Reduction in thermal conductivity of Bi–Te alloys through grain refinement method

Ternary alloys of thermoelectric materials Bi–Sb–Te and Bi–Se–Te of molecular formula, Bi_0·5Sb_1·5Te₃ (p type) and Bi_0·36Se_0·064Te_0·576 (n type), were prepared by mechanical alloying method. The preparation of materials by mechanical alloying method has effectively reduced the thermal conductivity by generating a large number of induced grain boundaries with required degree of disorder. The process of frequent milling was adapted for grain refinement. Substantial reduction in thermal conductivity was achieved due to nano-structuring of these alloys. Thermal conductivity values were found to be very low at room temperature, 0·5 W/mK and 0·8 W/mK, respectively for p and n type materials.     

Single Crystals of p-Type Solid Solutions between Bismuth and Antimony Chalcogenides for Cooling to T < 150 K

Sb₂Te₃–Bi₂Te₃ crystals (25–60 mol % Bi₂Te₃) doped with Bi₂Se₃ and excess Te were studied with the aim of identifying the optimal compositions for the p-legs of low-temperature coolers. The crystals were grown by the floating-crucible technique. Their transport properties were studied in the range 100–400 K. By measuring axial thermopower profiles, it was shown that increasing the Bi₂Te₃ and Bi₂Se₃ contents of the crystals has an adverse effect on their homogeneity. Crystals were prepared with a carrier concentration in the range (1–5) × 10¹⁹ cm^–3 and a thermoelectric power above 200 V/K at room temperature and the highest thermoelectric figure of merit at temperatures below 200 K. The maximum temperature drops and thermoelectric figures of merit were calculated for low-temperature stages of magnetothermoelectric coolers with hot-junction temperatures of 200 and 170 K and Bi–Sb n-legs.     

Effect of Annealing on Virgin and Recycled Carbon Fiber Electrochemically Deposited with N-type Bismuth Telluride and Bismuth Sulfide

N-type thermoelectric bismuth telluride (Bi₂Te₃) and bismuth sulfide (Bi₂S₃) were deposited on virgin carbon fiber (VCF) and recycled carbon fiber (RCF) substrates by electrodeposition. The effects of annealing on the surface morphology and the Seebeck coefficient of the Bi₂Te₃ and Bi₂S₃ films were investigated. A nearly stoichiometric N-type Bi₂Te₃ was obtained from an electrolyte solution of 8 mM of Bi(NO₃)₃.5H₂O and 10 mM of TeO₂, which displayed the highest Seebeck coefficient of ?20.01 and ?13.0 µV/K for VCF and RCF, respectively. The deposition of Bi₂S₃ was slightly off-stoichiometry, but the improvement was still significant with a Seebeck coefficient of ?16.3 and ?12.4 µV/K for VCF and RCF, respectively. The effect of varying the annealing temperature (275°C and 350°C) and annealing time (2 and 3 hours) was studied on a nearly stoichiometric N-type Bi₂Te₃. The result shows an improvement in the Seebeck coefficient by 1.51–1.24 times at 350°C for 2 hours.     

Effects of excess Te on the thermoelectric properties of p-type 25% Bi2Te3-75% Sb2Te3 single crystal and hot-pressed sinter

Effects of excess Te on the thermoelectric properties of p-type 25% Bi₂Te₃-75% Sb₂Te₃ single crystal and hot-pressed sinter were characterized and understood with the micro-phase diagram near the stoichiometric composition obtained by measuring the equilibrium Seebeck coefficient. Thermoelectric properties of the 25% Bi₂Te₃-75% Sb₂Te₃ single crystal were varied with the amount of excess Te, as -phase of the single crystal becomes less Te-deficient with adding more excess Te. However, thermoelectric properties of the hot-pressed sinter were not varied with the amount of excess Te, because the composition of -phase is not changed with the amount of excess Te. While a maximum figure-of-merit of 2.39 × 10^–3/K at 300 K was obtained for the 25% Bi₂Te₃-75% Sb₂Te₃ single crystal by adding 6 wt % excess Te, the hot-pressed 25% Bi₂Te₃-75% Sb₂Te₃ sinter exhibited the figure-of-merit of 2.97 × 10^–3/K regardless of the excess Te amount.     

