Bi2Te3-based Materials Research Articles

Top-down approach using supercritical carbon dioxide ball milling for producing sub-10 nm Bi2Te3 grains

We compare Bi2Te3 powders prepared by conventional ball milling to powders milled in supercritical carbon dioxide (scCO2). We demonstrate that scCO2 milling overcomes size-reduction limitations reported for conventional milling. XRD and TEM reveal nanograins with smaller average sizes (< 10 nm) and narrower grain size distributions in the scCO2 milled case. scCO2 milling also preserves the crystallinity and shows less oxidation than conventional milling. This is the first report of Bi2Te3 with a sub-10 nm grain size whilst conserving high quality crystallinity, made using a top-down approach. Our study offers a route for developing unprecedentedly fine bulk nanostructured Bi2Te3-based thermoelectric materials.

Effects of thickness on thermoelectric properties of Bi0.5Sb1.5Te3 thin films

Bi2Te3-based materials have been widely utilized for commercial bulk thermoelectric (TE) devices. For the manufacture of miniaturized TE devices, the large-scale deposition of high-performance Bi2Te3-based thin films is crucial. However, it has not yet been effectively resolved. Herein, the Bi0.5Sb1.5Te3 films with different thicknesses were grown on sapphire substrates by controlling the co-sputtering time. We reveal that the film thickness has a significant impact on the electrical transport properties. Due to the mutual influence of electrical conductivity and Seebeck coefficient, there exists an optimal thickness with the maximum power factor is as high as 2900 μW m−1 K−2. Therefore, systematic research on the thickness-dependent TE characteristics of Bi2Te3-based films and the deposition of high-performance films will provide important information for the large-scale development of high-performance micro-TE devices.

Room temperature Bi2Te3-based thermoelectric materials with high performance

Several off-stoichiometric compositions Bi0.5Sb1.5+xTe3+δ (x = 0.2; δ = 0, 0.12, 0.14) were deliberately synthesized to produce in-situ composites based on compositional engineering approach. The structural characterization of these materials employing XRD, SEM, and HR-TEM reveals the formation of in-situ-composites containing Bi0.5Sb1.5Te3 as matrix phase and minor phases of either Sb rich or Te rich in different compositions. Thermoelectric properties of these Bi0.5Sb1.5+xTe3+δ (x = 0.2; δ = 0, 0.12, 0.14) composites were studied in a wide range of temperatures extending from room temperature to 500 K. The electronic transport of these composites exhibits p-type semiconducting materials. The lowest thermal conductivity of ~ 0.69 W/m K @310 K was observed for Bi0.5Sb1.7Te3.12 composite, which was noted to be 14% reduced thermal conductivity when compared with that of the state-of-the-art Bi0.5Sb1.5Te3 (κ$$=$$ 0.82 W/m K) material. In addition to this, an enhanced power factor was also observed in Bi0.5Sb1.7Te3.12 which is primarily due to increased electrical conductivity of these materials. This enhanced power factor of the composition of Bi0.5Sb1.7Te3.12 coupled with reduced thermal conductivity yields to high ZT ~ 1.13 at nearly room temperature, making these materials viable for large scale applications.

Parametric Optimization of Exergy Efficiency in Solar Thermoelectric Generators

This paper presents the optimization of the exergy efficiency of a solar thermoelectric generator (STEG) with respect to its hot-side temperature, length-weighted current density, thermoelement area ratio, length of the thermoelectric (TE) legs, and the thermal concentration ratio (CR). Lagrange multiplier technique was employed to perform the optimization. It was found that optimal hot-side temperatures exist at which maximum exergy efficiencies of STEGs are attained. In the absence of optical concentration, but with thermal CRs as large as 600–700, these exergy efficiencies were as much as 6–8% for a hot-side temperature range of 175–277 °C. The optimal hot-side temperatures were found to be independent of both the TE device geometry and the thermal CR, but varied with the TE device’s dimensionless figure of merit, as well as the properties of the STEG’s optical component and absorber. Moreover, optimal combination of these parameters gives the configuration that optimizes the efficiency. For instance, for a Bi2Te3-based TE material, with ZTm = 1, and under vacuum conditions, the optimal configuration is: a solar surface absorber area of 1300 mm2; cross-sectional area of 1 mm2 and height of 1 mm for both p and n legs; and an emittance of 0.05, giving a conversion efficiency of about 7%. So with the optimized configuration, complicated optical systems (which usually include tracking mechanisms) may be avoided, and the required amounts of TE material minimized. This ultimately results in substantial reductions in material, manufacturing and system costs.

