) semiconductor materials are widely used for miniaturized versatile cooling devices in a wide spectrum of equipments and energy generation in space vehicles. The bismuth telluride pseudobinary alloy family presents the best characteristics for room temperature TE cooling applications. Using appropriately oriented single crystals instead of the well known polycrystalline materials made by powder metallurgy methods, the efficiency of the TE device (Thermoelectric Cooler – TEC or Thermoelectric Generator – TEG) could be almost doubled. For having good quality TE material it is required to produce equally doped single crystals by the controlled crystallization process, namely with the Bridgman-Stockbarber method. Our experiments were made in the Universal Multizone Crystallizator Type UMC, developed by the ADMATIS Ltd., Miskolc, using a quartz tube under high vacuum conditions and automatically controlled thermal field parameters. The crystallographic analysis of the obtained samples was made by Scanning Electron Microscopy – (SEM), X Ray Diffraction – (XRD), and neutron diffraction (TOF spectrometry).

High-temperature thermoelectric properties of thallium-filled skutterudites

The high temperature thermoelectric (TE) properties of polycrystalline Tl-filled skutterudites TlxCo4Sb12 (x=0, 0.05, 0.10, 0.15, 0.20, and 0.25) were examined from room temperature to 750 K. All samples exhibited negative Seebeck coefficients. The electrical resistivity, absolute values of the Seebeck coefficient, and the lattice thermal conductivity were found to decrease with increasing Tl content. Tl0.25Co4Sb12 exhibited the best TE performance; the maximum value for the dimensionless figure of merit ZT was 0.90 at 600 K.

Nanostructured Silicon-based Composites for High Temperature Thermoelectric Applications

AbstractRecently nanostructured bulk silicon and silicon-germanium have achieved large increases in the thermoelectric figure of merit (ZT). The ZT enhancement is attributed to a significant reduction in the lattice thermal conductivity while maintaining relatively high carrier mobility. Silicon-based thermoelectric devices are attractive due to their low-toxicity, thermal stability, low density, relative abundance and low cost of production. Although significant enhancements in ZT have been achieved using the nanostructuring route, additional decoupling of the thermal and electric transport terms is still necessary in order for silicon-based materials to be viable for thermoelectric applications such as waste heat recovery or radioisotope thermoelectric generators. It is theorized that additional increases in ZT could be achieved by forming composites with nanostructured inert inclusions to further scatter the heat-carrying phonons. Here we present the impact of insulating and conductive nanoparticle composites on ZT. The nanostructured composites are formed via ball milling and high pressure sintering of the nanoparticles. The thermoelectric properties and microstructure of the silicon-based composites are discussed.

High Temperature Thermoelectric Properties of Nano-Bulk Silicon and Silicon Germanium

AbstractPoint defect scattering via the formation of solid solutions to reduce the lattice thermal conductivity has been an effective method for increasing ZT in state-of-the-art thermoelectric materials such as Si-Ge, Bi2Te3-Sb2Te3 and PbTe-SnTe. However, increases in ZT are limited by a concurrent decrease in charge carrier mobility values. The search for effective methods for decoupling electronic and thermal transport led to the study of low dimensional thin film and wire structures, in particular because scattering rates for phonons and electrons can be better independently controlled. While promising results have been achieved on several material systems, integration of low dimensional structures into practical power generation devices that need to operate across large temperature differential is extremely challenging. We present achieving similar effects on the bulk scale via high pressure sintering of doped and undoped Si and Si-Ge nanoparticles. The nanoparticles are prepared via techniques that include high energy ball milling of the pure elements. The nanostructure of the materials is confirmed by powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and dynamic light scattering. Thermal conductivity measurements on the densified pellets show a drastic 90% reduction in the lattice contribution at room temperature when compared to doped single crystal Si. Additionally, Hall effect measurements show a much more limited degradation in the carrier mobility. The combination of low thermal conductivity and high power factor in heavily doped n-type nanostructured bulk Si leads to an unprecedented increase in ZT at 1275 K by a factor of 3.5 over that of single crystalline samples. Experimental results on both n-type and p-type Si are discussed in terms of the impact of the size distribution of the nanoparticles, doping impurities and nanoparticle synthesis processes.

