Conventional Thermoelectric Research Articles

Development of High Performance Thermoelectric Polymers via Doping or Dedoping Engineering.

It is of great significance to develop high-performance thermoelectric (TE) materials, because they can be used to harvest waste heat into electricity and there is abundant waste heat on earth. The conventional TE materials are inorganic semimetals or semiconductors like Bi2Te3 and its derivatives. However, they have problems of high cost, scarce/toxic elements, high thermal conductivity, and poor mechanical flexibility. Organic TE materials emerged as the next-generation TE materials because of their merits including solution processability, low cost, abundant element, low intrinsic thermal conductivity, and high mechanical flexibility. Organic TE materials are mainly conducting polymers because of their high conductivity. Both the conductivity and Seebeck coefficient depend on the doping level, and they are interdependent. Hence, the TE properties of polymers can be improved through doping/dedoping engineering. There are three types of doping forms, oxidative (or reductive) doping, protonic acid doping, and charge transfer doping. Accordingly, they can be dedoped by different approaches. In this article, we review the methods to dope and dedope p-type and n-type TE polymers and the combination of doping and dedoping to optimize their TE properties. Secondary doping is also covered, since it can significantly enhance the conductivity of some TE polymers.

Fabrication of Bi2Te3 and Sb2Te3 Thermoelectric Thin Films using Radio Frequency Magnetron Sputtering Technique.

Through various studies on thermoelectric (TE) materials, thin film configuration gives superior advantages over conventional bulk TEs, including adaptability to curved and flexible substrates. Several different thin film deposition methods have been explored, yet magnetron sputtering is still favorable due to its high deposition efficiency and scalability. Therefore, this study aims to fabricate a bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) thin film via the radio frequency (RF) magnetron sputtering method. The thin films were deposited on soda lime glass substrates at ambient temperature. The substrates were first washed using water and soap, ultrasonically cleaned with methanol, acetone, ethanol, and deionized water for 10 min, dried with nitrogen gas and hot plate, and finally treated under UV ozone for 10 min to remove residues before the coating process. A sputter target of Bi2Te3 and Sb2Te3 with Argon gas was used, and pre-sputtering was done to clean the target's surface. Then, a few clean substrates were loaded into the sputtering chamber, and the chamber was vacuumed until the pressure reached 2 x 10-5 Torr. The thin films were deposited for 60 min with Argon flow of 4 sccm and RF power at 75 W and 30 W for Bi2Te3 and Sb2Te3, respectively. This method resulted in highly uniform n-type Bi2Te3 and p-type Sb2Te3 thin films.

Investigating thin-film thermoelectric generators: Leg shape, TEG configuration, and contact resistance analysis

Thermoelectric generators (TEGs) are made of parallel p-type and n-type semiconducting legs which can directly convert temperature gradients into electricity. Theoretical studies have extensively explored the impact of the thermoelectric (TE) leg size and shape on the conversion efficiency of such bulk TEGs. However, in recent years, geometrically thin and film-shaped TEGs have gained attention for integration into wearable and flexible electronic devices, tapping into body heat as a power source. There is currently a lack of research on film-shaped TEGs, particularly regarding the performance of asymmetric TE leg shapes. This study investigates seven different TE leg shapes in thin-film TEGs, analyzing their electrical performance using COMSOL Multiphysics software. Results show that certain asymmetric TE leg shapes can yield over a 30% improvement in the produced open circuit voltage, as compared to conventional rectangular-shape TE legs. Furthermore, various configurations of thin-film TEGs were designed and analyzed, aiming to maximize device output power. Additionally, the study delves into the influence of electrical contact resistance on the performance of the optimal asymmetric TEGs. The proposed thin TE leg shapes and TEG configurations ae easily manufacturable and scalable using printing technology.

Janus 2D B2P6: A promising anisotropic thermoelectric material with high power factor

A two-dimensional (2D) material with high thermoelectric (TE) performance is of great interest for the energy conversion device. In this study, by using first-principles calculations, we show that the Janus 2D B2P6 has an extremely high TE power factor of 0.10 and 0.07 W/mK2 for p-type and n-type at room temperature, respectively, which are higher than that of conventional 2D TE materials. We also investigate lattice thermal conductivity, κph, of B2P6 by using the Boltzmann transport theory with phonon–phonon interaction. With κph of 41.45 and 133.3 W/mK for the armchair and zigzag directions, respectively, the optimized values of the TE figure of merit are up to 0.7 at 900 K. Moreover, the Janus 2D B2P6 exhibits anisotropic mechanical properties, in which the ideal strength of B2P6 is 10.7 and 18.63 N/m for the armchair and zigzag directions, respectively. Thus, our results suggest that the Janus 2D B2P6 is the potentially promising flexible anisotropic TE material.

