Bi2Te3-based Thermoelectric Materials Research Articles

Enhanced thermoelectric properties of 3D printed Bi2Te2.7Se0.3 by atomic interface engineering

The performance of n-type Bi2Te3 materials prepared by selective laser melting (SLM) 3D printing technology tends to be lower than p-type materials. Herein, a bottom-up interface engineering strategy based on Atomic Layer Deposition (ALD) is presented to improve the properties of n-type Bi2Te2.7Se0.3 (BTS) materials prepared by SLM. To validate this strategy, a ZnO ultrathin layer was applied to the grain boundaries of BTS, synergistically optimizing carrier/phonon transport. In terms of electrical transport, the energy filtering effect triggered by the constructed ZnO/BTS interface effectively enhances the Seebeck coefficient of the Bi2Te3 material printed by SLM, substantially increasing the power factor. Additionally, the high thermal stability of the ALD-deposited ZnO ultrathin layer effectively suppresses the volatilization of Te in BTS, regulating the carrier concentration. Regarding thermal transport properties, the multiscale defects introduced by the non-equilibrium solidification process during SLM printing cause effective phonon scattering across a wide frequency range, which in turn reduces the thermal conductivity of the material. Atomic-level interface modification in SLM-printed BTS enhanced the ZT value to 1.25. Exploiting this superior thermoelectric performance, a thermoelectric module was constructed. It demonstrated a conversion efficiency of 5.7 % with a temperature difference of 200 K, comparable to the advanced Bi2Te3-based thermoelectric modules. The integration of an ALD interface modification strategy with SLM technology has not only demonstrated potential applications in Bi2Te3-based thermoelectric material development, but also bears significant importance for the accelerated advancement of high-performance thermoelectric materials in other systems.

Atomic-Resolution Interfacial Reaction Mechanism between Bi2Te3-Based Alloys and Ni Electrodes.

Interdiffusion and solid-solid phase reaction at the interface between thermoelectric (TE) materials and the electrode critically influence interfacial transport properties and the overall energy conversion efficiency during service. Here, the microstructural evolution and diffusion mechanisms at the interfaces between the most widely used Bi2Te3-based TE materials, n-type Bi2Te2.7Se0.3 (BTS) and p-type Bi0.5Sb1.5Te3 (BST), and Ni electrodes were investigated at atomic resolution using spherical aberration-corrected scanning transmission electron microscopy (STEM). The BTS(0001)/Ni and BST(0001)/Ni interfaces were constructed by depositing Ni nanoparticles on mechanically exfoliated BTS and BST bulk materials and subsequent annealing. The interfacial reaction is initially dominated by Ni diffusion into the TE matrix to form NiAs-type NiM intermetallics, while Ni trans-quintuple-layer diffusion only occurs in Sb-rich BST. The Bi-rich BTS is more influenced by the Ni-Te preferential reaction, resulting in NiM abnormal grain growth and the formation of tilted and rotated interfaces. Bi diffusion into the BTS matrix forms a Bi double layer at the interface or Bi2[Bi2(Te,Se)3] as the annealing temperature increases, while Bi diffusion into the Ni thin film greatly accelerates the interfacial reaction rate, as elucidated by in situ heating STEM. The results provide essential structural details to understand and prevent the degradation of TE device performance.

Enhancing Thermoelectric Performance of n-Type Bi2Te2.7Se0.3 through Incorporation of Amorphous Si3N4 Nanoparticles.

Bi2Te3-based thermoelectric (TE) materials are the state-of-the-art compounds for commercial applications near room temperature. Nevertheless, the application of the n-type Bi2Te2.7Se0.3 (BTS) is restricted by the comparatively low figure of merit (ZT) and intrinsic embrittlement. Here, we show that through dispersion of amorphous Si3N4 (a-Si3N4) nanoparticles both 14% increase in power factor (at 300 K) and 48% decrease in lattice thermal conductivity are simultaneously realized. The increased power factor comes from enhanced thermopower and reduced electrical resistivity while the reduced lattice thermal conductivity originates mainly from scattering of middle- and low-frequency phonons at the incorporated a-Si3N4 nanoparticles. As a result, a large ZTmax = 1.19 (at 373 K) and an average ZTave ∼ 1.12 (300-473 K) with better mechanical properties are achieved for the BTS/0.25 wt % Si3N4 sample. Present results demonstrate that the incorporation of a-Si3N4 is a promising way to improve TE performance.

