Low Seebeck Coefficient Research Articles

Thermal and electrical transport conductivities of novel ordered double two-dimensional MXenes via density functional theory

In this paper, we examined the effects of transition metal replacement and ordering in 2D M3C2O2 MXene on the electronic structure and thermal and electrical conductivities using the density functional theory and also Boltzmann’s transport theory within the constant relaxation time approximation. We derived the following oxygen-functionalized ordered double-metal MXenes Ti2YC2O2, Y2TiC2O2, Sc2YC2O2, and Y2ScC2O2 through the surface and sub-layer tuning. The results revealed that the ordered MXenes give rise to different electronic structure and transport properties. The electronic thermal conductivities obtained in terms of the relaxation time increase with increase in absolute temperature, while the electrical conductivities decrease with increase in temperature. The respective transport properties of Y2TiC2O2 and Sc2YC2O2 were found to be enhanced due to the increased squared band velocity, density of state, and low Seebeck coefficient compared to Ti2YC2O2 and Y2ScC2O2 MXenes. Generally, the MXene materials investigated in this work were found to possess metallic properties with the existence of energy overlap around the Fermi level. The computed formation energies support chemical stability in all the materials.

Thermoelectric Properties of Cu-added Polycrystalline Bi2O2Se Oxyselenide

Bi2O2Se oxyselenide has been actively studied as a potentialn-type thermoelectric material because of its intrinsically low thermal conductivity and high Seebeck coefficient (S). However, Bi2O2Se has very low electrical conductivity (σ), resulting in relatively poor thermoelectric performance. Herein, we investigate the effect of Cu addition on the electrical and thermal transport ofn-type polycrystalline Bi2O2Se. A series of CuxBi2O2Se (x = 0, 0.0025, 0.005, and 0.0075) polycrystalline samples were synthesized by a conventional solidstate reaction. Tetragonal Bi2O2Se was successfully synthesized, and its lattice parameters gradually decreased with the addition of Cu. Further, σ decreased and the magnitude of S increased with increasing Cu content, according to the trade-off relationship between these parameters. Consequently, a maximum power factor of 0.106 mW m-1 K-2 was achieved for the sample with x = 0.0025 at 300 K, owing to the increase in the magnitude of S. The Hall carrier concentration decreased exponentially with the addition of Cu, which is mainly attributed to the possible enlargement of the band gap of the Cu-added samples. The lattice thermal conductivity decreased with increasing x, which was attributed to point-defect phonon scattering via Cu addition. Therefore, a maximumzT of 0.222 was obtained at 790 K for the Cu0.0025Bi2O2Se (x = 0.0025) sample, which was approximately 8% higher than that of the pristine Bi2O2Se sample.

Thermoelectrics of type-I and type-II nodal line semimetals within the two-band model

Metals and semimetals are often considered poor thermoelectric (TE) materials due to their lowSeebeck coefficients. However, we will show that topological semimetals in the class of nodal-linesemimetals (NLSs) may potentially exhibit better performance as TE materials. The NLSs are semimetalswith an intersection between the conduction band and valence band in the forms of a line (thus called thenodal line). We construct a two-band model using an almost-linear conduction and parabolic (or Mexicanhat) valence bands that overlap each other near the band edge to represent a type-I (or type-II) NLS. Wecalculate TE properties of the NLSs using the semiclassical Boltzmann transport theory and the relaxationtime approximation. By varying the band parameters in our model, we find that the type-II NLS generallyhas better TE performance than the type-I NLS. The type-II NLS, in particular, possesses a Seebeckcoefficient with a value possibly larger than twice that of normal metals. The origin of this feature mightbe the presence of a discontinuity in the density of states due to the intersection of the valence andconduction bands.

