Bismuth Telluride-based Alloys Research Articles

Synergistic Combination of Sb2Si2Te6 Additives for Enhanced Average ZT and Single-Leg Device Efficiency of Bi0.4Sb1.6Te3-based Composites.

Thermoelectric materials are highly promising for waste heat harvesting. Although thermoelectric materials research has expanded over the years, bismuth telluride-based alloys are still the best for near-room-temperature applications. In this work, a ≈38% enhancement of the average ZT (300-473K) to 1.21 is achieved by mixing Bi0.4Sb1.6Te3 with an emerging thermoelectric material Sb2Si2Te6, which is significantly higher than that of most BiySb2-yTe3-based composites. This enhancement is facilitated by the unique interface region between the Bi0.4Sb1.6Te3 matrix and Sb2Si2Te6-based precipitates with an orderly atomic arrangement, which promotes the transport of charge carriers with minimal scattering, overcoming a common factor that is limiting ZT enhancement in such composites. At the same time, high-density dislocations in the same region can effectively scatter the phonons, decoupling the electron-phonon transport. This results in a ≈56% enhancement of the thermoelectric quality factor at 373 K, from 0.41 for the pristine sample to 0.64 for the composite sample. A single-leg device is fabricated with a high efficiency of 5.4% at ΔT = 164 K further demonstrating the efficacy of the Sb2Si2Te6 compositing strategy and the importance of the precipitate-matrix interface microstructure in improving the performance of materials for relatively low-temperature applications.

High Pressure Drives Microstructure Modification and zT Enhancement in Bismuth Telluride-Based Alloys.

Manipulating and integrating the microstructures at different scales is crucial to tune the electrical and thermal properties of a given compound. High-pressure sintering can modify the multiscale microstructures and thus empower the cutting-edge thermoelectric performance. In this work, the high-pressure sintering technique followed by annealing is adopted to prepare Gd-doped p-type (Bi0.2Sb0.8)2(Te0.97Se0.03)3 alloys. First, the high energy of high-pressure sintering promotes the reduction of grain size, thus increasing the content of 2D grain boundaries. Next, high-pressure sintering induces strong interior strain, where 1D dense dislocations are generated near the strain field. More interestingly, the rare-earth element Gd with a high melting temperature is dissolved into the matrix via high-pressure sintering, thus promoting the formation of 0D extrinsic point defects. This concurrently improves the carrier concentration and density-of-state effective mass, resulting in an enhanced power factor. In addition, the integrated 0D point defects, 1D dislocations, and 2D grain boundaries by high-pressure sintering strengthen phonon scattering, thereby achieving a low lattice thermal conductivity of 0.5 Wm-1 K-1 at 348 K. Consequently, a maximum zT value of ∼1.1 at 348 K is achieved in the 0.4 at % Gd-doped (Bi0.2Sb0.8)2(Te0.97Se0.03)3 sample. This work demonstrates that high-pressure sintering enables microstructure modification to enhance the thermoelectric performance of Bi2Te3-based and other bulk materials.

Room-Temperature Thermoelectric Performance of n-Type Multiphase Pseudobinary Bi2Te3-Bi2S3 Compounds: Synergic Effects of Phonon Scattering and Energy Filtering.

Bismuth telluride-based alloys possess the highest efficiencies for the low-temperature-range (<500 K) applications among thermoelectric materials. Despite significant advances in the efficiency of p-type Bi2Te3-based materials through engineering the electronic band structure by convergence of multiple bands, the n-type pair still suffers from poor efficiency due to a lower number of electron pockets near the conduction band edge than the valence band. To overcome the persistent low efficiency of n-type Bi2Te3-based materials, we have fabricated multiphase pseudobinary Bi2Te3-Bi2S3 compounds to take advantages of phonon scattering and energy filtering at interfaces, enhancing the efficiency of these materials. The energy barrier generated at the interface of the secondary phase of Bi14Te13S8 in the Bi2Te3 matrix resulted in a higher Seebeck coefficient and consequently a higher power factor in multiphase compounds than the single-phase alloys. This effect was combined with low thermal conductivity achieved through phonon scattering at the interfaces of finely structured multiphase compounds and resulted in a relatively high thermoelectric figure of merit of ∼0.7 over the 300-550 K temperature range for the multiphase sample of n-type Bi2Te2.75S0.25, double the efficiency of single-phase Bi2Te3. Our results inform an alternative alloy design to enhance the performance of thermoelectric materials.

