Multiple Roles of Unconventional Heteroatom Dopants in Chalcogenide Thermoelectrics: The Influence of Nb on Transport and Defects in Bi2Te3 (original) (raw)
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Advanced Electronic Materials, 2021
Bismuth chalcogenides are promising materials for thermoelectric (TE) application due to their high power factor (product of the square of the Seebeck coefficient and electrical conductivity). However, their high thermal conductivity is an issue of concern. Single doping has proven to be useful in improving TE performance in recent years. Here, it is shown that dual isovalent doping shows the synergistic effect of thermal conductivity reduction and electron density control. The insertion of large atoms in the layered Bi2Te3 structure distorts the crystal lattice and contributes significantly to phonon scattering. The ultralow thermal conductivity (KT = 0.35 W m−1 K−1 at 473 K) compensates for the low power factor and thus enhances TE performance. The density functional theory electronic structure calculation results reveal deep defects states in the valence band, which influences the electronic transport properties of the system. Therefore, the dual dopants (indium and antimony) sho...
Energy Conversion and Management, 2017
We report an improved thermoelectric performance of n-type Bi 2 Se 3 bulk alloys synthesized by vacuum melt method followed by vacuum hot-pressing. In the samples so prepared, the synergetic combination of ultra low thermal conductivity ($0.7 W/m K), high Seebeck coefficient ($À168 lV/K), and low electrical resistivity ($15 lX-m) has been observed to successfully lead to a high figure-of-merit (ZT) of $0.96 at 370 K. A detailed characterization of the samples reveals a presence of multiscale hierarchical defect structures i.e. atomic scale disorder arising from a multitude of factors such as large anharmonicity of Bi-Se bond due to electrostatic repulsion between the lone pair of Bi and charge of Se, nanoscale grains and dislocations trapped between mesoscale grains/grain boundaries accompanied by intrinsic layered structure of Bi 2 Se 3. This compact layered grain structure in its consequence offers a high charge carrier mobility and thereby results into a high power factor, while multiscale hierarchical architecture accounts for the scattering of a wider spectrum of phonons leading to an ultra low thermal conductivity. In view of this promising thermoelectric performance together with the presence of copiously available constituent namely Se, the hot-pressed Bi 2 Se 3 presents a technologically suitable and commercially viable alternative to the conventional Bi 2 Te 3 which is based on expensive and scarcely available Te.
Enhancement of thermoelectric properties in CuI-doped Bi2Te2.7Se0.3 by hot-deformation
Journal of Alloys and Compounds, 2018
Nanostructured Ni doped Bi 2 S 3 (Bi 2−x Ni x S 3 , 0 ≤ x ≤ 0.07) is explored as a candidate for telluride free thermoelectric material, through a combination process of mechanical alloying with subsequent consolidation by cold pressing followed with a sintering process. The cold pressing method was found to impact the thermoelectric properties in two ways: (1) introduction of the dopant atom in the interstitial sites of the crystal lattice which results in an increase in carrier concentration, and (2) introduction of a porous structure which reduces the thermal conductivity. The electrical resistivity of Bi 2 S 3 was decreased by adding Ni atoms, which shows a minimum value of 2.35 × 10 −3 Ω m at 300 °C for Bi 1.99 Ni 0.01 S 3 sample. The presence of porous structures gives a significant effect on reduction of thermal conductivity, by a reduction of ~ 59.6% compared to a high density Bi 2 S 3. The thermal conductivity of Bi 2−x Ni x S 3 ranges from 0.31 to 0.52 W/m K in the temperature range of 27 °C (RT) to 300 °C with the lowest κ values of Bi 2 S 3 compared to the previous works. A maximum ZT value of 0.13 at 300 °C was achieved for Bi 1.99 Ni 0.01 S 3 sample, which is about 2.6 times higher than (0.05) of Bi 2 S 3 sample. This work show an optimization pathway to improve thermoelectric performance of Bi 2 S 3 through Ni doping and introduction of porosity.
