Substrate temperature-dependent thermoelectric figure of merit of nanocrystalline Bi2Te3 and Bi2Te2.7Se0.3 prepared using pulsed laser deposition supported by DFT study

This work reports the pulsed laser deposition of n-type selenium (Se) doped bismuth telluride (Bi₂Te_2.7Se_0.3) and n-type bismuth telluride (Bi₂Te₃) nanostructures under varying substrate temperatures. The influence of the substrate temperature during deposition on the structural, morphological and thermoelectric properties for each phase was investigated. Density functional theory (DFT) simulations were employed to study the electronic structures of the unit-cells of the compounds as well as their corresponding partial and total densities of states. Surface and structural characterization results revealed highly crystalline nanostructures with abundant grain boundaries. Systematic comparative analysis to determine the effect of Se inclusion into the Bi₂Te₃ matrix on the thermoelectric properties is highlighted. The dependence of the thermoelectric figure of merit (ZT) of the nanostructures on the substrate temperatures during deposition was demonstrated. The remarkable room temperature thermoelectric power factor (PF) of 2765 μW/mK² and 3179 μW/mK² for pure and Se-doped Bi₂Te₃ compounds respectively, signifies their potential of being useful in cooling and power generation purposes. The room temperature ZT values of the Se-doped Bi₂Te₃ was found to be 0.92, about 30% enhancement as compared with the pure phase, which evidently results from the suppressed thermal conductivity in the doped species caused by phonon scattering at the interfaces.     

Boosting the thermoelectric performance of Bi2O2Se by isovalent doping

N‐type Bi₂O₂Se has a bright prospect for mid‐temperature thermoelectric applications on account of the intrinsically low thermal conductivity. However, the low carrier concentration of Bi₂O₂Se (~10¹⁵ cm^?3) severely limits its thermoelectric performance. Herein, the boosting of the carrier concentration to ~10¹⁹ cm^?3 can be realized in our La‐doped Bi₂O₂Se ceramic samples, which could be ascribed to the formation of isoelectronic traps and the narrowing of band gap, and contribute to a marked increase in the electrical conductivity (from 0.03 S cm^?1 to 182 S cm^?1). Our X‐ray absorption near‐edge structure spectra results reveal that a local disordering of oxygen atoms could be an important reason for the intrinsically low thermal conductivity of Bi₂O₂Se, and the point defects can also suppress the lattice thermal conductivity in La‐doped Bi₂O₂Se. The ZT value can be enhanced by a factor of ~4.5 to 0.35 at 823 K for Bi_1.98La_0.02O₂Se as compared to the pristine Bi₂O₂Se. The coordinated optimization of electrical and thermal properties demonstrates an effective method for the rational design of high‐performance thermoelectric materials.     

Preparation and thermoelectric properties of Bi2SexTe3−x bulk materials

Bi₂Te₃ and Bi₂Se₃ nanoplates were synthesized by a microwave-assisted wet chemical method, and Bi₂Se_xTe_3−x (x=1, 2, 3) bulk nanocomposites were then prepared by hot pressing the Bi₂Te₃ and Bi₂Se₃ nanoplates at 80 MPa and 723 K in vacuum. The phase composition and microstructures of the bulk samples were characterized by powder X-ray diffraction and field-emission scanning electron microscopy, respectively. The electrical conductivity of the Bi₂Se_xTe_3−x bulk nanocomposites increases with increasing Se content, and the Seebeck coefficient value is negative, showing n-type conduction. The absolute Seebeck coefficient value decreases with increasing Se content. A highest power factor, 24.5 µWcm⁻¹ K⁻², is achieved from the sample of x=1 at 369 K among the studied samples.     

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.     