Effect of cold and hot compactions on corrosion behavior of p- and n-type bismuth telluride-based alloys developed through microwave sintering process

Bismuth Telluride (BiTe) based p-type and n-type alloys exhibit superior thermoelectric (TE) performance covering energy requirements for specialized and home utilization. The main challenge nowadays is the sustainability of their adequate TE performance in corrosive environments, which might activate the corrosion reactions, leading to the degradation of p/n semiconductors, and then failure of the TE device. This study investigates the electrochemical responses of cold and hot compacted, microwave-sintered p- and n-type BiTe alloys in a saline medium (3.5 wt% NaCl solution). XRD analysis of microwave-sintered cold- and hot-compacted BiTe pellets confirmed their phase purity and uniform crystal structure. Potentiodynamic polarization (PDP) and Electrochemical Impedance Spectroscopy (EIS) data showed enhancements in the corrosion behavior of hot-compacted p-type and cold-compacted n-type BiTe pellets. The study also proposes a corrosion resistance mechanism with an equivalent electrical circuit (EEC) to fit the experimental EIS data of both BiTe pellets. FE-SEM analysis showed visible microstructural evolutions of the BiTe pellets and their passive films. It revealed a remarkable improvement in the microstructure and blocking effect caused by the formed passive films coating the surfaces of the pellets and acting as a physical barrier preventing the passing of destructive Cl- ions. EDX spectra have proved the presence of p-type and n-type BiTe alloys, each with the corresponding dopant element of Antimony (Sb) or Selenium (Se), respectively, and in the same weight compositions for either hot or cold compacted pellets. AFM analysis examined the surface topography of developed pellets. It showed an increment in the surface roughness-mean-square (RMS) values with the development of passive films on p- and n-type BiTe alloys.

Removing the Oxygen-Induced Donor-like Effect for High Thermoelectric Performance in n-Type Bi2Te3-Based Compounds.

Bismuth telluride-based alloys are the best performing thermoelectric materials near room temperature. Grain size refinement and nanostructuring are the core stratagems for improving thermoelectric and mechanical properties. However, the donor-like effect induced by grain size refinement strongly restricts the thermoelectric properties especially in the vicinity of room temperature. In this study, the formation mechanism for the donor-like effect in Bi2Te3-based compounds was revealed by synthesizing five batches of polycrystalline samples. We demonstrate that the donor-like effect in Bi2Te3-based compounds is strongly related to the vacancy defects (VBi‴ and VTe···) induced by the fracturing process and oxygen in air for the first time. The oxygen-induced donor-like effect dramatically increases the carrier concentration from 2.5 × 1019 cm-3 for the zone melting ingot and bulks sintered with powders ground under an inert atmosphere to 7.5 × 1019 cm-3, which is largely beyond the optimum carrier concentration and seriously deteriorates the thermoelectric performance. Moreover, it is found that both avoiding exposure to air and eliminating the thermal vacancy defects (VBi‴ and VTe···) via heat treatment before exposure to air can effectively remove the donor-like effect, producing almost the same carrier concentration and Seebeck coefficient as those of the zone melting ingot for these samples. Therefore, a defect equation of oxygen-induced donor-like effect was proposed and was further explicitly corroborated by positron annihilation measurement. With the removal of donor-like effect and improved texturing via multiple hot deformation (HD) processes, a maximum power factor of 3.5 mW m-1 K-2 and a reproducible maximum ZT value of 1.01 near room temperature are achieved. This newly proposed defect equation of the oxygen-induced donor-like effect will provide a guideline for developing higher-performance V2VI3 polycrystalline materials for near-room-temperature applications.

