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 Performance of p-Type (Bi85Sb15)1?x Sn x Materials Prepared by a Pressureless Sintering Technique

In this study, a series of Sn-doped (Bi₈₅Sb₁₅)_1?x Sn _x (x?=?0, 0.025, 0.05, 0.1, 0.2, 0.3) thermoelectric materials was fabricated through mechanical alloying followed by pressureless sintering. The crystal structure was characterized by x-ray diffraction. The electrical transport properties and thermal properties were measured in the temperature range from 77?K to 300?K. The electrical transport as a function of temperature appeared to be characteristic of a semimetal. The Seebeck coefficient gradually changed from negative to positive with increasing Sn doping, showing p-type electrical transport properties. It is found that the Seebeck coefficients of the p-type Bi-Sb alloys decrease with increasing dopant concentration of Sn, which may be due to increasing carrier concentration. Among the p-type alloys, the power factor of (Bi₈₅Sb₁₅)_0.975Sn_0.025 reached a maximum value of 1.3?×?10^?3?W/mK² at 265?K, and the optimum figure of merit value of 0.13 was obtained at 240?K. The results indicate that good p-type Bi-Sb alloys can be prepared by this synthesis procedure.     

Improved Thermoelectric Properties of p-Type Bismuth Antimony-Based Alloys Prepared by Spark Plasma Sintering

Bi₈₅Sb_15?x Pb _x (x = 0, 0.5, 1, 2, 3) alloys have been prepared by the mechanical alloying–spark plasma sintering (MA-SPS) method. X-ray diffraction and scanning electron microscopy were used to characterize the microstructure of the alloys. The effect of Pb content on the thermoelectric properties was investigated in the temperature range 77–300 K. The results showed that the electrical transport properties of the Bi–Sb alloys changed from n-type to p-type with substitution of Sb by Pb. The maximum power factor reached 1.6 × 10^?3 W/mK² at 190 K, a significant improvement on values reported elsewhere. This study demonstrated that high-performance p-type thermoelectric Bi–Sb materials can be obtained by spark plasma sintering.     

Effect of Sintering on the Thermoelectric Transport Properties of Bulk Nanostructured Bi0.5Sb1.5Te3 Pellets Prepared by Chemical Synthesis

Considerable research effort has gone into improving the performance of traditional thermoelectric materials such as Bi_2?x Sb _x Te₃ through a variety of nanostructuring approaches. Bottom-up, chemical approaches have the potential to produce very small nanoparticles (?100?nm) with narrow size distribution and controlled shape. For this study, nanocrystalline powder of Bi_0.5Sb_1.5Te₃ was synthesized using a ligand-assisted chemical method, and consolidated into pellets with cold pressing followed by sintering in Ar atmosphere. The thermoelectric transport properties were measured from 7?K to 300?K as a function of sintering temperature. Sintering is found to increase ZT and to move the maximum in ZT to lower temperatures due to a reduction in the free charge concentration. Hall mobility studies indicate that sintering increases the electron mean free path more than it increases the phonon mean free path up to sintering temperature of 598?K. A maximum ZT of 0.42 was measured at temperature of 275?K.     

Improvement of Thermoelectric Performance for Sb-Doped SnO2 Ceramics Material by Addition of Cu as Sintering Additive

The thermoelectric properties, such as the Seebeck coefficient value, S, electrical conductivity, σ, and thermal conductivity, κ, of Sb- and Cu-doped SnO₂, that is, (Sn_1?x?y Cu _x Sb _y )O₂, were investigated in detail. The addition of a small amount of CuO significantly improved the relative density of the SnO₂ ceramics. However, the relative density slightly decreased with the excessive addition of CuO (more than x = 0.03). The addition of Sb₂O₅ should cause an increase in the number of charge carriers, resulting in an increased σ value, and the addition of both CuO and Sb₂O₅ caused an increase in σ and |S| at high temperature. The improvement of the sintering performance caused the enhancement of the thermoelectric performance. As the temperature region measured in this study (293–1073 K), the maximum ZT value was 0.29 at 1073 K for (Sn_0.985Cu_0.005Sb_0.01)O₂.     

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.     

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.     

Thermoelectric Performance of Sb- and La-Doped Mg2Si0.5Ge0.5

La _x Mg_2?x Si_0.49Ge_0.5Sb_0.01 compounds (x?=?0, 0.005, 0.01, 0.02) were synthesized by solid-state reaction followed by spark plasma sintering. The thermoelectric properties, such as the Seebeck coefficient, the electrical and thermal conductivities, and ZT, of these compounds have been studied in the temperature range of 300?K to 823?K. The figure of merit of this n-type compound has been raised above unity at 823?K for the sample with x?=?0.01, a value 60% higher than that of Mg₂Si_0.49Ge_0.5Sb_0.01. The reduction of the thermal conductivity via increasing phonon scattering is considered as the main reason for the enhanced ZT. These observations demonstrate an opportunity to improve the thermoelectric performance of Mg₂Si_1?x Ge _x solid solutions.     

