Te3 Thermoelectric Materials Research Articles

Development of the high performance thermoelectric unicouple based on Bi2Te3 compounds

Bismuth telluride-based compounds are the most common thermoelectric (TE) materials for power generating systems, which can operate in the temperature range of 300–600 K. One of the main disadvantages of Bi2Te3-based TE modules is the low energy conversion efficiency (3–6%). For this reason, we offer a unique thermoelectric power generation unicouple based on the developed n-type Bi2Te2.7Se0.3 and cut perpendicularly to the main crystallographic axis p-type Bi0.5Sb1.5Te3 thermoelectric materials. The application of the optimized chemical compositions in combination with Spark Plasma Sintering (SPS) technique provided the high average thermoelectric figure of merit of up to 0.97 and 0.72 for n-Bi2Te2.7Se0.3 and pꓕ-Bi0.5Sb1.5Te3, respectively. A three-dimensional finite-element simulation was employed to account for the different transport properties of the n- and p-type materials. As a result of the optimized design, the outstanding energy conversion efficiency ηmax ∼8% was experimentally measured for the thermoelectric unicouple device at the relatively small temperature gradient of 250 K (Tc = 350 K). The output unicouple parameters were recorded using the developed herein high-precision original setup. Furthermore, a power generation TE unicouple with an electric power of 0.04 W at the 0.5 V was designed and fabricated offering significant industrial potential.

Enhanced thermoelectric performance at elevated temperature via suppression of intrinsic excitation in p-type Bi0.5−xSnxSb1.5Te3 thermoelectric material

Enhanced thermoelectric performance at elevated temperature via suppression of intrinsic excitation in p-type Bi0.5−xSnxSb1.5Te3 thermoelectric material

Point Defect Engineering: Co-Doping Synergy Realizing Superior Performance in n-Type Bi2 Te3 Thermoelectric Materials.

Bi2 Te3 has attracted great attention because of its excellent thermoelectric (TE) performance around room temperature. However, the TE property of the n-type Bi2 Te3 is still relatively low compared to the p-type counterpart, which seriously hinders its commercial application with a combination of the n-type and p-type materials. Herein, an effective process of Cl and W co-doping is employed into the n-type Bi2 Te3 materials to enhance its TE properties. The Bi1.996 W0.004 Te2.476 Cl0.024 Se0.5 sample achieves a peak and average ZT over 1.3and 1.2, respectively, at temperature range of 300-575 K. A 24-leg TE module of this n-type material and a home-made p-type Bi2 Te3 sample can produce a high efficiency over 6% at a temperature gradient of 235 K, which possesses a 71% improvement compared with a commercial Bi2 Te3 module. This high performance is ascribed to the effect of the Cl and W doping. This co-doping not only significantly increases the Grüneisen parameter but also successfully induces interstitial atoms in the van der Waals gap, which lead to a low lattice thermal conductivity (κl ) of 0.31W m-1 K-1 and a boosted charge transport. This finding represents an important step to promote the development of the n-type Bi2 Te3 materials.

Enhanced thermoelectric composite performance from mesoporous carbon additives in a commercial Bi0.5Sb1.5Te3 matrix

Composites were prepared, through hot pressing, using carbon materials with different pore size distributions as additives for commercial Bi0.5Sb1.5Te3 thermoelectric material (BST, p-type). Thermoelectric properties of the composites were measured in a temperature range of 298‒473 K. Thermal conductivity of the composites, especially lattice thermal conductivity, was effectively decreased due to the mesoporous properties of the incorporated carbon additives. The electrical conductivity of the composites slightly decreased due to the electron scattering at the interface between the carbon material and the commercial BST matrix. The composite with 0.2 vol.% mesoporous carbon powder (36% mesoporosity) exhibited a figure of merit value approximately 10.7% higher than that of commercial BST without additives. This behavior resulted in 116% improved output power in the composite block-based single element compared with a bare BST thermoelectric block. The enhanced figure of merit was attributed to the effective reduction of lattice thermal conductivity by acoustic phonons scattering at the interface between the BST matrix and the mesoporous carbon as well as at the pore surfaces within the mesoporous carbon. By utilizing mesoporous carbon materials used in this study, the shortcomings and economic difficulties of the composite process with low dimensional carbon additives (carbon nanotubes, graphene, and nanodiamond) can be overcome for extensive practical applications. Mesoporous carbon powder with a tailored porosity distribution revealed the validity of bulk-type carbon additives to enhance the figure of merit of commercial thermoelectric materials.

