Showing papers on "Seebeck coefficient published in 2021"

Carbon allotrope hybrids advance thermoelectric development and applications

Abstract: As an emission-free energy conversion technology, thermoelectric technology has been considered an essential component in solving the global energy crisis. Carbon allotrope hybrids, with relatively low cost, high performance and engineerable mechanical strength and flexibility, are attracting increasing research interest. The key challenge of developing carbon allotrope hybrid thermoelectric applications lies in material performance enhancement, which is further restricted by enhancing the electrical performance, refraining the lattice thermal conductivity and engineering the mechanical properties. Compositing carbon allotropes to enhance electrical transport properties should be mainly attributed to the material orientation effect which increases the carrier mobility or to the energy filtering effect which increases the Seebeck coefficient. The reduced lattice thermal conductivity due to the formation of carbon allotrope hybrid is derived from various additional phonon scattering features. Furthermore, carbon allotrope-compositing is also effective in enhancing the mechanical properties of thermoelectric materials, which can potentially further widen the variety of applications of these materials. A key opportunity in better utilizing the flexibility of carbon materials is deploying them as stents. Carbon allotrope hybrids can provide a pathway such that rigid thermoelectric materials can be designed into flexible thermoelectric materials. Finally, we point out future research directions for carbon-hybrid thermoelectric materials.

Entropy engineering promotes thermoelectric performance in p-type chalcogenides.

Abstract: We demonstrate that the thermoelectric properties of p-type chalcogenides can be effectively improved by band convergence and hierarchical structure based on a high-entropy-stabilized matrix. The band convergence is due to the decreased light and heavy band energy offsets by alloying Cd for an enhanced Seebeck coefficient and electric transport property. Moreover, the hierarchical structure manipulated by entropy engineering introduces all-scale scattering sources for heat-carrying phonons resulting in a very low lattice thermal conductivity. Consequently, a peak zT of 2.0 at 900 K for p-type chalcogenides and a high experimental conversion efficiency of 12% at ΔT = 506 K for the fabricated segmented modules are achieved. This work provides an entropy strategy to form all-scale hierarchical structures employing high-entropy-stabilized matrix. This work will promote real applications of low-cost thermoelectric materials. The synergism of entropy engineering and the typical optimization mechanisms in high-entropy-stabilized chalcogenide is unknown. Here, the authors find high-entropy-stabilized composition works as a promising matrix of applying synergistic effect to realize high thermoelectric performance.

Seebeck-driven transverse thermoelectric generation.

Abstract: When a temperature gradient is applied to a closed circuit comprising two different conductors, a charge current is generated via the Seebeck effect1. Here, we utilize the Seebeck-effect-induced charge current to drive 'transverse' thermoelectric generation, which has great potential for energy harvesting and heat sensing applications owing to the orthogonal geometry of the heat-to-charge-current conversion2-9. We found that, in a closed circuit comprising thermoelectric and magnetic materials, artificial hybridization of the Seebeck effect into the anomalous Hall effect10 enables transverse thermoelectric generation with a similar symmetry to the anomalous Nernst effect11-27. Surprisingly, the Seebeck-effect-driven transverse thermopower can be several orders of magnitude larger than the anomalous-Nernst-effect-driven thermopower, which is clearly demonstrated by our experiments using Co2MnGa/Si hybrid materials. The unconventional approach could be a breakthrough in developing applications of transverse thermoelectric generation.

Macroscopic weavable fibers of carbon nanotubes with giant thermoelectric power factor.

