Thermopower Values Research Articles

Spin state driven weighted mobility and thermal conductivity properties of electron (Hf) doped strongly correlated Mott insulator LaCoO3 for thermoelectric applications

Here, we report the temperature-dependent electrical resistivity and thermopower of hole (Sr) and electron (Hf) doped LaCoO3 in the range of 303–753 K. With increasing temperature, the insulating behavior (303–503 K) with dominance of small polaron hopping to metallic transition (>503 K) is observed. The electron doped sample shows an insulating behavior (19.5 Ω cm) and positive thermopower (139 μV K−1) value due to the spin state blockade, i.e., electron hopping from high spin Co2+ to low spin Co3+ is strongly inhibited. The calculated weighted mobility (μW) of 0.01 to 0.96 cm2 V−1 s−1 validates the observed spin blockade mechanism in electron doped LaCoO3. The fluctuation of spin/orbital ordering and point defect scattering results in the low thermal conductivity of 0.5 W m−1 K−1 for Hf doped LaCoO3. The spin state blockade observed in the electrical resistivity and low lattice thermal conductivity reveals that spin state transition drives the thermoelectric response in Mott insulator LaCoO3.

Impact of Oxygen Stoichiometry on the Thermoelectric Properties of Bi2Sr2Co2Oy Thin Films.

In this study, we present a comprehensive analysis of the thermoelectric (TE) properties of highly c-axis-oriented thin films of layered misfit cobaltates Bi2Sr2Co2Oy. The films exhibit a high c-axis orientation, facilitating precise measurements of electronic transport and TE properties along the a-b crystallographic plane. Our findings reveal that the presence of nearly stoichiometric oxygen content results in high thermopower with metallic conductivity, while the annealing of the films in a reduced oxygen atmosphere eliminates their metallic behavior. According to the well-established Heike's limit, the thermopower tends to become temperature independent when the thermal energy significantly exceeds the bandwidth, which provides a rough estimation of charge carrier density by using the Heike's formula. This observation suggests that the dominant contribution to the thermopower comes from the narrow Co-t2g bands near the Fermi energy. Our study demonstrates that the calculated thermopower value using Heike's formula, based on the Hall electron density of the Bi2Sr2Co2Oy thin films at 300 K, aligns well with the experimental results, shedding light on the intriguing TE properties of this family of layered cobaltate oxide films.

Thermoelectric generator and temperature sensor based on polyamide doped n-type single-walled nanotubes toward self-powered wearable electronics

Due to its ability to convert body heat into electricity, organic thermoelectric material is considered a promising and smart maintenance-free power source to charge wearable electronics. However, developing flexible n-type organic thermoelectric materials and wearable p/n junction thermoelectric devices remains challenging. In this work, two insulated polyamides (PA6 and PA66) that have been widely used as fiber materials are employed as novel dopants for converting p-type single-walled carbon nanotubes (SWCNTs) to n-type thermoelectric materials. Because of the electron transferability of the amide group, polyamide-doped SWCNTs exhibit excellent thermopower values as large as −56.0 µV K−1 for PA66, and −54.5 µV K−1 for PA6. Thermoelectric devices with five p/n junctions connected in series are fabricated. The testing device produces a thermoelectric voltage of 43.1 mV and generates 1.85 µW thermoelectric power under temperature gradients of approximately 80 K. Furthermore, they display charming capability for temperature recognition and monitoring human activities as sensors. These promising results suggest that the flexible polyamide-doped SWCNT composites herein have high application potential as wearable thermoelectric electronics.

Effect of Linker Group on Thermoelectric Properties of Dimer Zinc Porphyrin-Based Molecular Junctions

The discovery of high-efficiency organic thermoelectric materials has the potential to pave the path for various energy-harvesting technologies and the development of flexible, transparent thin-film materials for Peltier coolers. Our study involves a comparison of the electrical and thermoelectric characteristics of four zinc porphyrin configurations. The results show that changing the linker between zinc diner units has an essential effect on the electrical conductance and thermopower values of these structures. Consequently, the thermoelectric figure of merit ZT will change which means the thermoelectric efficiency change.

