Effect of Se Substitution on Structural and Electrical Transport Properties of Bi0.4Sb1.6Se3x Te3(1−x) Hexagonal Rods

In the current study, novel hexagonal rods based on Bi_0.4Sb_1.6Te₃ ingots dispersed with x amount of Se (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) in the form Bi_0.4Sb_1.6Se_3x Te_3(1?x) were synthesized via a standard solid-state microwave route. The morphologies of these rods were explored using field-emission scanning electron microscopy (FESEM). The crystal structure of the powders was examined by x-ray diffraction (XRD) analysis, which showed that powders of the 0.0 ≤ x ≤ 0.8 samples could be indexed to the rhombohedral phase, whereas the sample with x = 1.0 had an orthorhombic phase structure. The influence of variations in the Se content on the thermoelectric properties was studied in the temperature range from 300 K to 523 K. Alloying of Se into Bi_0.4Sb_1.6Te₃ effectively caused a decrease in the hole concentration and, thus, a decrease in the electrical conductivity and an increase in the Seebeck coefficient. The maximal power factor measured in the present work was 7.47 mW/mK² at 373 K for the x = 0.8 sample.     

Fabrication of Bismuth Telluride Thermoelectric Films Containing Conductive Polymers Using a Printing Method

We prepared a mixture of thermoelectric bismuth telluride particles, a conductive polymer [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)], poly(acrylic acid) (PAA), and several organic additives to fabricate thermoelectric films using printing or coating techniques. In the mixture, the organic components (PEDOT:PSS, PAA, and an additive) act as a binder to connect bismuth telluride particles mechanically and electrically. Among the organic additives used, glycerol significantly enhanced the electrical conductivity and bismuth telluride particle dispersibility in the mixture. Bi_0.4Te_3.0Sb_1.6 films fabricated by spin-coating the mixture showed a thermoelectric figure of merit (ZT) of 0.2 at 300 K when the Bi_0.4Te₃Sb_1.6 particle diameter was 2.8 μm and its concentration in the elastic films was 95 wt.%.     

Thermoelectric Properties of Nano/microstructured p-Type Bi0.4Sb1.6Te3 Powders Fabricated by Mechanical Alloying and Vacuum Hot Pressing

Two kinds of Bi_0.4Sb_1.6Te₃ powder with different particle and grain sizes were fabricated by high-energy ball milling. Powder mixtures with varied weight ratios were consolidated by vacuum hot pressing (HP) to produce nano/ microstructured composites of identical chemical composition. From measurements of the Seebeck coefficient, electrical resistivity, and thermal conductivity of these composites, a figure of merit (ZT) value of up to 1.19 was achieved at 373 K for the sample containing 40% nanograin powder. This ZT value is higher than that of monolithic nanostructured Bi_0.4Sb_1.6Te₃. It is further noted that the ZT value of this sample in the temperature range of 450 K to 575 K is in the range of 0.7 to 1.1. Such ZT characteristics are suitable for power generation applications as no other material with a similar high ZT value in this temperature range has been observed until now. The achieved high ZT value can probably be attributed to the unique nano/microstructure, in which the dispersed nanograin powder increases the number of phonon scattering sites, which in turn results in a decrease of the thermal conductivity while simultaneously increasing the electrical conductivity, owing to the existence of the microsized powder that can provide a fast carrier transportation network. These results indicate that the nano/microstructured Bi_0.4Sb_1.6Te₃ alloy can serve as a high-performance material for application in thermoelectric devices.     

Structure and Transport Properties of Bulk Nanothermoelectrics Based on Bi x Sb2−x Te3 Fabricated by SPS Method

Ball milling with subsequent spark plasma sintering (SPS) was used to fabricate bulk nanothermoelectrics based on Bi _x Sb_2?x Te₃. The SPS technique enables reduced size of grains in comparison with the hot-pressing method. The electrical and thermal conductivities, Seebeck coefficient, and thermoelectric figure of merit as functions of temperature and alloy composition were measured for different sintering temperatures. The greatest value of the figure of merit ZT = 1.25 was reached at the temperature of 90°C to 100°C in Bi_0.4Sb_1.6Te₃ for sintering temperature of 450°C to 500°C. The volume and quantitative distributions of size of coherent dispersion areas (CDA) were calculated for different sintering temperatures. The phonon thermal conductivity of nanostructured Bi _x Sb_2?x Te₃ was investigated theoretically taking into account phonon scattering on grain boundaries and nanoprecipitates.     