Abstract: Low-dimensional materials have recently attracted much interest as thermoelectric materials because of their charge carrier confinement leading to thermoelectric performance enhancement. Carbon nanotubes are promising candidates because of their one-dimensionality in addition to their unique advantages such as flexibility and light weight. However, preserving the large power factor of individual carbon nanotubes in macroscopic assemblies has been challenging, primarily due to poor sample morphology and a lack of proper Fermi energy tuning. Here, we report an ultrahigh value of power factor (14 ± 5 mW m−1 K−2) for macroscopic weavable fibers of aligned carbon nanotubes with ultrahigh electrical and thermal conductivity. The observed giant power factor originates from the ultrahigh electrical conductivity achieved through excellent sample morphology, combined with an enhanced Seebeck coefficient through Fermi energy tuning. We fabricate a textile thermoelectric generator based on these carbon nanotube fibers, which demonstrates high thermoelectric performance, weavability, and scalability. The giant power factor we observe make these fibers strong candidates for the emerging field of thermoelectric active cooling, which requires a large thermoelectric power factor and a large thermal conductivity at the same time. Preserving the large power factor of carbon nanotubes is challenging, due to poor sample morphology and a lack of proper Fermi energy tuning. Here, the authors achieve a value of power factor of 14 ± 5 mW m−1 K−2 originating from the preserved conductivity and the ability to tune Fermi energy.

Electrical and thermal generation of spin currents by magnetic bilayer graphene.

Abstract: Ultracompact spintronic devices greatly benefit from the implementation of two-dimensional materials that provide large spin polarization of charge current together with long-distance transfer of spin information. Here spin-transport measurements in bilayer graphene evidence a strong spin–charge coupling due to a large induced exchange interaction by the proximity of an interlayer antiferromagnet (CrSBr). This results in the direct detection of the spin polarization of conductivity (up to 14%) and a spin-dependent Seebeck effect in the magnetic graphene. The efficient electrical and thermal spin–current generation is the most technologically relevant aspect of magnetism in graphene, controlled here by the antiferromagnetic dynamics of CrSBr. The high sensitivity of spin transport in graphene to the magnetization of the outermost layer of the adjacent antiferromagnet, furthermore, enables the read-out of a single magnetic sublattice. The combination of gate-tunable spin-dependent conductivity and Seebeck coefficient with long-distance spin transport in a single two-dimensional material promises ultrathin magnetic memory and sensory devices based on magnetic graphene. Graphene promises long-distance transfer of spin information with concomitant high charge carrier mobility. Proximity coupling of bilayer graphene with the 2D interlayer antiferromagnetic CrSBr now enables active generation of spin currents in graphene both electrically and thermally.

n-type charge transport in heavily p-doped polymers

Abstract: It is commonly assumed that charge-carrier transport in doped π-conjugated polymers is dominated by one type of charge carrier, either holes or electrons, as determined by the chemistry of the dopant. Here, through Seebeck coefficient and Hall effect measurements, we show that mobile electrons contribute substantially to charge-carrier transport in π-conjugated polymers that are heavily p-doped with strong electron acceptors. Specifically, the Seebeck coefficient of several p-doped polymers changes sign from positive to negative as the concentration of the oxidizing agents FeCl3 or NOBF4 increase, and Hall effect measurements for the same p-doped polymers reveal that electrons become the dominant delocalized charge carriers. Ultraviolet and inverse photoelectron spectroscopy measurements show that doping with oxidizing agents results in elimination of the transport gap at high doping concentrations. This approach of heavy p-type doping is demonstrated to provide a promising route to high-performance n-type organic thermoelectric materials. A broad range of characterization techniques is used to understand the dominant electron conduction in various p-type doped π-conjugated polymers, which show p-type and n-type thermoelectric power factors depending on the dopant concentration.

Transverse thermoelectric generation using magnetic materials

Abstract: The transverse thermoelectric effect refers to the conversion of a temperature gradient into a transverse charge current, or vice versa, which appears in a conductor under a magnetic field or in a magnetic material with spontaneous magnetization. Among such phenomena, the anomalous Nernst effect in magnetic materials has been receiving increasing attention from the viewpoints of fundamental physics and thermoelectric applications owing to the rapid development of spin caloritronics and topological materials science. In this research trend, a conceptually different transverse thermoelectric conversion phenomenon appearing in thermoelectric/magnetic hybrid materials has been demonstrated, enabling the generation of a large transverse thermopower. Here, we review the recent progress in fundamental and applied studies on the transverse thermoelectric generation using magnetic materials. We anticipate that this perspective will further stimulate research activities on the transverse thermoelectric generation and lead to the development of next-generation thermal energy harvesting and heat-flux sensing technologies.

