Bi2Te3 Te Research Articles

GeTe–Bi2Te3–Te System

GeTe–Bi2Te3–Te System

Probing photocarrier dynamics in a Bi2Te3–Te eutectic p–n junction with a laser terahertz emission microscope

Bismuth telluride (Bi2Te3)-based heterostructures have attracted considerable attention owing to their interesting anisotropic properties and expected higher thermoelectric performance. Therefore, exploring the nature of the carrier dynamics in these heterostructures has been an important subject in the design and optimization of advanced materials. In the present study, hot carrier injection and its subsequent spatiotemporal behavior in a multilayered crystalline Bi2Te3–Tellurium (Te) eutectic composite were studied using a laser terahertz (THz) emission microscopy (LTEM). The THz emission electric fields at the Bi2Te3–Te interface were polarized perpendicular to the interface. The polarities of these waveforms reveal the direction of the electric field between the Bi2Te3 and Te regions, indicating the carrier types of these components and the p–n junction formed at the interface. In addition, in the Te region, a strong THz emission with an electric field polarized parallel to the interface was observed. This unique THz emission can be qualitatively explained through hot photocarrier anisotropic transport by considering the effective mass of electrons and holes. LTEM clarified the local carrier dynamics in the microstructures and revealed the potential distribution and anisotropic transport properties. These findings contribute to the exploration of eutectic heterostructures as new functional materials and provide new avenues for cutting-edge thermoelectric and photovoltaic devices.

Design and evaluation of a hybrid solar thermphotovoltaic-thermoelectric system

In a solar thermophotovoltaic (STPV) system, TPV cells only use part of the energy emitted by the emitter, and most of the energy is wasted in the form of high-temperature thermal radiation. To completely utilize the high-temperature thermal radiation to improve the efficiency of the STPV system, a hybrid solar thermophotovoltaic-thermoelectric (STPV-TE) system is proposed. The non-contact installation method of the TPV cell and TE module eliminates the temperature interaction between them. The theoretical model and analysis method of the hybrid STPV-TE system are established. The STPV-TE experimental system consists of a commercial GaSb TPV cell and Bi2Te3 TE module. The experimental results showed that the STPV-TE system can promote the efficiency of the STPV system. The total efficiency of the STPV-TE system amounts to 5.91%, of which the TPV cell efficiency of 1.06% and the total efficiency of the four TE modules efficiency is 4.85%. The efficiency of the solo STPV system is 1.44%. The design method of the hybrid STPV-TE system proposes a new method of constructing the STPV-TE system.

Thermoelectric Performance Enhancement of Film by Pulse Electric Field and Multi-Nanocomposite Strategy.

Thermoelectric (TE) film has wide potential application in low-grade waste heat recovery and TE generation due to its quick response and multifunctional integration. Multi-nanocomposite is a promising method to solve the difficulty of maintaining temperature difference and achieving a high figure of merit ZT. However, the depletion layer induced by the multi-nanocomposite typically degrades performance. This study presents a simple and convenient method to solve this problem by pulse electric field (PEF). Prototypical TE Bi2 Te3 is selected as the objective film. The strong current density effect of PEF removes the depletion layer among carbon nanotubes (CNT) and Bi2 Te3 grains. Thus, the CNT nanocomposite with PEF treatment breaks the trade-off between electrical conductivity and Seebeck coefficient, achieving a power factor of 4400 µW m-1 K-2 which stabilizes after annealing effect to 2920 µW m-1 K-2 , a record for Bi2 Te3 films. Simultaneously, the self-assembled porosity decreases thermal conductivity via phonon scattering while still maintaining a high electrical conductivity of 3130 S cm-1 . Thus, the porosity helps maintain the temperature difference and thereby enables a sharp increase in output power. These results indicate that the combination of PEF and multi-nanocomposite is a new method to enhance TE performance, which would have a potential application in the commercial field.

Thermodynamic Properties of the Chalcogenide Phases in the Bi–Te–S System

The Bi–S–Te system has been studied using emf measurements in the composition region Bi2S3–Bi2Te3–Te–S at temperatures in the range 300–450 K. We have calculated relative partial molar functions of bismuth in alloys and used the results to obtain self-consistent sets of the standard Gibbs free energy, standard enthalpy, and standard entropy of the Bi2S3, Bi2Te3, and Bi2Te2S compounds and a solid solution with the composition Bi2Te1.8S1.2, based on the last compound. The data obtained for Bi2S3 and Bi2Te3 have been compared to data available in the literature. The thermodynamic functions of Bi2Te2S and Bi2Te1.8S1.2 have been determined for the first time.