Enhanced thermoelectric performance of Mg and Se co-doped Bi2Te3 nanostructures

Bi2Te3-based thermoelectric materials are good candidates for near low and room temperatures. Here, Se and Mg co-doped Bi2Te3 nanostructures were prepared by the solvothermal method. They have rhombohedral crystal structures with space group R-3m as examined by XRD. The nanoplate-like morphology was observed by FESEM. The porosity of the samples was confirmed by FESEM and density measurement. XPS was used to study the valence states and binding energy of the samples. The vibrational modes A1u1, Eg2 and A1g2 are shifted to lower wavenumber with doping, as obtained by Raman spectroscopy. The Hall effect was employed to obtain the carrier concentration and mobility. The band gap energy was obtained in the range of 0.27–0.31 eV by Tauc plotting. In Bi1.94Mg0.06Te2.9Se0.1, the power factor increases to 8.8 × 10−4 Wm−1K−2 at 330 K due to increase in carrier concentration. An ultralow lattice thermal conductivity of ∼0.12 W/m/K was obtained in Bi1.94Mg0.06Te2.9Se0.1 sample, due to the strong phonon scattering at grain boundary and interfaces. The maximum ZT of ∼0.63 was obtained at 300 K for Bi1.97Mg0.03Te2.9Se0.1. The ZT value is about 85 % greater than ZT of pure Bi2Te3 (∼0.34 at 300 K).

Development of Cu2Se/Ag2(S,Se)-Based Monolithic Thermoelectric Generators for Low-Grade Waste Heat Energy Harvesting.

With the ongoing climate and energy crises, thermoelectric conversion has slowly emerged as a clean and reliable alternative energy source for small Internet of Things (IoT) devices. Commercially available thermoelectric generators (TEGs) are typically composed of expensive and toxic Bi2Te3-based thermoelectric materials and require complicated and energy-intensive device assembly processes. As an alternative solution, we have developed a Ag- and Cu-chalcogenide-based monolithic TEG using simple, quick, and low-energy-cost device fabrication processes for low-grade waste heat recovery for energy harvesting. We used ductile Ag2S0.55Se0.45 and overstoichiometric Cu2.075Se, both possessing excellent transport properties around room temperature, with a zT value of ∼0.5 at 300 K. By optimizing the device fabrication process, we were successfully able to assemble the monolithic TEGs without any significant Ag- or Cu-ion migration and obtained a dense and robust device. Strategic optimization of the device structure was able to reduce the electrical contact resistance of the device, which resulted in increased power output. A maximum power density of 0.68 mW/cm2 at a ΔT = 30 K was obtained, which is comparable to a similar Bi2Te3-based monolithic TEG. These results show the potential of chalcogenide-based monolithic TEG as a simple and low-cost alternative to Bi2Te3-based TEGs for energy harvesting applications.

3D Printing of Bi2Te3-Based Thermoelectric Materials with High Performance and Shape Controllability.

Thermoelectric (TE) energy conversion technology provides a promising way to improve the efficiency of fossil energy by generating electricity from low-grade waste heat. With regard to these applications, thermoelectric generators (TEGs) should be designed from system integration perspectives to simultaneously improve heat transfer efficiency and system simplification as well as the robust mechanical properties. However, typical TEGs fabricated by conventional methods barely accomplish such requirements. Herein, high-quality TEGs were assembled by combining the well-flowable spherical bismuth telluride (BT) powdered precursors and selective laser melting (SLM) technology. By optimizing the electronic and phonon transport properties through defect engineering driven by 3D printing, a high figure of merit was accomplished for 1.27 (p-type) and 1.13 (n-type) in BT. This achievement is primarily attributed to the nonequilibrium solidification mechanism, which leads to the formation of multiscale defects during the 3D printing process. The introduction of these multiscale defects enables the effective scattering of wide frequency phonons, leading to a substantial reduction in lattice thermal conductivity. Meanwhile, robust mechanical properties were obtained in the printed p-type/n-type BT TE materials parallel to the building direction (BD) with a compressive strength reaching 257/250 MPa by employing the fine grain structure and the high density of nanotwins introduced during the SLM process. A well shape-controllable and high-performance TEG was designed using 3D-printed BT half-rings, and an output power of 134 mW was achieved at a temperature gradient of 38.9 °C. Our study opens a new route for the great potential of TE materials based on standard commercial SLM 3D printing technology for low-grade waste heat emitted from structures with heterogeneous shapes.