Kirigami-inspired organic and inorganic film-based flexible thermoelectric devices with built-in heat sink

Thermoelectric (TE) devices can convert heat to electricity directly, which offers a unique opportunity to realize waste heat recovery. However, conventional TE devices inevitably use heat sinks, which are bulky, rigid and heavy, limiting practical applications. Herein, we propose a fully integrated film-based TE device with intrinsically built-in fins as heat sink in a hexagonal honeycomb device structure, that simultaneously achieves high TE performance and conformability, as confirmed by experiments and modelling. A flexible Kapton substrate with copper electrodes, integrating either carbon nanotube (CNT) veils or bismuth telluride (Bi2Te3) TE ‘legs’, both of n- and p-type, achieved a remarkable specific power of 185.4 nW K−2 for a Bi2Te3-based device and 53.1 nW K−2 for a CNT-based device, thanks to the heat dissipation effect granted by the built-in fins. Besides, the addition of oriented polymer films interconnects, contracting when above their glass transition temperature, allowed a single substrate two-dimensional (2D) TE device to self-fold into a three-dimensional (3D) hexagonal honeycomb structure, with built-in fins, contactlessly and autonomously. The demonstrated shape-programmed kirigami-inspired scalable TE device paves the way for realising self-powered applications comprising hundreds of TE legs with both inorganic (e.g., Bi2Te3) and organic (e.g. CNT veils) TE materials and integrated heat sinks.

Next-Generation Quantum Materials for Thermoelectric Energy Conversion

This review presents the recent advances in the search for thermoelectric (TE) materials, mostly among intermetallic compounds and in the enhancement of their TE performance. Herein, contemporary approaches towards improving the efficiency of heat–electricity conversion (e.g., energy harvesting and heat pumping) are discussed through the understanding of various emergent physical mechanisms. The strategies for decoupling the individual TE parameters, as well as the simultaneous enhancement of the TE power factor and the suppression of heat conduction, are described for nanoparticle-doped materials, high entropy alloys, and nanowires. The achievement of a superior TE performance due to emergent quantum phenomena is discussed for intermetallic chalcogenides and related systems (e.g., strong and weak topological insulators, Weyl and Dirac semimetals), and some of the most promising compounds within these classes are highlighted. It was concluded that high-entropy alloying provides a methodological breakthrough for employing band engineering methods along with various phonon scattering mechanisms towards significant TE efficiency improvement in conventional TE materials. Finally, topological semimetals and magnetic semimetals with several intriguing features, such as a violation of the Wiedemann–Franz law and outstanding perpendicular Nernst signals, are presented as strong candidates for becoming next-generation TE quantum materials.

Ionoelastomer electrolytes for stretchable ionic thermoelectric supercapacitors

Ionic thermoelectric supercapacitors (ITESCs) produce orders of magnitude higher voltages than those of conventional thermoelectrics (TEs) based on the thermo-diffusion of electrons/holes and are therefore attractive for converting low-grade heat into electricity. The stretchability and stability of the whole ITESC are important for wearable heat harvesting applications. Recent studies on ITESC have focused on stretchable ionic TE electrolytes with a giant Seebeck coefficient, but there are no reports of fully stretchable ITESCs for wearable heat harvesting devices due to the lack of stretchable electrodes and stretchable ionic TE electrolytes with stability. Herein, we present a fully stretchable ITESC composed of stable high-performance ionic thermoelectric elastomer (ITE) electrolyte and stretchable gold nanowire (AuNW) electrodes. The ITE shows excellent air stability (> 60 d) in comparison to hydrogel-based electrolytes that are susceptible to dehydration in ambient conditions. Furthermore, the ITE exhibits an apparent thermopower up to 38.9 mV K–1 and ionic conductivity of 3.76 × 10–1 mS cm–1, which both are maintained up to a tensile strain of 250%. Finally, a fully stretchable ITESC with AuNW electrodes is developed which can harvest energy from thermal gradients during deformations.

