A Comparison Between Numerical and Simplified Thermoelectric Cooler Models

A recent investigation into thermoelectric phenomenon under large temperature differences, such as a thermoelectric cooler (TEC) at no-load, maximum- temperature-drop conditions (ΔT _max), has raised questions about the simplified approach that assumes constant material properties. The accuracy of the simplified approach can be improved by using material properties averaged over the relevant temperature range by integration, creating a quasi-constant material property solution. A one-dimensional (1-D) numerical solution, with fully temperature-dependent material properties, of a typical Bi₂Te₃ TEC operating from an elevated ambient temperature under no-load, ΔT _max conditions yielded a cold-side temperature nearly 8°C lower than the quasi-constant material property solution. This same cooler was modeled in ANSYS, providing a full three-dimensional (3-D) thermoelectric solution that matched the 1-D numerical solution within 0.6°C. With further investigation, it was found that more than half of the difference between the temperature-dependent numerical and quasi-constant material property solutions was due to the Thomson effect term, and most of the remaining error was due to temperature-independent electrical resistivity approximations. It was found that the error due to the temperature-independent thermal conductivity approximation was negligible. Experimental results for a 24-couple TEC further confirm the accuracy of the numerical model with temperature-dependent material properties.     

Enhanced Thermoelectric Properties of Antimony Telluride Thin Films with Preferred Orientation Prepared by Sputtering a Fan-Shaped Binary Composite Target

p-Type antimony telluride (Sb₂Te₃) thermoelectric thin films were deposited on BK7 glass substrates by ion beam sputter deposition using a fan-shaped binary composite target. The deposition temperature was varied from 100°C to 300°C in increments of 50°C. The influence of the deposition temperature on the microstructure, surface morphology, and thermoelectric properties of the thin films was systematically investigated. x-Ray diffraction results show that various alloy composition phases of the Sb₂Te₃ materials are grown when the deposition temperature is lower than 200°C. Preferred c-axis orientation of the Sb₂Te₃ thin film became obvious when the deposition temperature was above 200°C, and thin film with single-phase Sb₂Te₃ was obtained when the deposition temperature was 250°C. Scanning electron microscopy reveals that the average grain size of the films increases with increasing deposition temperature and that the thin film deposited at 250°C shows rhombohedral shape corresponding to the original Sb₂Te₃ structure. The room-temperature Seebeck coefficient and electrical conductivity range from 101 μV K^?1 to 161 μV K^?1 and 0.81 × 10³ S cm^?1 to 3.91 × 10³ S cm^?1, respectively, as the deposition temperature is increased from 100°C to 300°C. An optimal power factor of 6.12 × 10^?3 W m^?1 K^?2 is obtained for deposition temperature of 250°C. The thermoelectric properties of Sb₂Te₃ thin films have been found to be strongly enhanced when prepared using the fan-shaped binary composite target method with an appropriate substrate temperature.     

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.     

Post-annealing Effect on Microstructures and Thermoelectric Properties of Bi0.45Sb1.55Te3 Thin Films Deposited by Co-sputtering

p-Type Bi_0.45Sb_1.55Te₃ thermoelectric (TE) thin films have been prepared at room temperature by a magnetron cosputtering process. The effect of postannealing on the microstructure and TE properties of Bi_0.45Sb_1.55Te₃ films has been investigated in the temperature range from room temperature to 350°C. x-Ray diffraction analysis shows that the annealed films have polycrystalline rhombohedral crystal structure, and the average grain size increases from 36?nm to 64?nm with increasing annealing temperature from room temperature to 350°C. Electron probe microanalysis shows that annealing above 250°C can cause Te reevaporation, which induces porous thin films and dramatically affects electrical transport properties of the thin films. TE properties of the films have been investigated at room temperature. The hole concentration shows a trend from descent to ascent and has a minimum value at the annealing temperature of 200°C, while the Seebeck coefficient shows an opposite trend and a maximum value of 245?μV?K^?1. The electrical resistivity monotonically decreases from 19.8?mΩ?cm to 1.4?mΩ?cm with increasing annealing temperature. Correspondingly, a maximum value of power factor, 27.4?μW?K^?2?cm^?1, was obtained at the annealing temperature of 250°C.     

Low-Temperature,Solution-Based,Scalable Synthesis of Sb2Te3 Nanoparticles with an Enhanced Power Factor

Nanostructured thermoelectric (TE) materials, for example Sb₂Te₃, PbTe, and SiGe-based semiconductors, have excellent thermoelectric transport properties and are promising candidates for next-generation TE commercial application. However, it is a challenge to synthesize the corresponding pure nanocrystals with controlled size by low-temperature wet-chemical reaction. Herein, we report an alternative versatile solution-based method for synthesis of plate-like Sb₂Te₃ nanoparticles in a flask using SbCl₃ and Te powders as raw materials, EDTA-Na₂ as complexing agent, and NaBH₄ as reducing agent in the solvent (distilled water). To investigate their thermoelectric transport properties, the obtained powders were cold compacted into cuboid prisms then annealed under a protective N₂ atmosphere. The results showed that both the electrical conductivity (σ) and the power factor (S ² σ) can be enhanced by improving the purity of the products and by increasing the annealing temperature. The highest power factor was 2.04 μW cm^?1 K^?2 at 140°C and electrical conductivity remained in the range 5–10 × 10³ S m^?1. This work provides a simple and economic approach to preparation of large quantities of nanostructured Sb₂Te₃ with excellent TE performance, making it a fascinating candidate for commercialization of cooling devices.     

