Transverse Thermoelectric Research Articles

Assessment of Artificial Anisotropic Materials for Transverse Thermoelectric Generators

By alternately stacking layers of two materials that differ in their Seebeck coefficient and electrical and thermal conductivity, a composite material with artificial anisotropy of thermal and electrical transport properties is formed. Due to the transverse Seebeck effect, a thermoelectric (TE) voltage is generated perpendicular to a temperature gradient ΔT, that is applied at a certain angle φ with respect to the stacked layers (0° < φ < 90°). The TE properties of layered artificial anisotropic materials are described analytically using existing concepts and extending the available definitions to develop a consistent image of anisotropic media for TE energy generation. Based on these analytical descriptions, the TE performance of ceramic oxide–metal composites and transverse TE generators (TTEG) made of them are numerically calculated and presented in contour plots. These so‐called micro‐ and macro‐Babin plots map the influence of internal geometric parameters, i.e., the layer thickness ratio and the angle φ of the applied temperature gradient with respect to the stacked layers. Based on these diagrams, the optimal TTEG geometry can be narrowed down in a simple and fast way. In addition, the diagrams are used for a material screening to evaluate the suitability of different oxide ceramics for use in a TTEG.

Large thermoelectric transport in magnetically coupled CrI3/1T-MoS2 vdW heterostructure via spin–charge interconversion

Low-dimensional materials with prominent thermoelectric (TE) effect play a pivotal role in realizing state-of-the-art nanoscale TE devices. The fusion of TE effect with the magnetism through seamless integration of TE and magnetic materials in the 2D limit offers access to control longitudinal as well as transverse TE properties via magnetic proximity effect. Herein, we design a van der Waals (vdW) heterostructure of metallic 1T-MoS2 with promising TE properties and a layer-dependent magnetic CrI3 material. The result highlights exotic electronic and magnetic configurations of the designed monolayer-CrI3/1T-MoS2 vdW heterostructure, which show magnetically-coupled TE characteristics. The observed remarkable magnetic proximity stems from large magnetic anisotropy energy and spin polarization, which are found to be 2.21 meV Cr−1 and 12.30%, respectively. To this end, the semiconducting CrI3 layer with intrinsic magnetism leads to efficient control and tunability of the observed spin-correlated anomalous Nernst effect. Moreover, a large dimensionless figure of merit of ∼6 and a power factor of ∼3.8×1011/τ∘ Wm−1K−2s−1 near the Fermi level at 300 K endorse the rejuvenated TE effect. The strong relativistic spin–orbit coupling validates the significant correlation of TE properties with intrinsic magnetic configuration. The present study underscores the significance of the magnetic proximity-governed TE effect in vdW heterostructures to engineer low-dimensional TE devices.

Ca3Co4O9-based transverse thermoelectric heat flux sensors with high sensitivity and fast response time

High-temperature thin film heat flux sensors have been fabricated by growing c-axis tilted epitaxial Ca3Co4O9 (CCO) thin films on 5° vicinal cut LaAlO3 (001) single crystal substrates. The layered structure of Ca3Co4O9 yields the significant Seebeck coefficient anisotropy between the ab-plane and c-axis, which could generate a voltage of the heat flux sensor via the transverse thermoelectric (TTE) effect of thin films. A sensitivity of 27.7 μV/(kW/m2) has been determined in such 5° tilted Ca3Co4O9 thin films, which is much larger than other reported ones based on the TTE effect from various materials. After a thermal treatment at 900 °C in air for 10 min, the sensitivity of such heat flux sensors is almost non-variable, which indicates that the temperature resistance of the CCO-based TTE heat flux sensor is as high as 900 °C. In addition, a fast response time of 45 μs has been identified in such CCO-based TTE heat flux sensors. These results demonstrate that the CCO is a promising candidate to manufacture the TTE heat flux sensors with the superiorly comprehensive performance, including the high temperature resistance, high sensitivity, and fast response.

Large transverse magneto-thermoelectric effect in narrow-band-gap polycrystalline Ag2−δTe

Polycrystalline Ag2−δTe is promising for transverse energy conversion applications due to its high transverse thermoelectric performance.

Hybrid Transverse Magneto‐Thermoelectric Cooling in Artificially Tilted Multilayers

AbstractIn artificially tilted multilayers comprising two different conductors that are alternately and obliquely stacked, transverse thermoelectric conversion occurs, in which charge and heat currents are interconverted in the orthogonal direction. Although transverse thermoelectric conversion also occurs in homogeneous materials as an intrinsic transport phenomenon owing to the effects of magnetic fields, magnetization, and spins on conduction carriers, such magneto‐thermoelectric effects are investigated independently of thermoelectrics for artificially tilted multilayers. Here, this study shows that the synergy of these different principles improves the performance of transverse thermoelectric conversion. Using lock‐in thermography techniques, transverse thermoelectric conversion processes are visualized in artificially tilted multilayers and the experiments clarify how nonuniform charge currents are converted into orthogonal heat currents. Through the measurements of temperature change under magnetic fields, the contributions of the magneto‐thermoelectric effects are quantified in the artificially tilted multilayers and magnetically enhanced hybrid transverse thermoelectric cooling is demonstrated. By replacing one of the conductors in the multilayer with permanent magnets, the same functionality is obtained even in the absence of magnetic fields, paving the way for the creation of “thermoelectric permanent magnets” that exhibit efficient transverse thermoelectric conversion together with spontaneous magnetization. This study provides a new material design guideline for transverse thermoelectrics.