Giant negative thermopower of ionic hydrogel by synergistic coordination and hydration interactions.

Abstract: The design of ultrasensitive ionic thermopiles is important for low-grade heat collection and temperature sensing. However, high-quality ionic thermoelectric materials with negative thermopower hav...

Demonstration of valley anisotropy utilized to enhance the thermoelectric power factor

Abstract: Valley anisotropy is a favorable electronic structure feature that could be utilized for good thermoelectric performance. Here, taking advantage of the single anisotropic Fermi pocket in p-type Mg3Sb2, a feasible strategy utilizing the valley anisotropy to enhance the thermoelectric power factor is demonstrated by synergistic studies on both single crystals and textured polycrystalline samples. Compared to the heavy-band direction, a higher carrier mobility by a factor of 3 is observed along the light-band direction, while the Seebeck coefficient remains similar. Together with lower lattice thermal conductivity, an increased room-temperature zT by a factor of 3.6 is found. Moreover, the first-principles calculations of 66 isostructural Zintl phase compounds are conducted and 9 of them are screened out displaying a pz-orbital-dominated valence band, similar to Mg3Sb2. In this work, we experimentally demonstrate that valley anisotropy is an effective strategy for the enhancement of thermoelectric performance in materials with anisotropic Fermi pockets.

Quantifying charge carrier localization in chemically doped semiconducting polymers.

Abstract: Charge transport in semiconducting polymers ranges from localized (hopping-like) to delocalized (metal-like), yet no quantitative model exists to fully capture this transport spectrum and its dependency on charge carrier density. In this study, using an archetypal polymer-dopant system, we measure the temperature-dependent electrical conductivity, Seebeck coefficient and extent of oxidation. We then use these measurements to develop a semi-localized transport (SLoT) model, which captures both localized and delocalized transport contributions. By applying the SLoT model to published data, we demonstrate its broad utility. We are able to determine system-dependent parameters such as the maximum localization energy of the system, how this localization energy changes with doping, the amount of dopant required to achieve metal-like conductivity and the conductivity a system could have in the absence of localization effects. This proposed SLoT model improves our ability to predict and tailor electronic properties of doped semiconducting polymers.

Band degeneracy enhanced thermoelectric performance in layered oxyselenides by first-principles calculations

Abstract: Band degeneracy is effective in optimizing the power factors of thermoelectric (TE) materials by enhancing the Seebeck coefficients In this study, we demonstrate this effect in model systems of layered oxyselenide family by the density functional theory (DFT) combined with semi-classical Boltzmann transport theory TE transport performance of layered LaCuOSe and BiCuOSe are fully compared The results show that due to the larger electrical conductivities caused by longer electron relaxation times, the n-type systems show better TE performance than p-type systems for both LaCuOSe and BiCuOSe Besides, the conduction band degeneracy of LaCuOSe leads to a larger Seebeck coefficient and a higher optimal carrier concentration than n-type BiCuOSe, and thus a higher power factor The optimal figure of merit (ZT) value of 146 for n-type LaCuOSe is 22% larger than that of 12 for n-type BiCuOSe This study highlights the potential of wide band gap material LaCuOSe for highly efficient TE applications, and demonstrates that inducing band degeneracy by cations substitution is an effective way to enhance the TE performance of layered oxyselenides

Highly Stretchable Carbon Nanotubes/Polymer Thermoelectric Fibers.

Abstract: Thermoelectric (TE) technology provides a new way to directly harvest and convert the heat continuously released from the human body. The greatest challenge for TE materials applied in wearable TE generators is compatible with the constantly changing morphology of the human body while offering a continuous and stable power output. Here, a stretchable carboxylic single-walled carbon nanotube (SWNT)-based TE fiber is prepared by an improved wet-spinning method. The stable Seebeck coefficient of the annealed carboxylic SWNT-based TE fiber is 44 μV/K even under the tensile strain of ∼30%. Experimental results show that the fiber can continue to generate constant TE potential when it is changed to various shapes. The new stretchable TE fiber has a larger Seebeck coefficient and more stretchability than existing TE fibers based on the Seebeck effect, opening a path to using the technology for a variety of practical applications.