In situ synthesis and thermoelectric properties of La-doped Mg2(Si, Sn) composites

Utilizing the peritectic reaction and the miscibility gap featured in the pseudo-binary phase diagram of Mg2Si and Mg2Sn, we fabricated environmentally friendly Mg2(Si, Sn) thermoelectric (TE) composites in which the Mg2Si-rich bulk grains were in situ coated by Mg2Sn-rich thin layers. The Mg2Sn-rich grain boundary phase was selectively doped with La. It was found that the La doping dramatically increased the electrical conductivity to thermal conductivity ratio in the composites. As a result, a dimensionless figure of merit ZT ∼ 0.81 has been attained at 810 K for the Mg2−xLax(Si, Sn) in situ composite with x = 0.005, significantly higher than the ZT ∼0.18 at 540 K for the undoped composite and comparable to ZT ∼ 0.8 of state-of-the-art PbTe intermediate temperature TE alloys.

Efficient room temperature thermoelectric characteristics of Ca3−xAgxCo4O9+δ/Agy composites

We report measurements of electrical resistivity, thermopower and thermal conductivity for a series of high-textured Ag-doped and Ag-added Ca3Co4O9+δ ceramic specimens. The results show that variable range hopping transport behaviour of the system transforms from two-dimensional to quasi-three-dimensional with Ag addition, which is isotropic to a certain extent. The thermoelectric (TE) performance of Ca3Co4O9+δ is improved effectively by Ag+ doping or/and Ag addition. The Ca2.7Ag0.3 Co4O9+δ/Ag0.3 composite exhibits quite a respectable room temperature ZT value of 0.1, suggesting that such a composite is a promising candidate for p-type room temperature TE cooling application.

Synthesis, Structure, and High Temperature Thermoelectric Properties of Yb11Sb9.3Ge0.5.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Synthesis, Structure, and High Temperature Thermoelectric Properties of Yb11Sb9.3Ge0.5

AbstractZintl phase compounds with large unit cells and complex anionic structures such as Yb11Sb10 hold potential for being good thermoelectric materials. Single crystals of Ge‐doped Yb11Sb10 were synthesized using a molten Sn‐flux technique. Single crystal X‐ray diffraction data were obtained and resulted in a composition of Yb11Sb9.3Ge0.5 which was verified by microprobe. Yb11Sb9.3Ge0.5 is isostructural to Ho11Ge10, crystallizing in a body‐centered, tetragonal unit cell, space group I4/mmm, with Z = 4. The unit cell parameters of Yb11Sb9.3Ge0.5 are a = 11.8813(4), c = 17.1276(13) Å with a volume of 2417.8(2) Å3. These parameters correlate well with the structural refinement of previously published Yb11Sb10. The structure consists of 16 isolated Sb3− anions, 8 $\rm Sb^{4-}_{2}$ dumbbells, 2 $\rm Sb^{4-}_{4}$ square planar rings and 44 Yb2+ cations. The Ge, doped in at 28 % occupancy, was found to be site specific, residing on the 2 $\rm Sb^{4-}_{4}$ square planar rings. Single crystal X‐ray diffraction is most consistent with the site that makes up the square ring being less than fully occupied. The doped compound is additionally characterized by X‐ray powder diffraction, differential scanning calorimetry and thermogravimetry. High temperature (300–1200 K) thermoelectric properties show that the doped compound has extremely low thermal conductivity (10–30 mW/cmK), lower than that of Yb11Sb10. Temperature dependent resistivity is consistent with a heavily doped semiconductor. Yb11Sb9.3Ge0.5 shows p‐type behavior increasing from ∼22 μV/K at room temperature to ∼31 μV/K at 1140 K. The low value and the temperature dependence of the Seebeck coefficient suggest that bipolar conduction produces a compensated Seebeck coefficient and consequently a low zT.