Modifying carbon fiber fabric for flexible thermoelectric energy conversion

Fibrous conductive materials, typically, carbon fiber fabric (CFF), with warp and weft weaved carbon-fiber (CF) structure, is one of the most desirable candidates for wearable electronics by playing the role as smart devices and sensors. However, CFF suffers from extremely low mobility and Seebeck coefficient, and has rarely been considered as a potential thermoelectric material. Here, a flexible and high-strength thermoelectric material comprising ordered Bi2Te3 crystals anchored on CF networks is reported, where there is a crystallographic relationship exists between the Bi2Te3 [001] orientation and CF axis. This material, has a power factor enhanced by 2 orders of magnitude, a fourfold tensile strength compared with CFF, and the highest flexibility figure of merit ever reported. The origin of the excellent mechanical properties and the thermoelectric performance of the Bi2Te3/CFF material is attributed, by experimental evidence, to its crystal orientation, interface, cross-linked core–shell structure. Our results provide insight into the design and fabrication of high-strength flexible thermoelectric materials.

Thermoelectric properties of a vertically aligned carbon nanotube array with embedded bismuth telluride

Recently, thermoelectric (TE) devices have attracted much attention because they have no moving parts, simple structures, high reliability, and environmental friendly, when compared to other green energy techniques. In this paper, we report a novel thermoelectric composite constructed one with a self-assembled highly oriented Sb doped Bi2Te3 and one without doping nanoflake layer deposited on regular vertically aligned checkerboard-patterned multi-walled carbon nanotube (MWCNT) arrays (500 nm squares and 1 µm pitch) on insulated SiO2/Si substrates. The height of Bi2Te3/Bi0.4Sb1.6Te3, MWCNTs and volumetric ratio of MWCNT to Bi2Te3/ Bi0.4Sb1.6Te3 are about 3 μm, 1.5 μm, and 25%, respectively. The blending of regular vertically aligned MWCNT patterns into Bi0.4Sb1.6Te3 results in a dramatically enhancement of Seebeck coefficient and electrical conductivity. The Seebeck coefficient and power factor of Bi0.4Sb1.6Te3-MWCNTs show a maximum value of 600 μV/K and 60 μW/cm-K2 at 160 K and gradually decrease to 409 μV/K, and about 14.1 μW/cm-K2 at 300 K, respectively. The significant records of the low temperature Seebeck coefficients and relative electrical properties are extremely important for the fundamental understanding of vertically aligned MWCNT embedded thermoelectric composites.

Synergistic optimizing thermoelectric performance of SnTe by the integrated Multi-strategy

SnTe has been regarded as a Lead-free substitution for the outstanding thermoelectrics PbTe, owing to their similar crystal and band structures. However, the performance of pristine SnTe is limited by its intrinsic high carrier concentration, high thermal conductivity and low Seebeck coefficient. In this work, the integrated multi-strategy, including band convergence, defect engineering and carrier manipulation, is applied to optimize the overall thermoelectric properties of SnTe. In practice, Cd-doping, Cu2Se nano-precipitate, and Sb-doping are introduced step by step to achieve the goals. Owing to the high efficiency of Cd-doping for band convergence and Cu2Se nano-precipitate for defect engineering, the obtained compounds exhibit the high carrier mobility, about 50 cm2 V−1 s−1. Finally, a high peak ZT of 1.52 at 833 K is achieved in (Sn0.86Cd0.04Sb0.1Te)0.96(Cu2Se)0.04, accompanied with a low thermal conductivity of 1.3 W m−1 K−1.

Sustainable Thermoelectric Materials Predicted by Machine Learning

AbstractUsing datasets from several sources, a list of more than 450 materials is generated and related them with their thermoelectric properties. This is obtained by generating a set of features using only the molecular formula. Subsequently, a machine learning algorithm classifies the materials in specific, binary classes, for example, possessing high or low Seebeck coefficients or electrical conductivity. After adjusting the threshold values and grouping the materials into clusters, the thermoelectric performance of more than 25k materials is predicted. Finally, the results are filtered to obtain only the sustainable materials, that is, neither toxic nor critical, (ideally) inexpensive, and isotropic with regard to their transport properties to simplify the preparation procedure.