Effect of Bismuth Nanotubes on the Thermoelectric Properties of BiSb Alloy Nanocomposites

Bismuth nanotubes have been synthesized and successfully included in Bi_1?x Sb _x nanoalloys to form composite structures. The nanotubes were synthesized by transformation of a β-BiI precursor with n-BuLi solution leading to tubular bismuth structures. The Bi_1?x Sb _x nanoalloys were produced by ball milling. Three series of composite structures were synthesized by including different fractions (0 wt.%, 3 wt.%, 5 wt.%) of nanotubes in nanoalloys of different composition x. Investigation of thermoelectric and structural properties revealed a decrease of the thermal conductivity of up to 40% for the composites in comparison with alloys without nanotube inclusions. This effect can be attributed to enhanced phonon scattering. Seebeck coefficients and electrical conductivities were both slightly enhanced in the composite series with 3 wt.% nanotube inclusions, leading to enhancement of $$ ZT \ \left(ZT=\frac {(S^2 \sigma)}{\kappa}\,{ {T}}\right) $$ throughout the series compared with the nanoalloy series without nanotube inclusions.     

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.%.     

Influence of ZnO Inclusions on the Low-Temperature Thermoelectric Properties of CoSb3

CoSb₃ composites with different amounts of ZnO nanoparticles (2?wt.% to 12?wt.%) were prepared from nanosized ZnO (commercial) and micron-sized CoSb₃ (obtained via solid-state reaction) particles mixed in solution and freeze dried. The resulting powders were densified by spark plasma sintering. The samples were characterized by x-ray diffraction and scanning electron microscopy. It was found that ZnO forms micron-sized clusters at the grain boundaries of the matrix material. The thermoelectric properties (electrical resistivity, thermopower, and thermal conductivity) were measured in the 2?K to 300?K temperature range. Both the electrical and thermal conductivities were observed to decrease with increasing ZnO content. The dimensionless figure of merit ZT was improved by up to 30% at 300?K for the sample containing 2?wt.% ZnO.     

Thermal Conductivity in Bi1−x Sb x Solid Solutions

Bi_1?x Sb _x solid solutions have attracted much attention as promising low-temperature thermoelectric materials. Previously, we observed distinct extrema in the isotherms of the transport and mechanical properties of polycrystalline Bi_1?x Sb _x and attributed their presence to the transition from diluted to concentrated solid solutions and to the reconstruction of the energy band structure under increasing Sb concentration. The goal of the present work is a detailed study of the concentration dependences of the thermal conductivity λ for Bi_1?x Sb _x polycrystalline solid solutions (x = 0 to 0.09) in the temperature range of 170 K to 300 K. It is established that the λ(x) dependences exhibit a nonmonotonic behavior: in certain concentration ranges an anomalous increase in λ with increasing x is observed. It is shown that the concentration dependences of the thermoelectric figure of merit calculated on the basis of the measured λ values are also nonmonotonic. The obtained data represent additional evidence in favor of our assumptions stated earlier about a significant effect of electronic phase transitions observed in Bi_1?x Sb _x solid solutions on the concentration dependences of their thermoelectric properties. These results should be taken into account when developing new Bi_1?x Sb _x -based materials.     

Three-Dimensional Bi2Te3 Nanocrystallites Embedded in 2D Bi2Te3 Films Grown by MOCVD

Two- (2D) and three-dimensional (3D) growth of nanostructured Bi₂Te₃ films was performed on 4° tilt (100) GaAs substrates using a metalorganic chemical vapor deposition system. To obtain 3D Bi₂Te₃ crystallites embedded in 2D planar film, we alternately changed the gas flow rate in the reactor. By repeating two steps, 3D Bi₂Te₃ crystallites embedded in 2D planar Bi₂Te₃ film were obtained. The thermoelectric properties in terms of the thermal conductivity, electrical conductivity, and Seebeck coefficient were investigated at room temperature. The thermal conductivities of the nanostructured Bi₂Te₃ films were from 0.63?W/(m?K) to 0.94?W/(m?K) at room temperature, which are low compared with that of film without nanostructure [1.62?W/(m?K)]. The thermal conductivity of the film was effectively decreased with the decrease of size and increase of density of 3D crystallites. The results of this study open up a new method to fabricate nanostructured thermoelectric films with high thermoelectric figure of merit.     

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.     

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.     

