Experimental estimation of the Lorenz number and scattering parameter for p-type bismuth antimony telluride via multiple doping under constant temperature conditions

The effects of the addition of lead as one of the multiple dopants in p-type Bi_0.3?xSb_1.7Te_3.0+0.01Pb_x (x = 0, 0.001, 0.002, 0.003) fabricated by mechanical alloying followed by hot pressing were investigated. Measurements by X-ray diffraction (XRD), differential thermal analysis (DTA), and scanning electron microscopy (SEM) showed the same matrix morphology. The second phase by doped elements was not confirmed by transmission electron microscopy (TEM). By using the lead addition results and previously studied tellurium doping effects, the Lorenz number L was evaluated to be 0.73–1.18 × 10^?8 W S^?1 K^?2. The scattering parameter γ and reduced Fermi energy η were estimated by using expressions on the basis of a one-electron approximation, measured Seebeck coefficients, and the estimated L at room temperature. The γ ranged approximately from ?1.06 to ?0.60 and showed a mutual effect of acoustic and optical phonon scattering. The relationship between a dimensionless figure of merit ZT and η was clarified. The optimum η was determined as ?1.25 at ZT = 1.26. From these results, multi-doped Bi_0.3Sb_1.7Te_3.0 could be applied to evaluate L, γ, and η at a constant temperature.     

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.     

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.     

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.     

Electrodeposition of P-type BixSb2−xTey thermoelectric film from dimethyl sulfoxide solution

The electrochemical behaviors of Bi(III), Te(IV), Sb(III) and their mixtures in DMSO solutions were investigated using cyclic voltammetry and linear sweep voltammetry measurements. On this basis, Bi_xSb_2−xTe_y film thermoelectric materials were prepared by potentiodynamic electrodeposition technique from mixed DMSO solution, and the compositions, structures, morphologies as well as the thermoelectric properties of the deposited films were also analyzed. The results show that Bi_xSb_2−xTe_y compound can be prepared in a very wide potential range by potentiodynamic electrodeposition technique in the mixed DMSO solutions. After anneal treatment, the deposited film prepared in the potential range of −200 to −400 mV shows the highest Seebeck coefficient (185 μV/K), the lowest resistivity (3.34 × 10⁻⁵ Ω m), the smoothest surface, the most compact structure and processes the stoichiometry (Bi_0.49Sb_1.53Te_2.98) approaching to the Bi_0.5Sb_1.5Te₃ ideal material most. This Bi_0.49Sb_1.53Te_2.98 film is a kind of nanocrystalline material and (0 1 5) crystal plane is its preferred orientation.     

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.     

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.     

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.     

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.     

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.     

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.     

Thermoelectric properties of Cu-dispersed bi<Subscript>0.5</Subscript>sb<Subscript>1.5</Subscript>te<Subscript>3</Subscript>

A novel and simple approach was used to disperse Cu nanoparticles uniformly in the Bi_0.5Sb_1.5Te₃ matrix, and the thermoelectric properties were evaluated for the Cu-dispersed Bi_0.5Sb_1.5Te₃. Polycrystalline Bi_0.5Sb_1.5Te₃ powder prepared by encapsulated melting and grinding was dry-mixed with Cu(OAc)₂ powder. After Cu(OAc)₂ decomposition, the Cu-dispersed Bi_0.5Sb_1.5Te₃ was hot-pressed. Cu nanoparticles were well-dispersed in the Bi_0.5Sb_1.5Te₃ matrix and acted as effective phonon scattering centers. The electrical conductivity increased systematically with increasing level of Cu nanoparticle dispersion. All specimens had a positive Seebeck coefficient, which confirmed that the electrical charge was transported mainly by holes. The thermoelectric figure of merit was enhanced remarkably over a wide temperature range of 323-523 K.     

