Thermoelectric properties of (Bi0.25Sb0.75)2Te3 alloys fabricated by hot-pressing method

Thermoelectric properties of the hot-pressed p-type (Bi_0.25Sb_0.75)₂Te₃ alloy were characterized with variation of the hot-pressing temperature and the starting powder size. The roles of the factors which affect the Seebeck coefficient of the hot-pressed (Bi_0.25Sb_0.75)₂Te₃ alloy has been elucidated in this study. The donor-like behavior of oxygen could be one of the possible explanations for the higher Seebeck coefficient of the hot-pressed (Bi_0.25Sb_0.75)₂Te₃ alloy. Te vacancies formed by mechanical deformation during the powdering process significantly promote the diffusion of second phase Te atoms into their lattice sites so that the matrix Te solubility approaches its equilibrium value at a given temperature in a relatively short length of time. Using the Seebeck coefficient at various hot-pressing temperatures, the micro-phase diagram near the stoichiometric composition of (Bi_0.25Sb_0.75)₂Te₃ was evaluated.     

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.     

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).     

Morphological evolution in single-crystalline Bi2Te3 nanoparticles, nanosheets and nanotubes with different synthesis temperatures

A general surfactant-assisted wet chemical route has been developed for the synthesis of a variety of bismuth telluride (Bi₂Te₃) single-crystalline nanostructures with varied morphologies at different temperatures in which hydrazine hydrate plays as an important solvent. Bi₂Te₃ sheet grown nanoparticles, nanosheets and nanotubes have been synthesized by a simplest wet chemical route at 50, 70 and 100 °C within 4 h. Bi₂Te₃ sheet grown nanoparticles are obtained in agglomerate state and they are found with many wrinkles. Various types of Bi₂Te₃ nanotubes are also found which are tapered with one end open and the other closed. X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) pattern and energy dispersive X-ray (EDX) spectroscopy were employed to characterize the powder product. It is found that all nanoparticles, nanosheets and nanotubes are well-crystallized nanocrystals and morphologies of the powder products are greatly affected by different synthesis temperatures. The formation mechanisms of bismuth telluride nanostructures are also discussed.     

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.     

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.     

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.     

Pushing the optimal <Emphasis Type="Italic">ZT</Emphasis> values of p-type Bi<Subscript>2−x</Subscript>Sb<Subscript>x</Subscript>Te<Subscript>3</Subscript> alloys to a higher temperature by expanding band gaps and suppressing intrinsic excitation

In this paper, the band gap and intrinsic excitation temperature of p-type Bi_2?xSb_xTe₃ alloys were effectively adjusted by controlling antimony content. The higher the intrinsic excitation temperature was (higher the antimony content was), faster the electrical conductivity decreased with temperature, and the Seebeck coefficient peaks also occurred at a higher temperature. In addition, the effect of ambipolar thermal conductivity could not be ignored when the temperature is over the intrinsic excitation temperature. The starting temperature of ambipolar thermal conductivity increased from 320 to 380 K with x from 1.52 to 1.64 for Bi_2?xSb_xTe₃ alloys, and meanwhile the contribution of the ambipolar thermal conductivity to the total thermal conductivity was also becoming smaller and smaller. As a result, the corresponding optimal peaks of the figure of merits (ZT) were pushed to a higher temperature with increasing antimony contents. The sample of Bi_0.44Sb_1.56Te₃ obtained the maximum ZT value of 1.22 at 340 K, and the mean ZT values of Bi_0.4Sb_1.6Te₃ and Bi_0.44Sb_1.56Te₃ were all over 1.0 in a wide range of temperature from 300 to 460 K, which would be very suitable for the low temperature thermoelectric power generation.     