Influence of annealing on the structural and electrical transport properties of Bi0.5Sb1.5Te3.0 thin films deposited by co-sputtering

Bi_0.5Sb_1.5Te_3.0 thin films were deposited on silicon substrates at room temperature by co-sputtering and the effects of annealing temperatures on structure and thermoelectric properties were investigated. The composition, crystallinity, and microstructure of these thin films were characterized by energy dispersive X-ray spectroscopy, X-ray diffraction, and scanning electron microscopy. The crystalline quality of the thin films was enhanced with a rising annealing temperature. When annealed at 573 K, the layered structure of the Bi_0.5Sb_1.5Te_3.0 thin films with a preferred orientation along the (00l) plane was formed. However, excessive high annealing temperature caused the thin films to become porous due to the separation of substantial Sb-rich precipitates. The electrical transport properties of the thin films, in terms of electrical conductivity and Seebeck coefficient were determined at room temperature. The carrier concentration and mobility were calculated from the Hall coefficient measurement. By optimizing the annealing temperature and time to 573 K for 6 h, the thermoelectric power factor was enhanced to 22.54 μW/(cm K²) at its maximum with a moderate electrical conductivity of 6.21 × 10² S/cm and a maximum Seebeck coefficient of 190.6 μV/K.     

Facile synthesis and thermoelectric studies of n-type bismuth telluride nanorods with cathodic stripping Te electrode

Bismuth telluride (Bi₂Te₃) nanorods (NRs) of n-type thermoelectric materials were prepared using an electrogenerated precursor of tellurium electrode in the presence of Bi³⁺ and mercapto protecting agent in aqueous solution under atmosphere condition. The optimal preparation conditions were obtained with ratio of Bi³⁺ to mercapto group and Te coulomb by photoluminescence spectra. The mechanism for generation of Bi₂Te₃ precursor was investigated via the cyclic voltammetry. The highly crystalline rhombohedral structure of as-prepared Bi₂Te₃ NRs with the shell of Bi₂S₃ was evaluated with high resolution transmission electron microscopy (HRTEM) and powder X-ray diffraction (XRD) spectroscopy. The near-infrared absorption of synthetic Bi₂Te₃ NRs was characterized with spectrophotometer to obtain information of electron at interband transition. The thermoelectric performance of Bi₂Te₃ NRs was assessed with the result of electrical resistivity, Seebeck coefficient, thermal conductivity, and the figure of merit ZT parameters, indicating that thermoelectric performance of as-prepared Bi₂Te₃ nanocrystals was improved by reducing thermal conductivity while maintaining the power factor.     

Nanostructures for Thermoelectric Applications: Synthesis,Growth Mechanism,and Property Studies

Both heterostructures and hollow nanostructures have been predicted as candidates with excellent thermoelectric performance. In this Research News areticle, recent advances with regard to synthetic strategies, growth mechanisms, and thermoelectric properties of one‐dimensional heterostructures (segmented and core/shell) and tubular nanostructures are reported. The thermoelectric property studies of Te/Bi core/shell heterostructured nanowires and Bi₂Te₃ nanotubes indicate that the Seebeck coefficient and thermal conductivity of these materials can be optimized to improve their thermoelectric performance. In addition, the current issues and future research directions for promising thermoelectric nanostructures will be discussed on the basis of these experimental results.     

Crystal growth and thermoelectric properties of tetradymite-like layered chalcogenides and (Bi2Te3)1−x−y (Sb2Te3) x (Sb2Se3) y solid solutions

Thermoelectric materials for segmented n-and p-legs of thermoelectric generators have been prepared by Czochralski growth with melt supply through a floating crucible. Two-segment ingots have been obtained using melt compositions corresponding to ternary layered compounds in the PbTe-Bi₂Te₃ and PbTe-Sb₂Te₃ systems, with (Bi₂Te₃)_1?x?y (Sb₂Te₃) _x (Sb₂Se₃) _y solid solutions as seed materials. Seeded growth of the ternary compounds makes it possible to fabricate legs without joining segments by soldering. Using scanning hot point microprobe measurements, we have studied the thermoelectric power distribution across the seed-crystal interface. The results attest to a steep thermoelectric power gradient across the seed-crystal interface in a narrow region. Quantitative analysis of the distribution of the number of measurements with respect to thermoelectric power has revealed peaks corresponding to individual segments.     