Effect of excess Ge and Te on thermoelectric performance of GeTe

GeTe is a medium-temperature thermoelectric material with excellent performance. The thermoelectric performance of GeTe is affected by the carrier concentration generated by Ge vacancy. Therefore, it is of important to study the effect of excess Ge or Te on the thermoelectric performance of GeTe. In this paper, Ge_xTe_y materials (x:y = 1:1.08, 1:1.06, 1:1.04, 1:1, 1.05:1, 1.075:1, and 1.1:1) were fabricated by high-pressure sintering (HPS) and spark plasma sintering (SPS), respectively, to study the effects of different Ge/Te atomic ratios and preparation process on the thermoelectric properties of polycrystalline GeTe. The composition and microstructure were investigated by an X-ray diffraction method (XRD) and field-emission scanning electron microscope (FESEM). The thermoelectric performance was tested from 303 to 703 K. The measurement results show that the Seebeck coefficient of Ge_xTe_y increases and the conductivity decreases with the decreasing in Te content or the increasing in Ge content. Ge₁Te₁ exhibits the highest power factor because its Seebeck coefficient and conductivity are at an average level. Owing to the presence of pure Ge and the decrease of Ge vacancy, the lattice thermal conductivities of samples with excess Ge are higher than that of Ge₁Te₁. Ge₁Te₁ sintered by HPS has the highest ZT_max value, reaching 1.37 at 723 K.     

Novel Ge-Sb-Te thermoelectric materials: A demonstration for an efficient diffusion couple technique in expediently exploiting new thermoelectric materials

An efficient diffusion couple technique is proposed for rapid screening of potential thermoelectric materials and it has been explored in a (GeTe)_m (Sb₂Te₃)_n pseudobinary system. In this study, a combinatorial sample library of GeTe-Sb₂Te₃ diffusion couple with rich compositions and structures was prepared by plasma activation sintering (PAS). The relationship among composition, structure and performance of the materials was revealed by analyzing the chemical composition, crystal structure and thermoelectric properties of the microdomains in the combinatorial sample library, so as to achieve the purpose for rapid identification of Ge-Sb-Te compounds with excellent thermoelectric properties. The scanning results of the Seebeck coefficient of the samples library show that the sample with potential high thermoelectric performance has a chemical composition near Ge_33.1Sb_13.7Te_53.2. The bulk materials with compositions in the vicinity of Ge_33.1Sb_13.7Te_53.2 were prepared by a conventional melting and PAS process. Among these bulk samples, the one with a nominal composition of Ge₃₈Sb_10.3Te_51.7 achieves a maximum ZT value of 0.55 at 673 K, indicating that the constituent phases Ge_0.77Sb_0.154Te₁ and Ge_3.37Sb_1.63Te₆ observed in this sample could be potential candidates with excellent thermoelectric properties. These two ternary compounds should be investigated for further optimization. The results of this study provide a reference for the application of combinatorial metrology for screening and optimizing new thermoelectric materials.     

Effects of anion and cation doping on the thermoelectric properties of n-type PbS

The PbCl_xS_1-x and Pb_1-xBi_xS (x? =?0–0.05) bulks were fabricated with a facile method of hydrothermal synthesis and microwave sintering, and the effect of anionic and cationic donors on the thermoelectric performance of PbS was investigated. Although Cl^? and Bi³⁺ both effectively improved the thermoelectric properties of n-type PbS, more excellent thermoelectric performance was obtained from Cl^? doped samples because of higher electrical property and lower thermal conductivity at higher temperature (T? >?600?K). The thermoelectric figure of merit (ZT) reaches 1.04 for PbCl_0.015S_0.985 at 800?K and increases with temperature increasing without sign of saturation, which is probably the highest value ever reported for single-phase polycrystalline n-type PbS. The results also indicate that the hydrothermal synthesis and microwave sintering can realize anion doping as well as cation doping for n-type PbS at low cost, and PbS should be a robust alternative for PbTe thermoelectric materials.     