Optimizing Electronic Quality Factor toward High-Performance Ge 1- x - y Ta x Sb y Te Thermoelectrics: The Role of Transition Metal Doping

Abstract: Owing to high intrinsic figure-of-merit implemented by multi-band valleytronics, GeTe-based thermoelectric materials are promising for medium-temperature applications. Transition metals are widely used as dopants for developing high-performance GeTe thermoelectric materials. Herein, relevant work is critically reviewed to establish a correlation among transition metal doping, electronic quality factor, and figure-of-merit of GeTe. From first-principle calculations, it is found that Ta, as an undiscovered dopant in GeTe, can effectively converge energy offset between light and heavy conduction band extrema to enhance effective mass at high temperature. Such manipulation is verified by the increased Seebeck coefficient of synthesized Ge1- x - y Tax Sby Te samples from 160 to 180 µV K-1 at 775 K upon doping Ta, then to 220 µV K-1 with further alloying Sb. Characterization using electron microscopy also reveals the unique herringbone structure associated with multi-scale lattice defects induced by Ta doping, which greatly hinder phonon propagation to decrease thermal conductivity. As a result, a figure-of-merit of ≈2.0 is attained in the Ge0.88 Ta0.02 Sb0.10 Te sample, reflecting a maximum heat-to-electricity efficiency up to 17.7% under a temperature gradient of 400 K. The rationalized beneficial effects stemming from Ta doping is an important observation that will stimulate new exploration toward high-performance GeTe-based thermoelectric materials.

Thermoelectric Converters Based on Ionic Conductors.

Abstract: Thermoelectric materials represent a new paradigm for harvesting low-grade heat, which would otherwise be dissipated to the environment uselessly. Relative to conventional thermoelectric materials generally composed of semiconductors or semi-metals, ionic thermoelectric materials are rising as an alternative choice which exhibit higher Seebeck coefficient and lower thermal conductivity. The ionic thermoelectric materials own a completely different thermoelectric conversion mechanism, in which the ions do not enter the electrode but rearrange on the electrode surface to generate a voltage difference between the hot and cold electrodes. This unique character has inspired worldwide interests on the design of ionic-type thermoelectric converters with attractive advantages of high flexibility, low cost, limited environmental pollution, and self-healing capability. Referring to the categories of ionic thermoelectric conversion, some representative ionic thermoelectric materials with their respective characteristics are summarized in this minireview. In addition, examples of applying ionic thermoelectric materials in supercapacitors, wearable devices, and fire warning system are also discussed. Insight into the challenges for the further development of ionic thermoelectric materials is finally provided.

First-principles study of lead-free double perovskites Rb2TeX6 (X = Cl, Br, and I) for solar cells and renewable energy

Abstract: The double perovskites are emerging materials for renewable energy and potential alternatives to the organic and lead-based solar cells. In the present work, we have elaborated on the optoelectronic, thermoelectric, and thermodynamic properties of new double perovskites Rb2TeX6 (X = Cl, Br, and I). The tolerance factor and enthalpy of formation have been reported for structural and thermodynamic stability. The Passion and Pugh's ratio has been calculated from elastic constants to distinguish the brittle and ductile character. Furthermore, the highly precise Tran-Blaha modified Becke-Johnson potential exchange-correlation potential is executed to elaborate the band gaps. The band gaps exhibit from ultraviolet to visible region (3.2 eV–1.80), which increases the performance of these double perovskites for optoelectronic devices and solar cell applications. The optical behavior has been elucidated by the absorption of photon energy, polarization, and optical losses. The absorption is shifted to lower energy due to tuning of the band gap by the replacement Cl with Br and I ions. Thermoelectric behavior has been described by electrical and thermal conductivities, Seebeck coefficient, and figure of merit, while thermodynamic performance is attributed by Debye temperature and Navier sound velocities.

