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.     

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.     

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.     

Optimization of chemical and electrochemical parameters for the preparation of n-type Bi2Te2.7Se0.3 thin films by electrodeposition

A 2^3–1 fractional factorial design comprising four runs and three centre points was applied in order to optimize the electrodeposition process to find a compound with the best stoichiometry leading to a Bi₂Te_2.7Se_0.3 thin film suitable for thermoelectric applications. The key factors considered were the deposition potential, the percentage of bismuth and the percentage of selenium in the solution. The Bi^III, Se^IV, Te^IV electrolyte mixtures in 1 M HNO₃ (pH 0), allowed deposition of ternary alloys to be achieved at room temperature on stainless steel substrates. The deposition mechanism was investigated by linear voltammetry. The films were characterized by micropobe analysis, X-ray diffraction, scanning electron microscopy and atomic force microscopy. The XRD patterns of the film show that the as-deposited are polycrystalline and isostructural to Bi₂Te₃. The SEM study shows that the film is covered by crystallites while the AFM image reveals a low level of roughness.     

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.     

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.     

Thermoelectric transport and magnetoresistance of electrochemical deposited Bi2Te3 films at micrometer thickness

Bismuth telluride (Bi₂Te₃) is so far the best thermoelectric material for applications near room temperature, and also exhibits large magnetoresistance. While the electrochemical deposition approach can achieve effective growth of the Bi₂Te₃ films at micrometer thickness, the magnetoresistance transportation behavior of the electrochemically deposited Bi₂Te₃ films is yet not clear. In this work, we demonstrate the thermoelectric and magnetoresistance behaviors of the micrometer thick Bi₂Te₃ films deposited via electrochemical deposition approach. The optimum thermoelectric power factor is observed in the Bi₂Te₃ sample with electrochemical deposition thickness of ~6 μm followed by rapid photon annealing treatment, reaching the magnitude of ~1 μWcm⁻¹K⁻² that is similar to the previous reports. In contrast to the single crystalline or vacuum deposited Bi₂Te₃ or Bi₂Se₃ films, the electronic transportations of the electrochemically deposited Bi₂Te₃ are more influenced by the carrier scatterings by the grain boundaries and lattice defect. As a result, their magnetoresistance (MR) shows a distinguished non-monotonic behavior when varying the magnetic field, while the magnitude of their MR exhibits a positive temperature dependence. These MR behaviors largely differ to the previously reported ones from the single crystalline or vacuum deposited Bi₂Te₃ or Bi₂Se₃, in which cases their MR monotonically increases with the magnetic field and exhibits negative temperature dependence. This work reveals the previously overlooked role of grain boundary that also regulates the transportation properties of bismuth chalcogenides in the presence of magnetic field.     

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.     

The underpotential deposition of Bi2Te3−ySey thin films by an electrochemical co-deposition method

Bi₂Te_3−ySe_y thin films were grown on Au(1 1 1) substrates using an electrochemical co-deposition method at 25 °C. The appropriate co-deposition potentials based on the underpotential deposition (upd) potentials of Bi, Te and Se have been determined by the cyclic voltammetric studies. The films were grown from an electrolyte of 2.5 mM Bi(NO₃)₃, 2 mM TeO₂, and 0.3 mM SeO₂ in 0.1 M HNO₃ at a potential of −0.02 V vs. Ag|AgCl (3 M NaCl). X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were employed to characterize the thin films. XRD and EDS results revealed that the films are single phase with approximate composition of Bi₂Te_2.7Se_0.3. SEM studies showed that the films are homogeneous and have micronsized granular crystallites.     

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.     

Combined hot extrusion and spark plasma sintering method for producing highly textured thermoelectric Bi2Te3 alloys

Hot extrusion is a promising method for producing high-performance thermoelectric bismuth telluride alloys because of its ability to create textured microstructures. However, hot extrusion is less favourable for scaling-up because of temperature and strain gradients along the radial direction, and only <110> -textured thermoelectric legs can be obtained because of the fibre-like texture. We suggest a way to overcome these disadvantages by implementing an additional spark plasma sintering process on a stack of extrudates. Using this combined process, we demonstrate the fabrication of 12 × 15 × 13 mm³ p-type (Bi_0.2Sb_0.8)₂Te₃ samples from extrudates that had originally been 3 mm in diameter. The evolution of sheet-like texture revealed by SEM, XRD, and EBSD allows us to obtain both <110> - and <001> -textured thermoelectric legs from a single specimen that are desirable for low- and high-temperature applications, respectively. Our results demonstrate the combined method as an industry-friendly process for fabricating high-performance thermoelectric materials.     

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.     

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.     

Shift of tellurium solid-solubility limit and enhanced thermoelectric performance of bismuth antimony telluride milled with yttria-stabilized zirconia balls and vessels

As a thermoelectric material, Bi_0.3Sb_1.7Te_3.0+x (x = 0‒0.05) was fabricated by mechanical alloying using yttria-stabilized zirconia (YSZ) ceramic balls and vessels, followed by hot pressing. The effects of the added tellurium on the thermoelectric properties of Bi_0.3Sb_1.7Te_3.0 fabricated with YSZ milling media were investigated. All sintered samples were isotropic and showed p-type conduction. The tellurium solid-solubility limit for Bi_0.3Sb_1.7Te_3.0 was determined to be x = 0.01 by differential thermal analysis (DTA). The solid-solubility limit of the sample fabricated using YSZ was narrower than that of the congener prepared with Si₃N₄ balls and stainless-steel metal vessels. Among the evaluated compositions, the Bi_0.3Sb_1.7Te_3.01 sintered disk had the highest dimensionless figure of merit, ZT = 1.30, at room temperature. This value was superior to that of Bi_0.3Sb_1.7Te_3.0+x fabricated using metal vessels. Thus, selection of the milling media affected the optimum doping amount and maximum ZT.     

