Maximum ZT Value Research Articles

Transport and thermoelectric properties of Hf-doped FeVSb half-Heusler alloys

Nearly single-phase FeVSb half-Heusler alloys with fine grains were obtained by induction melting followed by mechanical alloying (MA), spark plasma sintering (SPS) and annealing process. Hf as a heavy element was used as dopant to present point defects aiming at decreasing the material’s thermal conductivity during phonon scattering. Thermoelectric properties of the FeV1−xHfxSb samples (0.0 ≤ x ≤ 0.3) were investigated as a function of temperature in a range from 300 to 600 K. Microstructure investigations showed that grain size decreases with increasing the level of Hf doping (x). Seebeck coefficient of the parent FeVSb compound showed negative sign which refers to n-type conduction. Interestingly, the sign has changed to positive with introducing Hf in the FeVSb lattice at different concentrations (x ≥ 0.1), which is due to one less valence electron in Hf as compared to V. The difference between the host atom (V) and the impurity atom (Hf) in terms of mass and size has resulted in a mass fluctuation and consequently a disorder scattering. The absolute value of the Seebeck coefficient |S| of the FeVSb system was measured at 110 μV/K, while the thermal conductivity value was obtained at 10.46 Wm−1K−1 near room temperature. A maximum power factor of 1.07 mWm−1 K−2 at 420 K was recorded. The thermal conductivity decreased rapidly upon Hf doping due to increased point defect scattering caused by Hf introducing to the FeVSb system. A maximum ZT value of 0.08 at 573 K for FeV0.9Hf0.1Sb was recorded in this study.

Synergetic Tuning of the Electrical and Thermal Transport Properties via Pb/Ag Dual Doping in BiCuSeO

The influence of Pb/Ag dual doping on the thermoelectric performance of BiCuSeO was investigated. It reveals that the electrical conductivity can be obviously improved owing to the increased carrier concentration mostly caused by the divalent Pb2+ doping at trivalent Bi3+. The electrical conductivity improves from 7.29 S/cm for BiCuSeO to 397.40 S/cm for Pb0.06Bi0.94Cu0.94Ag0.06SeO and Pb0.06Bi0.94CuSeO at 327 K. Combined with the moderate Seebeck coefficient (>120 μV/K) due to the large effective mass, a large power factor with 834 μW/mK2 is achieved. Meanwhile, the lattice thermal conductivity is visibly decreased, mainly benefiting from the substitution of Ag+ at the Cu+ site since the phonon scattering can be enhanced by mass fluctuation and strain fluctuation between the two elements. Thus, the total thermal conductivity is suppressed effectively compared with the Ag-free sample Pb0.06Bi0.94CuSeO. Finally, a maximum ZT value with nearly 1.0 has been obtained for Pb0.06Bi0.94Cu0.94Ag0.06SeO at 873 K, which is ∼64% and ∼47% larger than those of the pristine sample BiCuSeO and Ag-free sample Pb0.06Bi0.94CuSeO. Additionally, the ZT is also larger than the maximum value for the reported Pb-free samples BiCu0.94Ag0.06SeO and Bi0.92Ag0.08CuSeO, suggesting Pb and Ag are effective codopants of BiCuSeO to synergetically tune its electrical and thermal transport properties.

Artificial porous structure: An effective method to improve thermoelectric performance of Bi2Te3 based alloys

Introduction of porous structure is a feasible and promising method for improving thermoelectric performances for thermoelectric materials. Meanwhile, the uniformity of pores distribution plays a decisive role in the objective evaluation of thermoelectric properties for porous structures. In this work, extremely uniform porous structures in p-type Bi0.4Sb1.6Te3 bulks were obtained by pre-mixing with NH4HCO3 combined with resistance pressing sintering (RPS) process. The thermoelectric performance was obviously affected by the porous structure. The increasing porosity resulted in a significant decrease in electrical and thermal conductivity. It was inspiring that the reduction of thermal conductivity could compensate for the deterioration of electrical conductivity, leading to the enhancement in ZT value. The maximum ZT value of 1.11 was obtained for the sample prepared by pre-mixing with 5 ​wt % NH4HCO3 at 343 ​K, which was 20% higher than that of the fully dense sample. Finally, the advantages on the homogeneity of pores, thermoelectric properties and mechanical properties of the sample prepared by this method are confirmed through comparing with the sample prepared without NH4HCO3, when their relative densities both are 90%.

