Composition-Dependent Thermoelectric Properties of n-Type Bi2Te2.7Se0.3 Doped with In4Se3

We present the effects of In₄Se₃ addition on thermoelectric properties of n-type Bi₂Te_2.7Se_0.3. In this study, polycrystalline (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x pellets were prepared by mechanical alloying followed by spark plasma sintering (SPS). The thermoelectric properties such as Seebeck coefficient and electrical and thermal conductivities were measured in the temperature range of 300 K to 500 K. Addition of In₄Se₃ into Bi₂Te_2.7Se_0.3 resulted in segregation of In₄Se₃ phase within Bi₂Te_2.7Se_0.3 matrix. The Seebeck coefficient of the (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x samples exhibited lower values compared with that of pure Bi₂Te_2.7Se_0.3 phase. This reduction of Seebeck coefficient in n-type (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x is attributed to the formation of unwanted p-type phases by interdiffusion through the interface between (In₄Se₃) _x and (Bi₂Te_2.7Se_0.3)_1?x as well as consequently formed Te-deficient matrix. However, the decrease in electrical resistivity and thermal conductivity with addition of In₄Se₃ leads to an enhanced thermoelectric figure of merit (ZT) at a temperature range over 450 K: a maximum ZT of 1.0 is achieved for the n-type (In₄Se₃)_0.03-(Bi₂Te_2.7Se_0.3)_0.97 sample at 500 K.     

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.     

Nanostructure, Excitations, and Thermoelectric Properties of Bi2Te3-Based Nanomaterials

The effect of dimensionality and nanostructure on thermoelectric properties in Bi₂Te₃-based nanomaterials is summarized. Stoichiometric, single-crystalline Bi₂Te₃ nanowires were prepared by potential-pulsed electrochemical deposition in a nanostructured Al₂O₃ matrix, yielding transport in the basal plane. Polycrystalline, textured Sb₂Te₃ and Bi₂Te₃ thin films were grown at room temperature using molecular beam epitaxy and subsequently annealed at 250°C. Sb₂Te₃ films revealed low charge carrier density of 2.6?×?10¹⁹?cm^?3, large thermopower of 130???V?K^?1, and large charge carrier mobility of 402?cm²?V^?1?s^?1. Bi₂(Te_0.91Se_0.09)₃ and (Bi_0.26Sb_0.74)₂Te₃ nanostructured bulk samples were prepared from as-cast materials by ball milling and subsequent spark plasma sintering, yielding grain sizes of 50?nm and thermal diffusivities reduced by 60%. Structure, chemical composition, as well as electronic and phononic excitations were investigated by x-ray and electron diffraction, nuclear resonance scattering, and analytical energy-filtered transmission electron microscopy. Ab?initio calculations yielded point defect energies, excitation spectra, and band structure. Mechanisms limiting the thermoelectric figure of merit ZT for Bi₂Te₃ nanomaterials are discussed.     

High‐Performance N‐type Mg3Sb2 towards Thermoelectric Application near Room Temperature

Se‐doped Mg_3.2Sb_1.5Bi_0.5‐based thermoelectric materials are revisited in this study. An increased ZT value ≈ 1.4 at about 723 K is obtained in Mg_3.2Sb_1.5Bi_0.49Se_0.01 with optimized carrier concentration ≈ 1.9 × 10¹⁹ cm^?3. Based on this composition, Co and Mn are incorporated for the manipulation of the carrier scattering mechanism, which are beneficial to the dramatically enhanced electrical conductivity and power factor around room temperature range. Combined with the lowered lattice thermal conductivity due to the introduction of effective phonon scattering centers in Se&Mn‐codoped sample, a highest room temperature ZT value ≈ 0.63 and a peak ZT value ≈ 1.70 at 623 K are achieved for Mg_3.15Mn_0.05Sb_1.5Bi_0.49Se_0.01, leading to a high average ZT ≈ 1.33 from 323 to 673 K. In particular, a remarkable average ZT ≈ 1.18 between the temperature of 323 and 523 K is achieved, suggesting the competitive substitution for the commercialized n‐type Bi₂Te₃‐based thermoelectric materials.     

Effects of Bi2Se3 Nanoparticle Inclusions on the Microstructure and Thermoelectric Properties of Bi2Te3-Based Nanocomposites

A series of thermoelectric nanocomposite samples were prepared by integrating Bi₂Se₃ nanoparticles into a bulk Bi₂Te₃ matrix. Primarily, spherical Bi₂Se₃ nanoparticles with diameter of ～30 nm were synthesized by combining bismuth acetate with elemental Te in oleic acid solution. Bi₂Te₃-based nanocomposite samples were prepared by consolidating the appropriate quantity of Bi₂Se₃ nanoparticles with the starting elements (Bi and Te) using typical solid-state synthetic reactions. The microstructure and composition of the Bi₂Te₃-based nanocomposites, as well as the effects of the Bi₂Se₃ nanoparticles on their thermoelectric properties, are investigated. Transmission electron microscopy observation of the Bi₂Te₃-based nanocomposites reveals two types of interface between the constituent materials, i.e., coherent and incoherent, depending on the Bi₂Se₃ concentration. The Bi₂Se₃ nanoparticles in the Bi₂Te₃ matrix act as scattering centers for a wider range of phonon frequencies, thereby reducing the thermal conductivity. As a result, the maximum ZT value of 0.75 is obtained for the Bi₂Te₃ nanocomposite with 10 wt.% Bi₂Se₃ nanoparticles at room temperature. It is clear that the reduction in the thermal conductivity plays a central role in the enhancement of the ZT value.     