Bonding and interfacial reaction between Ni foil and n-type PbTe thermoelectric materials for thermoelectric module applications

Integration of next generation thermoelectric materials in thermoelectric modules requires a novel or alternative approach for mating the brittle semiconducting thermoelectric materials and the ductile metal interconnects. In this study, pure Ni foil was directly bonded to PbTe-based thermoelectric materials using a rapid hot-press. The materials were sintered at 600 and 650 °C, under a pressure of 40 MPa and for various holding times. The resulting interfacial microstructures of the Ni/PbTe joints were investigated. Additionally, the distributions of elements and the phases formed at the Ni/PbTe interface were analyzed. The β₂ phase (Ni_3±x Te₂, 38.8–41 at.% Te) was identified at the Ni/PbTe joints bonded at both 600 and 650 °C. A ternary phase with approximate composition Ni₅Pb₂Te₃ was found at the Ni/PbTe joints bonded at 650 °C. Additionally, the PbTe(Ni) phase was observed along the Ni grain boundaries for both bonding temperatures. Thermodynamics calculation results indicate that only the β₂ phase can be formed at the Ni/PbTe interface at 900 K among the binary nickel tellurides.     

Solubility and formation of ternary Widmanstätten precipitates in PbTe in the pseudo-binary PbTe–Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> system

A unidirectional solidification experiment by Bridgman method has been performed for the Pb₁₄Bi_28.8Te_57.2 composition, which lies on the pseudo-binary PbTe–Bi₂Te₃ system, resulting in the formation of Widmanstätten precipitates of a ternary compound, most likely with the structure of PbBi₂Te₄ in the PbTe matrix. The formation of the precipitates is caused by the decrease of bismuth solubility in the PbTe phase with decreasing temperature. The PbTe-rich part of the PbTe–Bi₂Te₃ phase diagram was investigated from the compositional variations in the unidirectionally solidified sample and the diffusion couples. This proved that the solubility decreases with decreasing temperature: 15.6 ± 0.9 (583 °C) to \( 6. 2_{ - 1.7}^{ + 2.1} \) (450 °C) at.% Bi. The orientation relationship between the matrix and precipitates has been examined by electron backscatter diffraction technique; precipitation occurs on {111} habit planes in PbTe with orientation relationship (0001)_precipitate//{111}_PbTe and <11\( \overline{2} \)0>_precipitate//<110>_PbTe. The thermoelectric properties in PbTe with Widmanstätten precipitates as examined by the scanning Seebeck probe method is –46 ± 2 μVK^?1.     

Selective Dissolution-Derived Nanoporous Design of Impurity-Free Bi2Te3 Alloys with High Thermoelectric Performance

Thermoelectric technology, which has been receiving attention as a sustainable energy source, has limited applications because of its relatively low conversion efficiency. To broaden their application scope, thermoelectric materials require a high dimensionless figure of merit (ZT). Porous structuring of a thermoelectric material is a promising approach to enhance ZT by reducing its thermal conductivity. However, nanopores do not form in thermoelectric materials in a straightforward manner; impurities are also likely to be present in thermoelectric materials. Here, a simple but effective way to synthesize impurity-free nanoporous Bi_0.4Sb_1.6Te₃ via the use of nanoporous raw powder, which is scalably formed by the selective dissolution of KCl after collision between Bi_0.4Sb_1.6Te₃ and KCl powders, is proposed. This approach creates abundant nanopores, which effectively scatter phonons, thereby reducing the lattice thermal conductivity by 33% from 0.55 to 0.37 W m⁻¹ K⁻¹. Benefitting from the optimized porous structure, porous Bi_0.4Sb_1.6Te₃ achieves a high ZT of 1.41 in the temperature range of 333–373 K, and an excellent average ZT of 1.34 over a wide temperature range of 298–473 K. This study provides a facile and scalable method for developing high thermoelectric performance Bi₂Te₃-based alloys that can be further applied to other thermoelectric materials.     

Mg Compensating Design in the Melting-Sintering Method For High-Performance Mg3(Bi,Sb)2 Thermoelectric Devices