Microstructural control of bismuth tellurium alloys by solidification with undercooling

The solidification of a thermoelectric alloy melt with undercooling was studied experimentally with the aim of controlling the microstructure during solidification. These trials were conducted using bismuth tellurium alloys, and the crystal morphology, number density and growth direction were observed while varying the initial composition and temperature gradient. The solidified material consisted of bismuth telluride compound crystals having plate-like facets along with a eutectic matrix. The growth mechanism of these crystals was examined in relation to the temperature and concentration fields around the solid-liquid interface, based on microstructural observations and temperature measurements. The solidification process was found to consist of three elementary steps: (1) recalescence dissipation in association with thermal undercooling, (2) relaxation in association with constitutional undercooling and (3) conventional phase change via heat conduction. The directivity and fineness of structure improved as the temperature gradient was increased during undercooling. In addition, electron backscatter diffraction data showed that the c-axis of each hexagonal bismuth telluride crystal was oriented almost parallel to the cooling surface. As a result, the thermoelectric power factor was enhanced. These results suggest that controlling the microstructure in bismuth telluride-based alloys via the application of a non-uniform undercooling field has the potential to allow the fabrication of high performance thermoelectric materials with increased production efficiency.

Simultaneous Regulation of Electrical and Thermal Transport Properties of N-Type Bi2Te3 via Adding Excessive Te Followed by Se Doping

Thermoelectric devices based on bismuth telluride-based alloys have been applied widely for several decades. Our results demonstrate that excessive Te constructs large amounts of anti-site defects ...

Amorphous Framework in Electrodeposited CuBiTe Thermoelectric Thin Films with High Room-Temperature Performance.

Bismuth telluride-based alloys are the most efficient thermoelectric materials near room temperature and widely used in commercial thermoelectric devices. Nevertheless, their thermoelectric performance needs to be improved further for wide-scale implementation either as a thermoelectric generator or cooler. Here, we propose a simultaneous codeposition of CuBiTe thin films and their phase transition strategy via the traditional electrodeposition process. With just 13 atom % Cu doping, crystalline-to-amorphous phase transformation resulted for the electroplated CuBiTe alloy. A close look at the alloy composition revealed spike-shaped nanocrystalline Bi2Te3 embedded in the CuBiTe amorphous matrix. Our result shows an exceptionally high power factor (3.02 mW m–1 K–2), which comes from the enhanced Seebeck coefficient (−275 μV K–1) and high electrical conductivity (3.99 × 104 S m–1) of CuBiTe films. Therefore, it can be suggested that the adopted strategy to form a unique nanocrystallite-embedded amorphous framework provides a platform to develop next-generation high-performance thermoelectric materials with an extraordinary power factor.

High thermoelectric performance of Bi2-xSbxTe3 bulk alloys prepared from non-nanostructured starting powders

In this work, hot press and hot deformation were applied to p-type Bi2-xSbxTe3 (x = 1.5–1.8) starting powders that have not undergone nanostructuring processing. The effects of hot pressing and hot deformation, as well as Sb-alloying, on the microstructure and the thermoelectric properties of the Bi2-xSbxTe3 alloys were systematically investigated. The results of this study are compared to those of others, which have addressed similar issues in samples prepared with analogous compaction techniques. ZT value of 1.12 at 375 K for hot pressed Bi0.4Sb1.6Te3 and ZT value of 1.24 at 375 K for hot deformed Bi0.4Sb1.6Te3 were achieved. To our knowledge, the latter ZT value is only slightly lower than the highest ever reported (ZT~1.3) for p-type bismuth telluride-based alloys prepared using starting materials without nanostructuring, suggesting that simpler preparation techniques can be applicable in modules fabrication.