Effect of Chemical Composition on the Thermoelectric Properties of n-Type Bi2Te3 Alloys Fabricated by Gas Atomization and Hot Extrusion

In this research, n-type (Bi₂Te₃)_1?x (Bi₂Se₃) _x -based thermoelectric (TE) materials were produced through a gas atomization process, and subsequently hot extruded with an extrusion ratio of 10:1 at 400 °C. The effect of chemical composition on TE properties was investigated. The microstructure of all extruded bars showed a homogeneous and fine distribution of grains due to the dynamic recrystallization during the hot extrusion process. With increasing Bi₂Te₃ content, from 0.85 to 0.90, both electrical resistivity and Seebeck coefficient values were increased. The maximum figure of merit (ZT) 0.673 was obtained at room temperature for (Bi₂Te₃)₀.₉₀(Bi₂Se₃)_0.10 alloys due to them exhibiting higher seebeck coefficient and lower thermal conductivity than other compositions.     

Effects of Different Morphologies of Bi2Te3 Nanopowders on Thermoelectric Properties

Thermoelectric Bi₂Te₃ alloy nanopowders with different morphologies were synthesized by hydrothermal processes with different surfactants. The nanopowders were hot-pressed into pellets, and their thermoelectric properties were investigated. The results show that the morphologies of the nanopowders have remarkable effects on the thermoelectric properties of the hot-pressed bulk pellets. A suitable microstructure of the bulk pellet prepared from flower-like nanosheets was found, having a lower electrical resistivity, larger Seebeck coefficient, and lower thermal conductivity, resulting in a high figure of merit ZT ≈ 1.16. The effects of the nanopowders with different morphologies on the microstructure and thermoelectric properties of hot-pressed bulk pellets are discussed.     

Enhancement of the Thermoelectric Performance of Bi0.4Sb1.6Te3 Alloys by In and Ga Doping

We report an enhancement of the thermoelectric figure of merit in polycrystalline In- and Ga-doped Bi_0.4Sb_1.6Te₃ compounds. Via the controlled doping of In or Ga, the lattice thermal conductivity was effectively reduced by strong point-defect phonon scattering while the power factor was not significantly changed due to the similarity of the density of states near the valence-band maximum between undoped and In- or Ga-doped compositions. An enhanced ZT of 1.2 at 320 K was obtained in 0.5 at.% In-doped Bi_0.4Sb_1.6Te₃ compound by these synergetic effects.     

Effect of Chromium Doping on the Thermoelectric Properties of Bi2Te3: Cr x Bi2Te3 and Cr x Bi2−x Te3

The thermoelectric (TE) properties of Bi₂Te₃ compounds intercalated and substituted with Cr, namely Cr _x Bi₂Te₃ and Cr _x Bi_2?x Te₃, respectively, have been investigated to study the influence of chromium on the TE properties of Bi₂Te₃. The Seebeck coefficients were found to be positive for all the samples in the temperature range between 300 K and 550 K. Although no effective enhancement of the Seebeck coefficient was observed, doping with Cr by means of either substitution or intercalation clearly not only improved the electrical conductivity but also lowered the thermal conductivity of Bi₂Te₃. As a result of the improvement, the figure of merit ZT is increased up to 0.8 and 0.65 at 300 K for 1% intercalated and 1% substituted Bi₂Te₃, respectively.     

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.     

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.     

Specific thermoelectric properties of lightly doped Bi<Subscript>2</Subscript>(TeSe)<Subscript>3</Subscript> solid solutions

Electrical and thermoelectric properties of a lightly doped n-Bi₂Te_2.7Se_0.3 solid solution have been studied in the temperature range 77–300 K. The results are compared with data for the compound PbTe_0.9Se_0.1 with a similar magnitude of the Seebeck coefficient S at 84 K. Along with lower thermal conductivity, Bi₂Te_2.7Se_0.3 has a higher electrical conductivity σ and a much weaker temperature dependence. As a result, the power coefficient S ²σ in optimal samples begins to decrease only when the density of minority carriers becomes significant. In this case, |S| considerably exceeds the standard value of 200 μV/K. The reduction of the electron density reduces the thermoelectric figure of merit Z at its maximum and slightly lowers the temperature of the maximum; therefore, the expected effect on the average value of Z in the range 77–300 K is absent. Similar behavior is observed in Bi₂Te_2.88Se_0.12, although the effect is less pronounced. The experimental results are discussed taking into account possible changes in the dominant scattering mechanisms, carrier density, and electron energy spectrum. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 7, 2004, pp. 811–815. Original Russian Text Copyright ? 2004 by Konstantinov, Prokof’eva, Ravich, Fedorov, Kompaniets.     