Optimizing phase and microstructure of chemical solution-deposited bismuth ferrite (BiFeO3) thin films to reduce DC leakage

Polycrystalline bismuth ferrite (BiFeO₃ or BFO) thin films were prepared by chemical solution deposition to explore the impact of processing conditions including annealing temperature, percent excess bismuth, and gel drying temperature on film microstructure and properties. Incorporating 0–5 % excess Bi and annealing at 550 °C in air produced stoichiometric single-phase BiFeO₃ films. Deviation from this temperature yielded the bismuth-rich Bi₃₆Fe₂O₅₇ phase at temperatures below 550 °C or the bismuth-deficient Bi₂Fe₄O₉ phase at temperatures above 550 °C, both of which contributed to higher DC leakage. However, even single-phase BiFeO₃ films produced at 550 °C show high DC leakage (~1.2 × 10^?1 A/cm² at 140 kV/cm) due to a porous microstructure. We have thus investigated unconventional thermal treatments that significantly increase film densification while maintaining phase purity. Under these revised thermal treatment conditions, room temperature leakage current values are reduced by three orders of magnitude to ~1.0 × 10^?4 A/cm² at 140 kV/cm.     

Effect of substrate temperature on the structural,morphological and optical properties of copper bismuth sulfide thin films deposited by electron beam evaporation method

Copper bismuth sulfide thin films were deposited at 200 °C, 300 °C, 400 °C and 500 °C on the glass substrates by electron beam evaporation method. X-ray diffraction study revealed that the copper bismuth sulfide films of single and mixed phases were formed as a function of substrate temperatures. Substrate temperature of 300 °C and 400 °C formed single phase Cu₄Bi₄S₉ and Cu₄Bi₅S₁₀ films respectively whereas substrate temperature of 500 °C formed mixed phases of Cu₄Bi₄S₉ and Cu₄Bi₅S₁₀ film. Crystallite size, dislocation density and microstrain of the films were modified by the various substrate temperatures. Surface morphology of the film Cu₄Bi₅S₁₀ deposited at 400 °C examined by scanning electron microscopy showed the distribution of spherical shaped particles on the film surface. The presence of copper, bismuth and sulfur elements in the deposited films was confirmed using energy dispersive spectral studies. The calculated direct optical band gap energy of the films deposited at different substrate temperature varied from 1.47 to 1.64 eV and the absorption coefficient is in the order of 10⁶ cm^?1.     

Preparation and characterization of segmented stacking for thermoelectric power generation

Based on the Seebeck effect, thermoelectric generators can convert thermal energy directly into electrical power, which can be applied in waste heat recovery and clean energy generation. In this work, segmented thermoelectric legs were prepared with high-performance thermoelectric materials for the fabrication of multistage thermoelectric generators, which can be utilized in medium temperature energy harvesting. The P-type leg material was Pb_0.94Sr_0.04Na_0.02Te/Bi_0.5Sb_1.5Te₃, and the N-type leg material was Pb_0.94Ag_0.01La_0.05Te/Bi₂Te₃. The length ratio of the two segments was optimized based on the energy conversion efficiency under different working conditions. The segmented legs were measured with the four-probe method at different temperatures to evaluate their output performance. At a temperature difference of 420 K, the maximum output power density was 0.40 W/cm² for the P-type leg and 0.32 W/cm² for the N-type leg.     

Characterization of thermoelectric properties of layers obtained by pulsed magnetron sputtering

The pulsed magnetron sputtering technique was applied for the preparation of layers of Bi₂Te₃ and Sb₂Te₃. Target materials were synthesized in evacuated quartz ampoules by melting elemental powders mixed in stoichiometric proportions. The structure and microstructure of targets and prepared films were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. Thermoelectric properties were defined by the Seebeck coefficient and electrical conductivity measurements in the temperature range 320-430 K. The layers were deposited at various powers (0.09-0.20 kW) and currents (0.07-0.16 A) at an argon pressure of about 3.0 Pa. The efficiencies of thermoelectric power obtained for bismuth telluride and antimony telluride were 2-4×10⁻⁴ and 2-6×10⁻³ W K⁻² m⁻¹, respectively. The synthesized materials were used for the fabrication of thermoelectric couples with Bi₂Te₃ as the n-type material and Sb₂Te₃ as the p-type material. The thermocouples were annealed under vacuum to obtain optimum thermoelectric properties. The Seebeck coefficient of thermocouples was evaluated by a Seebeck scanning microprobe [Platzek D, Karpinski G, Stewie C, Muchilo D, Müller E. Proceedings of the second European conference on thermoelectrics, Poland, Cracow, September 15-17, 2004].     

