Conventional Thermoelectric Materials Research Articles

Layer-by-layer deposition of MnSi1.7 film with high Seebeck coefficient and low electrical resistivity

A good thermoelectric material should have a high Seebeck coefficient, a low electrical resistivity, and a low thermal conductivity. For conventional thermoelectric materials, however, increasing the Seebeck coefficient also leads to a simultaneous increase in the electrical resistivity. In this paper, a method of layer-by-layer deposition of MnSi1.7 film with high Seebeck coefficient and low electrical resistivity is developed. After deposition of the first MnSi1.7 sub-layer, the deposition process is interrupted for several minutes, and then continues for another MnSi1.7 sub-layer. Therefore, the MnSi1.7 film contains two sub-layers for one interruption, three sub-layers for two interruptions, and so on. It is found that the n-type MnSi1.7 film with two sub-layers has a higher Seebeck coefficient, −0.451 mV K−1, and a lower electrical resistivity, 19.4 mOhm-cm, at 483 K as compared to that of without deposition interruption, −0.152 mV K−1 and 44.3 mOhm-cm. The p-type MnSi1.7 film with three sub-layers also has a higher Seebeck coefficient, 0.238 mV K−1, and a lower electrical resistivity, 5.5 mOhm-cm, at 733 K in comparison with that of without deposition interruption, 0.212 mV K−1 and 10.4 mOhm-cm.

Flexible Power Fabrics Made of Carbon Nanotubes for Harvesting Thermoelectricity

Thermoelectric energy conversion is very effective in capturing low-grade waste heat to supply electricity particularly to small devices such as sensors, wireless communication units, and wearable electronics. Conventional thermoelectric materials, however, are often inadequately brittle, expensive, toxic, and heavy. We developed both p- and n-type fabric-like flexible lightweight materials by functionalizing the large surfaces and junctions in carbon nanotube (CNT) mats. The poor thermopower and only p-type characteristics of typical CNTs have been converted into both p- and n-type with high thermopower. The changes in the electronic band diagrams of the CNTs were experimentally investigated, elucidating the carrier type and relatively large thermopower values. With our optimized device design to maximally utilize temperature gradients, an electrochromic glucose sensor was successfully operated without batteries or external power supplies, demonstrating self-powering capability. While our fundamental study provides a method of tailoring electronic transport properties, our device-level integration shows the feasibility of harvesting electrical energy by attaching the device to even curved surfaces like human bodies.

Prospects for Implementation of Thermoelectric Generators as Waste Heat Recovery Systems in Class 8 Truck Applications

With the rising cost of fuel and increasing demand for clean energy, solid-state thermoelectric (TE) devices are an attractive option for reducing fuel consumption and CO2 emissions. Although they are reliable energy converters, there are several barriers that have limited their implementation into wide market acceptance for automotive applications. These barriers include: the unsuitability of conventional thermoelectric materials for the automotive waste heat recovery temperature range; the rarity and toxicity of some otherwise suitable materials; and the limited ability to mass-manufacture thermoelectric devices from certain materials. One class of material that has demonstrated significant promise in the waste heat recovery temperature range is skutterudites. These materials have little toxicity, are relatively abundant, and have been investigated by NASA-JPL for the past twenty years as possible thermoelectric materials for space applications. In a recent collaboration between Michigan State University (MSU) and NASA-JPL, the first skutterudite-based 100 W thermoelectric generator (TEG) was constructed. In this paper, we will describe the efforts that have been directed towards: (a) enhancing the technology-readiness level of skutterudites to facilitate mass manufacturing similar to that of Bi2Te3, (b) optimizing skutterudites to improve thermal-to-electric conversion efficiencies for class 8 truck applications, and (c) describing how temperature cycling, oxidation, sublimation, and other barriers to wide market acceptance must be managed. To obtain the maximum performance from these devices, effective heat transfer systems need to be developed for integration of thermoelectric modules into practical generators.