Innovative design of bismuth-telluride-based thermoelectric micro-generators with high output power

An innovative design of a thermoelectric micro-generator with integrated wavy-shaped Bi2Te3-based materials yields the highest output power achieved so far for an in-plane device.

Thermal Stability and Mechanical Response of Bi2Te3-Based Materials for Thermoelectric Applications

Bi2Te3-based materials are among the most mature thermoelectric materials and have found wide near-room-temperature applications in power generation and spot cooling. Their practical applications o...

Evolution of the Intrinsic Point Defects in Bismuth Telluride-Based Thermoelectric Materials.

In polycrystalline bismuth telluride-based thermoelectric materials, mechanical-deformation-induced donor-like effects can introduce a high concentration of electrons to change the thermoelectric properties through the evolution of intrinsic point defects. However, the evolution law of these point defects during sample preparation remains elusive. Herein, we systematically investigate the evolution of intrinsic point defects in n-type Bi2Te3-based materials from the perspective of thermodynamics and kinetics, in combination with positron annihilation measurement. It is found that not only the mechanical deformation but also the sintering temperature is vital to the donor-like effect. The mechanical deformation can promote the formation of cation vacancies and facilitate the donor-like effect, and the sintering process can provide excess energy for Bi antisite atoms to surmount the diffusion potential barrier. This work provides us a better understanding of the evolution law of intrinsic point defects in Bi2Te3-based alloys and guides us to control the carrier concentration by manipulating intrinsic point defects.

High Porosity in Nanostructured n-Type Bi2Te3 Obtaining Ultralow Lattice Thermal Conductivity.

Porous structure possesses full potentials to develop high-performance thermoelectric materials with low lattice thermal conductivity. In this study, the n-type porous nanostructured Bi2Te3 pellet is fabricated by sintering Bi2Te3 nanoplates synthesized with a facile solvothermal method. With adequate sublimations of Bi2TeO5 during the spark plasma sintering, homogeneously distributed pores and dense grain boundaries are successfully introduced into the Bi2Te3 matrix, causing strong phonon scatterings, from which an ultralow lattice thermal conductivity of <0.1 W m-1 K-1 is achieved in the porous nanostructured Bi2Te3 pellet. With the well-maintained decent electrical performance, a power factor of 10.57 μW cm-1 K-2 at 420 K, as well as the reduced lattice thermal conductivity, secured a promising zT value of 0.97 at 420 K, which is among the highest values reported for pure n-type Bi2Te3. This study provides the insight of realizing ultralow lattice thermal conductivity by synergistic phonon scatterings of pores and nanostructure in the n-type Bi2Te3-based thermoelectric materials.

Ultrahigh Power Factor and Electron Mobility in n-Type Bi2Te3-x%Cu Stabilized under Excess Te Condition.

The thermoelectric (TE) community has mainly focused on improving the figure of merit (ZT) of materials. However, the output power of TE devices directly depends on the power factor (PF) rather than ZT. Effective strategies of enhancing PF have been elusive for Bi2Te3-based compounds, which are efficient thermoelectrics operating near ambient temperature. Here, we report ultrahigh carrier mobility of ∼467 cm2 V-1 s-1 and power factor of ∼45 μW cm-1 K-2 in a new n-type Bi2Te3 system with nominal composition CuxBi2Te3.17 (x = 0.02, 0.04, and 0.06). It is obtained by reacting Bi2Te3 with surplus Cu and Te and subsequently pressing powder products by spark plasma sintering (SPS). The SPS discharges excess Te but stabilizes the high extent of Cu in the structure, giving unique SPS CuxBi2Te3.17 samples. The analyzed composition is close to "CuxBi2Te3". Their charge transport properties are highly unusual. Hall carrier concentration and mobility simultaneously increase with the higher mole fraction of Cu contrary to the typical carrier scattering mechanism. As a consequence, the electrical conductivity is considerably enhanced with Cu incorporation. The Seebeck coefficient is nearly unchanged by the increasing Cu content in contrast to the general understanding of inverse relationship between electrical conductivity and Seebeck coefficient. These effects synergistically lead to a record high power factor among all polycrystalline n-type Bi2Te3-based materials.