Thermoelectric properties of Bi0.5Sb1.4−xNaxIn0.1Te3 alloys

The Bi0.5Sb1.4−xNaxIn0.1Te3 (x = 0.02–0.20) alloys were fabricated by high vacuum melting and hot-pressing technique. The phase structures and morphology of the bulk samples were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. Effects of In and Na co-doping on the electrical and thermal transport properties were studied from room temperature to 500 K. Seebeck coefficient of the Bi0.5Sb1.5Te3 can be enhanced by substituting Sb with In and Na at near room temperature. The electrical conductivity of the In and Na co-doped samples is lower than that of the Bi0.5Sb1.5Te3 alloy from room temperature to 500 K. In and Na co-doping of appropriate percentage optimizes the thermal conductivity of the Bi0.5Sb1.5Te3 alloy. The minimum value of thermal conductivity of Bi0.5Sb1.36Na0.04In0.1Te3 alloy is 0.45 W·m−1·K−1 at 323 K, which leads to a great improvement in the thermoelectric figure of merit (zT). The maximum zT value reaches 1.42 at 323 K.

Preparation and Characterization of Ni/Bi0.5Sb1.5Te3 Heterogeneous Multilayered Thermoelectric Materials

It is difficult to achieve coordinated optimization in thermoelectric materials due to the strong coupling between the electrical and thermal transport properties. However, interface effects, especially those caused by heterogeneous interfaces, are promising to overcome this challenge. In this work, the Ni/Bi0.5Sb1.5Te3 (BST) heterogeneous multilayer structure thermoelectric materials were fabricated by the combination of vacuum evaporation deposition and spark plasma sintering. The influence of Ni layer on the phase composition, microstructure and thermoelectric performance along the different directions (0°, 30°, 60° and 90°, the angles between the performance measurement direction and the Ni layer) of Ni/BST materials were systematically investigated. The microstructural analysis indicates that the distinct heterogeneous interfaces were firmly bonded, and the interface reaction layer was composed of Ni and Te. As compared with the matrix, the electrical conductivity and Seebeck coefficient of the Ni/BST heterogeneous multilayer thermoelectric materials increased, and the thermal conductivity slightly reduced. For Ni/BST90° sample, the maximum ZT value of 1.05 was achieved at 370 K, increased by 19.1% compared with the BST90°. Our work demonstrates that the electron and phonon transport properties can be simultaneously optimized by introducing the ordered heterogeneous interfaces.

Enhanced thermoelectric figure of merit of individual Si nanowires with ultralow contact resistances

Low-dimensional silicon-based materials have shown a great potential for thermoelectric applications due to their enhanced figure of merit ZT and high technology compatibility. However, their implementation in real devices remains highly challenging due to the associated large contact resistances (thermal and electrical). Herein we demonstrate ultralow contact resistance silicon nanowires epitaxially grown on scalable devices with enhanced ZT. Temperature dependent figure of merit was fully determined for monolithically integrated individual silicon nanowires achieving a maximum value of ZT = 0.2 at 620 K. Sidewise, this work accounts for the first time nearly zero thermal and electrical contact resistances in monolithically integrated bottom-up nanowires.

Enhancement of the Thermoelectric Properties of BiCuSeO via In Doping and Powder Size Controlling

BiCuSeO is regarded as one of the most promising oxygenated thermoelectric materials because of its special natural superlattice structure and corresponding ultra-low thermal conductivity. In order to improve the thermoelectric performance, we should pay attention to both composition adjustment and structure adjustment. In this paper, we report a study which combines the two ways to improve the electrical transporting performance and suppress the thermal transporting performance, and we have effectively improved the thermoelectric performance. Firstly, by adjusting the composition with In doping at the Bi site, the band gap widens, the energy offset between heavy band and light band increases, and the mobility and electrical transporting performance improves correspondingly. The maximum ZT value is increased to 0.61. Based on this, the thermoelectric properties are further improved in the whole temperature range by increasing the mechanical alloying strength to refine the powders and adjusting the proportion of fine powder and coarse powder. Grain refinement increases the Cu vacancies and corresponding electrical properties. In addition, Grain refinement contributes to enhancing phonon scattering and effectively reduces the lattice thermal conductivity. The thermal conductivity decreases significantly in the whole temperature range because grain refinement can significantly enhance phonon scattering and reduce the lattice thermal conductivity. The appropriate proportion of coarse powder and fine powder can make full use of the fine grains to enhance phonon scattering while enlarging the frequency range of scattered phonons, thus further reducing the thermal conductivity. Finally, the ZT value increased to 1.08.