Preparation and Characterization of Bi0.4Sb1.6Te3 Bulk Thermoelectric Materials

This work focused on the preparation of p-type Bi_0.4Sb_1.6Te₃ bulk materials by combining mechanical alloying (MA) and hot extrusion, with emphasis on grain refinement and preferred grain orientation. Pure Bi, Sb, and Te powders were mechanically alloyed then hot extruded in the temperature range 360–450°C. Bi_0.4Sb_1.6Te₃ bulk materials were successfully prepared by MA and hot extrusion. All the samples had sound appearance, with single phases and high densities. The hot-extruded samples had small grain sizes, and the lower the extrusion temperature, the smaller the grain sizes. The results indicated that the extrudates had preferred orientation. The basal plane was predominantly oriented parallel to the direction of extrusion. Similar Seebeck coefficients were obtained when extrusion temperature was in the range 380–420°C. Electrical resistivity decreased with increasing extrusion temperature. Thermal conductivity was relatively low, even if the extrusion temperature was 450°C. As a result, a ZT value of 1.2 was obtained at room temperature for the sample extruded at 400°C. Therefore, combination of MA and hot extrusion results in significant improvement of both the thermoelectric and mechanical performance of Bi_0.4Sb_1.6Te₃ bulk materials.     

Lattice Thermal Conductivity Reduction Due to In Situ-Generated Nano-Phase in Bi0.4Sb1.6Te3 Alloys by Microwave-Activated Hot Pressing

The p-type Bi_0.4Sb_1.6Te₃ alloys are prepared using a new method of mechanical alloying followed by microwave-activated hot pressing (MAHP). The effect of sintering temperature on the microstructure and thermoelectric properties of Bi_0.4Sb_1.6Te₃ alloys is investigated. Compared with other sintering techniques, the MAHP process can be used to produce relatively compact bulk materials at lower sintering temperatures owing to its unique sintering mechanism. The grain size of the MAHP specimens increases gradually with the sintering temperature and a partially oriented lamellar structure can be formed in some regions of specimens obtained. The formation of the in situ-generated nano-phase is induced by the arcing effect of the MAHP process, which enhances the phonon scattering effect and decreases the lattice thermal conductivity. A minimum lattice thermal conductivity of 0.41 W/(m·K) and a maximum figure of merit value of 1.04 are obtained at 100°C for the MAHP specimen sintered at 325°C. This technique may also be extended to other functional materials to obtain ultrafine microstructures at low sintering temperatures.     

Improvement of Thermoelectric Properties in (Bi0.5Sb0.5)2Te3 Films of Nanolayered Pillar Arrays

In this work, it is found that unique pillar arrays with nanolayered structure can favorably influence the carrier and phonon transport properties of films. p-(Bi_0.5Sb_0.5)₂Te₃ pillar array film with (0 1 5) orientation was successfully achieved by a simple ion-beam-assisted technique at deposition temperature of 400°C, owing to the enhanced mobility of deposited atoms for more sufficient growth along the in-plane direction. The pillar diameter was about 250 nm, and the layered nanostructure was clear, with each layer in the pillar array being <30 nm. The properties of the oriented (Bi_0.5Sb_0.5)₂Te₃ pillar array were greatly enhanced in comparison with those of ordinary polycrystalline films synthesized at deposition temperature of 350°C and 250°C. The (Bi_0.5Sb_0.5)₂Te₃ pillar array film with (0 1 5) preferred orientation exhibited a thermoelectric dimensionless figure of merit of ZT = 1.25 at room temperature. The unique pillar array with nanolayered structure is the main reason for the observed improvement in the properties of the (Bi_0.5Sb_0.5)₂Te₃ film.     