Thermoelectric transport properties of n-type layered homologous (Bi2)m(Bi2Se3)n compounds

This work experimentally showed that the homologous heterostructure (Bi2)m(Bi2Se3)n compounds of Bi4Se3 (m = 1, n = 1), Bi8Se7 (m = 5, n = 7), BiSe (m = 1, n = 2), and Bi8Se9 (m = 1, n = 3) are promising TE materials, with low lattice thermal conductivity (<0.8 W m−1 K−1) due to the bismuth bilayer. Each compound with pure phase can be obtained by using the simple method of mechanical alloying and spark plasma sintering. All of them show a high electrical conductivity over 1000 S cm−1 and relatively low Seebeck coefficient below 100 μV K−1 due to the high carrier concentration. However, the electrical conductivity gradually decreases with reducing the ratio of m to n from Bi4Se3 (m = 1, n = 1) to Bi8Se7 (m = 5, n = 7), BiSe (m = 1, n = 2) and Bi8Se9 (m = 1, n = 3) due to the reduced proportion of the [Bi2] layers in per crystal cell, while reverse for the change of the Seebeck coefficient. Thus, a relative high power factor with ∼8.0 μW cm−1 K−2 were obtained for BiSe and Bi8Se9. Therefore, combining with the low thermal conductivity, a maximum ZT of ∼0.3 at 623 K was obtained for Bi8Se9. Higher ZT values can be expected by further properly optimizing their electrical transport property using doping or alloying.

Preparation and thermoelectric transport properties of BiSe and Sb-doped BiSe single crystals

<p indent="0mm">Exploring high thermoelectric performance in single crystal materials with intrinsic low lattice thermal conductivity has become an important field for thermoelectric research. BiSe, as a member of (Bi₂Se₃)_{<italic>m</italic>}(Bi₂)_{<italic>n</italic>} series superlattice compounds, has a very low intrinsic thermal conductivity due to the existence of Bi₂ bilayer in its layered crystal structure. Moreover, it is a new and promising thermoelectric material due to the non-toxic and environmentally friendly chemical compositions. The special layered structure of BiSe makes its thermoelectric transport properties significantly anisotropic. In this paper, BiSe single crystal was prepared by Bridgman method, and its anisotropic thermoelectric transport properties were investigated. The results show that the sample has higher conductivity and thermal conductivity in the in-plane direction, while the lower Seebeck coefficient makes the thermoelectric figure of merit <italic>ZT</italic> value in this direction lower than that in the interlamellar direction. The maximum values of <italic>ZT</italic> in the two directions are 0.11 <sc>(573 K)</sc> and 0.16 <sc>(623 K),</sc> respectively. Compared with polycrystalline BiSe, the single crystal sample exhibits lower electric transport performance, which can be attributed to the difference of scattering coefficient. The carrier concentration of BiSe single crystal is regulated by Sb doping, which effectively improves the Seebeck coefficient of the samples and optimizes the electrical transport performance. In the meanwhile, Sb doping can produce lattice distortion and reduce the lattice thermal conductivity. The final <italic>ZT</italic> value of Bi_0.8Sb_0.2Se single crystal increases by about 1 fold, reaching 0.34 <sc>(573 K)</sc> in the interlamellar direction and 0.20 <sc>(573 K)</sc> in the in-plane direction.

Highly Thermoelectric ZnO@MXene (Ti3C2Tx) Composite Films Grown by Atomic Layer Deposition.

Due to its unique high conductivity and flexibility, the two-dimensional MXene material (Ti3C2Tx) is expected to possess great potential in the thermoelectric field. However, the low thermoelectric performance from high thermal conductivity and a low Seebeck coefficient has limited its practical application. In this report, we demonstrate the uniform growth of ZnO layers on the laminar Ti3C2Tx membrane by atomic layer deposition (ALD). Benefiting from the low-temperature deposition characteristics of the ALD technique, the ZnO@Ti3C2Tx composite films maintain the basic apparent morphology of the original films after the deposition. We reveal that the Schottky barrier formed between ZnO and Ti3C2Tx exhibits an energy-filtering effect, significantly enhancing the Seebeck coefficient to result in more than a double increase in the power factor. Meanwhile, the strong phonon-interface scattering between ZnO and Ti3C2Tx is found to reduce the thermal conductivity of the composite films by a factor of four as compared to pure Ti3C2Tx ones, further improving the overall thermoelectric properties of the ZnO@Ti3C2Tx composite films. Our investigation provides an ALD-based strategy for growing wide band gap layers on the narrow band gap films to improve the thermoelectric performance of various MXene materials.