Thermoelectric Properties of Synthesized P-Type Bi-Te-Sb Bulk Materials By Oxide Reduction Process

Bismuth telluride-based alloys have long been in the spotlight as they show promising ZT and high thermal stability near the room-temperature rigion. Although there are existing fabrication methods, but they all have problems like long processing time, lower sample thermoelectric property. Hence there exists a need to develop material of high thermoelectric properties in room temperature by simple method. The present work consists of synthesis and characterization of Bi-Te-Sb nanocomposite. Nanocomposite in thermoelectric materials has been proven to be an effective paradigm for optimizing the high thermoelectric performance primarily by reducing the thermal conductivity. In addition, changes in thermal transport properties brought up by adding dopants,material nanostructuring and dispersed phase addition, in the Bi-Te−Sb material for this methodology are measured and discussed. For the synthesis of p-typeBi-Te−Sb thermoelectric materials, reagent-grade raw materials (>99.99%) were used, and Bi2O3 (>99.99%), TeO2 (>99.99%), and Sb2O3 (>99.99%) powders were used. Bi-Te-Sb composites were fabricated by a high energy ball milling followed by spark plasma sintering process. Bi-Te-Sb milled powders show agglomerated particles between 100nm and 300nm in size with almost faced or rounded shape. Reduction powders with irrelgularly facetted shape and about 500nm in size. As a field-assisted technique, spark plasma sintering (SPS) enables densification of specimens in a very short period of time compared to other sintering techniques. For high performance thermoelectric material synthesis, SPS is widely used to fabricate nanograin-structured thermoelectric materials by rapidly densifying the nanopowders suppressing grain growth. Phase of powders and bulk were analyzed by X-ray diffraction. The microstructures of powders and bulk were characterized by field emission scanning electron microscopy. Bi-Te-Sb bulk materials were analyzed electrical and thermal transport properties. The achieved higher ZT value is attributed to the unique nanostructures which increase phonon scattering in the nanostructured materials to effectively reduce thermal conductivity.. Keyword Bi2Te3, Thermoelectric, nanostructure, SPS

Texture-induced reduction in electrical resistivity of p-type (Bi,Sb)2Te3 by a hot extrusion

The electrical resistance of a thermoelectric module is an important parameter as well as the thermoelectric figure-of-merit for the application of both waste energy harvesters and solid-state cooling devices. High resistance causes energy dissipation through Joule heating, leading to the degradation of the device's generating or cooling performance. Therefore, it is highly desirable to reduce the electric resistivity of the thermoelectric materials. Here, we report a hot extrusion technique to fabricate p-type bismuth antimony tellurides that has low electrical resistivity. Extrusion is a promising tool to fabricate a fiber-like texture of the microstructure, where the 001 normal vector of grains is pointing perpendicular to the extrusion direction. Such a textured microstructure can enhance the mobility of electrical carriers along the extrusion direction due to the anisotropic nature of electrical transport in bismuth telluride-based alloys. Beyond bismuth telluride-based thermoelectric materials, we believe that our result will offer a simple tool to manipulate physical properties of any two-dimensional, layered materials by controlling microstructure.

Effect of Immersion Time on Corrosion Behavior of Single-Phase Alloy and Nanocomposite Bismuth Telluride-Based Thermoelectrics in NaCl Solution

The corrosiveness of bismuth telluride-based thermoelectric materials (n-type single-phase alloy and a nanocomposite with MoS2 nanoinclusions), in 0.1 molar solution of sodium chloride (NaCl), was investigated. The electrochemical impedance spectroscopy curves obtained after 1, 24, 48 and 72 h immersion time revealed the enhancement of the corrosion resistance of the nanocomposite specimen in a 0.1 molar NaCl solution in comparison with the single-phase bismuth telluride-based alloys, and the passivity increased by immersion time up to 72 h. The nanocomposite sample with submicron grains provided suitable nucleation sites for passive film nucleation that led to higher protective behavior.