Electrical Conductance and Thermopower of β-Substituted Porphyrin Molecular Junctions─Synthesis and Transport.

Molecular junctions offer significant potential for enhancing thermoelectric power generation. Quantum interference effects and associated sharp features in electron transmission are expected to enable the tuning and enhancement of thermoelectric properties in molecular junctions. To systematically explore the effect of quantum interferences, we designed and synthesized two new classes of porphyrins, P1 and P2, with two methylthio anchoring groups in the 2,13- and 2,12-positions, respectively, and their Zn complexes, Zn-P1 and Zn-P2. Past theory suggests that P1 and Zn-P1 feature destructive quantum interference in single-molecule junctions with gold electrodes and may thus show high thermopower, while P2 and Zn-P2 do not. Our detailed experimental single-molecule break-junction studies of conductance and thermopower, the latter being the first ever performed on porphyrin molecular junctions, revealed that the electrical conductance of the P1 and Zn-P1 junctions is relatively close, and the same holds for P2 and Zn-P2, while there is a 6 times reduction in the electrical conductance between P1 and P2 type junctions. Further, we observed that the thermopower of P1 junctions is slightly larger than for P2 junctions, while Zn-P1 junctions show the largest thermopower and Zn-P2 junctions show the lowest. We relate the experimental results to quantum transport theory using first-principles approaches. While the conductance of P1 and Zn-P1 junctions is robustly predicted to be larger than those of P2 and Zn-P2, computed thermopowers depend sensitively on the level of theory and the single-molecule junction geometry. However, the predicted large difference in conductance and thermopower values between Zn-P1 and Zn-P2 derivatives, suggested in previous model calculations, is not supported by our experimental and theoretical findings.

Thermoelectric Performance of Bi1.8Sb0.2Te3.0 Alloys under High Pressures


 The pressure effect on the thermopower has been investigated for the Bi1.8Sb0.2Te3.0 alloys. We have found an anisotropic pressure dependence on the thermopower for p- and n-type Bi1.8Sb0.2Te3.0 at room temperature. Along a-axis, for p-type, the thermopower value increased with increase of pressure at a rate of 13.9 %/Gpa, whereas, along c-axis, itkeeps approximately constant.. The absolue value of the thermopower decreased with pressure along both a- and c-axis for n-type. The power factor increased largely with increase of pressure for p-type Bi1.8Sb0.2Te3.0, while it is approximately constant for n-type.

Interplay between diffusion and magnon-drag thermopower in pure iron and dilute iron alloy nanowire networks

Results of measurements on the thermoelectric power of 45 nm diameter interconnected nanowire networks consisting of pure Fe, dilute FeCu and FeCr alloys and Fe/Cu multilayers are presented. The thermopower values of Fe nanowires are very close to those found in bulk materials, at all temperatures studied between 70 and 320 K. For pure Fe, the diffusion thermopower at room temperature, estimated to be around − 15 upmuV/K from our data, is largely supplanted by the estimated positive magnon-drag contribution, close to 30 upmuV/K. In dilute FeCu and FeCr alloys, the magnon-drag thermopower is found to decrease with increasing impurity concentration to about 10 upmuV/K at 10% impurity content. While the diffusion thermopower is almost unchanged in FeCu nanowire networks compared to pure Fe, it is strongly reduced in FeCr nanowires due to pronounced changes in the density of states of the majority spin electrons. Measurements performed on Fe(7 nm)/Cu(10 nm) multilayer nanowires indicate a dominant contribution of charge carrier diffusion to the thermopower, as previously found in other magnetic multilayers, and a cancellation of the magnon-drag effect. The magneto-resistance and magneto-Seebeck effects measured on Fe/Cu multilayer nanowires allow the estimation of the spin-dependent Seebeck coefficient in Fe, which is about − 7.6 upmuV/K at ambient temperature.