Enhancement of thermoelectric performance in n-type Si90Ge10-based alloy by metallic Zn doping

Silicon–germanium (SiGe) alloy has become one of the representative high-temperature thermoelectric (TE) materials due to its advantages of stability, non-toxicity, oxidation resistance, and high mechanical strength. However, the high thermal conductivity and expensive Ge greatly limit the enhancement of zT value and its application. In this paper, n-type Si90Ge10P2Znx nanocomposites were prepared by ball milling and spark plasma sintering. By adjusting the Zn content and sintering time, multiple phonon-scattering centers, such as Zn precipitates, nano-pores, and layered structures, have been introduced into the SiGe matrix. The thermal conductivity was significantly reduced to 2.59 W m−1 K−1 without deteriorate power factor (PF), thus leading to a high zT value of 1.23 at 873 K. At 323–873 K, the average zT value (zTavg) also reached 0.6, increased by approximately 25% in comparison to the reported value using the same ratio of Si90Ge10. Compared with the conventional radioisotope TE generator with Si80Ge20 composition, the zTavg value increased by nearly 30% with only half of Ge, giving strong impetus to the application of SiGe-based TE materials.

Multiparameter optimization of the thermoelectric module under the constant heat flux condition

Thermoelectric (TE) technology enables direct heat-to-electricity conversion and offers an alternative way to carbon neutralization. Conventional TE device design is usually conducted under the constant temperature difference (CTD) condition. However, the constant heat flux (CHF) condition dominates in realistic applications. Here, using finite element simulation, we investigate the relationship between the geometric parameter and the TE performance under the CHF condition. Our results reveal that the thermal properties are the governing factors under the CHF condition. The simulations suggest that both the maximum output power and conversion efficiency increase with the height, decrease with the fill factor, and share a parabolic tendency as the cross-sectional area ratio changes. Instead of using the module parameters optimized under the CTD condition, the TE performance can be enhanced by 43% after geometric optimization under the CHF condition, suggesting that the optimization under the CHF condition is of significance for realistic applications.

Complete magnetic control over the superconducting thermoelectric effect

Giant thermoelectric effects are known to arise at the interface between superconductors and strongly polarized ferromagnets, enabling the construction of efficient thermoelectric generators. We predict that the thermopower of such a generator can be completely controlled by a magnetic input signal: Not only can the thermopower be toggled on and off by rotating a magnet, but it can even be entirely reversed. This in situ control diverges from conventional thermoelectrics, where the thermopower is usually fixed by the device design.

Large Room Temperature Anomalous Transverse Thermoelectric Effect in Kagome Antiferromagnet YMn6 Sn6.

Kagome magnets possess several novel nontrivial topological features owing to the strong correlation between topology and magnetism that extends to their applications in the field of thermoelectricity. Conventional thermoelectric (TE) devices use the Seebeck effect to convert heat into electrical energy. In contrast, transverse thermoelectric devices based on the Nernst effect are attracting recent attention due to their unique transverse geometry, which uses a single material to eliminate the need for a multitude of electrical connections compared to conventional TE devices. Here, a large anomalous transverse thermoelectric effect of ≈2 µV K-1 at room temperature in a kagome antiferromagnet YMn6 Sn6 single crystal is obtained. The obtained value is larger than that of state-of-the-art canted antiferromagnetic (AFM) materials and comparable with ferromagnetic systems. The large anomalous Nernst effect (ANE) can be attributed to the net Berry curvature near the Fermi level. Furthermore, the ANE of the AFM YMn6 Sn6 exceeds the magnetization scaling relationship of conventional ferromagnets. The results clearly illustrate that AFM material YMn6 Sn6 is an ideal topological material for room-temperature transverse thermoelectric applications.

Composite Aerogel of Electropolymerized Polyaniline and SWCNTs with High Thermoelectric Performance

AbstractAerogels are promising in the preparation of high‐performance thermoelectric (TE) materials due to their ultralow thermal conductivity. However, the TE performance of aerogels remains unsatisfactory. Herein, polyaniline/single‐walled carbon nanotubes (PANI/SWCNT) composite aerogel with high thermoelectric performance is synthesized through dynamic three‐phase interfacial electrochemical polymerization of aniline and subsequent physical mixing of PANI with SWCNTs followed by liquid nitrogen quenching and freeze‐drying. By adjusting the content of SWCNTs, the PANI/SWCNT aerogel (50 wt% SWCNTs) achieves a high power factor (PF) of 73.33 ± 2.03 µW m−1 K−2. The ultralow thermal conductivity of around 2.30 × 10−2 W m−1 K−1 for the composite aerogel is superior to that of most conventional organic TE materials, rendering an excellent figure of merit (ZT) value of ≈0.95 at room temperature and great potential in improving TE performance of organic and organic/inorganic composites. Thermal treatment of the composite aerogel further enhances the PF to 96.28 ± 1.89 µW m−1 K−2. This work provides an effective method in the formation of PANI/SWCNT TE composites and it puts forward insights into the preparation of other high‐performance TE composites.