Enhanced thermoelectric performance of Bi2Te3 by carbon nanotubes and silicate aerogel co-doping toward ocean energy harvesting

In this work, we propose a doped-modified thermoelectric generator (TEG) film and apply it to an ocean energy harvesting prototype that primarily utilizes ocean thermal energy. Firstly, A new Bi2Te3-based thermoelectric material with improved performance was synthesized by doping Bi2Te3 with Carbon nanotubes (CNTs) for their high electrical conductivity and Silica aerogel for their low thermal conductivity. Subsequently, a performance-optimized, flexible thermoelectric film was fabricated using electron printing technology. By combining the flexible thermoelectric film with a Triboelectric nanogenerator (TENG) and Electromagnetic generator (EMG), a hybrid energy harvesting prototype was developed that can harvest marine renewable energy. The prototype demonstrates a relatively high output voltage of 61 V and an output power of 81.2 μW, corresponding to a power density of 8.1 W/m3 in simulated marine environmental experiments. This ocean-oriented hybrid energy prototype based on co-doped thermoelectric materials unveils new possibilities for self-powered devices in the field of ocean energy harvesting.

Reaction mechanism of room-temperature synthesized n-Bi2Te3 and high thermoelectric performance p-(Bi,Sb)2Te3–Te using nanosized precursor methods: A novel and energy-efficient production method

Bi2Te3-based thermoelectric materials remain the largest segment of sales in thermoelectric products. Therefore, it is desirable to manufacture Bi2Te3-based thermoelectric materials with energy-efficient synthesis conforming to the 6th principle for green chemistry. We present the formation mechanism for room-temperature synthesized n-Bi2Te3 nanocomposites involving interaction between atoms of Bi and Te with high yield (near 100%) without using any organic substance. It is found that nanosized Bi and Te would first form before the growth of n-Bi2Te3. As such, to shorten the growth period of 14 days, we develop a procedure of producing p-Bi0.45Sb1.55Te3 using nanosized precursors of Bi, Sb, and Te to speed up the process. These nanosized precursors are obtained in one day at room temperature, followed by heat treatment. As a result, the zT is 1.27 at 400 K for p-(Bi, Sb)2Te3–Te composite and zT = 1 for n-Bi2Te3 at 425 K.

Multi-scale hierarchical microstructure modulation towards high room temperature thermoelectric performance in n-type Bi2Te3-based alloys

Bi2Te3-based thermoelectric materials are widely used in near room temperature refrigeration filed. Although Bi2Te3-based polycrystals prepared by powder metallurgy method exhibit good mechanical properties, the low room temperature dimensionless figure of merit (zT) for n-type alloys restricts their further usage. In this work, the electrical and thermal transport properties are manipulated synergistically by multi-scale hierarchical microstructure modulations in n-type Ag2Te/Bi2Te3 composites, including atomic-scale extrinsic point defects, nano-scale low-angel grain boundaries, mesoscale precipitate, and texturing. The carrier concentration is optimized by extrinsic point defects and the carrier mobility is improved due to the strengthened texture, and thus the power factor reaches a high value of 3.6 × 10−3 Wm−1/K2, which is even higher than the hot deformed counterparts. Simultaneously, the phonon scattering is enhanced by the multi-scale defects and the lattice thermal conductivity is suppressed. A high room temperature zT of ∼1.0 and an average zT of 0.96 in the 300–500 K range with good mechanical properties are obtained without hot deformation. This work puts forward a paradigm of enhancing the thermoelectric properties via multi-scale hierarchical microstructure modulations and is significant to the industrialization of manufacturing Bi2Te3-based alloys and devices.