Effect of the crystal alignment and grain size on the thermoelectric properties of Bi0.4Sb1.6Te3 sintered materials

Sintered polycrystalline Bi0.4Sb1.6Te3 thermoelectric materials with various degrees of crystal alignment and grain sizes were prepared by pulse-current sintering under cyclic uniaxial pressure at sintering temperatures of 350–425 °C for holding times of 0–60 min to clarify the relationship between the microstructure and the thermoelectric properties. The degree of crystal alignment was enhanced with an increase of both the sintering temperature and holding time, and grain growth was also confirmed in the high temperature or long-time sintering process. The thermoelectric properties were measured perpendicular to the pressing direction, which corresponds to the crystal alignment direction. The electrical resistivity decreased and the thermal conductivity slightly increased with increasing sintering temperature and holding time. As a result, the figures of merit of the crystal-aligned samples sintered at high temperatures or for long holding times tended to reach 0.9–1 with large power factors because of the small electrical resistivity. The relationships between the quantified microstructure parameters and thermoelectric properties are discussed. The electrical resistivity decreased with increasing degree of crystal alignment and it saturated at a certain degree of crystal alignment, indicating that perfect crystal alignment is not necessary to obtain the lower limit of the electrical resistivity. Conversely, no significant change of the lattice thermal conductivity was observed in the grain size range 0.6–9.7 μm. This means that the lattice thermal conductivity of crystal aligned Bi0.4Sb1.6Te3 is almost independent of the degree of crystal alignment and grain size in the measured range.

Improvement of Sn-3Ag-0.5Cu Soldered Joints Between Bi0.5Sb1.5Te3 Thermoelectric Material and a Cu Electrode

A Bi0.5Sb1.5Te3 thermoelectric (TE) element was directly soldered to a Cu electrode using Sn-3Ag-0.5 Cu alloy. The interface was sound and the bonding strength was satisfactory (8.6 MPa). However, the solder layer was exhausted quickly during high-temperature storage (HTS) tests at 150°C for 300 h and 600 h, and the bonding strength drastically decreased to 1.5 MPa. The consumption of the solder was prevented by electroplating a Ni barrier layer on the TE element, though a low bonding strength of 1.9 MPa resulted. Adding a Sn-rich thin film and a Ni barrier layer onto the Bi0.5Sb1.5Te3 element led to a high bonding strength of 12.1 MPa, which decreased only slightly after HTS reliability tests at 150°C for 1000 h. The sound interfaces of the Bi0.5Sb1.5Te3/Cu joints maintained their stability even after HTS at 175°C for 1000 h.

Effect of the Crystal Alignment and Grain Size on the Thermoelectric Properties of Bi <sub>0.4</sub>Sb <sub>1.6</sub>Te <sub>3</sub> Sintered Materials