Thermal conductivity variation of Bi2Te3 nanofilm with interfacial defects using molecular dynamics

In this study, the thermal conductivity and thermal boundary resistance (TBR) of Bi2Te3 nanofilms with different interfacial defects were investigated using nonequilibrium molecular dynamics simulations, and the effects of temperature, defects (step junctions and grooves), and interfaces (amorphous and telluride) were assessed. The results show a strong temperature dependence of the thermal conductivity for Bi2Te3 nanofilms with an ideal structure; moreover, as the height of step-junction defects increased, the thermal conductivity decreased, exhibiting a linear dependency. In addition, the thermal conductivity gradually decreased by 36%–40% as the width of the interface defects increased. We also verified the self-assembly mechanism for nanoscale Bi2Te3 and found that the Bi2Te3–Te interface induces strong phonon scattering. In addition, the TBR decreased as the width of the amorphous or Te interface increased. Thus, interfacial defects in Bi2Te3 nanofilms affect the thermal conductivity and TBR. The results of this study may be useful for optimizing Bi2Te3 thermoelectric devices in the future.

Thermodynamic evaluation of irreversibility and optimum performance of a concentrated PV-TEG cogenerated hybrid system

In this paper, the thermodynamics evaluation of an irreversible CPV-TEG cogenerating system, having Siemens SP75 PV module and commercially available Bi2Te3 TE module, has been investigated for wide solar spectrum to encompass the winter and summer solar radiation. Modeling and simulation of the hybrid system has been carried out to understand the feasibility of the system and to determine the irreversibilities present in the hybrid system. The irreversibilities or exergy destruction caused by the irreversible conversion process of solar energy into electricity are calculated using exergy analysis based on second law of thermodynamics. The various exergy losses occurring in the hybrid system have been calculated using exergy analysis. The effects of the incident solar irradiance varying in a wide range from 100 W/m2 to 1000 W/m2, TEG junction temperature ratio, concentration ratio, TEG current, Thomson coefficient, irreversibilities or exergy destruction and the incoming solar radiation exergy have been studied. The results showed that the Thomson effect has adverse effect on the performance of the hybrid system and the irreversibilities increase with increase in concentration ratio, C. Further, the power output of hybrid system increases by 86% as C increases from 1 to optimum value of 3 and the exergy efficiency is higher than the energy efficiency by 8%. The higher values of the irreversibilities make the system less inefficient and therefore, significant improvements are required because the elevated temperature could result in hot spots formation. The results of this analysis may be helpful in designing of practical CPV-TEG hybrid system.

Enhancement in thermoelectric properties of Te-embedded Bi2Te3 by preferential phonon scattering in heterostructure interface

A comprehensive understanding of the nano-structural effects that cause reduction in thermal conductivity represents important challenges for the development of thermoelectric materials with an improved figure of merit ZT. Bismuth telluride (Bi2Te3)-based thermoelectric materials exhibit very low levels of thermal conductivity. In this study, a Te crystal-embedded Bi2Te3 (Te–Bi2Te3) thin film was formed by establishing a specific annealing temperature for a Te-rich Bi/Te multilayered structure. Modulations in structure and composition were observed at the boundaries between the two phases of Te and Bi2Te3. Furthermore, the samples contained regularly shaped nanometer-scale Bi2Te3 single grains. Therefore, we obtained a dramatic ZT value of 2.27 (+ 0.04, − 0.08) at 375 K from the Te–Bi2Te3 thin film. Finally, we confirmed that interface phonon scattering between the Te–Bi2Te3 boundaries plays an important role in inter-grain phonon transport, which results in a reduction in the lattice thermal conductivity.

A New Strategy for Realizing the Conversion of “Homo–Hetero–Homo” Heteroepitaxial Growth in Bi2Te3 and the Thermoelectric Performance

Uncovering the reason for structure-dependent thermoelectric performance still remains a big challenge. A low-temperature and easily scalable strategy for synthesizing Bi2 Te3 nanostring hierarchical structures through solution-phase reactions, during which there is the conversion of "homo-hetero-homo" in Bi2 Te3 heteroepitaxial growth, is reported. Bi2 Te3 nanostrings are obtained through the transformation from pure Bi2 Te3 hexagonal nanosheets followed by TeBi2 Te3 "nanotop" heterostructures to Bi2 Te3 nanostrings. The growth of Bi2 Te3 nanostrings appears to be a self-assembly process through a wavy competition process generated from Te and Bi(3+) . The conversion of homo-hetero-homo opens up new platforms to investigate the wet chemistry of Bi2 Te3 nanomaterials. Furthermore, to study the effect of morphologies and hetero/homo structures, especially with the same origin and uniform conditions on their thermoelectric properties, the thermoelectric properties of Bi2 Te3 nanostrings and TeBi2 Te3 heterostructured pellets fabricated by spark plasma sintering have been investigated separately.