Ni doping and rational annealing boost thermoelectric performance of nanostructured double perovskite Pr1.8Sr0.2CoFeO6

Pr2CoFeO6-based double perovskite exhibits intrinsic low thermal conductivity and high Seebeck coefficient, but its low electrical conductivity induces unsatisfied thermoelectric performance. Here, we employ a synergistic strategy of rational annealing and Ni doping to achieve an improved ZT of ∼0.085 at 573 K in Pr1.8Sr0.2CoFe0.95Ni0.05O6, which is ∼425 % higher than that of pristine Pr2CoFeO6. The annealing process effectively reduces pores in the as-sintered bulk material, leading to an enhanced electrical conductivity without damaging the Seebeck coefficient, and the Ni doping further effectively improves the hole carrier concentration and in turn enhances the electrical conductivity, leading to a peak power factor of ∼113 μW m−1 K−2 at 573 K. Besides, Ni doping induces localized crystal imperfections including point defects, dislocations, and lattice distortions, contributing to an ultra-low thermal conductivity of <0.6 W m−1 K−1. As a result, the as-achieved Pr1.8Sr0.2CoFe0.95Ni0.05O6 has a ZT of ∼0.085 at 573 K, which is highly competitive to reported thermoelectric double perovskites, indicating its potential applications in high-performance thermoelectric devices.

Highly enhanced thermoelectric properties of Bi2S3 via (Se, Cl)-co doping in hydrothermal synthesis process

Bi2S3-based materials with low thermal conductivity and high Seebeck coefficient has been regarded as promising thermoelectric materials for years. To optimize the thermoelectric properties of Bi2S3 materials, (Se, Cl)-co-doped nanostructured Bi2S3 bulks were prepared by hydrothermal method combined with spark plasma sintering. The doping contents of Se and Cl in Bi2S3 depend on the reaction time of the hydrothermal process. A high power factor of 450 μWm−1 K−2 was achieved for Bi2S3 sample with the reaction time of 24 h owing to the substitution of S by Se and Cl. Meanwhile, a low thermal conductivity of 0.35 Wm−1 K−1 is obtained for the Bi2S3 sample with the reaction time of 18 h, owing to the strong phonon scattering caused by the high density of grain boundaries, pores and point defect. Therefore, benefiting from the improved electrical transport properties and reduced thermal conductivity, a peak ZT value of ~ 0.57 was obtained at 673 K for the Bi2S3 sample with the reaction time of 24 h.

Cement-Based Thermoelectric Device for Protection of Carbon Steel in Alkaline Chloride Solution

The thermoelectric cement-based materials can convert heat into electricity; this makes them promising candidates for impressed current cathodic protection of carbon steel. However, attempts to use the thermoelectric cement-based materials for energy conversion usually results in low conversion efficiency, because of the low electrical conductivity and Seebeck coefficient. Herein, we deposited polyaniline on the surface of MnO2 and fabricated a cement-based thermoelectric device with added PANI/MnO2 composite for the protection of carbon steel in alkaline chloride solution. The nanorod structure (70~80 nm in diameter) and evenly dispersed conductive PANI provide the PANI/MnO2 composite with good electrical conductivity (1.9 ± 0.03 S/cm) and Seebeck coefficient (−7.71 × 103 ± 50 μV/K) and, thereby, increase the Seebeck coefficient of cement-based materials to −2.02 × 103 ± 40 μV/K and the electrical conductivity of cement-based materials to 0.015 ± 0.0003 S/cm. Based on this, the corrosion of the carbon steel was delayed after cathodic protection, which was demonstrated by the electrochemical experiment results, such as the increased resistance of the carbon steel surface from 5.16 × 102 Ω·cm2 to 5.14 × 104 Ω·cm2, increased charge transfer resistance from 11.4 kΩ·cm2 to 1.98 × 106 kΩ·cm2, and the decreased corrosion current density from 1.67 μA/cm2 to 0.32 μA/cm2, underlining the role of anti-corrosion of the PANI/MnO2 composite in the cathodic protection system.

