Thermoelectric Figure Of Merit ZT Research Articles

Self-contained calibration samples and measurements of the thermoelectric figure of merit: A method to improve accuracy

Despite more than seven decades of active research and development in thermoelectricity, the accurate measurement of the thermoelectric (TE) properties of bulk materials has remained a challenge, mainly because of the strong interrelation between thermal and electrical phenomena. This work highlights practical advancements in methods and instrumentation dedicated to the simultaneous measurements of TE properties such as the Seebeck coefficient (S), the thermal (κ), and electrical (σ) conductivities and the dimensionless TE figure of merit ZT = S2σT/κ. The accuracy of a Harman based approach, as implemented by the ZT-Scanner (TEMTE Inc.), applicable to the simultaneous measurement of the above TE properties, has been made possible by a self-contained calibration procedure, which is based on the availability of two samples of the same homogeneous material having different shape factors. It is of practical importance that this approach provides a simple procedure to obtain the calibration for the figure of merit ZT and the thermal conductivity in the temperature interval from 300 to 720 K. In addition, we show that a simplified Harman setup with no thermocouples attached to the sample can also be used for self-contained calibrated ZT measurements. It is concluded that the implemented steady-state approach decreases the relative error down to 1%–2% for ZT measurements and can be recommended for most applications not involving dynamical behavior. In particular, it is proposed that self-generated calibration samples can critically increase the quality and ease of comparison of TE measurements if they are adopted by the TE community.

Thermoelectric properties of the bismuth oxychalcogenides Bi2SO2, Bi2SeO2 and Bi2TeO2

We present a detailed theoretical study of the thermoelectric properties of the bismuth oxychalcogenides Bi2ChO2 (Ch = S, Se, Te). The electrical transport is modelled using semi-classical Boltzmann transport theory with electronic structures from hybrid density-functional theory, including an approximate model for the electron lifetimes. The lattice thermal conductivity is calculated using first-principles phonon calculations with an explicit treatment of anharmonicity, yielding microscopic insight into how partial replacement of the chalcogen in the bismuth chalcogenides impacts the phonon transport. We find very good agreement between the predicted transport properties and a favourable cancellation of errors that allows for near-quantitative predictions of the thermoelectric figure of merit ZT. Our calculations suggest recent experiments on n-doped Bi2SeO2 have achieved close to the largest ZT possible in bulk materials, whereas the largest reported ZT for Bi2TeO2 could be improved sixfold by optimising the carrier concentration. We also predict that much larger ZT > 2.5, competitive with the benchmark thermoelectric SnSe, could be obtained for Bi2SO2 and Bi2SeO2 with heavy p-type doping. This study demonstrates the predictive power of this modelling approach for studying thermoelectrics and highlights several avenues for improving the performance of the Bi2ChO2.

The electron transport properties and thermoelectric performance in graphene–boron–nitride-nanoribbon-based heterojunctions

The electron transport properties and thermoelectric performance in graphene–boron–nitride-nanoribbon-based heterojunctions (GNR-BNN heterojunctions) have been studied by first-principles calculation. It is found that the electronic transport properties and thermoelectric performance properties are highly sensitive to the geometry details of the GNR-BNN heterojunction. Notably, our calculations reveal that the GNR-BNN heterojunctions not only can present obvious negative differential resistance effects, but also can present high thermoelectric figure of merit ZT: the figure-of-merit ZT will be over 1 at room temperature T = 300 K. Moreover, the thermoelectric performance is significantly enhanced by the combination of defects and alternating boron-nitride structures. Therefore, the GNR-BNN heterojunctions will provide the possibilities for designing and fabrication of multifunctional molecular device design.

The Substitution of Rare-Earth Gd in BaScCuTe3 Realizing the Band Degeneracy and the Point-Defect Scattering toward Enhanced Thermoelectric Performance.

Zintl compounds have continuously received significant attention, primarily due to their structural characteristics that align with the properties of the electron crystal and phonon glass. In this study, the crystal structure and thermoelectric properties of the quaternary Zintl chalcogenide BaScCuTe3 are investigated. The band structure calculations for BaScCuTe3 reveal a slight energy split of 0.08 eV between the second valence band and the valence band maximum, suggesting the presence of multiband-transport behaviors. Substitution of rare earth Gd for Sc is conducted, which significantly increases the hole concentration from 4.1 × 1019 cm-3 to 8.2 × 1019 cm-3 at room temperature. Meanwhile, the Seebeck coefficient increases because of the participation of the second valence band. A maximum power factor of 6.56 μW/cm·K2 at 773 K is obtained, which is 72% higher than that of the pristine sample. Moreover, the lattice thermal conductivity decreases from 0.57 W/m·K for BaScCuTe3 to 0.48 W/m·K for BaSc0.97Gd0.03CuTe3 at 773 K, owing to the introduction of point-defect scattering. As a result, there is a noteworthy improvement in the thermoelectric figure of merit zT, increasing from 0.44 for the undoped sample to 0.85 for BaSc0.98Gd0.02CuTe3. Considering these findings, BaScCuTe3 exhibits great potential and holds promise for further investigation in the field of thermoelectric materials.