Sintered polycrystalline Bi0.4Sb1.6Te3 thermoelectric materials with various degrees of crystal alignment and grain sizes were prepared by pulse-current sintering under cyclic uniaxial pressure at sintering temperatures of 350–425 °C for holding times of 0–60 min to clarify the relationship between the microstructure and the thermoelectric properties. The degree of crystal alignment was enhanced with an increase of both the sintering temperature and holding time, and grain growth was also confirmed in the high temperature or long-time sintering process. The thermoelectric properties were measured perpendicular to the pressing direction, which corresponds to the crystal alignment direction. The electrical resistivity decreased and the thermal conductivity slightly increased with increasing sintering temperature and holding time. As a result, the figures of merit of the crystal-aligned samples sintered at high temperatures or for long holding times tended to reach 0.9–1 with large power factors because of the small electrical resistivity. The relationships between the quantified microstructure parameters and thermoelectric properties are discussed. The electrical resistivity decreased with increasing degree of crystal alignment and it saturated at a certain degree of crystal alignment, indicating that perfect crystal alignment is not necessary to obtain the lower limit of the electrical resistivity. Conversely, no significant change of the lattice thermal conductivity was observed in the grain size range 0.6–9.7 μm. This means that the lattice thermal conductivity of crystal aligned Bi0.4Sb1.6Te3 is almost independent of the degree of crystal alignment and grain size in the measured range.

Effect of Cyclic Uniaxial Pressure in Pulse-Current Sintering on Microstructure of Bi2Te3-Based Thermoelectric Materials

Various cyclic uniaxial pressure patterns have been applied to a powder of Bi0.4Sb1.6Te3 thermoelectric material during the pulse-current sintering process. The cyclic uniaxial pressure pattern (0 MPa to 100 MPa) was varied by changing the number of pressure cycles and the ratio of the pressing time to the time during which no pressure was applied (i.e., pressing time/pressureless time). Sintering was performed at 400°C for 10 min in vacuum. Electron backscattered diffraction analysis revealed that both the number of pressure cycles and the pressing time ratio strongly affected the microstructure of the sintered materials. Increasing the number of pressure cycles led to a monotonic increase in the intensity of the c-plane orientation. Increasing the pressing time ratio also enhanced the intensity of the c-plane orientation, although the intensity of the c-plane orientation decreased at larger pressing time ratios. These results indicate that constant pressure and pressureless sintering are not effective for attaining c-plane orientation and that the cyclic uniaxial pressure pattern is important for controlling the microstructure of the sintered material. Consequently, the thermoelectric properties of sintered materials could be enhanced by varying the cyclic uniaxial pressure pattern.

Microstructure control of Bi0.4Sb1.6Te3 thermoelectric material by pulse-current sintering under cyclic uniaxial pressure

The effect of the sintering holding time on the microstructure and thermoelectric properties of Bi0.4Sb1.6Te3 prepared by pulse-current sintering under cyclic uniaxial pressure was investigated. The sintering was performed at 400 °C under a cyclic pressure of 0 MPa and 100 MPa, and the holding time was varied from 0 to 60 min. The microstructure of samples, including grain size, hexagonal c-plane texture, and high-angle grain boundary, changed with holding time, that is, grain size and c-plane texture were enhanced and the high-angle grain boundary was reduced with increasing holding time. The thermoelectric properties changed corresponding to the change in the microstructures. The Seebeck coefficient was independent of the sintering parameters and had a value of approximately 200 μV/K. Nevertheless, electrical resistivity decreased with holding time because of the increase in the c-plane texture. As a result, the power factor was enhanced with increasing holding time. In contrast, thermal conductivity was insensitive with the holding time and had almost constant value. As a result, a dimensionless figure of merit of approximately 1 at 300 K was achieved with a large power factor of 4.3 × 10−3 W/K2m. Therefore, in the PCS-cyclic process, the holding time is an important sintering parameter for controlling the microstructure and electrical properties of Bi0.4Sb1.6Te3 thermoelectric materials.