Low temperature reduction route to synthesise bismuth telluride (Bi2Te3) nanoparticles: structural and optical studies

Herein, we report the synthesis of highly yielded bismuth-telluride (Bi₂Te₃) nanoparticles at 50 °C by direct wet chemical route in which the bismuth and tellurium precursors have been dissolved in deionised water, ethylene glycol and hydrazine hydrate. This method is very facile, inexpensive and less hazardous and ensures almost complete yield of the precursors. The powder product was well characterised by powder X-ray diffraction, UV-Vis spectroscopy, Fourier transform infrared spectroscopy, energy dispersive X-ray diffraction, transmission electron microscopy and scanning electron microscopy. It is investigated that the synthesised powder has a rhombohedral structure of Bi₂Te₃ with average diameters of the particles about 35 nm. Thus, the synthesis process has been modified to design nanostructures of thermoelectric materials with related crystal structures.     

Synthesis of polycrystalline nanotubular Bi2Te3

Polycrystalline nanotubular Bi₂Te₃ could be prepared via a high-temperature solution process using nanoscale tellurium, decomposed from trioctylphosphine oxide (TOPO) extracted tellurium species (Te-TOPO), as sacrificial template. The formation of such tubular structure is believed to be the result of outward diffusion of Te during the alloying process. The electrical properties (Seebeck coefficient and electrical conductivity) of the polycrystalline nanotubular Bi₂Te₃ have been studied and the experimental results show that the electrical conductivity is approximately three orders of magnitude smaller than bulk bismuth telluride materials mainly due to the much larger resistance brought by the insufficient contact between the nanotubular structures.     

Microstructural features and deformation-induced lattice defects in hot-extruded Bi2Te3 thermoelectric compound

Bismuth telluride (Bi₂Te₃) bulk material was prepared by a powder hot-extrusion technique. The microstructure, texture evolution, and deformation-induced lattice defects, as well as the effect of heat treatment on its microstructure and thermoelectric properties were investigated. As-extruded Bi₂Te₃ compound was found to be characterized by a dynamically and statically recrystallized microstructure with fine-equiaxed grains and low dislocation density. The majority of boundaries in the extruded specimens were large-angle boundaries with average misorientation angles of more than 30°. There was no significant change in dislocation configuration, even after heat treatment at a temperature higher than the extrusion temperature. Moreover, the Seebeck coefficient and electrical resistivity of the extruded specimens could not be improved by annealing. Microstructural examinations indicated the existence of a deformation-induced stacking disorder with local atomic shift, which we term “phase disorder,” on the basal plane in the extruded Bi₂Te₃ compound. The presence of this structural defect may be one of the main reasons for the slightly lower values of thermoelectric properties for the hot-extruded Bi₂Te₃ compound, compared with a unidirectionally solidified specimen.     

Preparation and electrical transport properties of In filled and Te-doped CoSb3 skutterudite

Nanopowders with nominal compositions of Co₄Sb_11.5Te_0.5 and In_0.5Co₄Sb_11.5Te_0.5 were prepared via hydrothermal synthesis at 180 °C for 48 h, then heat treated and finally hot pressed at 625 °C and 80 MPa for 1 h in vacuum to form bulk samples. The phase compositions of the samples were determined by X-ray diffraction. Hall Effect measurement of the samples was carried out at room temperature. The fracture surface of the samples was observed by field emission scanning electron microscopy. The electrical conductivity and the Seebeck coefficient of the samples were measured from room temperature to around 748 K. The In-filled and Te-doped CoSb₃ sample with longer time annealing before hot pressing had much better electrical transport properties with the highest power factor of 38.4 μWcm^?1 K^?2 around 573 K.     