Polycrystalline MnBi as a transverse thermoelectric material

To assess the potential of polycrystalline MnBi as a transverse thermoelectric material, we have experimentally investigated its anomalous Nernst effect (ANE) by means of the heat flux method. We prepared MnBi samples by powder metallurgy; this technique allows the preparation of samples in arbitrary shapes with the possibility to tailor their magnetic properties. In the material exhibiting the highest remanent magnetization, we found a value of the ANE thermopower of −1.1 μV/K at 1 T, after the compensation of the ordinary Nernst effect from pure bismuth present inside the polycrystalline sample. This value is comparable with those reported in the literature for single crystals.

Low‐Cost and Biodegradable Thermoelectric Devices Based on van der Waals Semiconductors on Paper Substrates

We present a method to fabricate handcrafted thermoelectric devices on standard office paper substrates. The devices are based on thin films of WS2, Te, and BP (P‐type semiconductors) and TiS3 and TiS2 (N‐type semiconductors), deposited by simply rubbing powder of these materials against paper. The thermoelectric properties of these semiconducting films revealed maximum Seebeck coefficients of (+1.32 ± 0.27) mV K−1 and (−0.82 ± 0.15) mV K−1 for WS2 and TiS3, respectively. Additionally, Peltier elements were fabricated by interconnecting the P‐ and N‐type films with graphite electrodes. A thermopower value up to 6.11 mV K−1 was obtained when the Peltier element were constructed with three junctions. The findings of this work show proof‐of‐concept devices to illustrate the potential application of semiconducting van der Waals materials in future thermoelectric power generation as well as temperature sensing for low‐cost disposable electronic devices.

Thermal transport properties of IrSbSe

We report a thermal transport study of IrSbSe, which crystallizes in a noncentrosymmetric cubic structure with the $P2_13$ space group and shows a narrow-gap semiconducting behavior. The large discrepancy between the activation energy for conductivity [$E_\rho$ = 128(2) meV] and for thermopower [$E_S$ = 17.7(9) meV] from 200 to 300 K indicates the polaronic transport mechanism. Electrical resistivity varies as $exp(T_0/T)^{1/4}$ and thermopower varies as $T^{1/2}$ at low temperatures, indicating that it evolves into the Mott's variable-range hopping dominant conduction with decreasing temperature. IrSbSe shows relatively low value of thermal conductivity ($\sim$ 1.65 W/K$\cdot$m) and thermopower of about 0.24 mV/K around 100 K, yet poor electrical conductivity. On the other hand, high vacancy defect concentration on both Ir and Sb atomic sites of up to 15\%, suggests high defect tolerance and points to possibility of future improvement of carrier density by chemical substitution or defect optimization.

Correlation between thermopower and carrier mobility in the thermoelectric semimetal Ta2PdSe6

We have investigated the transport properties of single crystals of a thermoelectric semimetal Ta2PdSe6 and its niobium-substituted (Ta1−xNbx)2PdSe6 by means of resistivity, thermopower, and Hall resistivity measurements. The residual resistivity ratio systematically decreases by the Nb-substitution, indicating enhanced impurity scattering. The slope and peak-top value of thermopower also systematically decrease upon increasing x. We have analyzed the set of transport data by using a two-carrier model, then revealed strong correlation between the thermopower and carrier mobility in the titled compound. We propose that controlling carrier mobility is a possible route to achieve a high-performance thermoelectric semimetal.

Skin-Deep Aspect of Thermopower in Bi2Q3, PbQ, and BiCuQO (Q = Se, Te): Hidden One-Dimensional Character of Their Band Edges Leading to High Thermopower.