Enhanced thermoelectric performance of n-type bismuth-telluride-based alloys via In alloying and hot deformation for mid-temperature power generation

Bismuth telluride-based alloys are the most widely used commercial thermoelectric (TE) material for room temperature refrigeration. Here, we successfully shift up the optimum figure of merit of n-type bismuth-telluride-based TE materials for mid-temperature power generation. SbI3 doping is used to regulate the carrier concentration and Indium alloying to increase the bandgap, suppressing the detrimental bipolar conduction in the mid-temperature range. The lattice thermal conductivity is significantly reduced due to the multiscale microstructures induced via hot deformation. As a result, a peak zT of ∼1.1 was attained at 625 K for Bi1.85In0.15Te2Se + 0.25 wt% SbI3 alloy after hot deformation, showing a great application prospect of this alloy in mid-temperature TE power generation.

Pressure-induced insulator-to-metal transitions for enhancing thermoelectric power factor in bismuth telluride-based alloys.

First-principles calculations revealing insulator-to-metal transitions in Bi2Te3 and Bi2Te2Se, at 9 GPa and 12.5 GPa, respectively, match with prior experiments. Our electronic band structure calculations and accompanying Boltzmann transport calculations of thermoelectric properties for Bi2-xSbxTe2-ySey alloys explain and predict large power factor changes induced by pressure. Complex band degeneracy changes preceding insulator-to-metal transitions significantly alter the density of states near the Fermi level, and foster the disentangling of the unfavorable coupling between Seebeck coefficient and electrical conductivity. Our findings on pressure-induced changes in thermoelectric power factor provide insights for designing V2VI3-based high-performance thermoelectric materials through strategies such as alloying, high-pressure processing, and strain engineering.

Effect of Temperature and Environmental Aging on the Mechanical Reliability of n- and p-Type Thermoelectric Couples

Bismuth telluride-based alloys are the most widely used thermoelectric materials, which can obtain the conversion between heat energy and electricity. As a kind of lead-free solder alloy, Sn96.5Ag3.0Cu0.5 (SAC305) shows great potential in soldering between the thermoelectric element and electrode in thermoelectric couples. Cu/ SAC305 /Ni-plated Bi0.5Sb1.5Te3 (n-type) and Cu/ SAC305/Ni-plated Bi1.8Sb0.2Se0.15Te2.85 (p-type) couples were connected at 250°C by reflow soldering to investigate effects of temperature and thermall aging on the efficiency and reliability of thermoelectric device. Effects of temperature and environmental aging on shear force of thermoelectric couples were studied respectively in the temperature range of 20°C to 180°C. At room temperature of 20°C, n-type thermoelectric couples showed higher shear force than p-type thermoelectric couples. N-type thermoelectric couple showed higher shear reliability than p-type thermoelectric couples after synchronized environmental aging. The shear force of both types of thermoelectric couples decreased with the growth of thermal aging cycles. Finally, the fracture surfaces of two types of thermoelectric couples were investigated respectively using SEM to further analyse the failure mechanism.

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 (Bi0.2Sb0.8)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 (Bi0.2Sb0.8)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 (Bi0.2Sb0.8)2Te3, with reduction of the thermal conductivity which is more significant at temperatures above the cross point of the ZT values.

Shifting up the optimum figure of merit of p-type bismuth telluride-based thermoelectric materials for power generation by suppressing intrinsic conduction