Enhanced Mechanical Properties of Na0.02Pb0.98Te/MoTe2 Thermoelectric Composites Through in-Situ-Formed MoTe2.

Lead telluride (PbTe) is one of the best thermoelectric materials in the intermediate temperature range, which shows great potential for waste heat recycling. However, its low strength and high brittleness limit its large-scale application because the thermoelectric device usually undergoes mechanical vibration, mechanical and/or thermal cycling, and thermal shock in service. In this study, the enhanced mechanical properties and thermoelectric properties of PbTe are realized simultaneously through introducing dispersive transition-metal dichalcogenide MoTe2 (molybdenum telluride). The in-situ-formed MoTe2 precipitations with a size in the range from 2 to 5 μm and the tight and smooth interface between the PbTe matrix and precipitates contribute to the obvious crack deflection, crack bridging, and pull-out of long grains, dissipating more energy during crack propagation and resulting in a tortuous propagation path. Because of the toughening and the dispersion strengthening effect, the compressive strength, bending strength, and fracture toughness of the sample with a composite amount of 1% are 109 MPa and 50 MPa and 0.65 MPa·m1/2, respectively, which are increased by about 37, 117, and 67% compared to the Na0.02Pb0.98Te matrix. Additionally, the in situ MoTe2 precipitates intensify the interface phonon scattering and thus decrease the lattice thermal conductivity. As a result, the Na0.02Pb0.98Te-1%MoTe2 sample achieves a maximum ZT value of 1.46 at 700 K, which is 11% higher than that of Na0.02Pb0.98Te without any MoTe2 nanoprecipitation.

Lattice distortion and anisotropic thermoelectric properties in hot-deformed CuI-doped Bi2Te2·7Se0.3

We investigated anisotropic thermoelectric properties of (CuI)xBi2Te2·7Se0.3 (x = 0.0, 0.3, 0.6, and 0.9 mol.%) compounds, synthesized by the hot-press and hot-deformation process. In spite of polycrystalline compound, the hot-deformed compounds exhibit preferred orientation along the c-axis, parallel with the applied press direction. The sample of x = 0.3 mol.% shows the maximum power factor (3.8 mW m−1 K−2 at 300 K) and ZT value (0.97 at 423 K), which is relatively high thermoelectric performance in n-type thermoelectric materials as a mild-temperature operation. Notably, the in-plane lattice thermal conductivity of the x = 0.3% compound with covalent bonding layer has lower value than the one of out-of-plane lattice thermal conductivity with van der Waals bonding layer. From the high resolution transmission electron microscopy and electron diffraction measurements, we observe the lattice distortion of the x = 0.3% compound. Therefore, the unconventional anisotropic lattice thermal conductivity can be associated with the lattice distortion along the in-plane on the compound driven by the CuI doping.

Realization of n-type BiCuSeO through Co doping

We investigated the physical properties of the cobalt doped layered oxyselenides BiCu1-xCoxSeO. 5% Co doping led to a large increase in the Seebeck coefficient which should be attributed to the magnetic interactions with the magnetic moment carried by Co atoms. The carrier-type of the material is changed from p-type to n-type with 20% Co doping. We studied the thermoelectric property of Bi0.95Ca0.05Cu0.95Co0.05SeO. The dual-doping sample exhibits higher Seebeck coefficient than Ca single doped sample and the ZT was enhanced remarkably. A maximum ZT value of 0.11 was achieved for Bi0.95Ca0.05Cu0.95Co0.05SeO at 300 K.

Enhancement of thermoelectric properties of La-doped SrTiO3 bulk by introducing nanoscale porosity.