Microstructure Analysis and Thermoelectric Properties of Melt-Spun Bi-Sb-Te Compounds

In order to realize high-performance thermoelectric materials, a way to obtain small grain size is necessary for intensification of the phonon scattering. Here, we use a melt-spinning-spark plasma sintering process for making p-type Bi0.36Sb1.64Te3 thermoelectric materials and evaluate the relation between the process conditions and thermoelectric performance. We vary the Cu wheel rotation speed from 1000 rpm (~13 ms−1) to 4000 rpm (~52 ms−1) during the melt spinning process to change the cooling rate, allowing us to control the characteristic size of nanostructure in melt-spun Bi0.36Sb1.64Te3 ribbons. The higher wheel rotation speed decreases the size of nanostructure, but the grain sizes of sintered pellets are inversely proportional to the nanostructure size after the same sintering condition. As a result, the ZT values of the bulks fabricated from 1000–3000 rpm melt-spun ribbons are comparable each other, while the ZT value of the bulk from the 4000 rpm melt-spun ribbons is rather lower due to reduction of grain boundary phonon scattering. In this work, we can conclude that the smaller nanostructure in the melt spinning process does not always guarantee high-performance thermoelectric bulks, and an adequate following sintering process must be included.

Fabrication of Bi–Sb–Te Thermoelectric by Cold-Pressed Sintering for Motorcycle Exhaust.

This study was conducted on the Bi–Sb–Te thermoelectric material which is cold-pressed Sintering under 750 Mpa to make square thermoelectric pairs with size 8.2 mm × 8.2 mm and thicknesses 0.8 mm and 1.5 mm. The zone melting method was used to acquire P-type thermoelectric material Bi0.4Sb1.6Te3 and N-type thermoelectric material Bi2Te2.5Se0.5. At temperature 383 K, the measured Seebeck coefficient of Bi0.4Sb1.6Te3 is 222 μV/K, and its thermoelectric figure of merit ZT is 1.35. At temperature 400 K, the measured Seebeck coefficient of Bi2Te2.5Se0.5 is 210 μV/K, and its thermoelectric figure of merit ZT is 1.13. Using Solder paste Sn42Bi58 and copper electrode plate are in series connection with 16 pieces of P/N thermoelectric material to form thermoelectric modules. The thermoelectric module is actually pasted on the motorcycle waste heat source to be evaluated the performance, making the cold-end temperature dissipation heat can enhance the temperature difference between it so as to increase the output power. Increasing the leg thickness of thermoelectric module and making the about 35 °C temperature-difference of those can obviously enhance the performance of in terms of its voltage, its thermoelectric figure of merit ZT and output power of the thermoelectric modules.

Design and Preparation of Bi-Sb-Te Alloy Thermoelectric Power Generation from Motorcycle Exhaust

This study was conducted on design and preparation of the variable compositions bismuth antimony telluride (BiSbTe) alloys by mechanically alloying and the powders sintering to form the thermoelectric alloy materials. The powder of thermoelectric material is coldpressed under 766.4Mpa to make square thermoelectric pairs with size 8.2mm×8.2mm and thicknesses 0.8mm and 1.5mm. The zone melting method was used to acquire P-type thermoelectric material Bi0.4Sb1.6Te3 and N-type thermoelectric material Bi2Te2.55Se0.45. At temperature 381K, the measured Seebeck coefficient of Bi0.4Sb1.6Te3 is 223μV/K, and its thermoelectric figure of merit ZT is 1.36. At temperature 400K, the measured Seebeck coefficient of Bi2Te2.55Se0.45 is 206μV/K, and its thermoelectric figure of merit ZT is 1.03. Solder paste Sn42Bi58 and copper electrode plate are in series connection with 16 pieces of P/N thermoelectric material to form thermoelectric modules. The generating performance of a thermoelectric module is evaluated in terms of its voltage, current and output power of the thermoelectric modules.