Journal of Applied Physics, 2019
We use first-principles calculations to reveal the effects of divalent Pb, Ca, and Sn doping of Bi 2 Te 3 on the band structure and transport properties, including the Seebeck coefficient, α, and the reduced power factor, α 2 σ/τ, where σ is the electrical conductivity and τ is the effective relaxation time. Pb and Ca additions exhibit up to 60%-75% higher peak α 2 σ/τ than that of intrinsic Bi 2 Te 3 with Bi antisite defects. Pb occupancy and Ca occupancy of Bi sites increase σ/τ by activating high-degeneracy low-effective-mass bands near the valence band edge, unlike Bi antisite occupancy of Te sites that eliminates near-edge valence states in intrinsic Bi 2 Te 3. Neither Pb doping nor subatomic-percent Ca doping increases α significantly, due to band averaging. Higher Ca levels increase α and diminish σ, due to the emergence of a corrugated band structure underpinned by high-effective-mass bands, attributable to Ca-Te bond ionicity. Sn doping results in a distortion of the bands with a higher density of states that may be characterized as a resonant state but decreases α 2 σ by up to 30% due to increases in the charge carrier effective mass and decreases in both spin-orbit coupling and valence band quasidegeneracy. These results, and thermal conductivity calculations for nanostructured Bi 2 Te 3 , suggest that Pb or Ca doping can enhance the thermoelectric figure of merit ZT to values up to ZT ∼ 1.7, based on an experimentally determined τ. Our findings suggest that divalent doping can be attractive for realizing large ZT enhancements in pnictogen chalcogenides. Published under license by AIP Publishing. https://doi.
Nano Letters, 2010
The peak dimensionless thermoelectric figure-of-merit (ZT) of Bi 2 Te 3 -based n-type single crystals is about 0.85 in the ab plane at room temperature, which has not been improved over the last 50 years due to the high thermal conductivity of 1.65 W m -1 K -1 even though the power factor is 47 × 10 -4 W m -1 K -2 . In samples with random grain orientations, we found that the thermal conductivity can be decreased by making grain size smaller through ball milling and hot pressing, but the power factor decreased with a similar percentage, resulting in no gain in ZT. Reorienting the ab planes of the small crystals by repressing the as-pressed samples enhanced the peak ZT from 0.85 to 1.04 at about 125°C, a 22% improvement, mainly due to the more increase on power factor than on thermal conductivity. Further improvement is expected when the ab plane of most of the small crystals is reoriented to the direction perpendicular to the press direction and grains are made even smaller.
Journal of Alloys and Compounds, 2018
is p-type and narrow band gap semiconductor, but realization of hole transport is very challenging because of dominant electron donor Te vacancies and Te Bi anti site defects. In the present study, we report on the effect of Ge incorporation on structural and thermoelectric properties of Bi 2 Te 3. Carrier density (N p) of Bi 2 Te 3 increases by one order with Ge doping, revealing formation of acceptor states, consequently improving the electrical conductivity (σ). The p-type thermopower (α) enhances providing a significant gain in power factor (α 2 σ) from 0.32 x 10-3 Wm-1 K-2 to 2.52 x 10-3 Wm-1 K-2 for Bi 2 Te 3 to Bi 1.95 Ge 0.05 Te 3, respectively. Here, Ge being tetravalent contributes an extra hole to the Bi 2 Te 3 and leading to optimized, N p for improved thermoelectric properties. Thus, room temperature thermoelectric figure of merit ZT~0.95 have been achieved for Bi 1.95 Ge 0.05 Te 3. Additionally, the applicability of prepared Ge doped Bi 2 Te 3 materials for segmented thermoelectric devices has been discussed.