Electron-doped SrTiO3 is a well-known n-type thermoelectric material, although the figure of merit of SrTiO3 is still inferior compared with p-type metal oxide-based thermoelectric materials due to its high lattice thermal conductivity. In this study, we have used a different amount of the non-ionic surfactant F127 during sample preparation to introduce nanoscale porosities into bulk samples of La-doped SrTiO3. It has been observed that the porosities introduced into the bulk sample significantly improve the Seebeck coefficient and reduce the thermal conductivity by the charge carrier and phonon scattering respectively. Therefore, there is an overall enhancement in the power factor (PF) followed by a dimensionless figure of merit (zT) over a wide scale of temperature. The sample 20 at% La-doped SrTiO3 with 600 mg of F127 surfactant (SLTO 600F127) shows the maximum PF of 1.14 mW m−1 K−2 at 647 K which is 35% higher than the sample without porosity (SLTO 0F127), and the same sample (SLTO 600F127) shows the maximum value of zT is 0.32 at 968 K with an average enhancement of 62% in zT in comparison with the sample without porosity (SLTO 0F127).

Preparation and thermoelectric properties of Nd and Dy co-doped SrTiO3 bulk materials

To improve the thermoelectric properties of SrTiO3, Nd3+ and Dy3+ ions were doped in SrTiO3. Sr0.8Nd0.2TiO3 and Sr0.9Dy0.1TiO3 powders were prepared via a traditional solid state reaction, respectively. Two powders were mixed and followed the stoichiometric formula: Sr0.1x+0.8Dy0.1xNd0.2(1-x)TiO3 (x = 0, 0.3, 0.5, 0.7, 1). With the decreasing content of Dy, electrical conductivity of bulks is enhanced, but Seebeck coefficient decreases. The maximum electrical conductivity is 1435 S/cm at 423 K for Sr0.8Nd0.2TiO3 (x = 0) bulk. According to the Seebeck coefficient results, all samples are characterized by n-type semiconductor-like behavior. The maximum value of power factor is 1250 μW/mK2 at 573 K for Sr0.83Dy0.03Nd0.14TiO3 (x = 0.3) bulk. The maximum value of ZT, ZT = 0.19 at 673 K is obtained for Sr0.87Dy0.07Nd0.06TiO3 (x = 0.7) bulk with 7 mol% Dy and 6 mol% Nd, which is a result of the high power factor and low thermal conductivity.

Synergistically optimizing charge and phonon transport properties in n-type PbTe via introducing ternary compound AgSb(Se, Te)2

PbTe-based thermoelectric materials are considered to be a medium-temperature thermoelectric candidates and the high-performance n-type PbTe is urgently to be developed to match its p-type counterpart on account of practical application. Herein, we firstly optimized the carrier density of n-type PbTe through Sb doping, and obtained a maximum ZT value of ∼1.0. Then, based on the optimized carrier density, we introduced ternary compounds AgSb(Se, Te)2 to suppress the lattice thermal conductivity. Minimum lattice thermal conductivity of n-type PbTe was reduced as low as ∼0.53 W m−1K−1 and ∼0.62 Wm−1K−1 through introducing AgSbSe2 and AgSbTe2, respectively. Combining the maintained electrical properties with suppressed thermal conductivity, maximum ZT values ∼1.2 and ∼1.3 were obtained in PbTe+2%AgSbSe2 and PbTe+2%AgSbTe2, respectively. This work provides an effective strategy of synergistically balancing charge and phonon transports in n-type PbTe with ternary AgSb(Se, Te)2 compounds.

Predicting excellent anisotropic thermoelectric performance of the layered oxychalcogenides BiAgOCh (Ch = S, Se, and Te)

The layered oxychalcogenides BiCuOCh (Ch = S, Se, and Te) have gained extensive attention due to their low lattice thermal conductivity, moderate Seebeck coefficient, and tunable electrical conductivity. However, the thermoelectric properties of their Ag-based isomorphic compounds are still unclear. Here, by means of first‐principles calculations and Boltzmann transport theory, we identify that three materials exhibit intrinsically low lattice thermal conductivity. Because of large unit volumes and relatively low sound velocities, the lattice thermal conductivities for BiAgOS, BiAgOSe, and BiAgOTe are only 0.63, 0.58, and 0.49 W/mK, respectively. Combined with the analysis of electrical transport measurements results, the p-type BiAgOSe along a-direction shows preferable thermoelectric character with the maximum ZT value of 2.38 at 300 K. Besides, the ZT values for p-type BiAgOS and BiAgOTe at the same temperature along a-direction could reach to 1.75 and 1.90. Our results reveal that the family of oxychalcogenides BiAgOCh is potential p-type thermoelectric material. Especially, although n-type BiAgOCh cannot be a comparable thermoelectric material (ZT = 0.04–0.71), our results imply that optimum carrier concentration for n-type BiAgOS and BiAgOSe could be proved as an effective regulation strategy for relatively large ZT value. This is mainly due to the fact that ZT value has more than one extreme points from low carrier concentration (1 × 1018 cm−3) to high carrier concentration (1 × 1022 cm−3). Furthermore, this finding opens a new avenue for these oxychalcogenides compounds and significantly accelerates the process of the applications in oxides as potential high temperature thermoelectric materials.