Two-dimensional carbon nitride C6N nanosheet with egg-comb-like structure and electronic properties of a semimetal.

Abstract: In this study, the structural, electronic and optical properties of theoretically predicted C6N monolayer structure are investigated by means of Density Functional Theory-based First-Principles Calculations. Phonon band dispersion calculations and molecular dynamics simulations reveal the dynamical and thermal stability of C6N single-layer structure. We found out that the C6N monolayer has large negative in-plane Poissons ratios along both X and Y direction and the both values are almost four times that of the famous-pentagraphene. The electronic structure shows that C6N monolayer is a semi-metal and has a Dirac-point in the BZ. The optical analysis using the RPA method constructed over HSE06 illustrates that the first peak of absorption coefficient of the C6N monolayer along all polarizations is located in the IR range of spectrum, while the second absorption peak occurs in the visible range, which suggests its potential applications in optical and electronic devices. Interestingly, optically anisotropic character of this system is highly desirable for the design of polarization-sensitive photodetectors. Thermoelectric properties such as Seebeck coefficient, electrical conductivity, electronic thermal conductivity and power factor are investigated as a function of carrier doping at temperatures 300 K, 400 K, and 500 K. In general, we predict that the C6N monolayer could be a new platform for study of novel physical properties in two-dimensional semi-metal materials, which may provide new opportunities to realize high-speed low-dissipation.

Versatile Vanadium Doping Induces High Thermoelectric Performance in GeTe via Band Alignment and Structural Modulation

Abstract: Owing to the moderate energy offset between light and heavy band edges of the rock‐salt structured GeTe, its figure‐of‐merit (ZT) can be enhanced by the rational manipulation of electronic band structures. In this study, density functional theory calculations are implemented to predict that V is an effective dopant for GeTe to enlarge the bandgap and converge the energy offset, which suppresses the bipolar conduction and increases the effective mass. Experimentally, V‐doped Ge1−xVxTe samples are demonstrated to have an enhanced Seebeck coefficient from ≈163 to ≈191 µV K−1. Extra alloying with Bi in Ge1−x−yVxBiyTe can optimize the carrier concentration to further enhance the Seebeck coefficient up to ≈252 µV K−1, plus an outstanding power factor of ≈43 µW cm−1 K−2. Comprehensive structural characterization results also verify the refinement of grain size by V‐doping, associated with highly dense grain boundaries, stacking faults, nanoprecipitates, and point defects, reinforcing the wide‐frequency phonon scattering and in turn, securing an ultralow thermal conductivity of ≈0.59 W m−1 K−1. As a result, the Ge0.9V0.02Bi0.08Te sample shows a peak ZT of >2.1 at 773 K, with an average plateaued average ZT of >2.0 from 623 and 773 K, which extends better thermoelectric behavior for GeTe over a wider temperature range. This study clarifies the multiple benefits of V‐doping in GeTe‐based derivatives and provides a framework for a new‐type of high‐performance middle‐temperature thermoelectric material.

Ionic thermoelectric materials for near ambient temperature energy harvesting

Abstract: Ionic thermoelectric (i-TE) materials, using ions as the energy carrier, can generate a voltage under a temperature difference, bearing similarities to the Seebeck effect of electrons and holes in solid-state materials. Recent experiments have demonstrated large thermopower of quasi-solid-state i-TE materials, which are attractive for harvesting ambient heat as large enough voltage can be generated under a small temperature difference to match the voltage input needs of sensors for internet-of-things applications. In this perspective article, we discuss similarities and differences of i-TE materials from electronic-based thermoelectric materials and also different i-TE thermoelectric effects including the thermodiffusion (Soret) effect and the thermogalvanic effect, in which the latter includes redox reaction entropy and the Soret effect. Strategies to improve performances of materials and devices are elaborated, together with needs for future research in understanding microscopic origins of different effects.