Modeling critical thermoelectric transports driven by band broadening and phonon softening

Critical phenomena are one of the most captivating areas of modern physics, whereas the relevant experimental and theoretical studies are still very challenging. Particularly, the underlying mechanism behind the anomalous thermoelectric properties during critical phase transitions remains elusive, i.e., the current theoretical models for critical electrical transports are either qualitative or solely focused on a specific transport parameter. Herein, we develop a quantitative theory to model the electrical transports during critical phase transitions by incorporating both the band broadening effect and carrier-soft TO phonon interactions. It is found that the band-broadening effect contributes an additional term to Seebeck coefficient, while the carrier—soft TO phonon interactions greatly affects both electrical resistivity and Seebeck coefficient. The universality and validity of our model are well confirmed by experimental data. Furthermore, the features of critical phase transitions are effectively tuned. For example, alloying S in Cu2Se can reduce the phase transition temperature but increase the phase transition parameter b. The maximum thermoelectric figure of merit zT is pushed to a high value of 1.3 at the critical point (377 K), which is at least twice as large as those of normal static phases. This work not only provides a clear picture of the critical electrical transports but also presents new guidelines for future studies in this exciting area.

Crystal structure and thermoelectric properties of bulk polycrystalline (SbxBi1-x)2Se3 alloys

Samples of (SbxBi1-x)2Se3 compounds were synthesized from pure elements via simple melting in evacuated quartz tubes. Internal crystal structure, distribution of constituent elements and surface morphology were examined by X-ray diffraction analysis, energy dispersive x-ray spectroscopy and scanning electron microscope, respectively. Over a temperature range of 300–480 K, electrical and thermoelectrical properties were investigated. The electrical conductivity showed semi-metallic behavior against temperature increasing till a certain point, thereafter, exhibited semiconducting trend. A significant increase was observed due to Sb-doping. Seebeck coefficient exhibited notable increase as temperature increases, reaching its maximum. However, a serious decrease was observed with further temperature increasing. The most Sb-doped compound (Sb0.1Bi0.9)2Se3 showed a maximum power factor of 13.29 μW cm−1 K−2 at 373 K. Temperature dependence of the thermoelectric figure of merit ZT was studied, showing that the values increase with temperature increasing and then decrease. For the pristine Bi2Se3 compound, at room temperature, ZT increased from 0.014 to 0.68 at 363 K and then decreased again to 0.13 at 473 K. Doping with Sb at x = 0.1 resulted in remarkable increase in the ZT values over the entire temperature range. The improvement in electrical conductivity and the power factor are responsible for the greatly enhanced ZT throughout the whole temperature range. The maximum ZT was found at 0.87 for (Sb0.1Bi0.9)2Se3 at 373 K.

Thermoelectric properties of cement mortar doped with steel fiber under heating conditions

In order to supply the multifunctional demand for building materials in modern construction industry, research on the thermoelectric properties of cement mortar mixed with steel fibers (diameter 0.22 mm) for engineering purposes has been carried out in this paper. The self-built thermoelectric response test system is used to collect the thermoelectric power, surface temperature and conductivity of the cement-based composite specimens in real time. Based on the comparative measurement method, the thermal conductivity of large-sized specimens was obtained. The effect of different SF content on thermal conductivity, Seebeck coefficient and ZT of the specimens were investigated. The results show that the specimens doped with steel fiber is p-type thermoelectric material, and the maximum value of thermoelectric figure of merit ZT of the cement mortar doped with 7.5 wt% steel fiber at 50℃ is 0.175. The ZT of cement mortar specimens mixed with steel fibers is relatively low, but it has good application prospects as a low-cost and widely used building material.

First-principles modelling of the thermoelectric properties of n-type CaTiO3, SrTiO3 and BaTiO3

We benchmark a fully ab initio modelling workflow for predicting the electrical and thermal transport properties and thermoelectric figure of merit ZT on three oxide perovskites, viz. orthorhombic CaTiO3, tetragonal SrTiO3 and rhombohedral BaTiO3.