Measurements of electrical resistivity and Seebeck coefficient for disc-shaped samples

Herein we develop a methodology to measure the resistivity of discs by deriving a mathematic formula between resistivity and resistance from solving the electrostatic Laplace equation in polar coordinates. Then the resistivity and Seebeck coefficient of disc samples of p- and n-type bismuth telluride are measured experimentally either in nitrogen or helium atmosphere. The validity of Seebeck coefficient is demonstrated by the excellent linearity between Seebeck voltages and temperature differences. The bar samples are also measured for comparison. Finite element simulation is utilized to display the two-dimensional potentials and currents and have an error analysis. Furthermore, the resistivity error due to the probe distance error is discussed analytically based on the mathematic formula, and the probe distance can be optimized to minimize the resistivity error. The disclosed approach would extend the applicability of the present instruments to the disc-shaped samples and be useful in the emerging transverse thermoelectricity.

High sensitivity and fast response self-powered PbSe ultraviolet pulsed photodetectors based on the transverse thermoelectric effect

Light-induced transverse thermoelectric (TTE) effect has attracted extensive attentions because of the great potential application in self-powered photodetectors. In this paper, we report the ultraviolet pulsed light-induced TTE effect in the inclined PbSe thin films. Without any external voltage applied, gigantic open-circuit voltage signals with ultrafast responses in the order of nanoseconds were obtained when the film surface was irradiated by a 308 nm pulsed laser. The voltage sensitivity Rs can reach up to ∼7.75 V/mJ and the figure of merit Fm is as high as ∼350 mV/ns, both of which are higher than the best values reported in most of complex oxide thin films. In addition, the experimental results reveal that the polarity of the induced voltage will immediately reverse when the film is irradiated from the substrate side and the amplitudes of the voltage are linearly proportional to sin2α (α is the inclination angle of the films). Here, the anisotropy of the Seebeck coefficient is used to explain the origin of the TTE voltage. The results demonstrate the great potential application of PbSe thin film for the high sensitivity and fast response self-powered ultraviolet pulsed photodetectors.

Polycrystalline bismuth nanowire networks for flexible longitudinal and transverse thermoelectrics.

This paper reports on the preparation and the characterization of structural, electrical and thermoelectric properties of nanocomposite films formed from three-dimensional networks of polycrystalline bismuth (Bi) nanowires (NWs). The samples were fabricated by electrodeposition within polycarbonate (PC) templates with crossed cylindrical nanopores, yielding self-supported networks of Bi crossed nanowires (CNWs) with mean diameter values ranging from 23 nm to 230 nm. Temperature changes in electrical resistance and thermopower were studied by considering electric and thermal currents flowing in the plane of the films. While the values of the Seebeck coefficient are close to those of polycrystalline Bi for diameters greater than 100 nm, a progressive decrease in thermopower appears at smaller diameters, due to an increasing contribution of surface charge carriers as the diameter decreases. Transverse thermoelectricity based on the Nernst effect was also demonstrated on a network of Bi CNWs 230 nm in diameter. Such hierarchical architectures based on Bi CNWs are extremely robust, offering a reliable solution for the next generation of flexible thermoelectric devices.

Proximity induced longitudinal and transverse thermoelectric response in graphene-ferromagnetic CrBr3 vdW heterostructure

The integration of longitudinal and transverse thermoelectric (TE) fosters various new opportunities in tuning the charge transport behaviour and opens a platform for efficient thermopower devices. The presence of asymmetric electronic structure supposed to accomplish large thermopower and electronic figure of merit. Herein, we investigate magnetic proximity coupled longitudinal and transverse TE behaviour in heterostructure of monolayer semimetal, graphene and a monolayer ferromagnet, CrBr3 under the framework of ab initio-based calculations and employed constant relaxation time approximation (CRTA).The integrated density of states is elevated and asymmetric near Fermi energy region due to seamless proximity integration, depicting mixed character of graphene and CrBr3. The asymmetric nature of electronic structure significantly affects the Seebeck coefficients (S) and electrical conductivity (σ/τ) of heterostructure. The consistent step-like conductance spectrum influences interfacial polarization due to agile proximity integration. The magnitude of Seebeck coefficient (S) is found to be 653 µV K−1 near Fermi level. The heterostructure observes higher electrical conductivity and power factor in n-type region of the order of 106 S m−1 and 1020 cm−3 at room temperature. The dimensionless electronic figure of merit (zT e) advocates the heterostructure system to be an ideal TE material. Alongside longitudinal TE, we also find the heterostructure system is sensitive to anomalous Nernst effect (ANE) (transverse TE) with oscillatory nature. The Seebeck and ANE shows high degree of tunability with applied external electric field. The synergistic existence of Seebeck and ANE due to proximity integration in van der Waals atomic crystal at room temperature will provide realistic approach to experimentally fabricate and develop real-time thermopower devices.