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.     

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.     

Modeling a Thermoelectric Generator Applied to Diesel Automotive Heat Recovery

Thermoelectric generators (TEGs) are outstanding devices for automotive waste heat recovery. Their packaging, lack of moving parts, and direct heat to electrical conversion are the main benefits. Usually, TEGs are modeled with a constant hot-source temperature. However, energy in exhaust gases is limited, thus leading to a temperature decrease as heat is recovered. Therefore thermoelectric properties change along the TEG, affecting performance. A thermoelectric generator composed of Mg₂Si/Zn₄Sb₃ for high temperatures followed by Bi₂Te₃ for low temperatures has been modeled using engineering equation solver (EES) software. The model uses the finite-difference method with a strip-fins convective heat transfer coefficient. It has been validated on a commercial module with well-known properties. The thermoelectric connection and the number of thermoelements have been addressed as well as the optimum proportion of high-temperature material for a given thermoelectric heat exchanger. TEG output power has been estimated for a typical commercial vehicle at 90°C coolant temperature.     

Thermoelectric Properties of Bi2Te2Se Compensated by Native Defects and Sn Doping

In Bi₂Te₂Se the defect chemistry involves native defects that compete such that they can either exchange dominance or else significantly compensate each other. Here we show how the net carrier concentration, n ? p, which depends on the relative amounts of these defects and is readily obtained from Hall data, can be used as a fundamental materials parameter to describe the varied behavior of the thermoelectric properties as a function of compensation. We report the effects of tuning this parameter over multiple orders of magnitude by hole-doping the n-type material Bi₂Te₂Se_0.995, which is already significantly compensated because of its Se deficiency. Crystals with different levels of hole doping were achieved by two separate approaches, namely by selecting pieces from different locations in an undoped crystal in which a systematic carrier concentration gradient had been induced by its growth conditions, and alternatively by doping with Sn for Bi. The thermoelectric power factors for Bi_2?x Sn _x Te₂Se_0.995 for x = 0, 0.002, 0.005, 0.010, and 0.040 are reported, and the dependence of the transport properties on the extent of compensation is discussed.     

Thermoelectric Properties of Highly Deformed and Subsequently Annealed p-Type (Bi0.25Sb0.75)2Te3 Alloys

The effects of mechanical deformation and subsequent annealing on the thermoelectric properties and microstructure have been investigated for p-type (Bi_0.25Sb_0.75)₂Te₃ alloys prepared by melting followed by quenching. The mechanically deformed pellets were prepared by repetition of cold-pressing of quenched samples at room temperature. Cold-pressed pellets were then annealed at 300°C in vacuum, and the thermoelectric properties and microstructure were traced through the course of the heat treatment. For the heavily deformed samples, the Seebeck coefficient rapidly increased at the very early stage of annealing and did not change as the annealing time increased, due to recrystallization of a new δ-phase which equilibrated at the annealing temperature of 300°C (δ₃₀₀-phase). At the initial stage of annealing (recovery stage), the electrical resistivity sharply increased, probably due to the interaction of antistructural defects with vacancies produced during the cold-pressing treatment. However, for the lightly deformed samples, recrystallization occurred only at some portion of the grain boundaries, and the newly generated δ₃₀₀-phase slowly replaced the original, as-solidified δ_ingot-phase.     

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

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

Effect of Heat Treatment on the Thermoelectric Properties of Bismuth–Antimony–Telluride Prepared by Mechanical Deformation and Mechanical Alloying

In this work, p-type 20%Bi₂Te₃–80%Sb₂Te₃ bulk thermoelectric (TE) materials were prepared by mechanical deformation (MD) of pre-melted ingot and by mechanical alloying (MA) of elemental Bi, Sb, and Te granules followed by cold-pressing. The dependence on annealing time of changes of microstructure and TE properties of the prepared samples, including Seebeck coefficient, electrical resistivity, thermal conductivity, and figure-of-merit, was investigated. For both samples, saturation of the Seebeck coefficient and electrical resistivity were observed after annealing for 1 h at 380°C. It is suggested that energy stored in samples prepared by both MA and MD facilitated their recrystallization within short annealing times. The 20%Bi₂Te₃–80%Sb₂Te₃ sample prepared by MA followed by heat treatment had higher a Seebeck coefficient and electrical resistivity than specimens fabricated by MD. Maximum figures-of-merit of 3.00 × 10^?3/K and 2.85 × 10^?3/K were achieved for samples prepared by MA and MD, respectively.     