Enhanced Thermoelectric Properties of p-type Bi0.5Sb1.5Te3 Thermoelectric Materials by Mechanical Alloying and Spark Plasma Sintering

In this research, the microstructure and transport properties of p-type Bi0.5Sb1.5Te3 thermoelectric materials were investigated as a function of milling time. The p-type Bi0.5Sb1.5Te3 alloys were fabricated by mechanical alloying of elemental chunks of bismuth, antimony, and tellurium. This was followed by plasma spark sintering at 673 K. The micro-Vickers hardness (98.7 Hv) was considerably improved in the 90-min sample due to the presence of fine grains in the matrix that prevented crack propagation via grain-boundary hardening. The lowest lattice thermal conductivity (0.63 W/mK) was obtained for the 90-min sample, a value slightly lower than the minimum total thermal conductivity (0.872 ± 0.5 W/mK at 300 K) due to strong scattering of phonons and carriers owing to the completely randomness of the distribution of the fine-grain structure in the bulk samples. The maximum figure-of-merit (ZT = 0.98 ± 0.5 at 300 K) was obtained for the 90-min sample due to its superior power factor values.

Low-Temperature Bonding of Bi0.5Sb1.5Te3 Thermoelectric Material with Cu Electrodes Using a Thin-Film In Interlayer

A Bi0.5Sb1.5Te3 thermoelectric material electroplated with a Ni barrier layer and a Ag reaction layer was bonded with a Ag-coated Cu electrode at low temperatures of 448 K (175 °C) to 523 K (250 °C) using a 4-μm-thick In interlayer under an external pressure of 3 MPa. During the bonding process, the In thin film reacted with the Ag layer to form a double layer of Ag3In and Ag2In intermetallic compounds. No reaction occurred at the Bi0.5Sb1.5Te3/Ni interface, which resulted in low bonding strengths of about 3.2 MPa. The adhesion of the Bi0.5Sb1.5Te3/Ni interface was improved by precoating a 1-μm Sn film on the surface of the thermoelectric element and preheating it at 523 K (250 °C) for 3 minutes. In this case, the bonding strengths increased to a range of 9.1 to 11.5 MPa after bonding at 473 K (200 °C) for 5 to 60 minutes, and the shear-tested specimens fractured with cleavage characteristics in the interior of the thermoelectric material. The bonding at 448 K (175 °C) led to shear strengths ranging from 7.1 to 8.5 MPa for various bonding times between 5 and 60 minutes, which were further increased to the values of 10.4 to 11.7 MPa by increasing the bonding pressure to 9.8 MPa. The shear strengths of Bi0.5Sb1.5Te3/Cu joints bonded with the optimized conditions of the modified solid–liquid interdiffusion bonding process changed only slightly after long-term exposure at 473 K (200 °C) for 1000 hours.

Electrochemical Reduction Process of BiIII, TeIV and SbIII with Complexing Agent in Hydrochloric Acid Bath

Results are presented on addition of novel complexing agents citric acid monohydrate (C6H8O7·H2O) and potassium citrate monohydrate (K3C6H5O7·H2O) in hydrochloric acid aqueous solution. The electrochemical reduction process of BiIII, TeIV and SbIII single ions with complexing agent citric acid monohydrate (C6H8O7·H2O) and potassium citrate monohydrate (K3C6H5O7·H2O) in hydrochloric acid bath was investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements separately. The results indicate that the presence of citric acid monohydrate (C6H8O7·H2O) and potassium citrate monohydrate (K3C6H5O7·H2O) not only make the three ions stable coexistence in the solution but also make the deposition potential of the three ions closer to each other. The results were discussed facing co-deposition of Bi–Sb–Te three elements in hydrochloric acid bath to prepare p-type Bi0.5Sb1.5Te3 thermoelectric materials.

Fabrication of the Bi0.5Sb1.5Te3 thermoelectric material by fusion-mechanical milling and spark plasma sintering

This study is focused on the fabrication of the Bi0.5Sb1.5Te3 by fusion-mechanical milling and spark plasma sintering. Bi0.5Sb1.5Te3 powder with sub-micron particle size was produced by fusion-mechanical milling. Upon sintering the powder, it is found that the thermal and electrical characteristics of the sintered body varied with sintering temperature. As the sintering temperature increased, the Seebeck coefficient and electrical resistivity of the sintered body decreased and the thermal conductivity increased. In addition, the figure of merit (ZT) decreased with increasing sintering temperature. The sintered body, when sintered for 180s at 573K, displayed a figure of merit of approximately 1.15 in the range between room temperature and 473K.