Research Progress of Interfacial Design between Thermoelectric Materials and Electrode Materials.

Intensive efforts have been conducted to realize the reliable interfacial joining of thermoelectric materials and electrode materials with low interfacial contact resistance, which is an essential step to make thermoelectric materials into thermoelectric devices for industrial application. In this review, the roles of structural integrity, interdiffusion, and contact resistance in long-term reliabilities of thermoelectric modules are outlined first. Then interfacial reactions of near-room-temperature Bi2Te3-based thermoelectric materials and various electrode materials are reviewed comprehensively. We also summarized the joining behavior of the mid-temperature PbTe-based thermoelectric materials and commonly used electrode materials. Subsequently, for other thermoelectric materials systems, i.e., SiGe, CoSb3, and Mg3Sb2, previous attempts to join with some electrode materials are also recapitulated. Finally, some future prospects to further improve the joint reliability in thermoelectric device manufacturing are proposed. We believe that this review will provide guidance for preparing thermoelectric devices and optimizing thermoelectric device design.

Knowledge extraction and performance improvement of Bi2Te3-based thermoelectric materials by machine learning

Major advances in materials research often require serendipity and chemical intuition. Traditional trial-and-error-based experimental studies are becoming insufficient for designing novel high-performance materials because of staggering degree of freedom in materials' processing and composition. This work employed machine learning to aid the design of hot-extruded Bi2Te2.85Se0.15 bulk thermoelectric materials based on a small amount of in-house experimental data. Surprisingly, it was found that a combination of processing/composition design strategy with higher extrusion temperatures, more Cu dopants, and deficiency of Te, which is contradictory to experimental experiences, could enhance the materials’ figure of merit (ZT). Experimental validation demonstrated that such a processing/composition solution effectively suppressed formation of point defects, refined microstructure, and promoted evolution of a “fiber texture”, thus simultaneously improving thermoelectric and mechanical properties. The data-driven strategy breaks the rules of thumb in traditional experimental studies and extracts new knowledge to guide the design of high-performance Bi2Te3-based bulk thermoelectric materials.

Low-cost preparation of highly-efficient thermoelectric BixSb2-xTe3 nanostructured powders via mechanical alloying

In this paper, a low-cost and facile method for preparing highly-efficient Bi2Te3-based thermoelectric materials is demonstrated. P-type BixSb2-xTe3 (x ​= ​0.2, 0.3, 0.4, 0.5) nanostructured powders were fabricated via mechanical alloying, and consequently consolidated into pellets by hot-press sintering. The effect of Bi/Sb ratio on the thermoelectric properties was investigated. The higher hole density and mobility in samples with lower Bi concentration resulted in higher electrical conductivity and power factor values. The absence of preferred grain orientation and the high density of grain boundaries in these nanostructured materials led to significantly low lattice thermal conductivity. As a result, a peak ZT of 1.13 ​at 355 ​K and an average ZT of 1.01 over the investigated temperature range was achieved for Bi0.3Sb1.7Te3, suggesting that mechanical alloying can be considered as an advantageous technique for single-step synthesis of highly-efficient thermoelectric BixSb2-xTe3 alloys.

Direct ink writing of Bi2Te3-based thermoelectric materials induced by rheological design

Waste heat recovery systems built with thermoelectric (TE) materials offer promising means to generate electricity from waste heat directly, if TE materials or devices with controllable shapes are created through direct ink writing. However, the nature of TE inks used for direct ink writing must be modified with appropriate rheology to maintain stability and facilitate extrusion, as well as proper mechanical properties to resist deformation. Here, we report Bi2Te3-based inks modified with additives that allow direct printing of tunable architectures. The polyelectrolyte additives-induced adsorbed layers on the surface of TE particles are shown to significantly improve the stability and viscoelasticity of inks. Besides, the improved inks modified with methylcellulose additives exhibit an apparent enhancement of strength properties, thereby enabling the predesigned shapes to hold the successive layers printed above. Using charge control and framework reinforcement, the Bi2Te3-based inks print a series of architectures with porous structures, which largely reduce thermal conductivities. As a result, our three-dimensional-printed materials display high figures of merit ZT of 0.65 and 0.53 for p- and n-type. This work explores how additives can influence both rheological properties, printability, and microstructures, which pave the way to construct complex architectures and improve TE performance for structural and functional applications.

