Enhanced thermoelectric performance of n‐type Bi2O2Se by Cl‐doping at Se site

The n‐type polycrystalline Bi₂O₂Se_1?xCl_x (0≤x≤0.04) samples were fabricated through solid‐state reaction followed by spark plasma sintering. The carrier concentration was markedly increased to 1.38×10²⁰ cm^?3 by 1.5% Cl doping. The maximum electrical conductivity is 213.0 S/cm for x=0.015 at 823 K, which is much larger than 6.2 S/cm for pristine Bi₂O₂Se. Furthermore, the considerable enhancement of the electrical conductivity outweighs the moderate reduction of the Seebeck coefficient by Cl doping, thus contributing to a high power factor of 244.40 μ·WK^?2·m^?1 at 823 K. Coupled with the intrinsically suppressed thermal conductivity originating from the low velocity of sound and Young's modulus, a ZT of 0.23 at 823 K for Bi₂O₂Se_0.985Cl_0.015 was achieved, which is almost threefold the value attained in pristine Bi₂O₂Se. It reveals that Se‐site doping can be an effective strategy for improving the thermoelectric performance of the layered Bi₂O₂Se bulks.     

Enhanced thermoelectric properties of Na and Mg co−doped BiCuSeO

A series of Bi_0.97?xNa_0.03Mg_xCuSeO (0 ≤ x ≤ 0.12) was fabricated by a two?step solid?state reaction and spark plasma sintering (SPS), and the influence of Mg²⁺ doping on the thermoelectric properties of Bi_0.97Na_0.03CuSeO was systematically investigated. The SPS processed?Bi_0.97?xNa_0.03Mg_xCuSeO had a ZrSiCuAs?type tetragonal crystal structure (space group P4/nmm). The Mg²⁺ doping appreciably enhanced the electrical conductivity due to the increase in hole concentration. Furthermore, the Mg²⁺ doping increased the grain boundary areas and bulk porosity and induced the strain field and mass fluctuations, thereby reducing the phonon thermal conductivity. We significantly improved the thermoelectric performance of Bi_0.97?xNa_0.03Mg_xCuSeO (0 ≤ x ≤ 0.12) by enhancing the thermoelectric power factor and by reducing the thermal conductivity.     

Effects of sulfur substitution for oxygen on the thermoelectric properties of Bi2O2Se

In the present work, the thermoelectric properties of S-doped Bi₂O_2-xS_xSe at the temperatures from 320 to 793 K have been studied. The results show that the solubility limit of S is around x = 0.01 and S-doping is helpful to the sintering and grain growth of Bi₂O₂Se. Moreover, S-doping reduces the band gap of Bi₂O_2-xS_xSe remarkably as x rises. As a result, a thousand times promotion of electrical conductivity at x = 0.02 is obtained, leading to a nearly 3 times increase of power factor at 787 K. By virtue of the intrinsically low thermal conductivity, a peak ZT of 0.29 at 793 K with an average of 0.21 has been achieved for Bi₂O_1.98S_0.02Se, which is nearly 3 and 6 times larger than that of the pristine one. This study indicates that a small amount of S substitution for O could improve the thermoelectric properties of Bi₂O₂Se effectively.     

Influence of Ba2+ doping on the thermoelectric properties of BiCuSeO fabricated by spark plasma sintering

We studied the influence of Ba²⁺ doping on the thermoelectric properties of the p-type Bi_1–xBa_xCuSeO (0?≤?x?≤?0.21) fabricated by spark plasma sintering. The substitution of Ba²⁺ for Bi³⁺ gradually increased the electrical and thermal conductivities and decreased the Seebeck coefficient, which were due to the increased hole concentration. The largest value of dimensionless figure-of-merit (0.57) was obtained for the Bi_0.86Ba_0.14CuSeO at 500?°C, which was over three times greater than that of pristine BiCuSeO (0.18) at 500?°C. We believe that the thermoelectric properties of BiCuSeO were substantially enhanced through the partial substitution of Ba²⁺ for Bi³⁺.     