Periodic heating amplifies the efficiency of thermoelectric energy conversion

We show that the use of a periodic heat source, instead of a constant heat source, can improve the conversion efficiency of a thermoelectric power generator (TPG). A periodic heat source drives a periodic temperature difference across the thermoelectric with an amplitude ΔT. While the time average of ΔT is identical to the temperature difference under a constant heat source with equivalent energy input, the time average of (ΔT)2 is larger, resulting in improved conversion efficiency. Here we present experimental measurements on a commercial thermoelectric device (bismuth telluride based) to validate analytical and numerical models. These models show that maximum efficiency is achieved when the period of the heat source is much larger than the thermal time constant of the TPG. Under this quasi-steady condition, the thermoelectric figure of merit ZT is still the relevant parameter for material optimization. A conventional thermoelectric material with ZT = 1, operated with sinusoidal and square-wave heat sources (ΔT = 100 K, TCold = 300 K), can achieve 140% and 180% of the constant heat source efficiency; or otherwise stated, can perform like advanced materials with ZT of 1.6 and 2.8. Even greater improvement, inaccessible through materials-based ZT enhancements, can be achieved with low duty cycle heat sources.

Harvesting energy from low-grade heat based on nanofluids

Conventional thermoelectric materials have limited capability of scavenging electrical energy from low-grade heat (LGH). Based on the capacitive effect of liquid–solid interface in a nanoconfinement, we investigate a novel energy harvesting mechanism which is based on the thermally sensitive ion/charge distribution of electrolytes confined in nanopores. The mechanism is elucidated using comprehensive molecular dynamics (MD) simulations. The effective thermal sensitivity, effective figure of merit, and thermal-to-electric energy conversion efficiency of the nanofluidic system compare favorably with respect to the conventional thermoelectric materials. The result of a preliminary thermal-to-electrical energy conversion experiment on a nanoporous carbon is presented, to qualitatively show the feasibility of the approach.

Thermoelectric properties of copper selenide with ordered selenium layer and disordered copper layer

Thermoelectric figure-of-merit (ZT) of ∼1.6 at 700°C is achieved in β-phase copper selenide (Cu2Se) made by ball milling and hot pressing. The β-phase of such material possesses a natural superlattice-like structure that combines ordered selenium (Se) and disordered copper (Cu) layers in its unit cell, resulting in a low lattice thermal conductivity of 0.4–0.5Wm−1K−1. A λ-shaped specific heat peak indicates a phase transition from cubic β-phase to tetragonal α-phase at around 140°C upon cooling and vice versa. An abnormal decrease in specific heat with increasing temperature was also observed due to the increased random motion of disordered Cu atoms. These features indicate that β-phase Cu2Se could be potentially an interesting thermoelectric material, competing with other conventional thermoelectric materials.

Simultaneous Increases in Electrical Conductivity and Seebeck Coefficient of PEDOT:PSS Films by Adding Ionic Liquids into a Polymer Solution

The electrical conductivity and Seebeck coefficient of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films were simultaneously improved by adding an ionic liquid (IL) into a polymer solution of the polymers. The maximum electrical conductivity of such a PEDOT:PSS/IL film reached 174 S cm−1, more than an order of magnitude higher than that of pure PEDOT:PSS film, and the maximum Seebeck coefficient was up to 30 μV K−1, more than twice the value for pure PEDOT:PSS film. This behavior is different from conventional thermoelectric (TE) materials, whose TE properties are strongly correlated, such as increasing electrical conductivity with increasing carrier concentration, usually resulting in a logarithmic decrease in Seebeck coefficient. Atomic force microscopy images of the PEDOT:PSS/IL films indicated that the ILs induced formation of a particular three-dimensional structure of highly conducting PEDOT grains, resulting in improvement of the TE performance of PEDOT:PSS films.

Electronic structure and low temperature thermoelectric properties of In24 M8 O48 (M = Ge4+, Sn4+, Ti4+, and Zr4+)

The electronic structure and transport properties of In₂₄M₈O₄₈ (M = Ge(4+), Sn(4+), Ti(4+), and Zr(4+)) have been studied by using the full-potential linearized augmented plane-wave method and the semiclassical Boltzmann theory, respectively. It is found that the magnitude of powerfactor with respect to relation time follows the order of In₂₄Sn₈O₄₈ > In₂₄Zr₈O₄₈ > In₂₄Ge₈O₄₈ > In₂₄Ti₈O₄₈. The largest powerfactor is 2.7 × 10¹² W/K² ms for In₂₄Sn₈O₄₈ at 60 K, which is nearly thirty times larger than those of conventional n-type thermoelectric materials. The origin of the different thermoelectric behavior for these compounds is discussed from the electronic structure level. It is found that, at low temperature, the dopant strongly affect the bands near the Fermi level, which consequently leads to their different thermoelectric properties. The electronic configuration and the difference in atomic number between the dopant and the host atom also play an important role on the thermoelectric properties of In₂₄M₈O₄₈. Our calculations give a valuable insight on how to enhance the thermoelectric performance of In₃₂O₄₈.