Enhanced thermoelectric performance of p-type Bi2Te3-based materials by suppressing bipolar thermal conductivity

Thermal excitation causes a surge in carrier concentration in narrow bandgap semiconductors that seriously limit its application in the high-temperature zone. Therefore, suppressing bipolar thermal conductivity and broadening the application temperature zone is vitally important for commercial Bi2Te3-based thermoelectric materials. Here, Cu0.6Ni0.4 (CN) nanoparticles synthesized via hydrothermal method are introduced into Bi0.4Sb1.6Te3 (BST) commercial materials. Then, the electrical and thermal properties of BST-CN (with x wt%, x = 0, 0.1, 0.2, 0.4, BST-x-CN) materials are systematically investigated. The results exhibited that the weighted mobility could be improved and the bipolar thermal conductivity could be suppressed for all dealt samples. The maximum and average ZT (figure of merit) values were 1.3 at 393 K and 1.17 in the temperature range of 303–483 K for BST-0.1-CN sample, which exhibited an enhancement by 28.7% and 28.6% than that of the pure sample, respectively. And the optimal ZT values are attained at higher temperatures with the CN nanoparticles contents increasing. Meanwhile, the sintered technique was optimized to enhance electron transport properties, the highest room-temperature power factor of 5.08 mW/m/K2 was attained for the BST-0.1-CN cycle-2 sintered sample. And the optimal ZT value is 1.35 at 393 K for the BST-0.1-CN cycle-1 sintered sample. Finally, the results indicated that the thermoelectric performance could be improved and the application temperature zone could be broadened by suppressing bipolar thermal conductivity owing to the existence of CN nanoparticles.

Predicting the thermal conductivity of Bi2Te3-based thermoelectric energy materials: A machine learning approach

Bi2Te3-based materials are remarkable thermoelectric renewable energy harvesters. The measurement of their thermal conductivity (κ) is a critical phase toward the realization of the material's energy conversion efficiency. However, the experimental techniques involved in the measurements of κ, particularly for thin films, are incredibly challenging. Herein, we introduce a pioneering technique using support vector regression and decision tree regression machine learning models where the values of κ can be predicted based on the structural crystal lattice constants of the material and its electrical properties. A decision tree regression (DTR) and support vector regression (SVR) models using both radial basis function (RBF) and polynomial kernels were developed. The performance of the models was evaluated based on the correlation coefficient (CC) between the predicted and actual values of κ, R2 values, mean absolute error (MAE), and mean square error (MSE). Our results revealed that the DTR outperforms the SVR models in estimating the values of κ with CC of 98.7% and R2 of 97.5% for the testing phase. The models were validated by solving some real-world problems such as predicting the thermal conductivity of transition metal-doped Cu–Bi2Te3, effects of doping non-metals (Se–Bi2Te3), and the role of toxic elements (Pb–Bi2Te3) on the values of κ. The model was further employed to investigate the effects of pulsed laser deposition substrate temperature on the thermal conductivity of Bi2Te3. The performance of the models in predicting the thermal conductivity of Bi2Te3-based materials makes it a useful tool for thermoelectric energy research.

