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1. N-Type Bismuth Telluride Nanocomposite Materials Optimization for Thermoelectric Generators in Wearable Applications

   https://doi.org/10.3390/ma12091529  

   Nozariasbmarz, Amin ; Krasinski, Jerzy S. ; Vashaee, Daryoosh ( May 2019 , Materials)

   Thermoelectric materials could play a crucial role in the future of wearable electronic devices. They can continuously generate electricity from body heat. For efficient operation in wearable systems, in addition to a high thermoelectric figure of merit, zT, the thermoelectric material must have low thermal conductivity and a high Seebeck coefficient. In this study, we successfully synthesized high-performance nanocomposites of n-type Bi2Te2.7Se0.3, optimized especially for body heat harvesting and power generation applications. Different techniques such as dopant optimization, glass inclusion, microwave radiation in a single mode microwave cavity, and sintering conditions were used to optimize the temperature-dependent thermoelectric properties of Bi2Te2.7Se0.3. The effects of these techniques were studied and compared with each other. A room temperature thermal conductivity as low as 0.65 W/mK and high Seebeck coefficient of −297 μV/K were obtained for a wearable application, while maintaining a high thermoelectric figure of merit, zT, of 0.87 and an average zT of 0.82 over the entire temperature range of 25 °C to 225 °C, which makes the material appropriate for a variety of power generation applications. 

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2. Machine learning-assisted ultrafast flash sintering of high-performance and flexible silver–selenide thermoelectric devices

   https://doi.org/10.1039/D2EE01844F  

   Saeidi-Javash, Mortaza ; Wang, Ke ; Zeng, Minxiang ; Luo, Tengfei ; Dowling, Alexander W. ; Zhang, Yanliang ( October 2022 , Energy & Environmental Science)

   Flexible thermoelectric generators (TEGs) have shown immense potential for serving as a power source for wearable electronics and the Internet of Things. A key challenge preventing large-scale application of TEGs lies in the lack of a high-throughput processing method, which can sinter thermoelectric (TE) materials rapidly while maintaining their high thermoelectric properties. Herein, we integrate high-throughput experimentation and Bayesian optimization (BO) to accelerate the discovery of the optimum sintering conditions of silver–selenide TE films using an ultrafast intense pulsed light (flash) sintering technique. Due to the nature of the high-dimensional optimization problem of flash sintering processes, a Gaussian process regression (GPR) machine learning model is established to rapidly recommend the optimum flash sintering variables based on Bayesian expected improvement. For the first time, an ultrahigh-power factor flexible TE film (a power factor of 2205 μW m −1 K −2 with a zT of 1.1 at 300 K) is demonstrated with a sintering time less than 1.0 second, which is several orders of magnitude shorter than that of conventional thermal sintering techniques. The films also show excellent flexibility with 92% retention of the power factor (PF) after 10 3 bending cycles with a 5 mm bending radius. In addition, a wearable thermoelectric generator based on the flash-sintered films generates a very competitive power density of 0.5 mW cm −2 at a temperature difference of 10 K. This work not only shows the tremendous potential of high-performance and flexible silver–selenide TEGs but also demonstrates a machine learning-assisted flash sintering strategy that could be used for ultrafast, high-throughput and scalable processing of functional materials for a broad range of energy and electronic applications. 

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3. The Thermoelectric Properties of Bismuth Telluride

   https://doi.org/10.1002/aelm.201800904  

   Witting, Ian T. ; Chasapis, Thomas C. ; Ricci, Francesco ; Peters, Matthew ; Heinz, Nicholas A. ; Hautier, Geoffroy ; Snyder, G. Jeffrey ( April 2019 , Advanced Electronic Materials)