Thermoelectric Properties of Cu-doped Bi<Subscript>0.4</Subscript>Sb<Subscript>1.6</Subscript>Te<Subscript>3</Subscript> Prepared by Hot Extrusion

Cu_0.003Bi_0.4Sb_1.6Te₃ alloys were prepared by using encapsulated melting and hot extrusion (HE). The hot-extruded specimens had the relative average density of 98%. The (00l) planes were preferentially oriented parallel to the extrusion direction, but the specimens showed low crystallographic anisotropy with low orientation factors. The specimens were hot-extruded at 698 K, and they showed excellent mechanical properties with a Vickers hardness of 76 Hv and a bending strength of 59 MPa. However, as the HE temperature increased, the mechanical properties degraded due to grain growth. The hot-extruded specimens showed positive Seebeck coefficients, indicating that the specimens have p-type conduction. These specimens exhibited negative temperature dependences of electrical conductivity, and thus behaved as degenerate semiconductors. The Seebeck coefficient reached the maximum value at 373 K and then decreased with increasing temperature due to intrinsic conduction. Cu-doped specimens exhibited high power factors due to relatively higher electrical conductivities and Seebeck coefficients than those of undoped specimens. A thermal conductivity of 1.00 Wm^?1 K^?1 was obtained at 373 K for Cu_0.003Bi_0.4Sb_1.6Te₃ hot-extruded at 723 K. A maximum dimensionless figure of merit, ZT _max = 1.05, and an average dimensionless figure of merit, ZT _ave = 0.98, were achieved at 373 K.     

Synthesis of Bi0.5Sb1.5Te3 Thermoelectric Powder Using an Oxide-Reduction Process

The present study focused on synthesis of Bi_0.5Sb_1.5Te₃ thermoelectric powder using an oxide-reduction process. The phase structure and particle size of the synthesized powders were analyzed using x-ray diffractometry and scanning electron microscopy. The synthesized powder was sintered using the spark plasma sintering method. The thermoelectric properties of the sintered body were evaluated by measuring the Seebeck coefficient, electrical resistivity, and thermal conductivity. Bi_0.5Sb_1.5Te₃ powder was synthesized using a combination of mechanical milling, calcination, and reduction processes, using a mixture of Bi₂O₃, Sb₂O₃, and TeO₂ powders. The sintered body of the oxide-reduction-synthesized Bi_0.5Sb_1.5Te₃ powder showed p-type thermoelectric characteristics. The thermoelectric properties of the sintered bodies depended on the reduction time. After being reduced for 2 h at 663 K, the sintered body of the Bi_0.5Sb_1.5Te₃ powder showed a figure of merit of approximately 1.0 at room temperature.     

Thermoelectric Power Generation Characteristics of a Thin-Film Device Consisting of Electrodeposited n-Bi2Te3 and p-Sb2Te3 Thin-Film Legs

A thermoelectric thin-film device of the cross-plane configuration was fabricated by flip-chip bonding of the top electrodes to 242 pairs of electrodeposited n-type Bi₂Te₃ and p-type Sb₂Te₃ thin-film legs on the bottom substrate. The electrodeposited Bi₂Te₃ and Sb₂Te₃ films of 20-μm thickness exhibited Seebeck coefficients of ?59 μV/K and 485 μV/K, respectively. The internal resistance of the thin-film device was measured as 3.7 kΩ, most of which was attributed to the interfacial resistance of the flip-chip joints. The actual temperature difference ΔT _G working across the thin-film legs was estimated to be 10.4 times smaller than the apparent temperature difference ΔT applied across the thin-film device. The thin-film device exhibited an open-circuit voltage of 0.294 V and a maximum output power of 5.9 μW at an apparent temperature difference ΔT of 22.3 K applied across the thin-film device.     

Effect of Suppression of Grain Growth of Hot Extruded (Bi0.2Sb0.8)2Te3 Thermoelectric Alloys by MoS2 Nanoparticles