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.     

Improved Thermoelectric Performance of Free-Standing PEDOT:PSS/Bi2Te3 Films with Low Thermal Conductivity

Free-standing poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS)/Bi₂Te₃ thermoelectric (TE) composite films have been successfully prepared by a simple physical mixing method with different contents of Bi₂Te₃. x-Ray diffraction (XRD) and scanning electron microscopy were used to analyze the phase composition and microstructure of the composite films. Their TE performance from 100 K to 300 K was systematically investigated. The maximum electrical conductivity of the composite polymer film reached up to 421 S/cm when the film contained 10 wt.% Bi₂Te₃, corresponding to the highest power factor of 9.9 μW/m/K², while their Seebeck coefficient fluctuated smoothly in a tiny range (14.2 μV/K to 18.6 μV/K). In addition, a relatively low thermal conductivity of 0.07 ± 0.02 W/m/K has been obtained. The maximum figure of merit of the composite reached up to 0.04 at room temperature, which is a relatively high value in the organic TE field.     

Printed Se-Doped MA n-Type Bi2Te3 Thick-Film Thermoelectric Generators

In this work, we highlight new materials processing developments and fabrication techniques for dispenser-printed thick-film single-element thermoelectric generators (TEG). Printed deposition techniques allow for low-cost and scalable manufacturing of microscale energy devices. This work focuses on synthesis of unique composite thermoelectric systems optimized for low-temperature applications. We also demonstrate device fabrication techniques for high-density arrays of high-aspect-ratio planar single-element TEGs. Mechanical alloyed (MA) n-type Bi₂Te₃ powders were prepared by taking pure elemental Bi and Te in 36:64 molar ratio and using Se as an additive. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were used to characterize the as-milled powders to confirm the Bi₂Te₃ phase formation and particle size below 50???m. Thermoelectric properties of the composites were measured from room temperature to 100°C. We achieved a dimensionless figure of merit (ZT) of 0.17 at 300?K for MA n-type Bi₂Te₃?Cepoxy composites with 2?wt.% Se additive. A 20 single-leg TEG prototype with 5?mm?×?400???m?×?120???m printed element dimensions was fabricated on a polyimide substrate with evaporated gold contacts. The prototype device produced a power output of 1.6???W at 40???A and 40?mV voltage for a temperature difference of 20°C.     

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.     

The Effect of Grain Size and Density on the Thermoelectric Properties of Bi2Te3-PbTe Compounds

The effects of microstructure on thermoelectric properties were investigated in Bi₂Te₃-PbTe compounds of different grain size and density. Powders of two different sizes [0.1 μm to 1 μm (micropowder) and <50 nm (nanopowder)] were prepared from Bi₂Te₃-PbTe ingots by ball milling and high-energy ball milling. Three different samples were spark plasma sintered from each powder and the mixture of the two powders. The grain size and relative density of the sintered samples varied from 100 nm to a few micrometers and 89.7% to 97.3%, respectively. The dimensionless figure of merit zT of the sample sintered from nanopowder was about 0.50 at 500 K, being about 3.3 times larger than that of the sample sintered from micropowder (～0.15 at 500 K), when the relative density of the former and the latter were 89.7% and 97.3%, respectively. The improved thermoelectric performance of the samples may originate from the decrease of the thermal conductivity, which was caused by the decrease of the grain size and the increase of the amount of pores.     

Point Defect Engineering of High‐Performance Bismuth‐Telluride‐Based Thermoelectric Materials

Developing high‐performance thermoelectric materials is one of the crucial aspects for direct thermal‐to‐electric energy conversion. Herein, atomic scale point defect engineering is introduced as a new strategy to simultaneously optimize the electrical properties and lattice thermal conductivity of thermoelectric materials, and (Bi,Sb)₂(Te,Se)₃ thermoelectric solid solutions are selected as a paradigm to demonstrate the applicability of this new approach. Intrinsic point defects play an important role in enhancing the thermoelectric properties. Antisite defects and donor‐like effects are engineered in this system by tuning the formation energy of point defects and hot deformation. As a result, a record value of the figure of merit ZT of ≈1.2 at 445 K is obtained for n‐type polycrystalline Bi₂Te_2.3Se_0.7 alloys, and a high ZT value of ≈1.3 at 380 K is achieved for p‐type polycrystalline Bi_0.3Sb_1.7Te₃ alloys, both values being higher than those of commercial zone‐melted ingots. These results demonstrate the promise of point defect engineering as a new strategy to optimize thermoelectric properties.     

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.     

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.%.