Impact of tellurium on glass transition and crystallization in the Ge-Se-Te-Bi system

Samples with compositions of Ge₂₀Se_67−xTe_xBi₁₃ (x=0, 5, 10, 15 and 20 mol%) were prepared and characterized by XRD, Raman spectroscopy and DSC. XRD patterns and Raman spectra present consistent evidences of glass transition and crystallization in association with the variation of the structure network due to the substitution of Te for Se. Precipitation of thermoelectric (TE) crystals experiences an evolution from Bi₂Se₂Te (BST) to Bi₂SeTe₂ (BTS) phases with the increasing Te content. Based on DSC data performed at five heating rates, crystallization kinetics is analyzed by using Kissinger's relation, Ozawa model and Augis-Bennett approximation. Different methods show the identical evaluations of characteristic activation energies (E_c), while additional parameters of frequency factor (K₀) and Avrami exponent (n) are available with AB method. In particular, calculated E_c and K₀ demonstrate that with the substitution of Se by Te the crystallization process of the target TE BST crystal phase is much enhanced while that of the undesired GeSe₂ phase is slightly hindered.     

A thermoelectric generator comprising selenium-doped bismuth telluride on flexible carbon cloth with n-type thermoelectric properties

Carbon cloth was used as a flexible substrate for bismuth telluride (Bi₂Te₃) particles to provide flexibility and improve the overall thermoelectric performance. Bi₂Te₃ on carbon cloth (Bi₂Te₃/CC) was synthesized via a hydrothermal reaction with various reaction times. After over 12 h, the Bi₂Te₃ particles showed a clear hexagonal shape and were evenly adhered to the carbon cloth. Selenium (Se) atoms were doped into the Bi₂Te₃ structure to improve its thermoelectric performance. The electrical conductivity increased with increasing Se-dopant content until 40% Se was added. Moreover, the maximum power factor was 1300 μW/mK² at 473 K for the 30% Se-doped sample. The carbon cloth substrate maintained its electrical resistivity and flexibility after 2000 bending cycles. A flexible thermoelectric generator (TEG) fabricated using the five pairs of 30% Se-doped sample showed an open-circuit voltage of 17.4 mV and maximum power output of 850 nW at temperature difference ΔT = 30 K. This work offers a promising approach for providing flexibility and improving the thermoelectric performance of inorganic thermoelectric materials for wearable device applications using flexible carbon cloth substrate for low temperature range application.     

Studies on surface preparation and smoothness of nanostructured Bi2Te3-based alloys by electrochemical and mechanical methods

Significant improvements in the dimensionless thermoelectric figure-of-merit (ZT) for nanostructured bismuth telluride, Bi₂Te₃, and its alloys have been demonstrated. In designing high-performance thermoelectric devices, variations in the thermal and electrical contact resistances due to interfacial effects between the nanostructured alloy and the metallic electrodes remain a significant issue. Smooth scratch-free surfaces should provide a baseline for contact resistance studies. In this paper, the root mean square roughness over a 10 μm² of nanostructured bismuth tellurium based alloys was reduced from 133 nm to 1.9 nm by a procedure consisting of electrolysis, mechanical polishing, and chemical mechanical polishing (CMP). Post-CMP cleaning was also developed to yield a wettable surface for the subsequent conformable metallization.     

Effects of Bi2Se3 Amount in Thermoelectric Performance of Bi2(TeSe)3 Materials Fabricated by High‐Energy Ball Milling

Amount of Bi₂Se₃ has significant role in controlling thermoelectric properties of n‐type Bi₂(TeSe)₃ material. In this study, effects of Se alloying amount in Bi₂(TeSe)₃ thermoelectric materials fabricated by high‐energy ball milling and spark plasma sintering were studied and compared with other fabrication methods. Amount of Bi₂Se₃ (5%, 10%, 15%, and 20%) did not have any significant effect over fabricated powder size, grains of consolidated bulks, and mechanical properties; however, electrical properties and thermoelectric efficiency were noticeably influenced. Both carrier concentration and carrier mobility decreased with increase in Se amount. In total, 20% Se alloying was effective in improving thermoelectric figure of merit ZT value by almost 40% compared with only 5% Se alloying.     

A mechanistic study of electrodeposition of bismuth telluride on stainless steel substrates

Miniaturization of Bi₂Te₃ compounds is of great interest in semiconductor industries due to their distinct anisotropic thermoelectric properties at room temperature. The aim of the present work was to investigate the mechanism of the electrodeposition of Bi₂Te₃ compounds on stainless steel substrates and relate the morphology and composition of the resulting deposits to experimental parameters. Cyclic voltammetry (CV) experiments in acidic solutions containing Bi³⁺ and/or HTeO₂⁺ ions show that the deposition potential for the Bi₂Te₃ compound is more positive than either of the single elements alone. A detailed mechanism of the co-deposition was obtained by varying the concentrations of the two elements and evaluating the corresponding morphological and compositional changes of the deposits. The results show that the deposition of Te is kinetically hindered and that Bi deposition plays a major role during the co-deposition.