Fabrication and electrical properties of Bi2-xSbxTe3 ternary nanopillars array films

Highly oriented Bi_2-xSb_xTe₃ (x?=?0, 0.7, 1.1, 1.5, 2) ternary nanocrystalline films were fabricated using vacuum thermal evaporation method. Microstructures and morphologies indicate that Bi_2-xSb_xTe₃ films have pure rhombohedral phase with well-ordered nanopillars array. Bi, Sb and Te atoms uniformly distributed throughtout films with no precipitation. Electrical conductivity of Bi_2-xSb_xTe₃ films transforms from n-type to p-type when x?>?1.1. Metal-insulator transition was observed due to the incorporation of Sb in Bi₂Te₃. Bi_2-xSb_xTe₃ film with x?=?1.5 exhibits optimized electrical properties with maximum electrical conductivity σ of 2.95?×?10⁵ S?m^?1 at T?=?300?K, which is approximately ten times higher than that of the undoped Bi₂Te₃ film, and three times higher than previous report for Bi_0.5Sb_1.5Te₃ films and bulk materials. The maximum power factor PF of Bi_0.5Sb_1.5Te₃ nanopillars array film is about 3.83?μW?cm^?1 K^?2 at T?=?475?K. Highly oriented (Bi,Sb)₂Te₃ nanocrystalline films with tuned electronic transport properties have potentials in thermoelectric devices.     

Achieving weak anisotropy in N-type I-doped SnSe polycrystalline thermoelectric materials

SnSe is a very strong anisotropic material; sometimes, strong anisotropy is unenviable for producing parts of thermoelectric (TE) devices. In order to study the efficient preparation of high-performance n-type polycrystalline SnSe with weak anisotropy, in this work, we combine mechanical alloying at 450 RPM for 10 h and spark plasma sintering at 773 K under 50 MPa pressure for the preparation of polycrystalline SnSe _0.95-xI_x (x = 0,0.01,0.02,0.03) samples, and investigate the TE properties. The prepared samples show very weak anisotropy. With iodine doping, increased carrier concentration is observed, in agreement with DFT calculations. A peak ZT ≈ 1.02 at 723 K is observed with I-doping of x = 0.02, which is about 225% higher than that of undoped sample with ZT ≈ 0.31 at 723 K in parallel direction, mainly attributed to the enhanced power factor and about 56% reduced thermal conductivity from 0.68 Wm^?1K^?1 to 0.30 Wm^?1K^?1. TE properties in both directions are not much different, and the ratios of electrical and thermal conductivities in both directions are very close to unity.     

Thermoelectric properties of Bi2O3-added WO3 ceramics

Tungsten trioxide (WO₃) ceramics were prepared by firing Bi₂O₃-added WO₃ compacts with atomic ratios of Bi/W?=?0.00, 0.01, 0.03, or 0.05, in which Bi₂O₃ was mixed as a sintering agent. Dense ceramics consisting of remarkably grown WO₃ grains were obtained for Bi-containing samples with Bi/W?=?0.01, 0.03, and 0.05. The grain growth was enhanced by the liquid phase of Bi₂W₂O₉ formed among the WO₃ grains while firing. The XRD patterns did not show evidence for Bi inclusion into the WO₃ lattice, but the SEM-EDX showed an intensive distribution of Bi into the grain boundaries. Electrical conductivity σ and Seebeck coefficient S were measured in a temperature range of 373–1073?K. The temperature dependences indicated that the Bi₂O₃-added WO₃ ceramics were n-type semiconductors. It was considered that the electron carriers were generated from oxygen vacancies included into the WO₃ grains. The thermoelectric power factors S²σ for the ceramics ranged from 1.5?×?10^?7 W?m^?1 K^?2 to 2.8?×?10^?5 W?m^?1 K^?2, and the highest value occurred at 970?K for the ceramic with Bi/W?=?0.01.     

Enhanced Thermoelectric Properties of Bi2O2Se Ceramics by Bi Deficiencies

Polycrystalline Bi_2?xO₂Se ceramics were synthesized by spark plasma sintering process. Their thermoelectric properties were evaluated from 300 to 773 K. All the samples are layered structure with a tetragonal phase. The introduction of Bi deficiencies will cause the orientation alignment and change of effective mass. As a result, a significant enhancement of thermoelectric performance was achieved. The maximum of Seebeck coefficient is ?568.8 μV/K for Bi_1.9O₂Se at 773 K, much larger than ?445.6 μV/K for pristine Bi₂O₂Se. Featured with very low thermal conductivity [~0.6 W·(m·K)^?1] and an optimized electrical conductivity, ZT at 773 K is significantly increased from 0.05 for pristine Bi₂O₂Se to 0.12 for Bi_1.9O₂Se by introducing Bi deficiencies, which makes it a promising candidate for medium temperature thermoelectric applications.     

