High Thermoelectric Performance Research Articles

High thermoelectric performance of multiwalled carbon nanotubes based ionogels

Ionogels have emerged as promising thermoelectric materials with Seebeck coefficient 2–3 orders of magnitude higher than Seebeck coefficient of their inorganic counter parts. However, they suffer from the problem of low ionic conductivity, which can be improved with the addition of inorganic nanofillers to the ionogels. In the present work, thermoelectric performance of multiwall carbon nanotubes (MWCNTs) based ionogels (IGs) has been investigated. IGs were synthesized via in situ radical polymerization of polyethylene glycol 200 dimethacrylate (PEG200DMA) difunctional monomer in the presence of 1-butyl-3-methyl imidazolium tetrafluoroborate (an ionic liquid) and MWCNTs. Three composites namely MWCNTs-0.25, MWCNTs-0.5 and MWCNTs-1 were prepared having the concentration of MWCNTs by 0.25, 0.5 and 1 wt% respectively. A remarkable 75.3% enhancement in ionic conductivity was achieved for the MWCNTs-1 wt% ionogel compared to the base IG at 40 °C. This substantial improvement can be attributed to the "breathing polymer chain model," which describes the dissociation of ion aggregates due to the interaction between the ionic liquid and polymer chains. In terms of thermoelectric performance amongst the MWCNT ionogels, 0.25 wt% MWCNT-based ionogels was the optimized concentration with very high Seebeck coefficient of 1.70 mV/K and power factor of 4.1 µW/m. K along with excellent thermal stability up to 386 °C. These high-performing ionogels hold great promise for efficient utilization of low-grade thermal energy.

Ultra-low thermal conductivity and high thermoelectric performance of two-dimensional Ga2O2: A comprehensive first-principles study

Gallium oxide such as two-dimensional (2D) Ga2O3 monolayers has been widely used in lasers, dielectric coatings for solar cells, low-dimensional electronic and optoelectronic device applications due to their excellent electrical optical and thermal properties. In this work, we predicted a new 2D gallium oxide Ga2O2 and systematically investigated its stability, electronic properties, and thermoelectric performance. The phonon dispersion, elastic tensor and ab initio molecular dynamics simulations show that the Ga2O2 monolayer is dynamically, mechanics and thermally stable. We find the Ga2O2 monolayer is an indirect semiconductor with a wide band gap of 2.77 eV and high electron mobility. In particular, 2D Ga2O2 has extremely low thermal conductivity and the optical phonons dominate the thermal conductivity. By analyzing its harmonic and anharmonic properties, we find that the weak interaction between Ga atoms is the main reason for this phenomenon. At last, we predict that the maximum ZT values of 2D Ga2O2 are evaluated up to 6.5 at 500 K and 3.3 at 300 K, respectively. With ultra-low thermal conductivity and high thermoelectric performance, 2D Ga2O2 is considered a potential candidate for thermoelectric materials.

Exploring the versatile properties of Cs2BʹGeF6 (Bʹ: Sn, Pb) double perovskites: Insights into their mechanical stability, optoelectronic potential, and high thermoelectric performance

The present study investigates the structural, elastic, electronic, optical, and thermoelectric properties of the halide double perovskites Cs2SnGeF6 and Cs2PbGeF6. Density functional theory (DFT) calculations have been performed using the generalized gradient approximation (GGA), and spin–orbit coupling (SOC). The investigation reveals that both compounds exhibit mechanical stability. Notably, the studied compounds display weak elastic anisotropy. Electronic properties were further examined, including the electronic band structure and density of states. The compounds show a semiconductor nature with a direct bandgap (Γ-Γ). Additionally, a comprehensive analysis of optical properties was conducted, encompassing the complex dielectric constant, refractive index, reflectivity, extinction coefficient, electron energy loss, absorption coefficient, and optical conductivity function, spanning up to 13.0 eV. Furthermore, the research provides insights into the thermoelectric properties of these materials, revealing an increase in thermoelectric behavior with rising temperature. This underscores the semiconducting nature of the compounds, with electrons identified as the majority carriers.

CdSe Quantum Dots Enable High Thermoelectric Performance in Solution-Processed Polycrystalline SnSe.