T-Shaped Bi2Te3–Te Heteronanojunctions: Epitaxial Growth, Structural Modeling, and Thermoelectric Properties

Novel T-shaped Bi2Te3–Te heteronanojunctions composed of a rhombohedral structured Bi2Te3 nanoplate and a trigonal structured Te nanorod were fabricated by a simple and facile solvothermal method. A unique crystallographic relationship of [2110]Bi2Te3//[2110]Te and [0001]Bi2Te3//[0001]Te has been observed for this epitaxial growth. Such epitaxial nature between Bi2Te3 nanoplates and Te nanorods is caused by their negligible lattice mismatches in the corresponding atomic planes. The interfaces between Bi2Te3 nanoplates and Te nanorods lead to a low thermal conductivity in these heteronanojunctions, so that a promising figure of merit (ZT) value is obtained (0.73 ± 0.04 at 320 K). This study provides a new method and opportunity to engineer heteronanojunctions for high-efficiency thermoelectric devices for power generation.

Design Principle of Telluride-Based Nanowire Heterostructures for Potential Thermoelectric Applications

We present a design principle to develop new categories of telluride-based thermoelectric nanowire heterostructures through rational solution-phase reactions. The catalyst-free synthesis yields Te-Bi(2)Te(3) "barbell" nanowire heterostructures with a narrow diameter and length distribution as well as a rough control over the density of the hexagonal Bi(2)Te(3) plates on the Te nanowire bodies, which can be further converted to other telluride-based compositional-modulated nanowire heterostructures such as PbTe-Bi(2)Te(3). Initial characterizations of the hot-pressed nanostructured bulk pellets of the Te-Bi(2)Te(3) heterostructure show a largely enhanced Seebeck coefficient and greatly reduced thermal conductivity, which lead to an improved thermoelectric figure of merit. This approach opens up new platforms to investigate the phonon scattering and energy filtering.

Micro- and Nano-Technology: A Critical Design Key in Advanced Thermoelectric Cooling Systems

Advanced, thermoelectric cooling technologies now are receiving more research attention to provide cooling in advanced vehicles and residential systems to assist in increasing overall system energy efficiency and reduce greenhouse gas impacts from leakage by current R-134a systems. This work explores the systems-related impacts, barriers, and challenges of using micro-technology solutions integrated with advances in nano-scale thermoelectric materials in advanced TE cooling systems. Integrated system-level analyses that simultaneously account for thermal energy transport into / dissipation out of the TE device, environmental effects, temperature-dependent TE and thermo-physical properties, thermal losses, and thermal and electrical contact resistances are presented to establish accurate optimum system designs using both BixSb2-xTe3 / Bi2Te3 TE systems and Bi2Te3 TE systems. This work established the design trends and identified optimum design regimes and metrics for these types of systems that will minimize system mass, volume and cost to maximize their commercialization potential in vehicular and residential applications. The relationships between important design metrics, like coefficient of performance, specific cooling capacity and cooling heat flux requirements, upper limits, and critical differences in these metrics in BixSb2-xTe3 / Bi2Te3 TE systems and Bi2Te3 TE systems are explored and quantified. Finally, the work discusses the critical role that micro-technologiesmore » and nano-technologies can play in enabling miniature TE cooling systems in advanced vehicle and residential applications and gives some key relevant examples.« less

Thermoelectric properties and crystal structure of ternary compounds in the Ge(Sn,Pb)Te–Bi2Te3 systems

Polycrystalline samples of stoichiometric ternary compounds in the quasi-binary systems GeTe–Bi2Te3, SnTe–Bi2Te3 and PbTe–Bi2Te3 have been prepared and characterised by X-ray powder diffraction analysis. Lattice parameters have been determined and compared with literature data. At room temperature all the samples exhibit a high carrier concentration in the range 3.4×1019–2.6×1020cm−3. All AIVBi4Te7 and PbBi2Te4 compounds possess negative type of conductivity, while for other compounds it is positive. The Seebeck coefficient and electrical resistivity have been measured over the temperature range 100–800K. The calculated values of the energy band gap of Ge(Sn)Bi4Te7 and Ge(Sn)Bi2Te4 compounds were 0.19–0.22eV as determined from the temperature dependence of electrical resistivity in the intrinsic region. The density of state effective masses m∗ have been estimated for Ge- and Pb-based ternary compounds and are in the range 0.6–1.1m0. An assessment of the potential of these materials for thermoelectric application has also been made.