High Temperature Thermoelectric Properties of Mo3Sb7‐xTex for x = 1.6 and 1.5.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Atomic Ordering, Electronic Structure, and Transport Properties of LAST-m Systems

AbstractIn recent years, LAST-m (AgPbmSbTem+2) and related materials have emerged as potential high performance high temperature thermoelectrics. These compounds are obtained by starting from PbTe, and replacing pairs of Pb2+ ions by (Ag1+, Sb3+) pairs. One example is LAST-18. When optimally doped, this compound has thermoelectric figure of merit ZT=1.7 at 700K. This large ZT is most likely due to very low lattice thermal conductivity, caused by phonon scattering from nanostructures. These nanostructures involve clustering and ordering of Ag, Sb, and Pb ions. Possible origins of this atomic ordering and how the presence of nanostructures affects the electronic structure near the band gap region are discussed. The temperature (T) dependence of electrical conductivity σ (∼T2.2 in the range 300K <T< 900K) in n-type PbTe is analyzed in terms of the T-dependence of different physical quantities contributing to transport. We find that the dominant contribution comes from the explicit T-dependence of relaxation time rather than its energy dependence. The T-dependence of chemical potential is also significant in the concentration range of interest. Electronic thermal conductivity for constant field (κel,E) and for constant current (κel,J) are found to differ considerably at high temperatures and the Weidemann-Franz (WF) law κel,J = LoσT, where Lo =2x10−8WΩ/K is the Lorentz number, overstimates κel,J by nearly 60% at 800K for carrier concentration n=5x1019/cm3. As a result, one tends to underestimate the lattice contribution κlatt = κexp - κel,J. We give theoretical values of effective Lorentz number L = κel.J/σT for different n and T.

Defect Clustering and Nanostructure Formation in PbTe-based Bulk Thermoelectrics

AbstractIn recent years, LAST-m (AgPbmSbTem+2) and related materials have emerged as potential high performance high temperature thermoelectrics. One example is LAST-18. When optimally doped, this compound has thermoelectric figure of merit ZT=1.7 at 700K. This large ZT is most likely due to the low lattice thermal conductivity, caused by phonon scattering from nanostructures. These nanostructures involve clustering and ordering of Ag, Sb, and Pb ions. The origin of these nanostructures has been studied using Monte Carlo (MC) simulation of an ionic model and ab initio studies of pair interaction energies. Effects of these substitutions on the band structure near the gap and their implications on transport properties are briefly discussed.

Preparation and Characterization of High Temperature Thermoelectrics Based on Metal/Oxide Nanocomposites

AbstractThermoelectric devices based on “n-type” oxide semiconductors and metal/oxide nanocomposites are being considered for high temperature thermocouples, heat flux sensors and energy harvesting devices. In terms of energy harvesting, preliminary 2D thermoelectric calculations indicated that enough electrical energy can be generated from the large thermal gradients that exist within a gas turbine engine to power active wireless devices. Several promising bi-ceramic junctions based on this concept were investigated in terms of their high temperature thermoelectric properties. The most promising bi-ceramic junction was based on indium tin oxide (ITO) and a NiCrCoAlY/alumina nanocomposite. The thermoelectric responses of these individual elements were evaluated relative to a platinum reference electrode. A maximum emf of 77 mV was achieved for a NiCrCoAlY/alumina nanocomposite/platinum thermocouple for an imposed temperature gradient of 1111 °C. The thermoelectric power for this couple was 78 μV/°C. When this NiCrCoAlY/alumina nanocomposite was combined with ITO to form a bi-ceramic junction, thermoelectric powers on the order of 700 μV/°C were obtained. A maximum electromotive force of 291mV was achieved for a hot junction temperature of 1100 °C. The thermoelectric response after repeated thermal cycling to 1200 °C was both repeatable and reproducible. The ITO was prepared in varying nitrogen, oxygen and argon partial pressures, which was used to control the charge carrier concentration, stability and thermoelectric response of the bi-ceramic junctions. The thermoelectric response decreased with increasing nitrogen partial pressure and increased with oxygen partial pressure in the plasma with the argon partial pressure constant. The relationship between the sputtering parameters and thermoelectric properties was investigated and the application of these bi-ceramic junctions as thermocouples and energy harvesting devices is discussed.