Here, a high peakZTof ≈2.0 is reported in solution-processed polycrystalline Ge and Cd codoped SnSe. Microstructural characterization reveals that CdSe quantum dots are successfully introduced by solution process method. Ultraviolet photoelectron spectroscopy evinces that CdSe quantum dots enhance the density of states in the electronic structure of SnSe, which leads to a large Seebeck coefficient. It is found that Ge and Cd codoping simultaneously optimizes carrier concentration and improves electrical conductivity. The enhanced Seebeck coefficient and optimization of carrier concentration lead to marked increase in power factor. CdSe quantum dots combined with strong lattice strain give rise to strong phonon scattering, leading to an ultralow lattice thermal conductivity. Consequently, high thermoelectric performance is realized in solution-processed polycrystalline SnSe by designing quantum dot structures and introducing lattice strain. This work provides a new route for designing prospective thermoelectric materials by microstructural manipulation in solution chemistry.

High thermoelectric performance in polycrystalline Yb3Ge5 thin films

The development of eco-friendly thin-film thermoelectric generators for microenergy harvesting applications is highly desired. Ge-based materials have recently attracted significant research interest because of their superior performance at room temperature as human-compatible thermoelectric constituents. We examined the synthesis of Yb3Ge5 thin films and assessed their thermoelectric properties, considering the exceptional ability of Yb as a heavy metal to scatter phonons. The components of the resulting thin film were strongly dependent on the Yb/Ge deposition ratio and the subsequent annealing temperature. An increase in the Yb content augmented the formation of Yb2O3, whereas an increase in the Ge content facilitated the formation of Ge crystals. These trends were more evident at higher annealing temperatures. By controlling the YbGe composition, a power factor of 1470 µW m−1 K−2 was obtained at an annealing temperature of 700 °C, along with a low thermal conductivity of 3.1 W m−1 K−1. Machine-learning-facilitated data analysis revealed that the high power factor originated from Yb3Ge5. Notably, the power factor reached 1370 µW m−1 K−2 even at a temperature of 500 °C, causing the expansion of heat-resistant plastic films. Thus, we established the impressive potential of Yb3Ge5 as a suitable material for eco-friendly thin-film thermoelectric generators.

Improving thermoelectric properties of Bi2Te3 by straining under high pressure: Experiment and DFT calculation

Bi2Te3 intermetallic compounds show high thermoelectric performance near room temperature and are widely used for cooling and power generation applications. It is still necessary to improve its dimensionless figure of merit zT for large-scale industrial use. In this study, high-pressure torsion processing is applied to Bi2Te3 powder samples to investigate the crystal structure and enhance their thermoelectric properties. The processed Bi2Te3 samples show reduced cell volume, higher electrical conductivity, enhanced Seebeck coefficient and higher zT value compared with the original powder sample, even if the grain refinement is not remarkable. First-principles calculations are also used to investigate the electronic structure and thermoelectric properties to clarify the influence of high-pressure torsion processing.

High Thermoelectric Performance in Solution-Processed Semicrystalline PEDOT:PSS Films by Strong Acid-Base Treatment: Limitations and Potential.

Thermoelectric (TE) generation with solution-processable conducting polymers offers substantial potential in low-temperature energy harvesting based on high tunability in materials, processes, and form-factors. However, manipulating the TE and charge transport properties accompanies structural and energetic disorders, restricting the enhancement of thermoelectric power factor (PF). Here, solution-based strong acid-base treatment techniques are introduced to modulate the doping level of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films with preserving its molecular orientation, enabling to achieve a remarkably high PF of 534.5µWm-1 K-2 . Interestingly, theoretical modeling suggested that further de-doping can increase the PF beyond the experimental value. However, it is impossible to reach thisvalue experimentally, even without any degradation of PEDOT crystallinity. Uncovering the underlying reason for the limitation, an analysis of the relationship among the microstructure-thermoelectric performance-charge transport property revealed that inter-domain connectivity via tie-chains and the resultant percolation for transport are crucial factors in achieving high TE performance, as in charge transport. It is believed that the methods and fundamental understandings in this work would contribute to the exploitation of conducting polymer-based low-temperature energy harvesting.

Low Thermal Conductivity and High Thermoelectric Performance of Nb-Doped Quarternary Mixed Crystal Nb0.05W0.95-xMox(Se1-xSx)2.