Bi2Te3-MWCNT nanocomposite: An efficient thermoelectric material

Bi₂Te₃–MWCNT nanocomposite has been synthesized by hydrothermal technique and demonstrate the role of MWCNT for thermoelectric properties. Herein, MWCNT has been used as conducting filler, which leads to the enhancement in the electrical conductivity in the case of nanocomposite. Bi₂Te₃–MWCNT nanocomposite shows ~22% decrease in the thermal conductivity as compared to Bi₂Te₃ nanostructures, which is attributed to the enhanced phonon scattering at the interfaces of Bi₂Te₃–MWCNT nanocomposite. Due to the increase in the electrical conductivity and decrease in the thermal conductivity, the overall enhancement in the figure of merit is ~45% in Bi₂Te₃–MWCNT nanocomposite as compared to Bi₂Te₃ nanostructures.     

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.     

Comprehensive study of tellurium based glass ceramics for thermoelectric application

Tellurium based glasses have interesting thermoelectric characteristics. However, their high electrical resistivity is still an obstacle to considering them for thermoelectric applications. In this work, the (Te₈₅Se₁₅)_60???0.6xAs_40???0.4xCu_x glass system was studied. This revealed that Cu can act as glass former and increase both glass thermal stability and electrical conductivity. The best candidate, (Te₈₅Se₁₅)₄₅As₃₀Cu₂₅, was chosen to prepare composites with Bi_0.5Sb_1.5Te₃ using spark plasma sintering. These glass ceramic samples exhibited a much better thermoelectric performance. Glass ceramics with 50?mol. % of Bi_0.5Sb_1.5Te₃ show a maximum ZT value equal to 0.37 at 413?K. Meanwhile, the advantages of glass including low sintering temperature and high formability are well maintained.     

The influence of CNTs on the thermoelectric properties of a CNT/Bi2Te3 composite

CNT/Bi₂Te₃ composites were prepared from composite powders in which CNTs were implanted in the Bi₂Te₃ matrix powders by a novel chemical route. It was found that the fabricated composite had a microstructure of a homogeneous dispersion of CNTs in the Bi₂Te₃ matrix due to interfacial bonding agents of oxygen atoms attaching to the surface of CNTs. The dimensionless figure of merit (ZT) of the composite shows significantly increased values compared to those of pure binary Bi₂Te₃ in the temperature range of 298–498 K and a maximum ZT of 0.85 was obtained at 473 K. It is considered that the improved thermoelectric performance of the composite mainly originated from thermal conductivity that was reduced by active phonon-scattering at the CNT/Bi₂Te₃ interface.     

Europium (III) doped bismuth telluride decorated on carbon-based materials for enhancing thermoelectric performance

Scalable synthetic approach for superior performance of thermoelectric (TE) materials is a crucial step for the TE technology progress. Herein, reduced graphene oxide (RGO), carbon nitride (g-C ₃N₄) and europium (Eu) are utilized as additives to bismuth telluride (Bi₂Te₃) matrix to prepare various novel nanocomposites (NCs): (RGO@Bi_1.8Te₃Eu_0.2) and (RGO-g-C₃N₄@Bi_1.8Te₃Eu_0.2) with an enhanced TE performance. The novel NCs were synthesized via solvothermal method, physiochemically characterized and consolidated into pellets of 1 mm thickness to measure their TE properties. The new additives potentially affected the physicochemical and TE properties of Bi₂Te₃. Nanostructured hexagonal nanoplatelets with 12.5 nm thickness were observed by scanning and transmission electron microscopy (SEM and TEM) of the synthesized Bi₂Te₃. This thickness shrinked to 5.7 and 5.2 nm upon the formation of (RGO@Bi_1.8Te₃Eu_0.2) and (RGO-g-C₃N₄@Bi_1.8Te₃Eu_0.2) NCs, respectively. Energy dispersive X-ray Spectroscopy (EDS) of NCs proved the existence of Bi, Te, C, Eu and N atoms. Raman and Fourier-transform infrared (FT-IR) spectra confirmed the NC formation that led to narrowing the energy band gap of Bi₂Te₃ as displayed by UV–Vis spectra. Brunauer–Emmett–Teller showed specific surface area expansion of Bi₂Te₃ from 6.78 to 19.00 and 16.75 m²g^-1 of (RGO@Bi_1.8Te₃Eu_0.2) and (RGO-g-C₃N₄@Bi_1.8Te₃Eu_0.2) NCs, respectively. The electrical conductivity of Bi₂Te₃ rose by 56 and 69 times, whereas its thermal conductivity significantly dropped by 1.6 and 1.7 times upon (RGO@Bi_1.8Te₃Eu_0.2) and (RGO-g-C₃N₄@Bi_1.8Te₃Eu_0.2) NCs formation. Owing to extra channels of carrier transfer and phonon scattering induced by NCs heterointerfaces. Novel combination of carbon-based materials and Eu with Bi₂Te₃ matrix boosts its TE performance resulting in a worthy candidate for power generation applications at room-temperature.     