Dwelling deep into the anisotropic transport of Zintl Ca3Sb2

Phonon-glass-electron-crystal (PGEC) is a class of materials that exert electron crystal like pronounced electronic conduction and glass like supressed thermal conduction leading to astonishing thermoelectric (TE) performance. The density functional theory based investigation of Zintl Ca3Sb2 is presented. We find that a narrow band gap of 0.35 eV and the spatially dispersed phonon branches of Ca3Sb2 result in improved power factor and ultra-low thermal conductivity. Comparing with conventional TE materials PbTe, Bi2Te3 and Mg3Sb2, the Ca3Sb2 possesses significantly low average thermal conductivity (κL) of 0.26 Wm-1K−1 at 300 K. The root-cause of such a feeble κL is reciprocated in terms of the acoustic modes with supressed vibrational frequency that when coupled with lower lying optic phonon modes, results in significantly high phonon scattering rates and phonon anharmonicity. Interestingly, the thermal conduction along the cross-plane direction is found to be much more supressed than the in-plane direction suggesting appreciable anisotropy. The TE efficiency of Ca3Sb2 is envisaged from the temperature dependent profile of figure of merit ZT which is found to attain magnitude of ∼ 0.72 (in z-direction) within temperature range 300 K-600 K for both charge carriers. Besides providing in-depth insight to the thermal conduction, the study provides motivation to investigate this yet-to-be-synthesized potential Zintl compound.

High thermoelectric performance in metal phosphides MP2 (M = Co, Rh and Ir): a theoretical prediction from first-principles calculations.

Although metal phosphides have good electronic properties and high stabilities, they have been overlooked in general as thermoelectrics based on expectation of high thermal conductivity. Here we propose the metal phosphides MP2 (M = Co, Rh and Ir) as promising thermoelectrics through first-principles calculations of their thermoelectric properties. By using lattice dynamics calculations within unified theory of thermal transport in crystal and glass, we obtain the lattice thermal conductivities κl of MP2 as 0.63, 1.21 and 1.81 W m−1 K at 700 K for M = Co, Rh and Ir, respio ectively. Our calculations for crystalline structure, phonon dispersion, Grüneisen parameters and cumulative κl reveal that such low κl originates from strong rattling vibrations of M atoms and lattice anharmonicity, which significantly suppress heat-carrying acoustic phonon modes coupled with low-lying optical modes. Using mBJ exchange–correlation functional, we further calculate the electronic structures and transport properties, which are in good agreement with available experimental data, evaluating the relaxation time of charge carrier within deformation potential theory. We predict ultrahigh thermopower factors as 10.2, 7.1 and 6.4 mW m−1 K2 at 700 K for M = Co, Rh and Ir, being superior to the conventional thermoelectrics GeTe. Finally, we estimate their thermoelectric performance by computing figure of merit ZT, finding that upon n-type doping ZT can reach ∼1.7 at 700 K specially for CoP2. We believe that our work offers a novel materials platform to search for high-performance thermoelectrics using metal phosphides.

Transverse thermal energy conversion using spin and topological structures

Conversion of thermal to electrical energy has been a subject of intense study for well over two centuries. Despite steady progress throughout the past several decades, solid-state thermoelectric (TE) energy conversion devices remain adequate only for niche applications. One appealing option for circumventing the limits of conventional TE physics is to utilize phenomena where flows of heat and charge are perpendicular, the so-called “transverse” geometry. In this Tutorial, we discuss recent advances behind new ways to generate large transverse thermoelectric voltages, such as the spin Seebeck and Nernst effects, as well as Weyl physics. We provide suggestions for how these mechanisms might be enhanced and implemented into high-efficiency, next generation transverse TE devices. We also discuss best practices for accurate measurement and reporting of transverse thermoelectric material properties, including a case study of a round robin spin Seebeck coefficient measurement.

Manifestation of the thermoelectric properties in Ge-based halide perovskites

In spite of intensive studies on the chalcogenides as conventional thermoelectrics, it remains a challenge to find a proper material with high electrical but low thermal conductivities. In this work, we introduced a new class of thermoelectrics, Ge-based inorganic halide perovskites \ce{CsGeX3} (X = I, Br, Cl), which were already known as a promising candidate for photovoltaic applications. By performing the lattice-dynamics calculations and solving the Boltzmann transport equation, we revealed that these perovskites have ultralow thermal conductivities below 0.18 W m$^{-1}$ K$^{-1}$ while very high carrier mobilities above 860 cm$^2$ V$^{-1}$ s$^{-1}$, being much superior to the conventional thermoelectrics of chalcogenides. These results highlight the way of searching high-performance and low-cost thermoelectrics based on inorganic halide perovskites.