Transition-metal dichalcogenide WSe2 has attracted increasing interest due to its large thermopower (S), low-cost, and environment-friendly constituents. However, its thermoelectric figure of merit, ZT, of WSe2 is limited due to its large lattice thermal conductivity (κL) and low electrical conductivity. In view of WSe2 and MoS2 having the same crystal structure, here we designed and prepared Nb-doped quarternary mixed crystal (MC) Nb0.05W0.95-xMox(Se1-xSx)2 (0 ≤ x ≤ 0.095). The results indicate that the κL of the MC can reach as low as 0.12 W m K-1 at 850 K, being 93% smaller than that of WSe2. Our analysis reveals that its low κL originates chiefly from intense scattering of both high-frequency phonons from point defects (mainly alloying elements) and mid/low-frequency phonons from MoS2 inclusions residual within MC. In addition, the alloying of WSe2 with MoS2 causes a 5-fold increase in cation vacancies (VW‴'), leading to a large increase in hole concentration and electrical conductivity, which gives rise to a ∼7.5 times increase in power factor (reaching 4.2 μ W cm-1 K-2 at 850 K). As a result, a record high ZTmax = 0.63 is achieved at 850 K for the MC sample with x = 0.076, which is 20 times larger than that of WSe2, demonstrating that MC Nb0.05W0.95-xMox(Se1-xSx)2 is a promising thermoelectric material.

Origin of off-centering effect and the influence on heat transport in thermoelectrics

Recently, off-centering behavior has been discovered in a series of thermoelectric materials. This behavior indicates that the constituent atoms of the lattice displace from their coordination centers, leading to the locally distorted state and local symmetry breaking, while the material still retains its original crystallographic symmetry. This effect has been proved to be the root cause of ultralow thermal conductivity in off-centering materials, and is considered as an effective tool to regulate the thermal conductivity and improve the thermoelectric performance. Herein, we present a collection of recently discovered off-centering compounds, discuss their electronic origins and local coordination structures, and illuminate the underlying mechanism of the off-centering effect on phonon transport and thermal conductivity. This paper presents a comprehensive view of our current understanding to the off-centering effect, and provides a new idea for designing high performance thermoelectrics.

Construct Amorphous Polymer Interface to Enhance the Thermoelectric Performance of Commercial Bi0.5Sb1.5Te3 Materials.

Interfaces, such as grain boundaries and phase boundaries in thermoelectric (TE) materials, play a crucial role in the carrier/phonon transport. Accurate control of the features of interfaces, including composition, crystalline nature, and thickness may give rise to a promising pathway to break the trade-off between phonon and carrier transport properties, which is essential to design high-performance TE materials. In this work, the amorphous polymer interface (API) layer is introduced to the p-type commercial Bi0.5Sb1.5Te3 (BST) TE material by the liquid-phase sintering process. Due to the larger mismatch in the acoustic impedance or phonon spectra between the amorphous polymer layer and the BST phase, the additional interfacial thermal resistance is introduced, which results in a large decrease in lattice thermal conductivity. It is found that the interfacial thermal resistance at the API is much higher than that of normal grain boundary and hetero interface reported in the literature. Conversely, taking advantage of the strong electron and phonon scattering, a large net get of ZT was achieved. A maximum ZT of ∼1.22 at 350 K was obtained in the BST/polyimide-0.5% sample, which is considerably greater than that of the commercial BST matrix (∼0.99 at 350 K). Furthermore, the optimized BST/polymer sample also exhibited almost 20% enhancement in hardness compared with the pure BST sample. This work has opened a new window for designing high-performance TE composites, which may extend to other material systems.

A facile in-situ reaction method for preparing flexible Sb2Te3 thermoelectric thin films

Inorganic p-type Sb2Te3 flexible thin films (f-TFs) with eco-friendly and high thermoelectric performance have attracted wide research interest and potential for commercial applications. This study employs a facile in-situ reaction method to prepare flexible Sb2Te3 thin films by rationally adjusting the synthesized temperature. The prepared thin films show good crystallinity, which enhances the electrical conductivity of ~1,440 S·cm-1 due to the weakened carrier scattering. Simultaneously, the optimized carrier concentration, through adjusting the synthesis temperature, causes the intermediate Seebeck coefficient. Consequently, a high-power factor (16.0 μW·cm-1·K-2 at 300 K) is achieved for Sb2Te3 f-TFs prepared at 623 K. Besides, the f-TFs also exhibit good flexibility due to the slight change in resistance after bending. This study specifies that the in-situ reaction method is an effective route to prepare Sb2Te3 f-TFs with high thermoelectric performance.