Nanostructured bulk materials are regarded as a means of enhancing the performance of thermoelectric (TE) materials and devices. Powder metallurgy has the distinct advantage over conventional synthesis that it can start directly from nanosized particles. However, further processing, for example extrusion, usually requires elevated temperatures, which lead to grain growth. We have found that introduction of semiconductor nanoparticles of molybdenum disulfide (MoS₂), a well-known solid lubricant, suppresses grain growth in bismuth telluride-based alloys, thus improving the extrusion process. Scanning electron microscope images show that adding MoS₂ particles at concentrations of 0.2, 0.4, and 0.8 wt% to p-type (Bi_0.2Sb_0.8)₂Te₃, under otherwise identical extrusion conditions, reduces average grain size by a factor of four. Scherer’s formula applied to x-ray diffraction data indicates that average crystallite sizes (～17 nm) of powders are not significantly different from those of alloys extruded with MoS₂ (～18 nm), which is in stark contrast with those for conventional alloy (Bi_0.2Sb_0.8)₂Te₃ extruded under the same conditions (～80 nm). Harman measurements of TE properties reveal a decrease of the thermal conductivity accompanied by reduction of the room-temperature figure of merit (ZT) from 0.9 to 0.7, because of a lower power factor. Above 370 K, however, the performance of alloys containing MoS₂ surpasses that of (Bi_0.2Sb_0.8)₂Te₃, with reduction of the thermal conductivity which is more significant at temperatures above the cross point of the ZT values.     

Fabrication of a Flexible Bismuth Telluride Power Generation Module Using Microporous Polyimide Films as Substrates

In this study, we investigated the effect of the structure of microporous p-type (Bi_0.4Te₃Sb_1.6) and n-type (Bi_2.0Te_2.7Se_0.3) BiTe-based thin films on their thermoelectric performance. High-aspect-ratio porous thin films with pore depth greater than 1 μm and pore diameter ranging from 300 nm to 500 nm were prepared by oxygen plasma etching of polyimide (PI) layers capped with a heat-resistant block copolymer, which acted as the template. The cross-plane thermal conductivities of the porous p- and n-type thin films were 0.4 W m^?1 K^?1 and 0.42 W m^?1 K^?1, respectively, and the dimensionless figures of merit, ZT, of the p- and n-type BiTe films were estimated as 1.0 and 1.0, respectively, at room temperature. A prototype thermoelectric module consisting of 20 pairs of p- and n-type strips over an area of 3 cm × 5 cm was fabricated on the porous PI substrate. This module produced an output power of 0.1 mW and an output voltage of 0.6 V for a temperature difference of 130°C. The output power of the submicrostructured module was 1.5 times greater than that of a module based on smooth BiTe-based thin films. Thus, the thermoelectric performance of the thin films was improved owing to their submicroscale structure.     

Interface Microstructure and Performance of Sb Contacts in Bismuth Telluride-Based Thermoelectric Elements

A thermoelectric joint composed of p-type Bi_0.5Sb_1.5Te₃ (BiSbTe) material and an antimony (Sb) interlayer was fabricated by spark plasma sintering. The reliability of the thermoelectric joints was investigated using electron probe microanalysis for samples with different accelerated isothermal aging time. After aging for 30 days at 300°C in vacuum, the thickness of the diffusion layer at the BiSbTe/Sb interface was about 30 μm, and Sb₂Te₃ was identified to be the major interfacial compound by element analysis. The contact resistivity was 3 × 10^?6 ohm cm² before aging and increased to 8.5 × 10^?6 ohm cm² after aging for 30 days at 300°C, an increase associated with the thickness of the interfacial compound. This contact resistivity is very small compared with that of samples with solder alloys as the interlayer. In addition, we have also investigated the interface behavior of Sb layers integrated with n-type Bi₂Se_0.3Te_2.7 (BiSeTe) material, and obtained similar results as for the p-type semiconductor. The present study suggests that Sb may be useful as a new interlayer material for bismuth telluride-based power generation devices.     

Influence of Element Substitution on Corrosion Behavior of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Based Compounds