Synthesis of n-type Bi2Te1-xSex compounds through oxide reduction process and related thermoelectric properties

We propose a new process for the fabrication of n-type Bi₂Te_3-xSe_x (x = 0, 0.25, 0.4, 0.7) compounds. The compounds could be synthesized successfully using only oxide powders as the starting materials via the mechanical milling, oxidation, reduction, and spark plasma sintering processes. The controllability of the Se content could be ascertained by structural, electrical, and thermal characterizations, and the highest thermoelectric figure of merit (ZT) of 0.84 was achieved in Bi₂Te_2.6Se_0.4 compound at 423 K without any intentional doping. This process provides a new route to fabricate n-type Bi₂Te_3-xSe_x compounds with competitive ZTs using all oxide starting materials.     

Enhanced Thermoelectric Performance of <Emphasis Type="Italic">c</Emphasis>-Axis-Oriented Epitaxial Ba-Doped BiCuSeO Thin Films

We reported the epitaxial growth of c-axis-oriented Bi_1?xBa_xCuSeO (0?≤ x ≤?10%) thin films and investigated the effect of Ba doping on the structure, valence state of elements, and thermoelectric properties of the films. X-ray photoelectron spectroscopy analysis reveal that Bi³⁺ is partially reduced to the lower valence state after Ba doping, while Cu and Se ions still exist as +?1 and ??2 valence state, respectively. As the Ba doping content increases, both resistivity and Seebeck coefficient decrease because of the increased hole carrier concentration. A large power factor, as high as 1.24 mWm^?1 K^?2 at 673 K, has been achieved in the 7.5% Ba-doped BiCuSeO thin film, which is 1.5 times higher than those reported for the corresponding bulk samples. Considering that the nanoscale-thick Ba-doped films should have a very low thermal conductivity, high ZT can be expected in the films.     

Improved thermoelectric properties of n-type Bi2S3 via grain boundaries and in-situ nanoprecipitates

To improve the thermoelectric properties of n-type Bi₂S₃ materials, a certain amount of SbCl₃ were added into Bi₂S₃ materials by a conventional melting method combined with plasma activated sintering (PAS) process. The Bi₂S₃-based materials evolve from the lamellar- to particle-like structures after SbCl₃ doping. The phonon scattering has strong enhancement through the increased grain boundaries and in-situ Bi₂S₃ nanoprecipitates, resulting in the low lattice thermal conductivity. Meanwhile, the high power factor is achieved because of the marked increase in the electrical conductivity. Hence, the synergistic effect of antimony and chlorine substitutions not only contribute to reduce the thermal conductivity but also tune the electrical transport properties, yielding a peak ZT value of ∼ 0.65 at 773 K for the Bi₂S₃-1%SbCl₃ sample.     

Realizing tremendous electrical transport properties of polycrystalline SnSe2 by Cl-doped and anisotropy

SnSe₂ is regarded as an attractive thermoelectric material for its structural and chemical analogy to SnSe that is claimed with the highest ZT in single crystal. In this study, the pure and Cl-doped SnSe₂ polycrystals (3%, 6%, 9% and 12% molar Cl content) were fabricated in four steps that are hydrothermal synthesis, heating purification, diffusion doping, and spark plasma sintering. The phase structure, lamellar morphology and crystallite orientation were studied for the synthesized SnSe₂ powder and the sintered pellets. The structural evolution was traced from the SnSe₂ hexagonal plates of powders to the (001) oriented grains in pellets. The Cl doping into SnSe₂ was verified by phase composition, lattice parameter, element distribution, and chemical valance. The doped Cl increased both the carrier concentration and the mobility. The anisotropic thermoelectric properties of SnSe₂ bulk materials were investigated as functions of temperature from 50?°C to 300?°C and the doping amount, respectively. The Seebeck coefficient was less anisotropic than the electrical and thermal conduction. The grain orientation influenced the anisotropy of the electrical and thermal conductivity at a similar ratio. The power factors were less dependent on temperature with an optimum in-plane 1.06?mW?m^?1 K^?2 and out-of-plane 0.41?mW?m^?1 K^?2. The highest ZTs of 0.3 were attained at 300?°C in both directions.     