In situ generation of flower-like and microspherical dendrites to improve thermoelectric properties of p-type Bi0.46Sb1.54Te3

In order to obtain excellent thermoelectric properties, constructing three-dimensional flower-like dendrite and microspherical flocculated crystal nanostructures are considered to be an effective method to improve the performance. In this work, Bi0.46Sb1.54Te3-x wt% AgCuTe composite samples are synthesized through mechanical alloying. By SEM, we can directly observe the in situ generated 3D flower-like dendrites and micro-spherically flocculated crystals, that can provide a large number of nano interfaces and then reduce the thermal conductivity. At the same time, lattice defects of different sizes are formed in Bi0.46Sb1.54Te3 composites, resulting in further reduction of the lattice thermal conductivity. In addition, the presence of Ag and Cu can adjust the carrier concentration and enlarge the band gap, which results in a significantly enhanced Seebeck coefficient at higher temperature and a wider temperature range application of the material. Finally, a high ZT value of 1.46 at 425 K and the average ZT value of 1.31 at 300–500 K were achieved for Bi0.46Sb1.54Te3-0.1 wt% AgCuTe sample. This work opened up a new path for the commercial application of Bi2Te3-based thermoelectric materials.

Enhanced performance of Bi2Te3-based thermoelectric materials by incorporating Bi2Fe4O9 magnetic nanoparticles

Thermoelectric (TE) materials have been extensively investigated to solve the energy crisis and alleviate environmental pollution. Currently, n-type and p-type Bi2Te3 based are the only commercialized TE materials. However, their application has been severely limited by the low TE figure of merit (ZT) of p-type Bi2Te3-based TE materials. Here, we demonstrate that the TE performance of p-type materials could be improved by incorporating Bi2Fe4O9 magnetic nanoparticles. Bi2Fe4O9 magnetic nanoparticles could effectively scatter low-energy carrier and improve the carrier quality. The synergistic effect of boundary scattering and magnetic particles scattering lead to the increase of Seebeck coefficient and decrease of thermal conductivity. Finally, the maximum ZT value 1.1 is obtained at 393 K, which is 13% higher the pure sample. The average ZT value of the nanocomposite consisting Bi2Fe4O9 magnetic nanoparticles is 0.98 in the temperature range of 303–483 K, exhibiting an increase of 11%. Overall, the results indicate that magnetic nanoparticles with low saturation magnetic moment have positive effect on the performance of TE materials.

Direct Ink Writing of Bi2te3-Based Thermoelectric Materials Induced by Rheological Design

Direct Ink Writing of Bi2te3-Based Thermoelectric Materials Induced by Rheological Design

Correction: Direct ink writing of high-performance Bi2Te3-based thermoelectric materials using quasi-inorganic inks and interface engineering

Correction for ‘Direct ink writing of high-performance Bi2Te3-based thermoelectric materials using quasi-inorganic inks and interface engineering’ by Zhengshang Wang et al., J. Mater. Chem. A, 2022, https://doi.org/10.1039/d2ta02289c.

Enhancing the room temperature thermoelectric performance of n-type Bismuth-telluride-based polycrystalline materials by low-angle grain boundaries

Bi2Te3-based thermoelectric materials are widely used in solid-state refrigeration near room temperature. However, the room temperature figure of merit (zT) of n-type Bi2Te3-based polycrystals produced by once sintering is always lower than 0.8. Herein, low-angle grain boundaries (LAGBs) are introduced in n-type Bi2Te2.7+xSe0.3 by a simple step-hot-pressing procedure (once sintering) and well characterized by scanning transmission electron microscopy. LAGBs consist of dislocation arrays that can effectively scatter the medium-frequency phonons and thus suppress the lattice thermal conductivity. Although LAGBs also serve as scattering centers of low-energy electrons and deteriorate the carrier mobility, they could contribute to the enhanced Seebeck coefficient owing to the increased scattering factor. Overall, the samples with LAGBs own lower electronic thermal conductivity at the same power factor level. Finally, a high room temperature zT of 0.94 is obtained in n-type Bi2Te2.7Se0.3, which is comparable to those produced by multiple-time sintering. Moreover, step-hot-pressing is also found to be effective in promoting the room temperature zT of p-type Bi2Te3-based polycrystals. This work puts forward a new and simple method to construct LAGBs in Bi2Te3-based alloys that enhances their zTs, and sheds light on the underlying mechanisms about how the LAGB affects the electrical and thermal transport properties.