Band Modulation and Strain Fluctuation for Realizing High Average zT in GeTe

AbstractRecently, GeTe has emerged as a very promising thermoelectric material. However, the pristine GeTe exhibits intrinsically low Seebeck coefficient and high thermal conductivity. Therefore, resolving these issues is critical for further promoting its thermoelectric performance. Herein, PbTe and AgSbTe2 are chosen to form a solid solution with GeTe. Benefitting from the converged electronic bands and reduced thermal conductivity after AgSbTe2 alloying, the thermoelectric performance of GeTe‐based materials is effectively improved. In addition, PbTe alloying can optimize the carrier concentration and further improve the effective mass, and thus a remarkable enhancement in the Seebeck coefficient is realized. Moreover, intense phonon scattering occurs due to the introduced point defects at the Ge site, and the lattice thermal conductivity is effectively reduced to be as low as ≈0.47 W m−1 K−1. As a result, a high zT of ≈0.7 at 323 K and ≈2.4 at 723 K can be achieved in (Pb0.15Ge0.85Te)0.8(AgSbTe2)0.2, leading to a record‐high average zT of ≈1.8 and a high conversion efficiency of ≈14.8% under the temperature difference of 500 K.

High thermoelectric performance in metastable phase of silicon: A first-principles study

In this work, both thermal and electrical transport properties of diamond-cubic Si (Si-I) and metastable R8 phases of Si (Si-XII) are comparatively studied by using first-principles calculations combined with the Boltzmann transport theory. The metastable Si-XII shows one magnitude lower lattice thermal conductivity than stable Si-I from 300 to 500 K, attributed from the stronger phonon scattering in three-phonon scattering processes of Si-XII. For electronic transport properties, although Si-XII with smaller bandgap (0.22 eV) shows a lower Seebeck coefficient, the electrical conductivities of anisotropic n-type Si-XII show considerable values along the x axis due to the small effective masses of electrons along this direction. The peaks of the thermoelectric figure of merit (ZT) in n-type Si-XII are higher than that of p-type ones along the same direction. Owing to the lower lattice thermal conductivity and optimistic electrical conductivity, Si-XII exhibits larger optimal ZT compared with Si-I in both p- and n-type doping. For n-type Si-XII, the optimal ZT values at 300, 400, and 500 K can reach 0.24, 0.43, and 0.63 along the x axis at carrier concentrations of 2.6×1019, 4.1×1019, and 4.8×1019 cm−3, respectively. The reported results elucidate that the metastable Si could be integrated to the thermoelectric power generator.

Structural, morphological and thermoelectric properties of copper deficient and excessive Cu2-xS nanoparticles with (x=0-0.3)

Cu2-xS emerged as a potential thermoelectric material for use in various temperature ranges. The abundance, low price, the safety of its elements, and strong performance at relatively high temperatures are among its benefits. In this study, Cu2-xS (x= 0, 0.1, 0.2, and 0.3) nanoparticles are synthesized using a hydrothermal method. The effect of copper content on the structural and thermoelectric properties has been investigated. The copper atoms have changed the phase of the Cu2-xS crystal structure due to changes in the kinetic release rate of copper. The lower Seebeck coefficient and high electrical conductivity with increasing x value or decreasing copper concentration in the Cu2-xS matrix were observed. The copper vacancies created more charge carrier concentration. The maximum Seebeck coefficient value in the Cu2.0S sample is achieved by carrier localization. The Cu1.9S sample was found to have the best thermoelectric power generation performance, with a maximum power factor value of 1.72 × 10−3 Wm−1C−2.

Enhancing Thermoelectric Properties of (Cu2Te)1-x-(BiCuTeO)x Composites by Optimizing Carrier Concentration.