Unusual properties of icosahedral boron-rich solids

Icosahedral boron-rich solids are materials containing boron-rich units in which atoms reside at an icosahedron's 12 vertices. These materials are known for their exceptional bonding and the unusual structures that result. This article describes how the unusual bonding generates other distinctive and useful effects. In particular, radiation-induced atomic vacancies and interstitials spontaneously recombine to produce the “self-healing” that underlies these materials’ extraordinary radiation tolerance. Furthermore, boron carbides, a group of icosahedral boron-rich solids, possess unusual electronic, magnetic and thermal properties. For example, the charge carriers, holes, localize as singlet pairs on icosahedra. The unusual origin of this localization is indicated by the absence of a concomitant photo-ionization. The thermally assisted hopping of singlet pairs between icosahedra produces Seebeck coefficients that are unexpectedly large and only weakly dependent on carrier concentration. These properties are exploited in devices: (1) long-lived high-power high-capacity beta-voltaic cells, (2) very high temperature thermoelectrics and (3) solid-state neutron detectors.

Electronic structure of defects and defect clusters in narrow band-gap semiconductorPbTe

Lead chalcogenide salts PbSe and PbTe are IV–VI narrow gap semiconductorswhose study over last several decades has been motivated by their importancein infrared detectors, lasers and thermoelectrics. Recently a class of systems,AgSbPb2n−2Ten, has been found to be excellent high temperature thermoelectrics. Since electronicproperties of semiconductors in general and thermoelectric behaviour in particular, aredominated by defects, we have carried out ab initio electronic structure calculations of Agand Sb substitutional defects in PbTe. We find that these defects give rise to deep defectstates and the electronic structure near the gap depends sensitively on the micro structuralarrangements of these defects. The modified electronic structure may be responsible for theobserved high temperature thermoelectric properties of the above compounds.

High Temperature Thermoelectric Properties of NiZrSn Half‐Heusler Compounds.

AbstractFor Abstract see ChemInform Abstract in Full Text.

High Temperature Thermoelectric Properties of Ca1-xYxMnO2.98 (0.05≤x≤0.30)

High temperature thermoelectric properties of Ca 1-x Dy x MnO 2.98 (x=0, 0.05, 0.10, 0.15, 0.20) have been measured in the temperature range from 300 to 1273 K. Ca 1-x Dy x MnO 2.98 (x=0.10, 0.15, 0.20) exhibited the metal-insulator transition in the relation between electrical resistivity and temperature, and the transition temperature increased with increasing x. Electrical resistivity in the metallic region decreased with increasing x content, reflecting the increase of Mn 8+ ions, and the absolute value of Seebeck coefficient (a) also decreased. Thermal conductivity (λ) did not considerably change with increasing x content. Among substituted CaMnO 3 , the highest figure of merit of Z=1.05 × 10 -4 K -1 was obtained for x=0.05 and 0.10 at 773 K, while a Z=1.63 × 10 -4 K -1 was obtained for x=0.20 at 1273 K.

고온용 ZnO계 열전 재료의 방전플라즈마 소결 특성 및 미세구조

고온용 ZnO계 열전 재료의 방전플라즈마 소결 특성 및 미세구조

High Temperature Electrical Transport Properties of Eu and Yb-doped Skutterudites

ABSTRACTSkutterudites are a class of materials that show promise for thermoelectric applications due to their high power factor and the ability to “tune” the thermal conductivity due to the addition of “rattling” atoms into the novel structure of these materials. Thermopower and resistivity is measured and reported for a series of Eu and Yb-doped skutterudites over a temperature range of approximately 100 K – 700 K using the High Temperature Thermoelectric Probe. Sample measurement techniques are briefly discussed. Data from various dopings is presented and compared in hope of showing trends that point towards improvements in these skutterudites.