Hole-doped M4SiTe4 (M = Ta, Nb) as an efficient p-type thermoelectric material for low-temperature applications

Solid-state thermoelectric cooling is expected to be widely used in various cryogenic applications such as local cooling of superconducting devices. At present, however, thermoelectric cooling using p- and n-type Bi2Te3-based materials has been put to practical use only at room temperature. Recently, M4SiTe4 (M = Ta, Nb) has been found to show excellent n-type thermoelectric properties down to 50 K. This paper reports on the synthesis of high-performance p-type M4SiTe4 by Ti doping, which can be combined with n-type M4SiTe4 in a cooling device at low temperatures. The thermoelectric power factor of p-type M4SiTe4 reaches a maximum value of approximately 60 μW cm−1 K−2 at 210 K and exceeds the practical level in a wide temperature range of 130–270 K. A finite temperature drop by Peltier cooling was also achieved in a cooling device made of p- and n-type Ta4SiTe4 whisker crystals. These results clearly indicate that M4SiTe4 is promising to realize a practical thermoelectric cooler for use at low temperatures, which is not covered by Bi2Te3-based materials.

Improving thermoelectric performance of (Bi0.2Sb0.8)2(Te0.97Se0.03)3 via Sm-doping

We herein report the effects of Sm doping on the thermoelectric performance of (Bi0.2Sb0.8)2(Te0.97Se0.03)3 compounds prepared by a growth-from-the-melt spark-plasma-sintering procedure. In conjunction with micro-morphology and compositional analyses, the electrical conductivity, Seebeck coefficient, and thermal conductivity of (Bi0.2-xSmxSb0.8)2(Te0.97Se0.03)3 samples were measured as a function of temperature (298–473 K) and the Sm doping ratio (x = 0%, 0.2%, 0.4%, and 0.8%). We found that (i) there was no discernible crystal lattice symmetry change upon Sm doping; (ii) Sm doping generally increased the electric conductivity and practically retained the Seebeck coefficient, thereby yielding higher power factors; and (iii) specially, a minimum thermal conductivity ∼ 0.92 Wm−1K−1 was attained in the x = 0.4% sample at 423 K, resulting in a state-of-the-art ZT value ∼ 1.22, which is ∼25% higher than the maximum ZT value of pristine sample. Contrary to the customary view that Sm dopant adopts a trivalent state Sm3+, the variations of lattice constant pointed toward a lower valence state of Sm. The results demonstrated the efficacy and deepened our understanding of rare earth doping in thermoelectric study of Bi2Te3-based materials.

High switching ratio variable-temperature solid-state thermal switch based on thermoelectric effects

The switching ratio, the ratio between the thermal conductance in its off and on states, of a thermal switch is a critical design parameter of such a device. Here, we propose Peltier modules as a type of all-solid-state thermal switch that can operate over a wide temperature range and with an adjustable switching ratio. Calculations show that the switching ratio depends only on thermoelectric figure of merit and temperature bounds. A Peltier couple is constructed from Bi2Te3-based material and characterized by its thermal conductance and response time in the open condition and under an optimal activation current. The switching ratio diverges at small temperature differences with record-high measured values, an order of magnitude larger than achieved by other solid-state devices near room temperature. Materials with a high figure of merit could produce even larger switching ratios over higher temperature differences.

Unusual n-type thermoelectric properties of Bi2Te3 doped with divalent alkali earth metals

Bi2Te3-based materials are a representative thermoelectric system operating near ambient temperature. Their n-type family is very limited and underperforms the p-type counterpart, which is a major concern in the thermoelectric community. Here we report that alkali earth metals (AE = Mg, Ca, Sr, Ba) in Group 2 unusually induce n-type thermoelectric properties in Bi2Te3. Mg is the most efficient electron donor among them. The x = 0.01 sample of MgxBi2-xTe3 (x = 0.005, 0.01, 0.015) system exhibits remarkably enhanced power factor and ZT of ~0.8 at 350 K in comparison with pristine Bi2Te3. The improved performance is attributed to simultaneously enhanced electrical conductivity and reduced lattice thermal conductivity. Remarkably, MgxBi2-xTe3 materials show larger Seebeck coefficients than those expected by the theoretical Pisarenko relation.