   Bismuth telluride is the working material for most Peltier cooling devices and thermoelectric generators. This is because Bi₂Te₃(or more precisely its alloys with Sb₂Te₃for p‐type and Bi₂Se₃for n‐type material) has the highest thermoelectric figure of merit,zT, of any material around room temperature. Since thermoelectric technology will be greatly enhanced by improving Bi₂Te₃or finding a superior material, this review aims to identify and quantify the key material properties that make Bi₂Te₃such a good thermoelectric. The largezTcan be traced to the high band degeneracy, low effective mass, high carrier mobility, and relatively low lattice thermal conductivity, which all contribute to its remarkably high thermoelectric quality factor. Using literature data augmented with newer results, these material parameters are quantified, giving clear insight into the tailoring of the electronic band structure of Bi₂Te₃by alloying, or reducing thermal conductivity by nanostructuring. For example, this analysis clearly shows that the minority carrier excitation across the small bandgap significantly limits the thermoelectric performance of Bi₂Te₃, even at room temperature, showing that larger bandgap alloys are needed for higher temperature operation. Such effective material parameters can also be used for benchmarking future improvements in Bi₂Te₃or new replacement materials.

    

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4. High Thermoelectric Performance in Supersaturated Solid Solutions and Nanostructured n‐Type PbTe–GeTe

   https://doi.org/10.1002/adfm.201801617  

   Luo, Zhong‐Zhen ; Zhang, Xiaomi ; Hua, Xia ; Tan, Gangjian ; Bailey, Trevor P. ; Xu, Jianwei ; Uher, Ctirad ; Wolverton, Chris ; Dravid, Vinayak P. ; Yan, Qingyu ; et al ( June 2018 , Advanced Functional Materials)

   Sb‐doped and GeTe‐alloyed n‐type thermoelectric materials that show an excellent figure of meritZTin the intermediate temperature range (400–800 K) are reported. The synergistic effect of favorable changes to the band structure resulting in high Seebeck coefficient and enhanced phonon scattering by point defects and nanoscale precipitates resulting in reduction of thermal conductivity are demonstrated. The samples can be tuned as single‐phase solid solution (SS) or two‐phase system with nanoscale precipitates (Nano) based on the annealing processes. The GeTe alloying results in band structure modification by widening the bandgap and increasing the density‐of‐states effective mass of PbTe, resulting in significantly enhanced Seebeck coefficients. The nanoscale precipitates can improve the power factor in the low temperature range and further reduce the lattice thermal conductivity (κ_lat). Specifically, the Seebeck coefficient of Pb_0.988Sb_0.012Te–13%GeTe–Nano approaches −280 µV K⁻¹at 673 K with a low κ_latof 0.56 W m⁻¹K⁻¹at 573 K. Consequently, a peakZTvalue of 1.38 is achieved at 623 K. Moreover, a high averageZT_avgvalue of ≈1.04 is obtained in the temperature range from 300 to 773 K for n‐type Pb_0.988Sb_0.012Te–13%GeTe–Nano.

    

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5. Antibonding induced anharmonicity leading to ultralow lattice thermal conductivity and extraordinary thermoelectric performance in CsK 2 X (X = Sb, Bi)

   https://doi.org/10.1039/D2TC03356A  

   Yuan, Kunpeng ; Zhang, Xiaoliang ; Chang, Zheng ; Tang, Dawei ; Hu, Ming ( November 2022 , Journal of Materials Chemistry C)

   Full Heusler compounds have long been discovered as exceptional n-type thermoelectric materials. However, no p-type compounds could match the high n-type figure of merit ( ZT ). In this work, based on first-principles transport theory, we predict the unprecedentedly high p-type ZT = 2.2 at 300 K and 5.3 at 800 K in full Heusler CsK 2 Bi and CsK 2 Sb, respectively. By incorporating the higher-order phonon scattering, we find that the high ZT value primarily stems from the ultralow lattice thermal conductivity ( κ L ) of less than 0.2 W mK −1 at room temperature, decreased by 40% compared to the calculation only considering three-phonon scattering. Such ultralow κ L is rooted in the enhanced phonon anharmonicity and scattering channels stemming from the coexistence of antibonding-induced anharmonic rattling of Cs atoms and low-lying optical branches. Moreover, the flat and heavy nature of valence band edges leads to a high Seebeck coefficient and moderate power factor at optimal hole concentration, while the dispersive and light conduction band edges yield much larger electrical conductivity and electronic thermal conductivity ( κ e ), and the predominant role of κ e suppresses the n-type ZT . This study offers a deeper insight into the thermal and electronic transport properties in full Heusler compounds with strong phonon anharmonicity and excellent thermoelectric performance. 

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