Because of the high carrier concentration, copper telluride (Cu2Te) has a relatively low Seebeck coefficient and high thermal conductivity, which are not good for its thermoelectric performance. To simultaneously optimize carrier concentration, lower thermal conductivity and improve the stability, BiCuTeO, an oxygen containing compound with lower carrier concentration, is in situ formed in Cu2Te by a method of combining self-propagating high-temperature synthesis (SHS) with spark plasma sintering (SPS). With the incorporation of BiCuTeO, the carrier concentration decreased from 8.1 × 1020 to 3.8 × 1020 cm−3, bringing the increase of power factor from ~1.91 to ~2.97 μW cm−1 K−2 at normal temperature. At the same time, thermal conductivity reduced from 2.61 to 1.48 W m−1 K−1 at 623 K. Consequently, (Cu2Te)0.95-(BiCuTeO)0.05 composite sample reached a relatively high ZT value of 0.13 at 723 K, which is 41% higher than that of Cu2Te.

Free-standing p-Type SWCNT/MXene composite films with low thermal conductivity and enhanced thermoelectric performance

Single-walled carbon nanotubes (SWCNTs) show great potential as flexible thermoelectric (TE) materials, however, the low Seebeck coefficients and high thermal conductivities restrict their applications in this field. In this work, taking the advantages of one-dimensional (1D) SWCNTs and two-dimensional (2D) MXenes, a series of three-dimensional (3D) hollow structured SWCNT/MXene films had been prepared for the first time as potent p-Type TE materials. First, compared to that of SWCNTs, the Seebeck coefficients of the composites can be increased due to energy-filtering effect at the SWCNT/MXene interfaces. Holes in the SWCNTs were compensated by electrons from MXenes, leading to a decreased hole concentration, which can also increase the Seebeck coefficients. Second, 3D hollow structures were formed in the composite films, which can decrease the thermal conductivity significantly. Third, contact junctions between SWCNTs and MXenes were formed in the composites, which were beneficial for electron transfer. As a result, at MXene loading of 10 wt%, SWCNT/MXene-10 displayed the highest TE performance. The Seebeck coefficient and ZT value is about 2 times and 20 times higher than that of the neat SWCNTs, respectively. At last, a TE device consists of 10 SWCNT/MXene-10 legs was prepared with a maximum output power of 1.54 µW at temperature difference of 117.3 K. This work provides a facile strategy in designing of SWCNT-based TE materials with high performances.

A novel high-entropy perovskite ceramics Sr0.9La0.1(Zr0.25Sn0.25Ti0.25Hf0.25)O3 with low thermal conductivity and high Seebeck coefficient

In this work, a novel high-entropy n-type thermoelectric material Sr0.9La0.1(Zr0.25Sn0.25Ti0.25Hf0.25)O3 with pure perovskite phase was prepared using a conventional solid state processing route. The results of TEM and XPS show that various types of crystal defects and lattice distortions, such as oxygen vacancies, edge dislocations, in-phase rotations of octahedron and antiparallel cation displacements coexist in this high-entropy ceramic. At 873 K, the high-entropy ceramics showed both a low thermal conductivity (1.89 W/m/K) and a high Seebeck coefficient (393 μV/K). This work highlights a way to obtain high-performance perovskite-type oxide thermoelectric materials through high-entropy composition design.

Feasibility of high performance in p‐type Ge1−xBixTe materials for thermoelectric modules

AbstractGeTe is a promising candidate for the fabrication of high‐temperature segments for p‐type thermoelectric (TE) legs. The main restriction for the widespread use of this material in TE devices is high carrier concentration (up to ∼ 1021 cm−3), which causes the low Seebeck coefficient and high electronic component of thermal conductivity. In this work, the band structure diagram and phase equilibria data have been effectively used to attune the carrier concentration and to obtain the high TE performance. The Ge1−xBixTe (x = 0.04) material prepared by the Spark plasma sintering (SPS) technique demonstrates a high power factor accompanied by moderate thermal conductivity. As a result, a significantly higher dimensionless TE figure of merit ZT = 2.0 has been obtained at ∼ 800 K. Moreover, we are the first to propose that application of the developed Ge1−xBixTe (x = 0.04) material in the TE unicouple should be accompanied by SnTe and CoGe2 transition layers. Only such a unique solution for the TE unicouple makes it possible to prevent the negative effects of high contact resistance and chemical diffusion between the segments at high temperatures.