High performance thermoelectric composites via scalable and low-cost ink processing

Despite significant increases in thermoelectric Figure of merit zT achieved in the past two decades, the lack of scalable and low-cost device manufacturing methods has remained a major barrier for...

Weak electron-phonon coupling contributing to enhanced thermoelectric performance in n-type TiCoSb half-Heusler alloys

n-type TiCoSb half-Heusler (HH) alloys show lower thermoelectric performance may be notably owing to the low valley degeneracy of the conduction band. Here, we show that this drawback can be counterbalanced by a decrease in the deformation potential coefficient and hence, weak electron-phonon coupling strength driven by the substitution of Nb for Ti in the n-type alloys Ti1-xNbxCoSb0.96Bi0.04. The combined substitutions of Nb and Bi yield a reduction of ∼86% in the deformation potential of the conduction band minimum, with the lowest value of ∼5 eV at 300 K achieved in Ti0.85Nb0.15CoSb0.96Bi0.04. This effect leads to enhanced power factor from ∼0.018 mW m−1 K−2 for x = 0 to ∼1.69 mW m−1 K−2 for x = 0.15 at RT due to the resulting strong increase in electron mobility by one order of magnitude. Both the isovalent Bi and aliovalent Nb substitutions further contribute to decrease the lattice thermal conductivity owing to enhanced mass and strain field fluctuations. The beneficial combined effects of weaker electron-phonon coupling and enhanced point-defect phonon scattering results in a higher dimensionless thermoelectric figure of merit ZT, with a peak value ∼ 0.37 at 870 K in Ti0.85Nb0.15CoSb0.96Bi0.04, representing a ∼ 375% improvement with respect to pristine TiCoSb.

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.

Combining direct ink writing with reactive melt infiltration to create architectured thermoelectric legs

We present a new additive-reactive synthesis method where inks – cast into molds or 3D-additively extruded into architectured shapes – are reacted into intermetallic thermoelectric compounds. The new method, as demonstrated for equiatomic TiNiSn, combines: (i) extrusion printing (or casting) of inks containing Ni and Ti powders, (ii) debinding and reactive sintering to form a porous NiTi network, (iii) network infiltration with liquid Sn and subsequent reaction to synthesize the TiNiSn phase. Thin plates, created through this method, show high phase purity and low residual porosity. A thermoelectric figure of merit zT = 0.47 ± 0.05 is achieved at 800 K, within the broad range of values (zT = 0.3–1.0) reported for TiNiSn createdviatraditional powder hot-pressing methods. Layered TiNiSn microlattices – after 3D ink-extrusion printing and infiltration – exhibit high relative densities and minimal undesirable secondary phase content, with a Seebeck coefficient on par with the ink-cast TiNiSn plates, demonstrating that thermoelectric legs with far-ranging architectural freedom can be created additively with this novel reactive manufacturing method. Numerous other thermoelectric (and other) compounds, currently limited to basic geometries due to brittleness, are amenable to this new method based on a sequence of reactive sintering/infiltration of cast or ink-extruded precursors.

Thermoelectric Properties of High‐Entropy Wolframite Oxide: (CoCuNiFeZn)1−xGaxWO4

Herein, the synthesis of high‐entropy wolframite oxide (CoCuNiFeZn)1‐xGaxWO4 through standard solid‐state route followed by spark plasma sintering and their structural, microstructural, and thermoelectric (TE) properties are investigated. X‐ray diffraction pattern followed by patterns matching refinement shows a monoclinic structure with the volume of the unit cell decreasing with increasing Ga content. The optical bandgap for these oxides shows a cocktail effect in high‐entropy configuration. The Seebeck coefficient indicates electrons as dominating charge carriers with a nondegenerate behavior. The electrical resistivity decreases with increasing temperature depicting a semiconducting nature. Thermal conductivity in high‐entropy samples (κ ≈ 2.1 W m−1 K−1 @ 300 K) is significantly lower as compared to MgWO4 (κ ≈ 11.5 W m−1 K−1 @ 300 K), which can be explained by the strong phonon scattering due to large lattice disorder in high‐entropy configuration. The TE figure of merit zT increases with Ga doping via modifying all three TE parameters positively.