Intensified Phonon Scattering in ZrCoBi Half-Heusler by Noble Metals Doping.

ZrCoBi-based half-Heuslers have great potential in power generation applications because of their high thermoelectric performance in both p- and n-type constituents. In this work, n-type ZrCoBi with improved thermoelectric performance has been realized by intensifying the phonon scattering via noble metal doping, e.g., Pd and Pt doping. The carrier concentration was effectively tuned to the optimal range, and the lattice thermal conductivity was greatly suppressed via the strong strain field and mass fluctuation scattering brought about by the large difference in atomic size and mass between Pd or Pt and Co. Consequently, the state-of-art figure of merit zT ∼1 was achieved in Pd- or Pt-doped ZrCoBi. In addition, the average zTavg values for ZrCo0.95Pd0.05Bi and ZrCo0.925Pt0.075Bi have reached 0.58 and 0.51, respectively, which are higher than those of most of the reported n-type ZrCoBi-based and ZrCoSb-based half-Heusler alloys.

Polaron interfacial entropy as a route to high thermoelectric performance in DAE-doped PEDOT:PSS films.

Enhancing the thermoelectric transport properties of conductive polymer materials has been a long-term challenge, in spite of the success seen with molecular doping strategies. However, the strong coupling between the thermopower and the electrical conductivity limits thermoelectric performance. Here, we use polaron interfacial occupied entropy engineering to break through this intercoupling for a PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate)) thin film by using photochromic diarylethene (DAE) dopants coupled with UV-light modulation. With a 10-fold enhancement of the thermopower from 13.5 μV K-1 to 135.4 μV K-1 and almost unchanged electrical conductivity, the DAE-doped PEDOT:PSS thin film achieved an extremely high power factor of 521.28 μW m-1 K-2 from an original value of 6.78 μW m-1 K-2. The thermopower was positively correlated with the UV-light intensity but decreased with increasing temperature, indicating resonant coupling between the planar closed DAE molecule and PEDOT. Both the experiments and theoretical calculations consistently confirmed the formation of an interface state due to this resonant coupling. Interfacial entropy engineering of polarons could play a critical role in enhancing the thermoelectric performance of the organic film.

Grand herringbone architecture securing the high thermoelectric performance of GeTe

This work demonstrates a unique two-stage cooling procedure for synthesizing high-performance thermoelectric (TE) GeTe alloys with a maximum zT value of 2.0 (720 K) without assistance of chemical doping. The efficiency breakthrough stems from the grand herringbone structure resulted from the annealing step intentionally inserted near the phase transition temperature Tc of GeTe when cooling. Compared with the samples prepared by conventional methods, the two-stage cooled GeTe processes not only lower carrier concentrations p (4.7–5.3 × 1020 cm−3) but also higher mobility (∼125 cm2 V−1 s−1) that prevent electrical conductivity and TE power factor from declining as p decreases. The twinned domains in the two-stage cooled samples are further analyzed and classified by the compatibility theory. Our calculations show that the twinning structures in the two-stage cooled samples fit well with the compatibility criterion for twin variant selections and are thermodynamically in a stable state. These results not only attest to the efficacy of using the two-stage cooling procedure to manipulate the microstructures of GeTe but also provides a new paradigm for optimizing the performance of GeTe-based TE materials.

Developing a Multiband Electronic Band Structure Model and Predictive Maps for Bismuth-Rich Mg3(Sb1-xBix)2 Thermoelectric Materials.