Evaluation of CO2 production for enhanced oil recovery from four power plants

Abstract Four coal-based power plants were evaluated with respect to their capacity to produce CO2 for Enhanced Oil Recovery (EOR). The plant characteristics were evaluated using energy and exergoeconomic criteria and a robust coal gasification/combustion mathematical model that can predict temperature, converted fraction and particle size distribution for solids have been used for a high pressure fluidized bed. Other models based on Python, Aspen Hysys and Microsoft Excel have been used too. Integrating carbon sequestration reduces the global energy and exergy efficiencies of all power plants (up to 10%). However, the Integrated Gasification Combined Cycle (IGCC) is a promising technology utilizing coal for generating electrical energy and direct compression of CO2 (11–20 MPa). Similarly, integrating gasification with Solid Oxide Fuel Cells (SOFC), allows for the pre-combustion capture of CO2, with the advantage of lower initial investment costs. The oxy-fuel combustion (OXY) plant offers high energy and exergy efficiencies, but the exergoeconomic cost of CO2 is increased by 31 USD/t as compared to IGCC. The conventional thermoelectric (CT) plant exhibit disadvantages due to their simple power cycle and the elevated initial investment costs. This suggests that coal-gasification based plants are the best alternatives for CO2 production for EOR and co-generated electrical power.

Experimental investigation of novel integrated photovoltaic-thermoelectric hybrid devices with enhanced performance

A combined photovoltaic (PV) cell and thermoelectric (TE) device can effectively expand the utilization of the solar spectrum, and it has been confirmed that effective heat transfer can improve the performance of PV-TE hybrid devices. In this study, a series of novel integrated PV-TE hybrid devices with enhanced heat transfer capabilities have been manufactured by removing the upper ceramic plate of conventional TE devices. An insulating layer was deposited on the back of the PV cell to avoid an electrical connection between the PV cell and the TE device. An exhaustive experimental study has been conducted on the integrated PV-TE hybrid device, using a system that includes a solar simulator and cooling equipment. Experimental results indicate that the integrated hybrid design can improve the electrical output of the PV cell. In addition, thermal greases with different thermal conductivities are used to decrease the thermal resistance between the PV cell and TE device. The use of the thermal lubricant significantly improves the performance of the integrated PV-TE hybrid device by enhancing the heat dissipation of the PV cell, and the heat absorption of the hot side of TE device. The thickness of the insulating layer may be controlled with the use of different application times, and this has been demonstrated. Experimental results show that a thinner insulating layer can increase heat transfer significantly and improve the performance of the integrated PV-TE hybrid device. Consequently, the design utilizes a more compatible and efficient integrated couple involving a PV cell and TE device.

A flexible thermoelectric device based on a Bi2Te3-carbon nanotube hybrid

Thermoelectric (TE) materials and devices have attracted great attention due to their ability to convert waste heat to electrical power and active cooling. However, the conventional bulk TE materials are inorganic semiconductors with inherent brittleness and rigidity. They cannot closely contact curved heat sources and sinks, which limits their application in modern electronics. It remains a big challenge to fabricate high-performance TE materials and devices with good flexibility. Here, we report a flexible TE device comprised of a single wall carbon nanotube (SWCNT) network and (000l)-textured Bi2Te3 nanocrystals prepared by a magnetron sputtering technique. The unique Bi2Te3-SWCNT hybrid structure has a TE figure of merit (ZT) value of ∼0.23 at ∼330 K. A prototype TE device made of this hybrid gives a maximum output power density of ∼0.93 mW cm−2 under a temperature difference of 25 K at ambient temperature and shows good flexibility under bending. Our results open up a new way to the development of flexible TEs and their application in self-powered portable devices.

Three-terminal interface as a thermoelectric generator beyond the Seebeck effect

We investigate thermoelectric transport through interfaces with inelastic scatterings by developing a quantum theory, which has been extensively validated by existing theories. We find that under temperature bias, while a two-terminal conductor-insulator interface behaves only as a thermal resistor, a three-terminal conductor-insulator-conductor interface can function as an electricity generator caused by phonon-mediated electron scatterings with heat-charge current separation. Unlike conventional thermoelectrics which is a property of a bulk caused by the Seebeck effect, this thermoelectric behavior is a property of an interface driven by electron-phonon scatterings.