High thermoelectric performance of (Bi1-xPrx)2(Te0.9Se0.1)3 alloys prepared by high-pressure sintering method

In this article, n-type (Bi_1-xPr_x)₂(Te_0.9Se_0.1)₃ (x = 0, .002, .004, .008) alloys were fabricated by high-pressure sintering (HPS) method together with annealing. The effect of high pressure and Pr contents on the microstructure and thermoelectric performance of samples were explored in detail. The results show that the HPS samples are composed of nanoparticles. Pr doping has significant impacts on the electrical and thermal transport properties of the Bi₂Te_2.7Se_0.3 alloys. The HPS sample with x = .004 shows the maximum ZT value of .31 at 473 K, which is enhanced by 41% to compare with the Pr-free sample. Annealing can improve the thermoelectric properties by increasing the electrical transport properties and decreasing the thermal conductivity simultaneously. As a result, the highest ZT value of 1.06 is achieved for the annealed sample with x = .004 at 373 K, which is beneficial to the thermoelectric power generation.     

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.     

High-throughput optimization and fabrication of Bi2Te2.7Se0.3-based artificially tilted multilayer thermoelectric devices

Artificially tilted multilayer thermoelectric devices (ATMTDs) have attracted growing attention due to their ease in miniaturization and high flexibility in device design. However, most of these devices are inefficient due to the lack of effective strategy to optimize their material matching and geometrical configurations. Herein, a high-throughput optimization approach is employed to screen high-performance Bi₂Te_2.7Se_0.3-based ATMTDs from a material genome database covering 230 kinds of candidates. 14 kinds of ATMTDs are found to have ZT_zx,max values exceeding 0.3 and tilt angles greater than 15°. Bi_0.1Sb_1.9Te₃/Bi₂Te_2.7Se_0.3 ATMTD is screened out and fabricated because of its excellent transverse figure of merit, large tilt angle, and good interface compatibility. Consequently, transverse figure of merit over 0.3, thermal sensitivity greater than 0.11 mV·K^?1, and power density up to 1.1 kW·m^?2 are recorded in Bi_0.1Sb_1.9Te₃/Bi₂Te_2.7Se_0.3 ATMTD. This indicates that ATMTDs have great potential for application in the fields of temperature detection and power generation.     

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.     

Enhancement of thermoelectric performance of N-type Bi2Te3 based thin films via in situ annealing during magnetron sputtering

In this work, n-type Bi₂Te₃ based thin films were prepared in 300 °C via DC magnetron sputtering, and influences of sputtering power and annealing time on thermoelectric properties of films were investigated. The raise of sputtering power brings about the improvement of deposited rate and enhancement of grain size. Taking the consideration that the large-sized grains are to phonon scattering, we determine the medial power of 30 W as the basic technical parameters for the purpose of further optimizing performance through an in situ annealing process. Subsequently, thin-film treated by in situ annealing process acts out an obvious reduction in electrical conductivity attributed to the decrease in carrier concentration. Especially, the film annealed for 40 min shows an enhancement in the Seebeck coefficient and leads to a maximum power factor 0.82 m W m⁻¹ K⁻² at 543 K.     