Atmospheric water may condense on the surface of Bi₂Te₃-based compounds constituting the Peltier module, depending on the operating environment used. In the stage of disposal, Bi₂Te₃-based compounds may come into contact with water in waste disposal sites. There are very few publications about the influence of condensed water on Peltier modules. Bi₂Te₃-Sb₂Te₃ or Bi₂Te₃-Bi₂Se₃ pseudo binary system compounds are used as p-type material or n-type material, respectively. The lattice distortion will be induced in the crystal of Bi₂Te₃-based compounds by element substitution due to the reduction in their thermal conductivity. However, the influence of element substitution on the corrosion behavior of Bi₂Te₃-based compounds remains unclear. In this study, the influence of element substitution on the corrosion behavior of Bi₂Te₃-based compounds with practical compositions has been investigated. Bi_0.5Sb_1.5Te₃ or Bi₂Te_2.85Se_0.15 was prepared by the vertical Bridgman method. The electrochemical properties at room temperature were evaluated by cyclic voltammetry in a standard three-electrode cell. The working electrolyte was a naturally aerated 0.6 or 3.0 mass% NaCl solution. From the tendency for corrosion potential for all the samples, the corrosion sensitivity of ternary compounds was slightly higher than that of binary compounds. From the trend of current density, it was found that Bi_0.5Sb_1.5Te₃ had a corrosion resistance intermediate between Bi₂Te₃ and Sb₂Te₃. On the other hand, corrosion resistance was affected despite a small amount of Se substitution, and the corrosion resistance of Bi₂Te_2.85Se_0.15 was close to or lower than that of Bi₂Se₃. From the observation results of the corrosion products, the trends of morphology and composition of corrosion products for Bi_0.5Sb_1.5Te₃ or Bi₂Te_2.85Se_0.15 were consistent with those of Sb₂Te₃ or Bi₂Se₃, respectively. From the results of x-ray photoelectron spectroscopy for the electrolyte after testing, the possibility that a corrosion product diffuses to the environment including the salt was suggested in Bi_0.5Sb_1.5Te₃. However, the amount of dissolved corrosion product was very low, and the chemical stability of the corrosion product was not changed or improved by element substitution.     

Effects of Reaction Temperature on Thermoelectric Properties of p-Type Nanostructured Bi2−x Sb x Te3 Prepared Using Hydrothermal Method and Evacuated-and-Encapsulated Sintering

We report fabrication of nanostructured Bi_2?x Sb _x Te₃ using hydrothermal method followed by cold-pressing and evacuated-and-encapsulated sintering techniques. To obtain lower resistivity, the reaction temperature in the hydrothermal synthesis is investigated, and the effects on the ZT values of Bi_2?x Sb _x Te₃ are reported. Both the x = 1.52 and 1.55 samples hydrothermally synthesized at 160°C show lower resistivity than the x = 1.55 sample hydrothermally synthesized at 140°C. However, the power factor is lower for the samples synthesized at 160°C due to the accompanying smaller thermopower. All three samples exhibit remarkably low thermal conductivity of around 0.41 W m^?1 K^?1 at room temperature. The peak ZT value occurs at 270 K for all three samples, being ZT = 1.75, 1.29, and 1.17 for x = 1.55 (synthesized at 140°C), 1.55 (synthesized at 160°C), and 1.52 (synthesized at 160°C), respectively.     

Enhancement of Thermoelectric Figure of Merit for Bi0.5Sb1.5Te3 by Metal Nanoparticle Decoration

Introducing nanoinclusions in thermoelectric (TE) materials is expected to lower the lattice thermal conductivity by intensifying the phonon scattering effect, thus enhancing their TE figure of merit ZT. We report a novel method of fabricating Bi_0.5Sb_1.5Te₃ nanocomposite with nanoscale metal particles by using metal acetate precursor, which is low cost and facile to scale up for mass production. Ag and Cu particles of ??40?nm were successfully near-monodispersed at grain boundaries of Bi_0.5Sb_1.5Te₃ matrix. The well-dispersed metal nanoparticles reduce the lattice thermal conductivity extensively, while enhancing the power factor. Consequently, ZT was enhanced by more than 25% near room temperature and by more than 300% at 520?K compared with a Bi_0.5Sb_1.5Te₃ reference sample. The peak ZT of 1.35 was achieved at 400?K for 0.1?wt.% Cu-decorated Bi_0.5Sb_1.5Te₃.     

The Effect of Cu Addition on the System Stability and Thermoelectric Properties of Bi2Te3

Cu-doped Bi₂Te₃ nanopowders with nominal composition Cu _x Bi₂Te₃ (x = 0, 0.01, 0.025, and 0.05) were synthesized by a gas-induced-reduction method using TeO₂, Bi(NO₃)₃·5H₂O and Cu(NO₃)₂·3H₂O as raw materials and then hot-pressed into bulk materials. x-Ray diffraction (XRD) analysis indicates that, when x ≠ 0, pure Cu _x Bi₂Te₃ phase was obtained, and that when x = 0, Bi₂Te₃ mixed with a small amount of Bi₂TeO₅ was obtained. Field emission scanning electron microscopy observation reveals that Cu addition significantly reduces the grain sizes of the materials. First-principle calculations show that the order of the free energies of the materials is: Cu-doped Bi₂Te₃ (substitution of Cu for Bi) < Cu intercalated Bi₂Te₃ < Bi₂Te₃. The electrical and thermal conductivities decrease and the Seebeck coefficient increases with Cu addition. The maximum figure of merit, ZT, reaches 0.67 at 500 K for a Cu_0.05Bi₂Te₃ sample.     