Ultrasonic-assisted hot pressing of Bi2Te3-based thermoelectric materials

In this study, n-type Bi2Te3-based bulk materials were prepared using the ultrasonic-assisted hot pressing. The effects of ultrasonic vibration on the mechanical and thermoelectric properties of the prepared samples were investigated. Field-emission scanning electron microscope (FESEM) tests revealed that the ultrasonic vibration refined the grain size to the submicron level. The grain size became smaller as the ultrasonic vibration duration is extended. The results showed that the sample with an ultrasonic vibration duration of 20 min (namely, Ultra 20) has a hardness of 64 HV0.3 and a flexural strength of 22.4 MPa, which represents an 82.9% and 50.2% improvement, respectively, over the untreated hot-pressed sample (namely, Ultra 0). As the ultrasonic vibration duration increased, the absolute value of the Seebeck coefficient was found to increase, whereas the electrical conductivity and the thermal conductivity decreased. The maximum dimensionless figure of merit (ZTmax) was 0.78 at 480 K for the Ultra 20 sample, which was 10.9% and 16.4% higher than the Ultra 10 and Ultra 0 samples, respectively. The results proved that both the mechanical properties and the thermoelectric properties of the Bi2Te2.7Se0.3 bulk materials are improved by ultrasonic vibration.

Characterization of thin Bi2Te3-based films and effects of heat treatment on their optical properties

Thin Bi2Te3 and based films were thermally evaporated onto highly cleaned glass substrate at high vacuum. Source materials are bulk alloys prepared by the standard melting method. X-ray diffraction analysis of the as-deposited films asserted a polycrystalline nature with a hexagonal type structure. Surface roughness and morphology were investigated by means of AFM and SEM respectively. Nano-crystallinity of the concerned materials was asserted by TEM investigations. Optical characteristics were defined for the synthesized films before and after annealing in air atmosphere for 3 h at 300 °C. Transmittance and reflectance spectra of the as prepared and annealed films were measured in the wavelength range 350–2700 nm using a double beam spectrophotometer. Absorption coefficient, refractive index and energy band gap values were derived based on the measured spectra and the film's thickness. It was found that Bi2Te3-based materials are highly refractive indexed material. Index of refraction was used for calculation and determination of optical dispersion energies, in terms of the single oscillator constants using the Wemple–DiDomenico method. The allowed optical transitions were observed as direct optical transitions with energy gap in the range of 0.56–0.82 eV. Real and imaginary parts of the optical dielectric function were estimated before and after annealing treatment, exhibiting very high values. Further increase was detected after annealing. The influence of thermal annealing on the optical properties of the Bi2Te3 and Se containing films is investigated and compared to the as-prepared corresponding samples in the present work.

Large thermoelectric power factor in one-dimensional telluride Nb4SiTe4 and substituted compounds

We found that whisker crystals of Mo-doped Nb4SiTe4 show high thermoelectric performances at low temperatures, indicated by the largest power factor of ∼70 μW cm−1 K–2 at 230–300 K, much larger than those of Bi2Te3-based practical materials. This power factor is smaller than the maximum value in the 5d analogue of Ta4SiTe4 but is comparable to that with a similar doping level. First principles calculation results suggest that the difference in thermoelectric performances between Nb and Ta compounds is caused by the much smaller bandgap in Nb4SiTe4 than that in Ta4SiTe4 due to the weaker spin-orbit coupling in the former. We also demonstrated that the solid solution of Nb4SiTe4 and Ta4SiTe4 shows a large power factor, indicating that their combination is promising as a practical thermoelectric material, as in the case of Bi2Te3 and Sb2Te3. These results advance our understanding of the mechanism of high thermoelectric performances in this one-dimensional telluride system, indicating the high potential of this system as a practical thermoelectric material for low temperature applications.