Preparation and thermoelectric properties of MoS2/Bi2Te3 nanocomposites

MoS₂ nanosheets with size of several-hundred nanometers were prepared by a hydrothermal intercalation/exfoliation method, then MoS₂/Bi₂Te₃ composite nanopowders were prepared by a microwave-assisted wet chemical method using the MoS₂ nanosheets, TeO₂, Bi(NO₃)₃·5H₂O, KOH and ethylene glycol as raw materials. Bulk MoS₂/Bi₂Te₃ nanocomposites were prepared by hot pressing the MoS₂/Bi₂Te₃ composite nanopowders with MoS₂ nanosheet content ranging from 0 to 17 wt% at 80 MPa and 648 K in vacuum. X-ray photoelectron spectroscopy and X-ray diffraction analyses indicate that MoS₂ and Bi₂Te₃ did not react each other during the hot pressing. FESEM observation reveals that the MoS₂/Bi₂Te₃ composite samples had a more compact microstructure than the pristine Bi₂Te₃ bulk sample. The MoS₂ phase was relatively randomly dispersed in the composite. At a given temperature, the electrical conductivity of the composites increases first then decreases as the MoS₂ content increases, whereas the Seebeck coefficient of the bulk nanocomposites does not change much. A highest power factor, ~18.3 μW cm⁻¹ K⁻² which is about 30% higher than that of pristine Bi₂Te₃ sample, at 319 K has been achieved from a nanocomposite sample containing 6 wt% MoS₂.     

Synergistically optimizing electrical and thermal transport properties of Bi2O2Se ceramics by Te‐substitution

Bi₂O₂Se oxyselenides, characterized with intrinsically low lattice thermal conductivity and large Seebeck coefficient, are potential n‐type thermoelectric material in the mediate temperature range. Given the low carrier concentration of ~10¹⁵ cm^?3 at 300 K, the intrinsically low electrical conductivity actually hinders further enhancement of their thermoelectric performance. In this work, the isovalent Te‐substitution of Se plays an effective role in narrowing the band gap, which notably increases the carrier concentration to ~10¹⁸ cm^?3 at 300 K and the electron conduction activation energy has been lowered significantly from 0.33 to 0.14 eV. As a consequence, the power factor has been improved from 104 μW·K^?2·m^?1 for pristine Bi₂O₂Se to 297 μW·K^?2·m^?1 for Bi₂O₂Se_0.96Te_0.04 at 823 K. Meanwhile, the suppressed lattice thermal conductivity derives from the introduced point defects by heavier Te atoms. The gradually decreased phonon mean free path reflects the increasingly intense phonon scattering. Ultimately, the ZT value attains 0.28 for Bi₂O₂Se_0.96Te_0.04 at 823 K, an enhancement by a factor of ~2 as compared to that of pristine Bi₂O₂Se. This study has demonstrated that Te‐substitution of Se could synergistically optimize the electrical and thermal properties thus effectively enhancing the thermoelectric performance of Bi₂O₂Se.     

Recent progress in electrodeposition of thermoelectric thin films and nanostructures

Thermoelectric power generators and coolers have many advantages over conventional refrigerators and power generators such as solid-state operation, compact design, vast scalability, zero-emissions and long operating lifetime with no maintenance. However, the applications of thermoelectric devices are limited to where their unique advantages outweigh their low efficiency. Despite this practical confine, there has been a reinvigorated interest in the field of thermoelectrics through identification of classical and quantum mechanical size effects, which provide additional ways to enhance energy conversion efficiencies in nanostructured materials. Although, there are a few reports which demonstrated the improvement of efficiency through nanoengineering, the successful application of these nanostructures will be determined by a cost-effective and high through-put fabrication method. Electrodeposition is the method of choice to synthesize nanoengineered thermoelectric materials because of low operating and capital cost, high deposition rates, near room temperature operation, and the ability to tailor the properties of materials by adjusting deposition conditions. In this paper, we reviewed the recent progress of the electrodeposition of thermoelectric thin films and nanostructures including Bi, Bi_1−xSb_x, Bi₂Te₃, Sb₂Te₃, (Bi_1−xSb_x)₂Te₃, Bi₂Se₃, Bi₂Te_3−ySe_y, PbTe, PbSe, PbSe_1−xTe_x and CoSb₃.     