The Effect of Adding Nano-Bi2Te3 on Properties of GeTe-Based Thermoelectric Material

The efficient thermoelectric materials (GeTe)_0.85?x (Mn_0.6Sn_0.4Te)_0.15(Bi₂Te₃) _x (0 ≤ x ≤ 0.05), in which Bi₂Te₃ is nanopowder, were prepared by hot pressing. The effect of adding neutral nano-Bi₂Te₃ content on the thermoelectric properties of germanium telluride was investigated. With increasing x, the thermal conductivity of the prepared samples decreased significantly and the Seebeck coefficient declined slightly, while there was no obvious change in electrical conductivity. In both electrical conductivity and Seebeck coefficient curves at different x values, there are inflection points around 600 K. The maximum dimensionless figure of merit ZT of the prepared materials is 1.54, attained in the temperature range from 700 K to 750 K for x = 0.03. The x-ray diffraction (XRD) pattern shows that Bi₂Te₃ has been alloyed into the GeTe-MnTe-SnTe alloy, which is consistent with the high-resolution scanning electron microscopy (HRSEM) images. Adding nano-Bi₂Te₃ to GeTe-based materials could also increase their performance stability at high temperature as a result of decreasing the phase-transition temperature T _c.     

Thermodynamic Re-modeling of the Sb-Te System Using Associate and Ionic Models

The Sb-Te system is re-modeled using the calculation of phase diagram (CALPHAD) technique. The liquid phase is modeled as (Sb, Sb₂Te₃, Te) using the associate model and as (Sb³⁺) _p (Te^2?,Te,Va) _q using the ionic model. The solution phases rhom(Sb) and hex(Te) are described as substitutional solutions. Two compounds, delta and gamma, are treated as (Sb)_0.4(Sb,Te)_0.6 according to their homogeneity ranges, while the compound Sb₂Te₃ follows a strict stoichiometry. A set of self-consistent thermodynamic parameters is obtained. Using these thermodynamic parameters, the experimental Sb-Te phase diagram, mixing enthalpies of liquid at 911 K and 935 K, activities of Sb and Te in liquid at 911 K and 1023 K, and Gibbs energy of liquid at 911 K, is well reproduced by the calculations. And the calculated enthalpy of formation, enthalpy of fusion, and heat capacity of Sb₂Te₃ are also in fairly good agreement with all the available experimental data.     

Thermoelectric Properties of n-Type Bi2Te3/PbSe0.5Te0.5 Segmented Thermoelectric Material

To investigate the effects of segmentation of thermoelectric materials on performance levels, n-type segmented Bi₂Te₃/PbSe_0.5Te_0.5 thermoelectric material was fabricated, and its output power was measured and compared with those of Bi₂Te₃ and PbSe_0.5Te_0.5. The two materials were bonded by diffusion bonding with a diffusion layer that was ～18 μm thick. The electrical conductivity, Seebeck coefficient, and power factor of the segmented Bi₂Te₃/PbSe_0.5Te_0.5 sample were close to the average of the values for Bi₂Te₃ and PbSe_0.5Te_0.5. The output power of Bi₂Te₃ was higher than those of PbSe_0.5Te_0.5 and the segmented sample for small ΔT (300 K to 400 K and 300 K to 500 K), but that of the segmented sample was higher than those of Bi₂Te₃ and PbSe_0.5Te_0.5 when ΔT exceeded 300 K (300 K to 600 K and 300 K to 700 K). The output power of the segmented sample was about 15% and 73% higher than those of the Bi₂Te₃ and PbSe_0.5Te_0.5 samples, respectively, when ΔT was 400 K (300 K to 700 K). The efficiency of thermoelectric materials for large temperature differences can be enhanced by segmenting materials with high performance in different temperature ranges.