ConspectusThermoelectric (TE) materials have received much attention because of their ability to convert heat energy to electrical energy. At a given temperature T, the efficiency of a TE material for this energy conversion is measured by the figure of merit zT, which is related to the thermopower (or Seebeck coefficient) S, the thermal conductivity κ, and the electrical conductivity σ of the TE material as zT = (S2σT)/κ. Bi2Q3 and PbQ (Q = Se, Te) are efficient TE materials with high zT, although they are not ecofriendly and their stability is poor at high temperature. In principle, a TE material can have a high zT if it has a low thermal conductivity and a high electrical conductivity, but the latter condition is hardly met in a real material because the parameters S, σ and κ have a conflicting dependence on material properties. The difficulty in searching for TE materials of high zT is even more exasperated because the relationship between the thermopower S and the carrier density n (hereafter, the S-vs-n relationship) for the well-known hole-doped samples of BiCuSeO showed that the hole carriers responsible for their thermopower are associated largely with the electronic states lying within ∼0.5 eV of its valence band maximum (VBM). Thus, the states governing the TE properties lie in the "skin-deep" region from the VBM. For electron-doped TE systems, the electron carriers responsible for their thermopower should also be associated with the electronic states lying within ∼0.5 eV of the conduction band minimum (CBM). This makes it difficult to predict TE materials of high zT. One faces a similar skin-deep phenomenon in searching for superconductors of high transition temperature because the transition from a normal metallic to a superconducting state involves the normal metallic states in the vicinity of the Fermi level EF. Other skin-deep phenomena in metallic compounds include the formation of charge density wave (CDW), which involves the electronic states in the vicinity of their Fermi levels. For magnetic materials of transition-metal ions, the preferred orientation of their spin moments is a skin-deep phenomenon because it is governed by the interaction between the highest-occupied and the lowest unoccupied d-states of these ions. In the present work we probe the issues concerning how to find the possible range of thermopower expected for a given TE material and hence how to recognize what experimental values of thermopower are expected or unusual. For these purposes, we analyze the accumulated S and n data on the three well-studied TE materials, Bi2Q3, PbQ, and BiCuQO (Q = Se, Te), as representative examples, in terms of the ideal theoretical S-vs-n relationships, which we determine for their defect-free Bi2Q3, PbQ, and BiCuQO structures using density functional theory (DFT) calculations under the rigid band approximation. We find that the general trends in the experimental S-vs-n relationships are reasonably well explained by the calculated S-vs-n relationships, and the carrier densities covering these relationships are associated with the states lying within ∼0.5 eV from their band edges confirming the skin-deep nature of their thermoelectric properties. Despite the fact that these TE materials are not one-dimensional (1D) in structure, they mostly possess sharp density-of-state peaks around their band edges because their band dispersions have a hidden 1D character so their thermopower is generally high in magnitude.

Colossal phonon drag enhanced thermopower in lightly doped diamond

Diamond is one of the most studied materials because of its unique combination of remarkable electrical, mechanical, thermal and optical properties. Using a fully self-consistent ab initio theory of coupled electron-phonon transport, we reveal another striking behavior: a huge drag enhancement of the thermopower of lightly doped diamond. Thermopower values of around 100,000 μV K−1 are found at 100 K, significantly exceeding the highest previously measured value in the correlated metal FeSb2, and occurring at much higher temperatures. The enormous thermopower in diamond arises primarily from exceptionally weak anharmonic phonon decay around and below 100 K that facilitates efficient momentum exchange between charge carriers and phonons through electron-phonon interactions. Exceedingly large thermoelectric power factors are also identified. This work gives insights into the physics of the coupled electron-phonon system in solids and advances our understanding of thermoelectric transport in the regime of strong drag.