Highly efficient n-(Bi, Sb)2Te3 thermoelectric materials for temperatures below 200 K

The possibility of using an n-type Bi_2?x Sb_xTe₃ solid solution in thermoelectric refrigerators at T<200 K is considered. It is shown that, if the material under consideration is optimized for the above temperature region, the temperature dependence of the Seebeck coefficient α becomes less pronounced, and the crystal-lattice thermal conductivity κ_L decreases as compared to what is observed in a conventional n-Bi₂Te_3?y Se_y solid solution. These factors and a high mobility of charge carriers μ₀ bring about an increase in the parameter β ～ ZT, where Z is the thermoelectric efficiency.     

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

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

Electrodeposition of Sb x Te y Thermoelectric Films from Dimethyl Sulfoxide Solution

The electrochemical behaviors of nonaqueous dimethyl sulfoxide solutions containing Te^IV and Sb^III were investigated using cyclic voltammetry. On this basis, Sb _x Te _y thermoelectric films were prepared by the potentiodynamic electrodeposition technique from nonaqueous dimethyl sulfoxide solution, and the composition, morphology, and thermoelectric properties of the films were analyzed. Sb _x Te _y thermoelectric films prepared under different potential ranges all possessed smooth morphology. After annealing treatment at 200°C under N₂ protection for 4?h, all the deposited films showed p-type semiconductor properties. Sb_1.87Te_3.13 thermoelectric film, which most closely approached the stoichiometry of Sb₂Te₃ and possessed the highest Seebeck coefficient, could be potentiodynamically electrodeposited in the potential range of ?200?mV to ?600?mV.     

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

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

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.     

Thermopower Enhancement of Bi2Te3 Films by Doping I Ions

The thermoelectric properties of I-doped Bi₂Te₃ films grown by metal-organic chemical vapor deposition have been studied. I-doped epitaxial (00l) Bi₂Te₃ films were successfully grown on 4° tilted GaAs (001) substrates at 360 °C. I concentration in the Bi₂Te₃ films was easily controlled by the variation in a flow rate of H₂ carrier gas for the delivery of an isopropyliodide precursor. As I ions in the as-grown Bi₂Te₃ films were not fully activated, they did not influence the carrier concentration and thermoelectric properties. However, a post-annealing process at 400 °C activated I ions as a donor, accompanied with an increase in the carrier concentration. Interestingly, the I-doped Bi₂Te₃ films after the post-annealing process also exhibited enhancement of the Seebeck coefficient at the same electron concentration compared to un-doped Bi₂Te₃ films. Through doping I ions into Bi₂Te₃, the thermopower was also enhanced in Bi₂Te₃, and a high power factor of 5 × 10^?3 W K^?2 m^?1 was achieved.     

Electrical and Thermal Conductivity and Conduction Mechanism of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> Alloy

Ge₂Sb₂Te₅ alloy has drawn much attention due to its application in phase-change random-access memory and potential as a thermoelectric material. Electrical and thermal conductivity are important material properties in both applications. The aim of this work is to investigate the temperature dependence of the electrical and thermal conductivity of Ge₂Sb₂Te₅ alloy and discuss the thermal conduction mechanism. The electrical resistivity and thermal conductivity of Ge₂Sb₂Te₅ alloy were measured from room temperature to 823 K by four-terminal and hot-strip method, respectively. With increasing temperature, the electrical resistivity increased while the thermal conductivity first decreased up to about 600 K then increased. The electronic component of the thermal conductivity was calculated from the Wiedemann–Franz law using the resistivity results. At room temperature, Ge₂Sb₂Te₅ alloy has large electronic thermal conductivity and low lattice thermal conductivity. Bipolar diffusion contributes more to the thermal conductivity with increasing temperature. The special crystallographic structure of Ge₂Sb₂Te₅ alloy accounts for the thermal conduction mechanism.     

Layered Sb2Te3 Nanoflakes as Chalcogenide Dielectrics

Dielectric nanoflakes of Sb₂Te₃ represent an important advance in science and technology due to their extraordinary properties. Polycrystalline layered Sb₂Te₃ nanoflakes have been successfully synthesized via a high-throughput chemical route at 60°C. The frequency and temperature dependence of the dielectric constant and dielectric loss of the layered Sb₂Te₃ nanoflakes have been measured in the frequency range from 30 Hz to 758,000 Hz and temperature range from 313 K to 373 K. As-synthesized Sb₂Te₃ nanoflakes are shown to be promising alternative dielectrics because of their high dielectric constant (ε′ ≈ 7.3 to 6022) and low dielectric loss (tan δ ≈ 0.2 to 9.2). These higher values of ε′ and lower values of tan δ of Sb₂Te₃ nanoflakes confirm that capacitors with capacity (C) of ～5.2 pF to 4336 pF may be fabricated for storing renewable energy. Raman spectroscopy confirms that the peak located at ～142 cm^?1 corresponds to one in-plane vibrational mode (E _g ² ) of layered Te–Sb–Te–Sb–Te lattice vibration.