Thermoelectric properties of ZnO nanowires: A first principle research

By means of ab-initio electronic structure calculation and one-dimensional Boltzmann transport equation solution, we investigate the size dependent thermoelectric (TE) properties of n-type ZnO nanowires (NWs) and surface passivation effects. As demonstrated by our calculations, largest figure of merit ZT achievable in thin NWs is larger than that in wide NWs, whereas being restrained by higher demand of n-type doping. Moreover, bare NWs are superior in TE application comparing with the passivated. To compete with conventional TE materials, lattice thermal conductivity of ZnO NWs should be at least 2 orders of magnitude lower than bulk value.

Crystal structure, electronic structure, and thermoelectric properties of Ca5Al2Sb6

The electronic structure and transport properties of Ca5Al2Sb6 are investigated by using first-principles calculations and Boltzmann transport theory, respectively. The results show that the partially filled valence band induces a high carrier concentration of about n = 5 × 1019 cm−3. There is a combination of heavy and light bands at the conduction band edge, which may lead to a combination of high Seebeck coefficient and reasonable conductivity. At mid-and-high temperature, the thermoelectric powerfactor, with respect to relaxation time of p-type Ca5Al2Sb6, is higher than that of the n-type within the carrier concentration ranging from −10 × 1021 cm−3 to 10 × 1021 cm−3, without considering the kinds of doping. But with decreasing temperature, the thermoelectric powerfactor with respect to relaxation time of n-type Ca5Al2Sb6 is higher than that of the p-type. The thermoelectric coefficient is increasingly sensitive to carrier concentration with the decreasing temperature. Most strikingly, at 30 K, the thermoelectric powerfactor, with respect to relaxation time, is nearly thirty-five times larger than that of conventional n-type thermoelectric materials. At 300 K, the thermoelectric powerfactor with respect to relaxation time of Ca5Al2Sb6 is equal to that of the conventional p-type thermoelectric materials. Our theoretical calculations give valuable insight on how to improve the thermoelectric performance of Ca5Al2Sb6 under different temperature and doping conditions.

Electronic structure and thermoelectric properties of In32−xGexO48 (x=, 1, 2, and 3) at low temperature

The electronic properties of In32−xGexO48 (x=0, 1, 2, 3, 4, 5, 6, and 7) are studied by using the density functional theory. The transport coefficients of In32−xGexO48 (x=0, 1, 2, and 3) are then calculated within the semiclassical Boltzmann theory. The largest Seebeck coefficient is nearly seven times larger than that of Bi2Te3. Most strikingly, the thermoelectric power factor with respect to relaxation time is nearly 70 times larger than that of conventional thermoelectric materials. Our theoretical calculations give a valuable insight on how to enhance the thermoelectric performance of In32−xGexO48.

ＲＦプラズマＣＶＤ法により作製したＤＬＣ薄膜の熱電性能評価

This paper presents an investigation of the thermoelectric performance of DLC films deposited on glass substrates using RF plasma CVD method. The respective thermoelectric performances of Si-doped DLC film and non-Si doped DLC film were evaluated and compared. The DLC films showed a Seebeck effect, and they had p-type semiconductor characteristics.The values of DLC films’ Seebeck coefficients were 1/5 - 1/100 compared to those of the conventional thermoelectric materials. At temperatures of 80-200°C, the Seebeck coefficients of Si-doped DLC and non-doped DLC were almost identical. The resistivity value of DLC films decreased exponentially with increasing temperature. Furthermore, the DLC film values were much larger than those of conventional thermoelectric materials: 105 to 1010 times larger. The thermal conductivity of DLC films was about one-half that of conventional thermoelectric materials at room temperature. The results presented above suggest that reducing DLC film resistivity through control of deposition conditions, doped element composition, and other means must be examined to raise the thermoelectric performance of DLC film.