Optimization in fabricating bismuth telluride thin films by ion beam sputtering deposition

N-type bismuth telluride (Bi₂Te₃) thermoelectric thin films were deposited on BK7 glass substrates by ion beam sputtering method. Various substrate temperatures were tried to obtain optimal thermoelectric performance. The influence of deposition temperature on microstructure, surface morphology and thermoelectric properties was investigated. X-ray diffraction shows that the films are rhombohedral with c-axis as the preferred crystal orientation when the deposition temperature is above 250 °C. All the films with single Bi₂Te₃ phase are obtained by comparing X-ray diffraction and Raman spectroscopy. Scanning electron microscopy result reveals that the average grain size of the film is larger than 500 nm when the deposition temperature is above 300 °C. Thermoelectric properties including Seebeck coefficient and electrical conductivities were measured at room temperature, respectively. It is found that Seebeck coefficients increase from − 28 μV k^− 1 to − 146 μV k^− 1 and the electrical conductivities increase from 1.87 × 10³ S cm^− 1 to 3.94 × 10³ S cm^− 1 when the deposition temperature rose to 250 °C and 300 °C, respectively. An optimal power factor of 6.45 × 10^− 3 Wm^− 1 K^− 2 is gained when the deposition temperature is 300 °C. The thermoelectric properties of bismuth telluride thin films have been found to be strongly enhanced by increasing the deposition temperature.     

Fabrication of three-dimensional snowflake-like bismuth sulfide nanostructures by simple refluxing

Three-dimensional snowflake-like bismuth sulfide nanostructures were successfully synthesized by simple refluxing at 160 °C in ethylene glycol, using bismuth citrate and thiourea as reactants. The crystal structures and morphologies of the products were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX). The Bi₂S₃ nanostructure was built up by highly ordered one-dimensional Bi₂S₃ nanorods, which was aligned in an orderly fashion. Ethylene glycol plays a critical role in the creation of bismuth sulfide three-dimensional nanostructures, which serves as an excellent solvent and structure director. Bismuth citrate, a linear polymer, also makes for the formation of the three-dimensional nanostructures.     

Rapid penetration of bismuth from solid Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> along grain boundaries in Cu and Cu-based alloys

As well known, bismuth rapidly penetrates into copper grain boundaries at about 550 °C and embrittles copper. In the experiments, the authors have used solid Bi₂Te₃ for the embrittlement of pure copper and copper-based solid solutions containing iron and silver. The investigated alloys were heated in the closed volume together with Bi₂Te₃ for a short time (5–90 min) at 570 °C in the hydrogen atmosphere. Bi₂Te₃ did not contact with copper samples during annealing. After that, the samples were bent and grain boundary cracks were formed (with the depth about 10–500 μm). Experiment showed that silver accelerates the embrittlement in the contrast to iron. The cracks in the silver–copper alloys were deeper than in the iron–copper ones. It was assumed that the depth of cracks is equal to the penetration depth. The reasons for this phenomenon were discussed in terms of the impurities effect on the grain boundary segregation.     

Preparation of Bi2S3 nanorods via a hydrothermal approach

Large-scale bismuth sulfide (Bi₂S₃) nanorods with uniform size have been prepared by hydrothermal method using bismuth chloride (BiCl₃) and sodium sulfide (Na₂S·9H₂O) as raw materials at 180 °C and pH = 1-2 for 12 h. The powder X-ray diffraction (XRD) pattern shows the Bi₂S₃ crystal belongs to the orthorhombic phase with calculated lattice constants a = 1.1187 nm, b = 1.1075 nm and c = 0.3976 nm. Furthermore, the quantification of X-ray photoelectron spectra (XPS) analysis peaks gives an atomic ratio of 1.9:3.0 for Bi:S. Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopic (TEM) studies reveal that the appearance of the as-prepared Bi₂S₃ is rod-like with typical lengths in the range of 2-5 μm and diameters in the range of 10-30 nm. Finally the influences of the reaction conditions are discussed and a possible mechanism for the formation of Bi₂S₃ nanorods is proposed.