Tweaking the interplay of the layers by substitution in Ca3Co4O9+δ for high efficiency thermoelectric materials

Ca3Co4O9 is a well-known high temperature thermoelectric material owing to its sandwich type layered structure consisting of both conducting and insulating slabs. In the present study we attempt to enhance the electrical conductivity without affecting the thermal conductivity by suitable doping in these layers. Compositions with a general formula Ca3−xSrxCo4−xMgx/2Tix/2O9+δ (0 ≤ x ≤ 0.15) were prepared by conventional solid state method and subjected to routine characterization techniques like XRD, SEM and XPS. Sr substitution in the insulating layer and Mg, Ti substitution in the conductive layer led to a decrease in the electrical resistivity from 17.86 mΩ cm to 9.74 mΩ cm, which resulted in an increase in the power factors. The dimensionless thermoelectric figure of merit ZT is found to be 0.13 at 873 K for x = 0.15, which is ̴2.1 times higher than the parent compound attributed to the excess oxygen in the doped composition.

General Figures of Merit ZQ for Thermoelectric Generators Under Constant Heat-In Flux Boundary.

The thermoelectric figure of merit ZT bridges the efficiency and material parameters for a thermoelectric device operating under constant temperature of the hot- and cold-source thermal boundary (Type-I TB). However, many application scenarios fall under the constant heat-in flux (qh ) and constant cold-source temperature (Tc ) thermal boundary (Type-II TB), for which a figure of merit is absent for more than half a century. This study aims to fill this gap and propose a figure of merit ZQD for the thermoelectric devices under the Type-II TB condition, defined as , where Z, h, and κ are the traditional figure of merit, leg height, and thermal conductivity, respectively. The effectiveness of ZQD is verified through both numerical calculations and experiments, which are more accurate and practical than ZT. Furthermore, a system-level figure of merit ZQS is suggested after considering the external thermal resistance. Finally, optimization strategies for thermoelectric systems based on ZQS are proposed, showing a 30% enhancement in the efficiency. ZQD and ZQS are expected to be widely used in the thermoelectric field.

Decreasing the Carrier Concentration of ZrNiSn: An Opposite Way to the Best N-Type Half-Heusler Thermoelectrics.

N-type ZrNiSn-based alloys reach a record thermoelectric figure of merit zT ≈1.2 by increasing the carrier concentration to 4-5 × 1020 cm-3 . In this work, It is reported that a comparable zT can also be realized in trace Ru-doped ZrNiSn-based alloy at even lower temperature by decreasing the carrier concentration. Compared to the previously reported Co doping, the doping of Ru results in a more effective reduction in carrier concentration, and thus higher Seebeck coefficient, lower electronic thermal conductivity, and enhanced thermoelectric performance. The electronic specific heat coefficient of the ZrNi1- x Rux Sn sample remains constant with increasing Ru content, indicating no obvious change in the density of states effective mass. Theoretical calculations show that the doping of Ru has negligible effect on the bottom of conduction band. The lattice thermal conductivity is further reduced by alloying Ti and Hf at the Zr site, and the bipolar diffusion is suppressed by doping of 0.5 at.% Sb. As a result, Ti0.25 Zr0.5 Hf0.25 Ni0.99 Ru0.01 Sn0.995 Sb0.005 reaches not only a zT value of 1.1 at 773 K but also a record average zT value of 0.8 in 300 to 873 K, demonstrating the effectiveness of trace Ru doping on boosting the thermoelectric performance of ZrNiSn-based alloys.

Vacancy Suppression Induced Synergetic Optimization of Thermoelectric Performance in Sb-Doped GeTe Evidenced by Positron Annihilation Spectroscopy.

Synergetic optimization of the electrical and thermal transport performance of GeTe has been achieved through Sb doping in this work, resulting in a high thermoelectric figure of merit ZT of 2.2 at 723 K. Positron annihilation measurements provided clear evidence that Sb doping in GeTe can effectively suppress the Ge vacancies, and the decrease of vacancy concentration coincides well with the change of hole carrier concentration after Sb doping. The decreased scattering by hole carriers and vacancies causes notable increase in carrier mobility. Despite this, the density of states effective mass is not enhanced by Sb doping, a maximum power factor of 4562 μW m-1 K-2 at 723 K is obtained for Ge0.94Sb0.06Te with an optimized carrier concentration of ∼3.65 × 1020 cm-3. Meanwhile, the electronic thermal conductivity κe is reduced because of the decreased electrical conductivity σ with the increase of the Sb doping amount. In addition, the lattice thermal conductivity κL is also suppressed due to multiple phonon scattering mechanism, such as the large mass and strain fluctuations by the substitution of Sb for Ge atoms, and also the unique microstructure including grain boundary, nano-pore, and dislocation in the samples. In conclusion, a maximum ZT of 2.2 is gained at 723 K, which contributes to preferable TE property for GeTe-based materials.