The abundance of low-temperature waste heat produced by industry and automobile exhaust necessitates the development of power generation with thermoelectric (TE) materials. Commercially available bismuth telluride-based alloys are generally used near room temperature. Materials that are composed of p-type bismuth telluride, which are suitable for low-temperature power generation (near 380 K), were successfully obtained through Sb-alloying, which suppresses detrimental intrinsic conduction at elevated temperatures by increasing hole concentrations and material band gaps. Furthermore, hot deformation (HD)-induced multi-scale microstructures were successfully realized in the high-performance p-type TE materials. Enhanced textures and donor-like effects all contributed to improved electrical transport properties. Multiple phonon scattering centers, including local nanostructures induced by dynamic recrystallization and high-density lattice defects, significantly reduced the lattice thermal conductivity. These combined effects resulted in observable improvement of ZT over the entire temperature range, with all TE parameters measured along the in-plane direction. The maximum ZT of 1.3 for the hot-deformed Bi0.3Sb1.7Te3 alloy was reached at 380 K, whereas the average ZTav of 1.18 was found in the range of 300–480 K, indicating potential for application in low-temperature TE power generation. Thermoelectric materials, which convert temperature differences and electric voltage into each other, serve in refrigeration or power generation applications. Currently, bismuth telluride (Bi2Te3) and its alloys are the most widely used thermoelectric materials. Tie-Jun Zhu, Xin-Bing Zhao and co-workers from Zhejiang University, China, have now investigated the effect of antimony (Sb) alloying on bismuth tellurides through a series of polycrystalline solid solutions of Bi2-xSbxTe3—where x varies between 1.4 and 1.8—prepared by hot deformation. Systematic tuning of the alloy composition showed that higher antimony content raised the material's optimal conversion temperature by repressing undesirable conduction. This effect arises from an increase in both the hole concentration and the band gap in the material. For a composition where x is 1.7, the alloy showed optimal performances at 380 kelvin—a suitable temperature for low-temperature power generation from the waste heat generated by industry or vehicles. The p-type bismuth telluride-based polycrystalline materials suiting for low-temperature power generations (near 380 K) have been obtained through Sb-alloying and HD, which suppresses the detrimental effect of intrinsic conduction at elevated temperature via increasing the hole concentration and band gap. The hot-deformed Bi0.3Sb1.7Te3 alloy, not usual composition Bi0.5Sb1.5Te3, shows a maximum ZT of 1.3 at 380 K, indicating a bright application potential in low-temperature power generations.

Improving thermoelectric properties of n-type bismuth–telluride-based alloys by deformation-induced lattice defects and texture enhancement

Repetitive hot deformation has been demonstrated as a new approach to obtain high-performance n-type bismuth–telluride-based alloys, benefiting from the deformation-induced lattice defects and texture enhancement. X-ray diffraction measurement showed that the oriented textures were greatly enhanced after repetitive hot deformation of the alloys with a quasi-layered crystal structure. The electrical conductivity was remarkably improved by the deformation-induced donor-like defect and texture enhancement, while the Seebeck coefficient remained almost unchanged, and consequently the room temperature power factor was significantly increased from 1.3Wm−1K2, before hot deformation, to 2.9Wm−1K2 after four hot deformations. The in-plane lattice thermal conductivity was also largely reduced by the generated high-density lattice defects during the hot-deformation process. The maximum ZT value for the repetitively hot-deformed samples reached 1.0 at 513K, suggesting that the simple new top-down method is very promising for large-scale production of high-performance bismuth–telluride-based polycrystalline bulk materials.

Effect of Processing Route on the Microstructure and Thermoelectric Properties of Bismuth Telluride-Based Alloys

Considerable work has been done to engineer materials with high efficiencies of thermoelectric heat-to-electricity conversion and the mechanical strength necessary to withstand the demands of practical applications. In particular, in the bismuth telluride system, extrusion pressing has been found to be effective for improving the mechanical strength of alloys via grain refinement. We review some of the literature relating to processing approaches for the bismuth telluride system. We also present preliminary data for a series of samples obtained by incorporating C60 via ball milling and spark plasma sintering into a matrix consisting of a (Bi,Sb)2Te3 alloy, with a focus on the texture of the composites and its relation to thermoelectric transport properties, in comparison to the parent material. The viability of improving the thermoelectric performance of bismuth telluride alloys by the insertion of nanoparticles into a composite is also considered.

Ultrasonic synthesis of nanocrystals of metal selenides and tellurides

A mild sonochemical route was used to prepare Bi2Te3 nanocrystals, and Bi2Se2Te and Bi2SeTe2 ternary nano-alloys in aqueous solution; an interfacial mechanism for this sonochemical reaction is discussed. The basic thermoelectric properties (Seebeck coefficient) of the as-prepared nanocrystals were studied, and this simple, easy and efficient synthetic route may further encourage wider application of bismuth telluride-based alloys in solid state thermoelectric cooling devices. Furthermore, the adopted synthetic route was extended to the preparation of other nanocrystals with uniform morphologies, as well as some common but useful semiconductors.