Influence of Central-Metallic Atoms of Molecular Junction on Electrical and Thermal Transport

Molecular structures of a series of metal-complexes molecules of bipyrimdine have interesting features in various applications such as industrial and scientific. The transition through metal complexes at molecular structures has a significant role in the metal-bridges interactions. Hence, the bipyrimdine's complex molecules were used to show the effect of the metal atoms on the electronics, electrical, and thermoelectric properties and to identify the π-conjugated system in all attempts before/ or after replaced one carbon with nitrogen atoms in one or two the pyridine rings. SIESTA code, and GOLLUM code programs were used in our calculations. The results showed that the energy gap decreased significantly with the type of metal atoms, which appear in electronic applications at various LUMO-HOMO energy gaps. The electrical and thermal results have confirmed the effect of metal-complexes center atoms in the transmission and Seebeck coefficients. From the position of the Fermi energy (E-EF=0 eV), the conductance and the thermopower values are determined, and thus the specificity figure of merit ZTe was computed. Therefore, we found the ZTe be a high value for bipyridine molecule about 1.7, which is several order of magnitude higher than of the set molecules. Whereas a low value of ZTe for Cr-metal-molecule.

Thermoelectric properties plus phonon and de Haas\u2013van Alphen frequencies of hole/electron-doped hbox {CeIn}_3

We investigate temperature, pressure, and localization dependence of thermoelectric properties, phonon and de Haas–van Alphen (dHvA) frequencies of the anti-ferromagnetic (AFM) CeIn_3 using density functional theory (DFT) and local, hybrid, and band correlated functionals. It is found that the maximum values of thermopower, power factor, and electronic figure of merit of this compound occur at low (high) temperatures provided that the 4f-Ce electrons are (not) localized enough. The maximum values of the thermopower, power factor, electronic figure of merit (conductivity parameters), and their related doping levels (do not) considerably depend on the localization degree and pressure. The effects of pressure on these parameters substantially depend on the degree of localization. The phonon frequencies are calculated to be real which shows that the crystal is dynamically stable. From the phonon band structure, the thermal conductivity is predicted to be homogeneous. This prediction is found consistent with the thermal conductivity components calculated along three Cartesian directions. In analogous to the thermoelectric properties, it is found that the dHvA frequencies also depend on both pressure and localization degree. To ensure that the phase transition at Néel temperature cannot remarkably affect the results, we verify the density of states (DOS) of the compound at the paramagnetic phase constructing a non-collinear magnetic structure where the angles of the spins are determined so that the resultant magnetic moment vanishes. The non-collinear results reveal that the DOS and whence the thermoelectric properties of the compound are not changed considerably by the phase transition. To validate the accuracy of the results, the total and partial DOSs are recalculated using DFT plus dynamical mean-field theory (DFT+DMFT). The DFT+DMFT DOSs, in agreement with the hybrid DOSs, predict the Kondo effect in this compound.

Enhancement of high temperature thermoelectric performance of cobaltite based materials for automotive exhaust thermoelectric generators

Thermoelectric (TE) generators can directly convert exhaust waste heat into electricity in vehicles. However, the low conversion efficiency of TE generators is the main obstacle to their commercialization in automotive. Their efficiency mainly depends on the performance of the used materials which is quantified by the figure of merit (ZT value). In the present study, single- and co-doped calcium cobaltites (CCO) with rare-earth (Tb) and transition metals (Cu, Fe, Ni, Mn, Cr) were produced using sol–gel technique in order to improve their high temperature TE properties for heat recovery in exhaust manifold applications. By the combined effect of doping approach and the production technique used in this study, a remarkable decrease in the grain size of CCO was obtained, and thus its thermal conductivity dramatically decreased. Besides, thermopower values were improved significantly. The reduction in thermal conductivity and the increase in thermopower led to an enhancement in ZT value of CCO ceramics. Among all the co-doped samples, Tb–Cu co-doped CCO displayed the maximum ZT value of 0.116 at 873 K which is 2.5 times larger than that of pure CCO. The high thermal stability and the enhanced TE performance make Tb–Cu co-doped CCO material a potential candidate for heat recovery in automotive exhaust TE generators.

Thermoelectric Properties of Hot-Pressed Ba3Cu14-δTe12.