N-type Mg₃(Bi, Sb)₂-based thermoelectric (TE) alloys show great promise for solid-state power generation and refrigeration, owing to their excellent figure-of-merit (ZT) and using cheap Mg. However, their rigorous preparation conditions and poor thermal stability limit their large-scale applications. Here, this work develops an Mg compensating strategy to realize n-type Mg₃(Bi, Sb)₂ by a facile melting-sintering approach. “2D roadmaps” of TE parameters versus sintering temperature and time are plotted to understand the Mg-vacancy-formation and Mg-diffusion mechanisms. Under this guidance, high weight mobility of 347 cm² V⁻¹ s⁻¹ and power factor of 34 µW cm⁻¹ K⁻² can be obtained for Mg_3.05Bi_1.99Te_0.01, and a peak ZT≈1.55 at 723 K and average ZT≈1.25 within 323–723 K can be obtained for Mg_3.05(Sb_0.75Bi_0.25)_1.99Te_0.01. Moreover, this Mg compensating strategy can also improve the interfacial connecting and thermal stability of corresponding Mg₃(Bi, Sb)₂/Fe TE legs. As a consequence, this work fabricates an 8-pair Mg₃Sb₂-GeTe-based power-generation device reaching an energy conversion efficiency of ≈5.0% at a temperature difference of 439 K, and a one-pair Mg₃Sb₂-Bi₂Te₃-based cooling device reaching −10.7 °C at the cold side. This work paves a facile way to obtain Mg₃Sb₂-based TE devices at low cost and also provides a guide to optimize the off-stoichiometric defects in other TE materials.     

Synthesis and thermoelectric properties of bismuth–antimony–tellurium-based nanopowders

Bismuth–antimony–tellurium-based nanopowders were fabricated by a chemical process in which dissolved Bi, Te and Sb salts were directly reduced in each element via surfactant-assistant polyol reducing agents. XRD patterns of the synthesized nanopowders showed that the formed phases correspond mainly to (Bi_0.5Sb_0.5)₂Te₃ and Bi_0.5Sb_1.5Te₃, respectively. The phases revealed that the three different elements were stably alloyed as ternary composition in one powder via the simple chemical route. The nanopowders were consolidated into bismuth telluride-based bulk materials that exhibited electrical resistivity above 5.6 × 10^?5 Ωm, 150 μ V/K of the Seebeck coefficient and 0.7 W/mK of thermal conductivity at room temperature. These results showed that p-type thermoelectric nanopowders obtained from a simplified chemical process could be used in making thermoelectric materials towards high performances.     

Ag_2Te掺杂对p型Bi_(0.5)Sb_(1.5)Te_3块状合金热电性能的影响

采用真空熔炼、机械球磨及放电等离子烧结技术(SPS)制备得到了(Ag2Te)x(Bi0.5Sb1.5Te3)1-x(x=0,0.025,0.05,0.1)系列样品,性能测试表明,Ag2Te的掺入可以显著改变材料的热电性能变化趋势,掺杂样品在温度为450~550K范围内具有较未掺杂样品更优的热电性能.适当量的Ag2Te掺入能够有效地提高材料的声子散射,降低材料的热导率.在测试温度范围内,(Ag2Te)0.05(Bi0.5Sb1.5Te3)0.95具有最低的晶格热导,室温至575K范围内保持在0.2~0.3W/(m·K)之间,在575K时,(Ag2Te)0.05(Bi0.5Sb1.5Te3)0.95试样具有最大热电优值ZT=0.84,相较于未掺杂样品提高了约20%.     

Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript>-4 mol % Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Single Crystals with Graded Carrier Concentration

A procedure is developed for the preparation of Bi_0.5Sb_1.5Te₃-4 mol % Bi₂Se₃ single crystals with a graded longitudinal carrier concentration profile. The thermoelectric power gradient in the crystals is 40–70 μV/K over a distance of 1–1.5 mm. The shape of the growth interface is determined. Graded legs are produced that have optimal carrier concentrations for different working temperatures of thermoelectric coolers (the thermoelectric power at the hot end is from 220 to 270 μV/K). Regions with low carrier concentration (α > 220 μV/K) are shown to contain Te-based eutectic precipitates along the cleavage plane. __________ Translated from Neorganicheskie Materialy, Vol. 41, No. 10, 2005, pp. 1200–1205. Original Russian Text Copyright ? 2005 by Ivanova, Granatkina, Petrova, Korzhuev.     

Optimization of thermoelectric properties on Bi2Te3 thin films deposited by thermal co-evaporation

The optimization of the thermal co-evaporation deposition process for n-type bismuth telluride (Bi₂Te₃) thin films deposited onto polyimide substrates and intended for thermoelectric applications is reported. The influence of deposition parameters (evaporation rate and substrate temperature) on film composition and thermoelectric properties was studied for optimal thermoelectric performance. Energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy confirmed the formation of Bi₂Te₃ thin films. Seebeck coefficient (up to 250 μV K^− 1), in-plane electrical resistivity (≈10 μΩ m), carrier concentration (3×10¹⁹-20×10¹⁹ cm^− 3) and Hall mobility (80-170 cm² V⁻¹ s^− 1) were measured at room temperature for selected Bi₂Te₃ samples.     