Structural, electronic and thermoelectric properties of PbTe-based chalcogenide compounds
2019
The basic physical properties of La2CuBiS5 are studied by the first-principle calculations and the semiclassical Boltzmann theory. Charge density difference calculations show that electrons accumulate between Bi-S atoms, indicating considerable covalent bonding of Bi and S atoms. A similar charge density difference indicates that the Cu-S bonds also exhibit covalent character. The calculated minimum thermal conductivity of La2CuBiS5 is low, which is conducive to its use as a thermoelectric material. Owing to a bipolar effect, induced by thermal excitation, the material's Seebeck coefficient decreases sharply at T = 800 K. For the n-type and p-type doping conditions, the largest values of S 2 σ/ / /τ were calculated as −1.71×10 11 and 1.837×10 11 W K −2 ms −1 , respectively. The combination of a large dispersion and a high band degeneracy along the Γ-Y direction in the band structure simultaneously induces the highest Sy value and a high σ/ / /τ y value. Thus, the thermoelectric performance of La2CuBiS5 is anisotropic and most favorable along the y direction.
Bi 2 Te 3-based compounds have long been studied as thermoelectric materials in cooling applications near room temperature. Here, we investigated the thermoelectric properties of CuI-doped Bi 2 Te 2.1 Se 0.9 compounds. The Cu/I codoping induces the lattice distortion partially in the matrix. We report that the charge density wave caused by the local lattice distortion affects the electrical and thermal transport properties. From the high-temperature specific heat, we found a first-order phase transitions near 490 and 575 K for CuI-doped compounds (CuI) x Bi 2 Te 2.1 Se 0.9 (x = 0.3 and 0.6%), respectively. It is not a structural phase transition, confirming from the high-temperature X-ray diffraction. The temperature-dependent electrical resistivity shows a typical behavior of charge density wave transition, which is consistent with the temperature-dependent Seebeck coefficient and thermal conductivity. The transmission electron microscopy and electron diffraction show a local lattice distortion, driven by the charge density wave transition. The charge density wave formation in the Bi 2 Te 3-based compounds are exceptional because of the possibility of coexistence of charge density wave and topological surface states. From the Kubo formula and Boltzmann transport calculations, the formation of charge density wave enhances the power factor. The lattice modulation and charge density wave decrease lattice thermal conductivity, resulting in the enhancement of thermoelectric performance simultaneously in CuI-doped samples. Consequently, an enhancement of thermoelectric performance ZT over 1.0 is achieved at 448 K in the (CuI) 0.003 Bi 2 Te 2.1 Se 0.9 sample. The enhancement of ZT at high temperature gives rise to a superior average ZT avg (1.0) value than those of previously reported ones.
Room temperature Bi2Te3-based thermoelectric materials with high performance
Journal of Materials Science: Materials in Electronics, 2020
Several off-stoichiometric compositions Bi 0.5 Sb 1.5+x Te 3+δ (x = 0.2; δ = 0, 0.12, 0.14) were deliberately synthesized to produce in-situ composites based on compositional engineering approach. The structural characterization of these materials employing XRD, SEM, and HR-TEM reveals the formation of in-situ-composites containing Bi 0.5 Sb 1.5 Te 3 as matrix phase and minor phases of either Sb rich or Te rich in different compositions. Thermoelectric properties of these Bi 0.5 Sb 1.5+x Te 3+δ (x = 0.2; δ = 0, 0.12, 0.14) composites were studied in a wide range of temperatures extending from room temperature to 500 K. The electronic transport of these composites exhibits p-type semiconducting materials. The lowest thermal conductivity of ~ 0.69 W/m K @310 K was observed for Bi 0.5 Sb 1.7 Te 3.12 composite, which was noted to be 14% reduced thermal conductivity when compared with that of the state-of-the-art Bi 0.5 Sb 1.5 Te 3 (κ = 0.82 W/m K) material. In addition to this, an enhanced power factor was also observed in Bi 0.5 Sb 1.7 Te 3.12 which is primarily due to increased electrical conductivity of these materials. This enhanced power factor of the composition of Bi 0.5 Sb 1.7 Te 3.12 coupled with reduced thermal conductivity yields to high ZT ~ 1.13 at nearly room temperature, making these materials viable for large scale applications.