Thermoelectric power of Bi<Subscript>92</Subscript>Sb<Subscript>8</Subscript> and Bi<Subscript>85</Subscript>Sb<Subscript>15</Subscript> thin films

The results of studying the galvanomagnetic and thermoelectric properties of thin block Bi₉₂Sb₈ and Bi₈₅Sb₁₅ films on mica and polyimide substrates are presented. The method used for measuring the thermoelectric power allowed us to study the temperature dependence the thermoelectric power, without introducing additional deformations into the substrate–film system. A significant difference in the temperature dependences of the galvanomagnetic and thermoelectric properties of films on mica and polyimide is found. The free charge-carrier concentrations and mobilities in the films on mica and polyimide and levels of the chemical potential for electrons and holes are calculated within the two-band approximation. The difference in the charge-carrier parameters for films on mica and polyimide is associated with strains in the film–substrate system.     

Anisotropy of electrical properties of lead doped extruded samples of Bi85Sb15

Thin films of Bi₈₅Sb₁₅ solid solution wich can be used in thermoelectric devices for cooling and stabilizing the temperature of Gann and Impatt diodes have been obtained by extrusion method. They have surface roughness in the range 10–80µm, dielectric lossestan δ ～ 10^?3 at 10 GHz, thermal conductivityβ ～ 4x10² W / (M · K). Anisotropy of electroconductivityσ, thermoelectric powerα and Hall coefficientsR _H of lead doped extruded Bi₈₅Sb₁₅ samples has been investigated in the temperature range between 77 K and 300 K and in the presence of magnetic fieldH up to ～ 74×10⁴ A/m. It is shown that the value and sign of the anisotropy coefficient essentially depend on heat treatment and impurity concentration. Experimental results are explained taking into account a crystal structure of Bi₈₅Sb₁₅, formation of texture and generation of deformation defects.     

Effect of Electric Current on the Performance of Bi<Subscript>1.2</Subscript>Sb<Subscript>4.8</Subscript>Te<Subscript>9</Subscript> Thermoelectric Material Prepared by a Field-Activated Pressure-Assisted Sintering Process

Field-activated pressure-assisted sintering (FAPAS) was applied to sinter Bi_1.2Sb_4.8Te₉ thermoelectric materials under different conditions, including no-current sintering (NCS), low-density current sintering (LCS), and high-density current sintering (HCS). The effect of the current density on the final thermoelectric performance of the products was investigated. Applying a higher-density electric current and shorter dwell time can improve the thermoelectric performance of the sample by increasing its electric conductivity and decreasing its thermal conductivity. The maximum figure of merit ZT values of the NCS, LCS, and HCS samples were 0.46, 0.48, and 0.57, respectively. Therefore, applying a high-density electric current in the sintering process may be an effective way to obtain Bi_1.2Sb_4.8Te₉ thermoelectric material with high ZT value.     

Percolation Conduction in Hybrid Thermoelectric Material Consisting of Bi0.88Sb0.12 and Barium Ferrite Particles

A hybrid material consisting of thermoelectric Bi_0.88Sb_0.12 and Ba ferrite Bi_0.88Sb_0.12(BaFe₁₂O₁₉) _x (x = 0, 0.025, 0.04, and 0.08) was synthesized using sintering. Powder x-ray diffraction patterns and scanning electron microscopy images of the hybrid indicate that the BaFe₁₂O₁₉ particles were well distributed in the host Bi_0.88Sb_0.12 phase. The temperature dependence of the electrical resistivity ρ of the host Bi-Sb exhibits metallic behavior. By the addition of Ba ferrite particles, the ρ at 300 K increases intensively, and ρ(Τ) then behaves similarly to a semiconductor. However, it is noted that the thermoelectric power S is unchanged. Inhibition of current and heat flows by a restricted conduction path and the unchanged electromotive force generated by the Seebeck effect in the conduction path can be understood based on a site-percolation model consisting of conducting Bi-Sb and insulating Ba ferrite. The critical volume fraction p _c of this system was estimated experimentally as p _c = 0.68.     

Characterizations of solid-state microwave-synthesized Sb2Te3-based alloys with various compositions of bismuth in Bi2xSb2(1−x)Te3

In this article, thermoelectric (TE) materials based on p-type Sb₂Te₃ samples and dispersed with x amounts of Bi (x=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) in the form Bi_2xSb_2(1−x)Te₃ were synthesized via a standard solid-state microwave route. The microstructure of the ingots was characterized by field emission scanning electron microscopy. As-synthesized ingots were formed by the assembly of micro-sheet grains. The phase composition of the powders was characterized by X-ray diffraction, revealing a rhombohedral structure. The influence of variations in Bi content (x) on the TE properties of the resulting alloy was studied in the temperature range of 303 K to 523 K. Increases in x caused a decrease in hole concentration and electrical conductivity and an increase in Seebeck coefficient. A maximum power factor of 4.96 mW/mK² was obtained at about 373 K for a Bi_2xSb_2(1−x)Te₃ sample with x=0.2.