High performance functionally graded and segmented Bi2Te3-based materials for thermoelectric power generation

Bi₂Te₃-based materials possess a figure of merit maximum over a narrow temperature range. When used in a generating mode over a large temperature difference the material operates at a substantially lower overall figure of merit than its maximum value. The conversion efficiency of a thermoelectric generator for low temperature waste heat recovery can be increased by employing functionally graded or segmented materials. In this work functionally graded p-type Bi₂Te₃-based thermoelectric materials have been prepared from melt by the Bridgman method using double doping technique. Segmented n-type thermoelement has been fabricated by joining two Bi₂Te₃-based materials with figure of merit maximum at 270 K and 380 K. The thermoelectric properties of the materials and a thermocouple comprised of p-type functionally graded and n-type segmented materials have been measured over a temperature range 200 K–450 K. The material efficiency of the thermocouple over the temperature gradient 223 K–423 K is estimated to be 10% compared with 8.8% for a standard Bi₂Te₃-based materials.     

Manipulating the Interfacial Band Bending For Enhancing the Thermoelectric Properties of 1T′-MoTe2/Bi2Te3 Superlattice Films

Interfacial charge effects, such as band bending, modulation doping, and energy filtering, are critical for improving electronic transport properties of superlattice films. However, effectively manipulating interfacial band bending has proven challenging in previous studies. In this study, (1T′-MoTe₂)_x(Bi₂Te₃)_y superlattice films with symmetry-mismatch were successfully fabricated via the molecular beam epitaxy. This enables to manipulate the interfacial band bending, thereby optimizing the corresponding thermoelectric performance. These results demonstrate that the increase of Te/Bi flux ratio (R) effectively tailored interfacial band bending, resulting in a reduction of the interfacial electric potential from ≈127 meV at R = 16 to ≈73 meV at R = 8. It is further verified that a smaller interfacial electric potential is more beneficial for optimizing the electronic transport properties of (1T′-MoTe₂)_x(Bi₂Te₃)_y. Especially, the (1T′-MoTe₂)₁(Bi₂Te₃)₁₂ superlattice film displays the highest thermoelectric power factor of 2.72 mW m⁻¹ K⁻² among all films, due to the synergy of modulation doping, energy filtering, and the manipulation of band bending. Moreover, the lattice thermal conductivity of the superlattice films is significantly reduced. This work provides valuable guidance to manipulate the interfacial band bending and further enhance the thermoelectric performances of superlattice films.     

Pressure-induced band cross-over in Pb1−x Sn x Te

Resistivity and thermoelectric power studies have been carried out on two semiconductor alloy systems viz Pb_0·8Sn_0·2Te and Pb_0·6Sn_0·4Te up to 35 kbar pressure. Thermoelectric power and resistivity data on Pb_0·8Sn_0·2Te indicate that the energy gapE _g=E _{L −} ⁶ −E _{L 6} ⁺ decreases with pressure resulting in a zero gap state near 35 kbar pressure. TEP studies on the alloy system Pb_0·6Sn_0·4Te provide direct evidence for a pressure induced L ₆ ⁻ →L ₆ ⁺ cross over transition.     

Effects of annealing on thermoelectric properties of Sb2Te3 thin films prepared by radio frequency magnetron sputtering

Antimony telluride (Sb₂Te₃) thin films were deposited on silicon substrates at room temperature (300 K) by radio frequency magnetron sputtering method. The effects of annealing in N₂ atmosphere on their thermoelectric properties were investigated. The microstructure and composition of these films were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction, respectively. The electrical transport properties of the thin films, in terms of electrical conductivity and Seebeck coefficient were determined at room temperature. The carrier concentration and mobility were calculated from the Hall coefficient measurement. Both of the Seebeck coefficient and Hall coefficient measurement showed that the prepared Sb₂Te₃ thin films were p-type semiconductor materials. By optimizing the annealing temperature, the power factor achieved a maximum value of 18.02 μW cm^?1 K^?2 when the annealing temperature was increased to 523 K for 6 h with a maximum electrical conductivity (1.17 × 10³ S/cm) and moderate Seebeck coefficient (123.9 μV/K).