Fabrication of bismuth telluride nanoparticles using a chemical synthetic process and their thermoelectric evaluations

Bismuth telluride nanoparticles for thermoelectric applications were successfully prepared via a water-based chemical reaction. In this process, we used both a complexing agent (ethylenediaminetetraacetic acid) and a reducing agent (ascorbic acid) to stabilize the bismuth precursor (Bi(NO₃)₃) in water and to favor the reaction with the reduced source of tellurium. The resulting powder was confirmed to range in size below ca. 100 nm with the crystalline structure corresponding to the rhmobohedral Bi₂Te₃. We sintered the nanocrystalline powder via a spark plasma sintering process, thus we obtained the sintered body composed of nano-sized grains. Then, we measured some important transport properties (electrical resistivity, Seebeck coefficient, and thermal conductivity) of the sintered body to calculate its thermoelectric performance, the figure of merit. Finally, we discussed the effect of the nanostructure in the sintered body on the thermal conductivity.     

Effect of Sintering Temperature on Thermoelectric Properties of p‐Bi2Te3 Alloys Produced by Gas Atomization

In this research, p‐type Bi₂Te₃–75% Sb₂Te₃ thermoelectric alloy powders were produced by gas atomization and subsequently sintered by hot pressing at different temperatures. The grain growth of the hot‐pressed samples was observed with increasing sintering temperature from 380°C to 460°C. The compressive strength increased with increasing hot‐pressing temperature due to the high relative density of bulk samples obtained at high temperatures. The effect of sintering temperature on thermoelectric (TE) properties was studied. The maximum power factor 3.48 mW/mK² was obtained for the sample hot pressed at 420°C due to the resulting high electrical conductivity and enhanced Seebeck coefficient values.     

Enhancing thermoelectric properties of p-type (Bi,Sb)2Te3 via porous structures

Bismuth telluride is a widely used commercial thermoelectric material with excellent thermoelectric performances near room temperature. Reducing thermal conductivity is one of the most effective ways to improve performances of thermoelectric materials. In this study, the thermal conductivity of the material was reduced by fabricating porous structures. Highly dense NaCl-(Bi,Sb)₂Te₃ composites were fabricated by a high-pressure technology. The NaCl phase was then removed from the composites by ultrasonic washing to produce porous structures. The produced (Bi,Sb)₂Te₃ porous materials possessed excellent thermoelectric properties. The porosity and pore size of the (Bi,Sb)₂Te₃ porous materials increased with the increasing NaCl content, decreasing the thermal conductivity significantly. An ultra-low lattice thermal conductivity of 0.21 Wm^?1K^?1 at 493 K was achieved when the porosity was 39%, almost the lowest lattice thermal conductivity reported for (Bi,Sb)₂Te₃ bulk materials. The figure of merit ZT value was enhanced to 1.05 at 493 K when the porosity was 25%. Compared with the most compacted samples (ZT = 0.79 and porosity of 10%) prepared under the same conditions, the ZT value of the porous samples increased by 33%. This study indicated that porous thermoelectric materials can be prepared simply, quickly and efficiently by high-pressure/ultrasonication washing to improve thermoelectric performances, which has evident reference values for preparing other thermoelectric pore materials with enhancing behaviors.     

Pulsed electrodeposition of (Bi1-xSbx)2Te3 thermoelectric thin films

(Bi_1-xSb_x)₂Te₃ thermoelectric thin films were deposited on stainless steel discs in 1 M perchloric acid and 0.1 M tartaric acid by pulse electrodeposition in order to optimize the grain growth. The influence of the electrolyte composition, the cathodic current density and the cathodic pulse time on film stoichiometry were studied. The results show that it is necessary to increase the Sb content in the electrolyte to obtain the (Bi_0.25Sb_0.75)₂Te₃ film stoichiometry. Pulse plating reduced the grain size and the roughness, compared with continuous plating. Thermoelectric and electrical properties were also studied and it was found that the Seebeck coefficient and electrical resistivity were related to two parameters: the cathodic pulse current density and the films thickness.     

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.