Preparation and Characterization of P-Type Bi<sub>0.45</sub>Sb<sub>1.55</sub>Te<sub>3</sub> Thin Film Using Pulsed CO<sub>2</sub> Laser

P-type Bi0.45Sb1.55Te3 thermoelectric material was synthesized using cold pressing process. The obtained sample was prepared in the form of pellet with a diameter of 10 mm and 2 mm thick and used as a target for laser ablation. The laser source was a pulsed CO2 laser working at a wavelength of 10.6 μm with a laser energy density of 2 J/cm2 per pulse. P-type Bi0.45Sb1.55Te3 thermoelectric thin films were deposited on Si substrates for different ablation times of 1, 2 and 3 h. The cross-section and surface morphologies of the thermoelectric films were investigated using field emission scanning electron microscopy (FE-SEM). The results show that the thickness and average particle size of the films increased from 35 to 58 nm, and 28 to 35 nm, respectively, when the ablation time was increased from 1 to 3 h. The crystalline structure of the TE films was investigated by X-ray diffraction (XRD).

Thermoelectric and mechanical properties of multi-walled carbon nanotube doped Bi0.4Sb1.6Te3 thermoelectric material

Since many thermoelectrics are brittle in nature with low mechanical strength, improving their mechanical properties is important to fabricate devices such as thermoelectric power generators and coolers. In this work, multiwalled carbon nanotubes (CNTs) were incorporated into polycrystalline Bi0.4Sb1.6Te3 through powder processing, which increased the flexural strength from 32 MPa to 90 MPa. Electrical and thermal conductivities were both reduced in the CNT containing materials, leading to unchanged figure of merit. Dynamic Young's and shear moduli of the composites were lower than the base material, while the Poisson's ratio was not affected by CNT doping.

Preparation of Bi0.5Sb1.5Te3 Thermoelectric Materials by Pulse-Current Sintering Under Cyclic Uniaxial Pressure

The effects of applying cyclic uniaxial pressure during the pulse-current sintering process on the crystal alignment and thermoelectric properties of p-type Bi0.5Sb1.5Te3 were investigated. Sintering was performed at 673 K using pulse-current heating under 70 MPa or 100 MPa of cyclic uniaxial pressure. x-Ray diffraction patterns and electron backscattered diffraction analyses showed that application of the cyclic uniaxial pressure enhanced crystal grain orientation. The texture consisted of flattened crystal grains stacked in the thickness direction of the sintered materials. The hexagonal c-plane strongly tended to align in the direction perpendicular to the uniaxial pressure. Owing to the crystal alignment, the Hall mobility in the direction perpendicular to the uniaxial pressure became larger than that of equivalent samples prepared with a constant uniaxial pressure. As a result of the increase in Hall mobility, the resistivity of the material was decreased while the equivalent Seebeck coefficient was maintained and the power factor was improved.

Nonmonotonic change in the structural grain size of the Bi0.4Sb1.6Te3 thermoelectric material synthesised by spark plasma sintering

Abstract A nonmonotonic variation of crystallite size in thermoelectric materials depending on spark plasma sintering (SPS) temperature was found in this work by using X-ray diffraction. The crystallite size grows with increasing temperature from 250 °C to 400 °C and decreases at temperatures from 400 °C to 500 °C. The transmission electron microscopy results suggest that the decrease in the average crystallite size is associated with an intense formation of fine grains at an SPS temperature of 450 °C as a result of repeated recrystallisation. New grains in the structure precipitate faster than the old grains grow. The total density of the defects including the twins decreases. The initiation of new repeated recrystallisation centres occurs on the grain boundaries, on the dislocation defects and in the grain bulk, most likely on the subgrains. The pattern that has been discovered solves the problem of nano state preservation at elevated SPS temperatures.