External magnetic field effects on the thermoelectric and thermodynamic properties of doped monolayer S-graphene: A theoretical study

This study explores the electronic, thermoelectric, and thermal properties of S-Graphene, a unique graphene allotrope with two Dirac cones and zero band gap, in the presence of magnetic field and doping. A full tight-binding model and Green function approach are utilized to study the effects of robustness, magnetic field, and doping on the thermoelectric properties of S-Graphene, including the heat capacity C(T), magnetic susceptibility χ(T), thermoelectric figure of merit ZT(T), and Seedbeck coefficients S(T) and the results are compared with those for T-Graphene. To including the robustness effects, four S1–S4 structures with different hopping parameters are selected. The results demonstrate that the temperature dependent of thermal properties of S-Graphene are strongly affected by the external parameters such as external field, chemical potential and Robustness effects. The C(T) exhibits a Schottky anomaly whose position is dependent on the magnetic field and doping parameters. Pure cases demonstrate minimum values for C(T) and χ(T), while electron/hole doping leads to increase their intensity due to enhanced density concentration of charge carriers. Additionally, the behavior of ZT(T) and S(T) depends on the structure type of S-Graphene and displays intriguing features in different temperature regions. The magnetic field significantly affects the number, position and intensity of peaks in S(μ). These findings provide important insights into the electronic, thermoelectric, and thermal properties of S-Graphene and offer possibilities for future device applications.

Phase formation and thermoelectric properties of FeSe2–CoSe2 system

The phase formation and thermoelectric transport properties of FeSe2–CoSe2 systems were investigated by synthesizing a series of (Fe1–xCox)Se2 polycrystalline alloys (x = 0, 0.25, 0.5, 0.75, and 1). It was observed that as x increased from 0 to 0.5, lattice parameters increased while the orthorhombic structure of FeSe2 was retained. The bipolar conduction behavior of FeSe2 (x = 0) changed to n-type conduction at all measured temperatures. The electrical conductivity is greatly increased with x, and the largely enhanced power factor of 1.37 mW/mK2 at 600 K is seen for Fe0·5Co0·5Se2 (x = 0.5) compared to 0.37 mW/mK2 for FeSe2. As x is further increased to 0.75 (Fe0·25Co0·75Se2), the cubic phase of CoSe2 started to form and no further enhancement of power factor is seen. Lattice thermal conductivity is gradually decreased to 3.18 and 1.79 W/mK Fe0·75Co0·25Se2 (x = 0.25) and Fe0·5Co0·5Se2 (x = 0.5) at 600 K, compared to 4.36 W/mK for x = 0 (FeSe2). Consequently, the thermoelectric figure of merit zT at 600 K for Fe0·75Co0·25Se2 (x = 0.25) and Fe0·5Co0·5Se2 (x = 0.5) is greatly enhanced to 0.14 and 0.16, respectively, compared to 0.05 for x = 0 (FeSe2).

Structure Evolution and Bonding Inhomogeneity toward High Thermoelectric Performance in Cu2CoSnS4-xSex Materials.

Lightweight diamond-like structure (DLS) materials are excellent candidates for thermoelectric (TE) applications due to their low costs, eco-friendly nature, and property stability. The main obstacles restricting the energy-conversion performance by the lightweight DLS materials are high lattice thermal conductivity and relatively low carrier mobility. By investigating the anion substitution effect on the structural, microstructural, electronic, and thermal properties of Cu2CoSnS4-xSex, we show that the simultaneous enhancement of the crystal symmetry and bonding inhomogeneity engineering are effective approaches to enhance the TE performance in lightweight DLS materials. Particularly, the increase of x in Cu2CoSnS4-xSex makes the DLS structure with the ideal tetrahedral bond angles of 109.5° favorable, leading to better crystal symmetry and higher carrier mobility in samples with higher selenium content. In turn, the phonon transport in the investigated DLS materials is strongly disturbed due to the bonding inhomogeneity between anions and three sorts of cations inducing large lattice anharmonicity. The increase of Se content in Cu2CoSnS4-xSex only intensified this effect resulting in a lower lattice component of the thermal conductivity (κL) for Se-rich samples. As a result of the enhanced power factor S2ρ-1 and the low κL, the dimensionless thermoelectric figure of merit ZT achieves a high value of 0.75 for Cu2CoSnSe4 DLS material. This work demonstrates that crystal symmetry and bonding inhomogeneity play an important role in the transport properties of DLS materials and provide a path for the development of new perspective materials for TE energy conversion.