The Effects of Polysilastyrene and Au Additions on the Thermoelectric Properties of β-SiC/Si Composites

Recently, Yamaguchi et al. proposed a self-cooling device that does not require additional power circuits for cooling because it is Peltier-cooled using its own current in conjunction with a thermoelectric material. Silicon carbide is a promising thermoelectric material for this technology since its electrical conductivity, thermal conductivity, and Seebeck coefficient are higher than those of conventional thermoelectric materials. This study investigates the effects of polysilastyrene and Au additions on the thermoelectric properties of p-type β-SiC/Si polycrystalline semiconductor composites in order to assess whether their addition improves the performance of self-cooling devices.

Thermal and Electrical Conductivity of Size‐Tuned Bismuth Telluride Nanoparticles

Quantum-confined semiconductors composed of heavy elements hold great promise as thermoelectric materials. An increase in the density of states near the Fermi level due to quantum confinement effects and an increased scattering of boundary phonons due to nanostructuring can lead to an increase in the dimensionless figure of merit, ZT, which is defined as s 2 STk � 1 . [1,2] Here, s is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and k is the thermal conductivity. To realize the highest ZTquantum-confined material possible from a conventional thermoelectric semiconductor material such as bismuth telluride, meeting several criteria are important. First, it is reasonable to expect that uniform, optimal size quantum domains will lead to the highest ZT at a given doping level. The electronic structure of the semiconductor, which determines the thermoelectric power, depends on the degree of quantum confinement and thus the domain size. Second, the interfacial electrical transport must be optimized. The presence of impurities between the domains/particles typically leads to barriers to transport that reduce the electrical conductivity. Third, while optimizing electrical transport between domains, it is important that the quantum architecture not be destroyed, or the phonon scattering will decrease and the thermal conductivity will increase. Fourth, it is well knownthatproperdopingiscriticaltotheperformanceofbulk

Modeling heat transfer in Bi 2Te 3–Sb 2Te 3 nanostructures

Bi 2Te 3–Sb 2Te 3 nanostructures are gaining importance for use in thermoelectric applications following the finding that the Bi 2Te 3–Sb 2Te 3 superlattice exhibits a figure of merit, ZT = 2.4, which is higher than conventional thermoelectric materials. In this paper, thermal transport in the cross-plane direction for Bi 2Te 3–Sb 2Te 3 nanostructures is simulated using the Boltzmann transport equation (BTE) for phonon intensity. The phonon group velocity, specific heat, and relaxation time are calculated based on phonon dispersion model. The interfaces are modeled using a combination of diffuse mismatch model (DMM), and the elastic acoustic mismatch model (AMM). The thermal conductivity for the Bi 2Te 3–Sb 2Te 3 superlattice is compared with the experimental data, and the best match is obtained for specularity parameter, p, of 0.9. The present model is extended to solve for thermal transport in 2-D nanowire composite in which Sb 2Te 3 wires are embedded in a host material of Bi 2Te 3. Unlike in bulk composites, the results show a strong dependence of thermal conductivity, temperature, and heat flux on the wire size, wire atomic percentage, and interface specularity parameter. The thermal conductivity of the nanowire is found to be in the range of 0.034–0.74 depending on the atomic percentage and the value of p.

Thermoelectric Properties of Bi<sub>x</sub>Sb<sub>y</sub>Te<sub>z</sub>Se<sub>w</sub> Nanocomposite Materials

Nanopowders of n-type (Bi0.95Sb0.05)2(Te0.95Se0.05)3 and p-type (Bi0.2Sb0.8)2Te3 have been synthesized by laser fracture of micron-sized powders in water. These alloys are the best conventional thermoelectric materials for use in room temperature applications. The nanopowders have been characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The nanopowders have been mechanically mixed in different ratios with the micron sized powders. These mixtures have then been cold pressed in order to perform thermoelectric characterization and to see the influence of nano-particle inclusions on the transport properties.