Enhanced thermoelectric properties in chimney ladder structured Mn(BxSi1-x)1.75 due to the dual lattice occupation of boron

Due to its abundant, inexpensive, and nontoxic features, higher manganese silicide MnSi1.75 has drawn intensive attention as a promising middle temperature thermoelectric material. In this study, we applied aberration-corrected scanning transmission electron microscopy to characterize its chimney ladder crystal structure and found the particular contrast along the [110] direction, contributed from the enhancement of helical Si layers on Mn layers. Moreover, it is demonstrated that the Seebeck coefficient and electrical conductivity of MnSi1.75 could be simultaneously reinforced via boron doping, giving a peak power factor value of 1.72 mW m−1 K−2 at 723 K for Mn(B0.04Si0.96)1.75. Interestingly, it is indicated that boron would occupy both Si lattice and interstitial sites, resulting in an optimized charge carrier concentration and a reduced lattice thermal conductivity at the same time. A maximum zT value of ∼0.55 at 773 K and a high zTave value of 0.4 from 300 to 823 K were eventually realized in the Mn(B0.04Si0.96)1.75 sample.

New Sb2Te3-xSex Monolayers with High Electron Mobilities and Wide Absorption Range.

As a traditional thermoelectric material with high thermoelectric performance at room temperature, antimony telluride (Sb2Te3) has been widely used in energy applications like power generation and refrigeration. By employing the "atomic transmutation" method, three new kinds of Sb2Te3-based monolayers of α-Sb2Te2Se, α-Sb2TeSe2, and β-Sb2TeSe2 are designed, which are expected to possess a high thermoelectric performance due to the quantum-confinement effects. In this work, by using the ab initio calculations, we systematically study electronic structures, vibration modes, optical, transport, and thermoelectric properties for the three kinds of monolayers and find that they are indirect-band-gap semiconductor materials with chemical and thermodynamic stability up to 700 K (α-Sb2TeSe2) or 900 K (α-Sb2Te2Se and β-Sb2TeSe2). The band gaps are around 1.0 eV with five nearly degenerate peaks in valence bands. The three Sb2Te3-xSex monolayers possess high electron mobilities larger than 1000 cm2/(V s), and the maximum zT values of α-Sb2Te2Se/α-Sb2TeSe2/β-Sb2TeSe2 are 0.70/0.71/0.65 at 300 K, respectively. For optical properties, the three Sb2Te3-xSex monolayers possess a wide absorption range from the blue region to the ultraviolet region. Compared with Sb2Te3, the three new kinds of monolayers possess a wider range of absorptions, higher mobilities, and thermoelectric performances, which may lead to promising applications in thermoelectric devices and saturable absorbers.

Characterization of multiple-filled skutterudites with high thermoelectric performance

High performance multiple-filled X–CoSb3 skutterudites (X = Yb、Ca、Ba、Al、Ga、In、La、Eu) was prepared by the long term annealing and hot-pressing approaches. Compared with the one-element filling method, multi-elements filling in CoSb3 can improve their thermoelectric values. The lattice constant of skutterudite filled with multiple types of atoms was increased to 9.076 Å, higher than 9.036 Å of the un-filled one. The electrical conductivity, thermal conductivity and Seebeck coefficient were analyzed from 300K to 823K. The filling fraction of Yb increase with the increasing Fe content in both the Fe-rich phase and the Fe-poor phase. For Ca atoms, a significant phase separation occurs in the Fe-rich phase and the Fe-poor phase, and the Ca filling fraction in the Fe-poor phase is much higher. With the increase of Fe contents, the Al atoms gradually increase in the Fe-poor phase and decrease in the Fe-rich phase. On the contrary, the filled Ga and In atoms have no obvious regularity in this statistical analysis. Compared with the referred Yb0.5Co4Sb12 sample, the lattice thermal conductivity is effectively reduced by multi-filling, the multi-filled skutterudite obtained the 47% reduce of the lattice thermal conductivity and the maximum ZT value was reached 1.22 at 823K.