The aim of this study was to investigate the thermoelectric properties of hot-pressed Ba3Cu14-δTe12 as well as its stability with regards to Cu ion movement. For the latter, two single crystals were picked from pellets after they were measured up to 573 and 673 K, which showed no significant changes in the occupancies of any of the Cu sites. All investigated Ba3Cu14-δTe12 materials displayed low thermal conductivity values (<1 W m-1 K-1) and appropriate electrical conductivity values (300-600 Ω-1 cm-1). However, the thermopower values were comparably low (<+65 μV K-1), resulting in uncompetitive zT values, with the highest being achieved for Ba3Cu13.175Te12, namely zT = 0.12 at 570 K. In an attempt to decrease the thermal conductivity, and thereby enhance the figure of merit, a brief alloying study with Ag was undertaken. The incorporation of Ag, however, did not produce any significant improvements.

Optimal carrier concentration for FeSb2 colossal thermopower

Crystals of FeSb2 correlated narrow-gap semiconductor host colossal thermopower values. By tuning the impurity level here, we show that electron–phonon scattering that transfers phonon momentum to electrons is efficient only for certain optimal carrier concentration in the low-mobility band. Phonon drag acting on such states in crystals with high phonon mean free path enhances thermopower to colossal values, whereas for different carrier concentration, dominant thermal transport mechanism is electronic diffusion. This highlights the dual nature of correlated in-gap states that take part in the phonon drag but also reduce phonon mean free path.

Magnetic critical behavior and anomalous Hall effect in 2H−Co0.22TaS2 single crystals

We report ferromagnetism in 2H-Co$_{0.22}$TaS$_2$ single crystals where Co atoms are intercalated in the van der Waals gap, and a systematic study of its magnetic critical behavior in the vicinity of $T_c \sim 28$ K. The obtained critical exponents $\beta$ = 0.43(2), $\gamma$ = 1.15(1), and $\delta = 3.54(1)$ fulfill the Widom scaling relation $\delta = 1+\gamma/\beta$ and follow the scaling equation. This indicates that the spin coupling in 2H-Co$_{0.22}$TaS$_2$ is of three-dimensional Hersenberg type coupled with long-range magnetic interaction, and that the exchange interaction decays with distance as $J(r)\approx r^{-4.69}$. 2H-Co$_{0.22}$TaS$_2$ exhibits a weak temperature-dependent metallic behavior in resistivity and negative values of thermopower with dominant electron-type carriers, in which obvious anomalies were observed below $T_c$ as well as the anomalous Hall effect (AHE). The linear scaling behavior between the modified anomalous Hall resistivity $\rho_{xy}/\mu_0H$ and longitudinal resistivity $\rho_{xx}^2M/\mu_0H$ implies that the origin of AHE in 2H-Co$_{0.22}$TaS$_2$ should be dominated by the extrinsic side-jump mechanism.

Structural stability and thermoelectric properties of new discovered half-Heusler KLaX (X = C, Si, Ge, and Sn) compounds

Structural, elastic, electronic, and thermoelectric properties of KLaX (X = C, Si, Ge and Sn) half Heusler compounds have been studied using the full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). It is shown that the KLaX half Heusler compounds are energetically and mechanically stable. The calculated band structure estimates with modified Becke-Johnson (mBJ) potential, indicates a semiconducting nature with direct band gaps of 0.76 eV, 0.79 eV, and 0.81 eV for KLaC, KLaSn, and KLaX (X = Si and Ge) respectively. The valence band of KLaX is more influenced by spin-orbit coupling than in the conduction band. The Seebeck coefficient (S), electronic conductivity (σ), electronic thermal conductivity (κe), figure of merit (ZT) and the power factor (PF) have been investigated using the semi-classical Boltzmann transport theory with rigid band theory. The KLaX half Heusler compounds exhibit high thermopower values and p-type charge carriers for their thermoelectric performance than electron doping. Including the spin-orbit coupling in our calculations increases the flatness of the valence band from X = Si to Sn. The high values of ZT and PF of KLaX half-Heusler compounds make it promising materials for thermoelectric applications.