In recent years, bismuth-rich Mg3(Sb1-xBix)2 (x = 0.5-0.8) compositions have generated significant interest due to their excellent thermoelectric (TE) performance near room temperature, making them potential applicants for recovery of low-grade waste heat. The superior performance in these materials is due to its complex electronic band structure (EBS) with presence of multiple near degenerate bands close to the conduction band edge. The position and curvature of these bands strongly depend on the alloy composition, doping amount as well as temperature. Thus, identifying optimal material compositions to get the best TE performance depends on an understanding of the temperature dynamics of EBS and forms the objective of this work. Mg3Sb0.6Bi1.4 (x = 0.7) is chosen for this study due to its reported high near room temperature performance, and compositions with varying doping concentrations (Te used as dopant) have been synthesized. EBS parameters like effective mass and deformation potential of bands, interband separation and band gap values have been estimated using a recently developed refinement approach. Refinement results indicate that the interband separation between conduction bands to be a function of both temperature and doping concentration. Further, thermal conductivity (κ) was estimated for all of the compositions. Utilizing the EBS and κ information, predictive 3D maps indicating the variation in zT (TE figure of merit) with doping concentration and temperature have been generated. The 3D maps reveal an interesting surface topography with a broad peak zT region. This observation explains why these materials have high TE performance and are less sensitive to doping inhomogeneities. Our results provide detailed EBS information and fundamental insights on the TE properties of Mg3Sb0.6Bi1.4. Further, the proposed technique can be utilized to probe other Mg3(Sb1-xBix)2 compositions and TE materials.

High entropy effect on thermoelectric properties of the nonequilibrium cubic phase of AgBiSe2-2xSxTex with x = 0-0.6.

Silver bismuth diselenide (AgBiSe2) has attracted much attention as an efficient thermoelectric material owing to its low thermal conductivity. However, AgBiSe2 exhibits multiple crystal structural transitions with temperature, and high thermoelectric performance was realized only in the high-temperature cubic phase. We previously reported the stabilization of the cubic phase in AgBiSe2-2xSxTex with x = 0.6-0.8 at room temperature using a high-entropy-alloy (HEA) approach. In this paper, we succeeded in stabilizing the cubic phase in AgBiSe2-2xSxTex with x = 0-0.6 using an ice-quenching method and investigated the HE effect on thermoelectric properties below room temperature to avoid the emergence of a hexagonal phase above room temperature. Cubic AgBiSe2-2xSxTex exhibited n-type conductivity at 10-300 K. The increase in electrical conductivity is due to a combined effect of increased charge density and electron mobility, depending on the sample. It is evident that charge carrier density strongly increases in the x = 0.4 sample while the mobility remains roughly the same compared to the x = 0.0 sample. In x = 0.2 and 0.6 samples, electronic conductivity increases primarily due to enhanced mobility. S and Te substitutions induced a variation in the band structure, resulting in carrier mobility enhancement. Furthermore, thermal conductivity showed reduction tendency with increasing amounts of S and Te due to the enhancement of phonon scattering. Simultaneous electronic conductivity increase and thermal conductivity reduction resulted in the systematic improvement of ZT values for HE-type cubic AgBiSe2-2xSxTex.

Robust nanotube-based nanosensor designed for the detection of explosive molecules.

The adequate determination and detection of explosive molecules is key to introducing improvements in areas related to safety, whose progress depends on an adequate and rapid determination of dangerous substances. To detect explosives down to the molecular level and accurately discriminate between different but somehow similar substances, it is necessary to design sensors that can differentiate them uniquely and efficiently. In this study, we present a new generation nanoscale sensor based on carbon nanotubes with an adapted nanopore shape that is capable of effectively discriminating between five types of explosive compounds (TATP, RDX, PENT, HMX and DNT). We show that the interaction of each compound with the walls of the nanotubes induces changes in transmission and current that allows clear differentiation of each type of molecule. Interestingly, the transport properties do not depend on the orientation of the molecules within the nanopore in most cases, making it a robust device with high reproducibility and stability. The results also show that these systems can lead to relatively high thermoelectric performances and, furthermore, the Seebeck coefficient can be used to discriminate between them.

Band and microstructure engineering toward high thermoelectric performance in SnTe

AgBiS2 alloying combining with Ge doping facilitates the band convergence and suppresses the phonon transport, leading to a competitive zT of 1.6 in SnTe.

Mn-doping method boosts Se doping concentration in Cu2S towards high thermoelectric performance

The Mn, Se co-doping method increases the doping limit of Se. High-concentration Se doping not only optimises the electrical transport but also decreases the lattice's thermal conductivity, resulting in the zT value of 0.97 at 723 K.

High thermoelectric performance in p-type ZnSb upon increasing Zn vacancies: an experimental and theoretical study

Zn-vacancy driven high power factor and strong coupling of acoustic and optical modes enhance the zT in p-type ZnSb.