Effective atomic interface engineering in Bi2Te2.7Se0.3 thermoelectric material by atomic-layer-deposition approach

Grain boundaries play a critical role in the carrier/phonon transport in thermoelectric materials. It remains a big challenge to control over both chemical composition and dimension of grain boundary precisely by traditional approaches. Herein, an bottom-up grain boundary engineering strategy based on atomic layer deposition (ALD) is first introduced to atomically control and modify the grain boundary of Bi2Te3-based thermoelectric materials. To demonstrate the effect of this strategy, ultrathin ZnO interlayer is deposited on the Bi2Te2.7Se0.3 (BTS) grain boundaries to optimize of the carrier/phonon transport for achieving high thermoelectric performance. In situ TEM experiments upon heating reveals that the ZnO interlayer will give rise to the precipitation of Te nanodot at ZnO/BTS interface, which can be atomically controlled by adjusting the thickness of ZnO layer. Benefited from the atomically precise modified grain boundary, a maximum ZT of 0.85 is obtained, approximately 1.8 times higher than that of the pure BTS. As a powerful interfacial modification strategy, ALD-based approach can be extended to other thermoelectric material system simply, which may contribute to the development of high performance thermoelectric material of great significance.

Effect of annealing on microstructure and thermoelectric properties of hot-extruded Bi–Sb–Te bulk materials

The effect of annealing on the microstructure, thermoelectric properties and hardness of the hot-extruded Bi–Sb–Te materials has been investigated systematically to optimize their thermoelectric and mechanical properties. The mechanically alloyed powder was consolidated by hot extrusion at either 340 or 400 °C, followed by annealing in a temperature range of 260–400 °C. The microstructure of the annealed samples contained submicron grains with preferred (001) texture. As annealing temperature increased, the small-angle grain boundaries (SAGBs) increased because the increased amount of Te-rich and Sb-rich phases inhibits the movements of dislocations and SAGBs. The submicron microstructure led to a low thermal conductivity, for example, ~ 0.9 W/mK after annealing at TA ≥ 380 °C. The Seebeck coefficient highly depended on carrier mobility in addition to carrier concentration. For the extruded samples prepared at a lower extrusion temperature of 340 °C, the mobility increased significantly after annealing, resulting in great enhancements in the Seebeck coefficient and electrical conductivity. A peak ZT value of 0.94 and high hardness were simultaneously obtained under the conditions of hot extrusion at 340 °C and annealing at 380 °C. It seems that the combination of low-temperature extrusion and high-temperature annealing is an effective route to prepare high-performance Bi2Te3-based materials.

Enhanced thermoelectric properties of polycrystalline Bi2Te3 core fibers with preferentially oriented nanosheets

Bi2Te3-based materials have been reported to be one of the best room-temperature thermoelectric materials, and it is a challenge to substantially improve their thermoelectric properties. Here novel Bi2Te3 core fibers with borosilicate glass cladding were fabricated utilizing a modified molten core drawing method. The Bi2Te3 core of the fiber was found to consist of hexagonal polycrystalline nanosheets, and polycrystalline nanosheets had a preferential orientation; in other words, the hexagonal Bi2Te3 lamellar cleavage more tended to be parallel to the symmetry axis of the fibers. Compared with a homemade 3-mm-diameter Bi2Te3 rod, the polycrystalline nanosheets’ preferential orientation in the 89-μm-diameter Bi2Te3 core increased its electrical conductivity, but deduced its Seebeck coefficient. The Bi2Te3 core exhibits an ultrahigh ZT of 0.73 at 300 K, which is 232% higher than that of the Bi2Te3 rod. The demonstration of fibers with oriented nano-polycrystalline core and the integration with an efficient fabrication technique will pave the way for the fabrication of high-performance thermoelectric fibers.

Wearable thermoelectric generator to harvest body heat for powering a miniaturized accelerometer

Wearable thermoelectric generators (TEGs) enable the conversion of human body heat into microwatts to milliwatt electricity, which can be utilized to power miniaturized electronic devices for motion detection and healthcare monitoring. This paper presents a novel wearable TEG with 52 pairs of cubic-shaped thermoelectric legs to harvest human body heat. The thermoelectric legs are made of P-type and N-type Bi2Te3-based powder materials, and are connected electrically in series through soldering. The flexible printed circuit board (FPCB) with special holes is designed and used as substrate to enhance the flexibility of the TEG for wearable applications. The performances of the TEG, including the bulk thermoelectric legs, are characterized. The results show that the TEG can generate an open-circuit voltage of 37.2 mV at ΔT = 50 K, and the internal resistance of the TEG is quite low at a value of 1.8 Ω. Then the TEG was worn on a human wrist to harvest body heat and power a 3-axis miniaturized accelerometer for detection of body motion at ΔT = 18 K. The results demonstrate that the developed wearable TEG features high output performance and could be utilized for powering electronics and/or sensors by harvesting human body heat.