Synergistic optimization of electrical-thermal properties of dual vacancy Bi1−x-yPbyCu1−xSeO by improving mobility and reducing lattice thermal conductivity

We fabricate Bi_1?x-yPb_yCu_1?xSeO (x = 0, 0.03, 0.06, y = 0, 0.10) samples via 4 min-microwave synthesis combined with 5 min-spark plasma sintering. The phase composition, microstructure, valence, and electrical and thermal transport properties of the samples are investigated at 298–873 K. Pb doping provides impurity carriers and increases the concentration to 0.9–3.0 × 10²⁰ cm^-³ . Bi and Cu vacancy could provide a carrier transport channel to reduce carrier scattering probability, leading to improved mobility. Twin crystals, stacking faults, and grain boundary segregation are observed in Bi_0.87Pb_0.10Cu_0.97SeO on scanning transmission electron microscopy. Bi and Cu vacancy increase the sample point defects in Pb-doped or undoped samples which results in a decrease in lattice thermal conductivity. The lattice thermal conductivity of Bi_0.87Pb_0.10Cu_0.97SeO is decreased to an extremely low value of 0.13 Wm^?1 K^?1 and a maximum ZT value of 1.09 is achieved at 873 K.     

Carrier concentration optimization for thermoelectric performance enhancement in n-type Bi2O2Se

The Bi₂O₂Se-based compounds with an intrinsically low thermal conductivity and relatively high Seebeck coefficient are good candidates for thermoelectric application. However, the low electrical conductivity resulted from carrier concentration of only 10¹⁵?cm^?3 for pristine material, which is too low for optimized thermoelectrics. As a result, the carrier concentration optimization of Bi₂O₂Se is important and useful to achieve higher power factor. In this work, the effect of Ge-doping at the Bi site has been investigated systematically, with expectations of carrier concentration optimization. It is found that Ge doping is an efficient method to increase carrier concentration. Due to the largely increased carrier concentration via Ge doping, the room temperature electrical conductivity rises rapidly from 0.03?S/cm in pristine sample to 133?S/cm in x?=?0.08 sample. Combined with the intrinsically low thermal conductivity, a maximum ZT value of 0.30 has been achieved at 823?K for Bi_1.92Ge_0.08O₂Se, which is the highest ZT value for Bi₂O₂Se-based thermoelectric materials.     

Fabrication and thermoelectric properties of Yb-doped ZrNiSn half-Heusler alloys

Half-Heusler (HH) alloys constitute an important class of materials that exhibit promising potential in high-temperature thermoelectric (TE) power generation. In this work, we synthesized Zr_1?x Yb _x NiSn (x?=?0, 0.01, 0.02, 0.04, 0.06 and 0.10) HH alloys using a time-efficient levitation melting and spark plasma sintering procedure. X-ray diffraction showed that the samples were predominantly single phased, and that the lattice constant increased systematically with increasing Yb doping ratio. The doping effects of Yb on the thermoelectric properties were studied. It was found that Yb doping consistently decreased the electrical and thermal conductivities. On the other hand, the effects of Yb doping on the Seebeck coefficient were found to be non-monotonic. The magnitude of the Seebeck coefficient (n-type) was increased upon Yb doping up to x?=?0.02, above which Yb doping introduced notable p-type conduction. As a result, the room-temperature Seebeck coefficient of the x?=?0.10 sample became positive although the magnitude was not high. The thermoelectric figure of merit, ZT, reached a maximum of ～0.38 at 900 K for the x?=?0.01 sample. Selective doping on the Ni and Sn sites are necessary to further optimize the TE performance of Zr_1?x Yb _x NiSn alloys.     

High-temperature thermoelectric performance of (W1−xTix)18O49

High-temperature thermoelectric properties of tungsten-based Magnéli phase oxide (W_1-xTi_x)₁₈O₄₉ (0 ≤ x ≤ 0.25) prepared by solid state reaction followed by densification via spark plasma sintering (SPS) were studied. The Ti substitution increased the Seebeck coefficient, the power factor, and decreased both the electronic and lattice thermal conductivity. The synergistic substitution effect on the electrical and thermal properties and inherently low total thermal conductivity of 1.46 ± 0.08 WK^?1 m^?1 originating from the tunnel-like crystal structure of the oxide led to a significantly high ZT of 0.50 ± 0.07 at 1073 K for the sample with x = 0.2.     