Effects of Ge substitution on the thermoelectric properties and pseudogap characteristics ofFe2VGa

We report the effects of partial substitution of Ge onto the Ga sites ofFe2VGa by measuring electrical resistivity, Seebeck coefficient and thermal conductivity as afunction of temperature. It is found that Ge substitution effectively dopes electrons to thesystem and thus causes a dramatic decrease in the electrical resistivity. The Seebeckcoefficient changes sign from positive to negative upon replacing Ga by Ge, which is ingood agreement with a simple band filling picture. In addition, the magnitudeof the Seebeck coefficient gradually increases and attains a maximum value of85 µV K−1 at around120 K for Fe2VGa0.9Ge0.1. Such a variation of Seebeck coefficient can be understood by means of rigidband-like shifting of the Fermi level across the pseudogap. The thermalconductivity is also reduced and a detailed analysis based on the Debyeapproximation indicates that the extrinsic disorder introduced by Ge substitution inFe2VGa has a minor contribution to the point defect scattering. Other lattice imperfections, such asantisite disorder, may be the main source for the point defect scattering whichshows no systematic variation with Ge concentration. While the thermoelectricperformance improves with the partial substitution of Ge, the largest figure-of-merit(ZT) value among these presently investigated alloys is still an order of magnitude lower thanthe conventional thermoelectric materials.

High-temperature thermoelectric properties of Nb-doped MNiSn (M=Ti, Zr) half-Heusler compound

Abstract The temperature dependence of electrical conductivity, Seebeck coefficient, Hall coefficient, and thermal conductivity of Nb-doped MNiSn (M = Ti, Zr) half-Heusler compounds were investigated at different temperature, ranging from room temperature to 1000 K. The power factor reached 4 × 10−3 Wm−1 K−2 above 600 K for both systems. The power factor for TiNiSn-based samples decreased above 700 K due to the narrower band gap. The Hall mobility was relatively small; however, estimated carrier effective mass was larger by one order of magnitude than that for conventional thermoelectric material. The thermal conductivity increased above 700 K due to the ambipolar diffusion effect. The ambipolar diffusion effect depended on the band gap width and the ratio of electron–hole conductivity. Heavy carrier-doping effectively suppressed the ambipolar diffusion effect, i.e. restrain the increase of thermal conductivity at high temperature. The maximum ZT value of 0.6 at 800 K was obtained for Zr0.98Nb0.02NiSn.

Thermoelectric power factor of LSCoO compounds

Electrical resistivity ρ(T) and Seebeck coefficient S(T) measurements of polycrystalline La0.8Sr0.2Co1-xMnxO3(0⩽x⩽0.1) (LSCoO) samples were carried out in the temperature range between 90 and 290K. The samples were grown by citrate complex method; Seebeck coefficient and electrical resistivity were measured by differential technique and four-probe method, respectively. Both S(T) and ρ(T) increase monotonically with the Mn content, the electrical resistivity exhibit a metallic-semiconducting temperature behavior, while Seebeck coefficient is positive in whole measured temperature range. From S(T) and ρ(T) data it was possible to calculate the thermoelectric power factor PF, which in the samples with x=0.08, reaches values close to 18μW/K2cm; these values for PF are comparable with those of conventional thermoelectric materials, becoming these oxides in promissory thermoelectric materials.

Thermoelectric properties of quaternary Heusler alloysFe2VAl1−xSix

We report the effects of Si substitution on the temperature-dependent electrical resistivity, Seebeck coefficient, as well as thermal conductivity in the Heusler-type compound ${\mathrm{Fe}}_{2}\mathrm{VAl}$. It is found that the substitution of Si onto the Al sites causes a significant decrease in the electrical resistivity and lattice thermal conductivity. A theoretical analysis indicated that the reduction of lattice thermal conductivity arises mainly from point-defect scattering of the phonons. With slight substitution, the Seebeck coefficient changes sign from positive to negative, accompanied by the appearance of a broad minimum at high temperatures. These features are associated with the change in the electronic band structure, where the Fermi level shifts upwards from the center of the pseudogap due to electron-doping effect. For $xg0.1$ in ${\mathrm{Fe}}_{2}{\mathrm{VAl}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}$, no broad minimum in the Seebeck coefficient appears, indicative of a dramatic modification in the band structure of these materials. While the thermoelectric performance improves with increasing Si concentration, the largest figure-of-merit $ZT$ value among these alloys is still an order of magnitude lower than conventional thermoelectric materials.