Largely Enhanced Seebeck Coefficient and Thermoelectric Performance by the Distortion of Electronic Density of States in Ge2Sb2Te5.

As one of the state-of-the-art phase-change materials, the stable Ge2Sb2Te5 hexagonal compound also exhibits decent thermoelectric performance with high electrical conductivity and low thermal conductivity. Nonetheless, the excessively high carrier concentration and low Seebeck coefficient are the bottlenecks to achieve high zT values. In this work, with the intention to optimize the electrical properties, indium was introduced as a potentially donor-like dopant in a series of Ge2-xInxSb2Te5 samples. The substitution of indium for germanium lowers the density of hole carriers and enhances the Seebeck coefficient. Noticeably, the room-temperature Seebeck coefficient of the doped samples can be three times as large as that of the pristine one, which obviously departures from the theoretically predicted Pisarenko relation based on the single parabolic band model. By virtue of DFT calculations and modeling, the remarkable enhancement of Seebeck coefficient was attributed to the doping-induced local distortion in the electronic density of states. Further insight reveals that indium doping amplifies the bonding character of Ge-Te adjacent to indium and enhances the atomic interaction along the c-axis. Due to the optimized electrical properties as well as the suppressed thermal conductivity, a maximal zT value of 0.78 was achieved in Ge1.85In0.15Sb2Te5 at 700 K, which is about 40% higher than that of the pristine sample.

First-principles insight on elastic, electronic, and thermoelectric transport properties of BAgX (X = Ti, Zr, Hf)

The geometrical structures of BAgX (X = Ti, Zr, Hf) are relaxed and their dynamical stability is confirmed by phonon dispersion curves, based on the first-principles density functional theory. Band energy structures, effective masses, relaxation time and carrier mobility of these structures are also obtained. The elastic constants and the deformation potential constants of each structure are calculated with the deformation potential theory. The lattice thermal conductivity is calculated by employing the Slack equation and elastic matrix. The electronic thermal conductivity, Seebeck coefficient, and power factors are obtained by solving the Boltzmann equation. Finally, the dimensionless figure of merit (ZT) is evaluated. The results demonstrate that a maximum ZT value of 0.8–1.2 could be achieved for the n-type BAgX, and the largest one is 1.2 for the n-type BAgZr at the carrier concentration value of 1.17 × 1020 with the temperature of 500 K, implying that the n-type BAgZr is a promising candidate for thermoelectric materials.

Ru Alloying Induced Enhanced Thermoelectric Performance in FeSi2-Based Compounds.

β-FeSi2 has been considered a promising material for thermoelectric applications, but its thermoelectric performance is greatly limited by the overhigh lattice thermal conductivity. In thermoelectrics, alloying an effective method to reduce the lattice thermal conductivity. In this study, we chose the Co-doped β-FeSi2 (Fe0.94Co0.06Si2) as the matrix and then prepared a series of Ru alloyed Fe0.94-xRuxCo0.06Si2 (x = 0, 0.005, 0.01, 0.02, and 0.05). X-ray characterizations show that all samples crystallize in the β-FeSi2 structure. The elemental mappings detect an inhomogeneous Ru distribution in Fe0.89Ru0.05Co0.06Si2, which is attributed to the different Ru solution contents in ε-FeSi and α-FeSi2+δ before the formation of β-FeSi2 and the slow diffusion behavior of Ru during the annealing process. The Ru-alloying obviously reduces the lattice thermal conductivity via introducing the mass and strain field fluctuations to interrupt the phonon transports, while it has a weak effect on electrical transport properties. Finally, a maximum zT value of 0.33 at 900 K has been obtained for Fe0.89Ru0.05Co0.06Si2, which is about 27% higher than that for Fe0.94Co0.06Si2.