Grain boundary engineering strategy for simultaneously reducing the electron concentration and lattice thermal conductivity in n-type Bi2Te2.7Se0.3-based thermoelectric materials

This study demonstrates atomic layer deposition (ALD) of an extremely thin Al₂O₃ layer over n-type Bi₂Te_2.7Se_0.3 to alleviate the adverse effects of multiple boundaries on their thermoelectric performance. Multiple boundaries reduce thermal conductivity (κ), but generate electrons, deviating from the optimum carrier concentration. Only one Al₂O₃ ALD cycle effectively suppresses Te volatilization at the grain boundaries, resulting in a decrease from 5.8 × 10¹⁹/cm³ to 3.6 × 10¹⁹/cm³ in the electron concentration. Concurrently, the one-cycle-Al₂O₃ coating produces fine grains, thus inducing numerous boundaries, ultimately suppressing the lattice κ from 0.64 to 0.33 W/m·K. A further increase in the number of Al₂O₃ cycles leads in a significant rise in the resistance, resulting in degradation of thermoelectric performance. Consequently, the ZT value is increased by 51 % as a result of Al₂O₃ coating with a single ALD cycle. Our approach offers new insights into the simultaneous reduction of the κ and electron concentration in n-type Bi₂Te₃-based materials.     

Enhanced thermoelectric performance in three-dimensional superlattice of topological insulator thin films

We show that certain three-dimensional (3D) superlattice nanostructure based on Bi₂Te₃ topological insulator thin films has better thermoelectric performance than two-dimensional (2D) thin films. The 3D superlattice shows a predicted peak value of ZT of approximately 6 for gapped surface states at room temperature and retains a high figure of merit ZT of approximately 2.5 for gapless surface states. In contrast, 2D thin films with gapless surface states show no advantage over bulk Bi₂Te₃. The enhancement of the thermoelectric performance originates from a combination of the reduction of lattice thermal conductivity by phonon-interface scattering, the high mobility of the topologically protected surface states, the enhancement of Seebeck coefficient, and the reduction of electron thermal conductivity by energy filtering. Our study shows that the nanostructure design of topological insulators provides a possible new way of ZT enhancement.     

Enhanced thermoelectric performance of p-type sintered BiSbTe-based composites with AgSbTe2 addition

Bismuth telluride-based materials have been widely used in the field of thermoelectric cooling near room temperature. However, the material utilization and device conversion efficiency were limited by the low thermoelectric performance and poor mechanical properties of commercial zone-melting materials. With an aim to optimize the comprehensive properties, we prepared the composite samples of Bi_0.48Sb_1.52Te₃ (BST)-x wt% AgSbTe₂ (x = 0, 0.05, 0.1, 0.2) via the hot pressing method. It was found that the AgSbTe₂ addition can effectively increase the carrier concentration and improve the power factor to 46 μW cm^?1 K^?2 at 300 K. Due to the introduction of dislocations, stress and Te inhomogeneities, the lattice thermal conductivity of the composite was significantly reduced to 0.69 W m^?1 K^?1 at 325 K. As a result, a maximum ZT of 1.15 at 325 K is obtained for the x = 0.1 sample. Interestingly, BST-0.1 wt% AgSbTe₂ exhibits roughly isotropic thermoelectric performance perpendicular to and parallel to the pressing direction. Our study suggests that the BST-AgSbTe₂ composite is very promising for the application of thermoelectric refrigeration near room temperature.