Influence of reactant type on the Sr incorporation grade and structural characteristics of Ba1−xSrxTiO3 (x=0−1) grown by sol–gel-hydrothermal synthesis

The influence of barium and strontium starting reactants used in different mole ratios, BaCl₂ and Ba(OH)₂, SrCl₂ and Sr(OH)₂, on the chemical and structural properties of Ba_1?xSr_xTiO₃ (x=0?1) (BST) nanoparticles prepared via sol–gel-hydrothermal synthesis in an oxygen atmosphere is discussed. The effect of the type of reactant on the relative amount of Sr incorporated in BST compound was also analysed. The synthesised BST nanoparticles showed differences in their structural and chemical characteristics, which were attributed to the presence of Cl^? or OH^? anions during the synthesis of the compound. The structure, morphology and oxidation state of the samples were studied by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy, respectively. In addition, theoretical calculations using cluster models were carried out to understand the possible phases formed of BST, the effect of the Sr incorporation and the possible presence of oxygen vacancies inside the BST structure.     

Enhancements of thermoelectric performance in n-type Bi2Te3-based nanocomposites through incorporating 2D Mxenes

Bi₂Te_2.7Se_0.3 compound has been considered as an efficient n-type room-temperature thermoelectric (TE) material. However, the large-scale applications for low-quality energy harvesting were limited due to its low energy-conversion efficiency. We demonstrate that TE performance of Bi₂Te_2.7Se_0.3 system is optimized by 2D Ti₃C₂T_x additive. Here, a 43% reduction of electrical resistivity is obtained for the nanocomposites at 380 K, originating from the increased carrier concentration. Consequently, the g = 0.1 sample shows a maximum power factor of 1.49 Wmm^?1K^?2. Meanwhile, the lattice thermal conductivity for nanocomposite samples is reduced from 0.77 to 0.41 Wm^?1K^?1 at 380 K, due to the enhanced phonon scattering induced by the interfaces between Ti₃C₂T_x nanosheets and Bi₂Te_2.7Se_0.3 matrix. Therefore, a peak ZT of 0.68 is achieved at 380 K for Bi₂Te_2.7Se_0.3/0.1 wt% Ti₃C₂T_x, which is enhanced by 48% compared with pristine sample. This work provides a new route for optimizing TE performance of Bi₂Te_2.7Se_0.3 materials.     

The effect of Er3+ doping on the structure and thermoelectric properties of CdO ceramics

The effect of Er³⁺ doping on the structure and thermoelectric transport properties of CdO ceramics was investigated. The solubility limit of Er³⁺ in CdO was very small and that additions of more than about 0.5 at% Er³⁺ resulted in the presence of Er₂O₃. With the addition of Er³⁺, the average grain size of Cd_1?xEr_xO (0 ≤ x ≤ 0.015) decreased and the carrier concentration as well as mobility increased at room temperature. A small amount of Er³⁺ doping resulted in a marked increase of electric conductivity and a moderate decrease of Seebeck coefficient. Although Er³⁺ doping also leaded to an increase in thermal conductivity, a large ZT of 0.2 was achieved in x = 0.005 sample at 723 K due to the obvious improvement of power factor. The results demonstrate that CdO:Er is a new promising n-type thermoelectric material.     

Electrochemically deposited thermoelectric n-type Bi2Te3 thin films

Electrochemically deposited n-type BiTe alloy thin films were grown from nitric acid baths on sputtered Bi_xTe_y/SiO/Si substrates. The film compositions, which varied from 57 to 63 at.% Te were strongly dependent on the deposition conditions. Surface morphologies varied from needle-like to granular structures depending on deposited Te content. Electrical and thermoelectric properties of these electrodeposited Bi_xTe_y thin films were measured before and after annealing and compared to those of bulk Bi₂Te₃. Annealing at 250 °C in reducing H₂ atmosphere enhanced thermoelectric properties by reducing film defects. In-plane electrical resistivity was highly dependent on composition and microstructure. In-plane Hall mobility decreased with increasing carrier concentration, while the magnitude of the Seebeck coefficient increased with increasing electrical conductivity to a maximum of −188.5 μV/K. Overall, the thermoelectric properties of electrodeposited n-type BiTe thin films after annealing were comparable to those of bulk BiTe films.