Large Magneto-Transverse and Longitudinal Thermoelectric Effects in the Magnetic Weyl Semimetal TbPtBi.

Magnetic topological semimetals provide new opportunities for power generation and solid-state cooling based on thermoelectric (TE) effect. The interplay between magnetism and nontrivial band topology prompts the magnetic topological semimetals to yield strong transverse TE effect, while the longitudinal TE performance is usually poor. Herein, it is demonstrated that the magnetic Weyl semimetal TbPtBi has high value for both transverse and longitudinal thermopower with large power factor (PF). At 300K and 13.5Tesla, the transverse thermopower and PF reach up to 214µVK-1 and 35µWcm-1 K-2 , respectively, which are comparable to those of state-of-the-art TE materials. Combining first-principles calculations, longitudinal magnetoresistance and planar Hall resistance measurements, and two-band model fitting, the large transverse thermopower and PF are attributed to both bipolar effect and large Hall angle. Moreover, the imperfectly compensated charge carriers and large transverse magnetoresistance induce the maximum magneto-longitudinal thermopower of 251µVK-1 with a PF of 24µWcm-1 K-2 at 150K and 13.5Tesla, which is two times higher than that at zero magnetic field. This work demonstrates the great potential of topological semimetals for TEs and offers a new excellent candidate for magneto-TEs.

High-frequency response heat flux sensor based on the transverse thermoelectric effect of inclined La1−xCaxMnO3 films

The heat flux sensors composed of inclined epitaxial La1−xCaxMnO3 (LCMO) films have been fabricated on c-axis miscut SrTiO3 (001) single crystal substrates. Based on the transverse thermoelectric (TTE) effect originated from the anisotropic Seebeck coefficient between ab-plane and c-axis of LCMO films, the obvious response voltages perpendicular to the temperature gradient are detected, and their amplitudes show good linearity with heat fluxes. The sensitivities of heat flux sensors are 8.64, 11.88, 12.66, and 14.25 μV/(kW/m2) for LCMO films with inclined 12°, 18°, 20°, and 24°, respectively. Additionally, the response frequency of the sensor reaches ∼300 kHz at −3 dB, which is comparable with the reported TTE heat flux sensors composed of YBa2Cu3O7-δ. The above results demonstrate the potential of the LCMO-based TTE heat flux sensors in high-frequency heat flux measurement.

Ultra-broadband light detection based on the light-induced transverse thermoelectric effect of epitaxial PbSe thin films with inclined structure

PbSe is a simple binary compound that has been studied extensively for use as a promising moderate-temperature thermoelectric material. In this Letter, we report the observation of the light-induced transverse thermoelectric (TTE) effect in c-axis inclined PbSe thin films that were grown epitaxially on c-axis miscut SrTiO3 single crystal substrates using the pulsed laser deposition technique. Because of the anisotropic Seebeck coefficient of these inclined PbSe thin films, high TTE voltage signals were detected when the film surfaces were irradiated using various different continuous-wave lasers with wavelengths ranging from the ultraviolet (360 nm) to the far infrared (10.6 μm). In addition, the amplitudes of the output voltage signals showed good linear dependence on both the radiation power density and the film inclination angle. The results above demonstrate the potential of PbSe for self-powered ultra-broadband light detection applications.

Conserved axis-dependent conduction polarity in NaSnAs polycrystalline bulk sample for transverse thermoelectric application

Layered semiconductor NaSnAs has a distinct property of large anisotropy in effective mass, resulting in axis-dependent conduction polarity. This property of NaSnAs is useful for transverse thermoelectric (TE) cooling and thermal energy harvesting with a single TE leg. In this study, we used a Sn flux method followed by hot pressing to synthesize NaSnAs polycrystalline bulk samples. Seebeck coefficient measurements revealed n-type polarity along in-plane and p-type polarity along cross-plane directions over a temperature range of 300–520 K. This finding indicates that axis-dependent conduction polarity is conserved in polycrystalline NaSnAs bulk samples with a preferred orientation. A significantly low lattice thermal conductivity of 0.8 W/mK at 520 K was obtained because of phonon scattering caused by strong anharmonicity in the lattice vibration. First-principles calculation suggests that axis-dependent conduction polarity occurs where the Fermi level lies within the bandgap. Furthermore, we predict that high-TE performance can be achieved by optimizing the carrier concentration because of their characteristic pudding mold type band structure, which leads to the coexistence of a high density of states and group velocity.

Giant anomalous Nernst signal in the antiferromagnet YbMnBi2

A large anomalous Nernst effect (ANE) is crucial for thermoelectric energy conversion applications because the associated unique transverse geometry facilitates module fabrication. Topological ferromagnets with large Berry curvatures show large ANEs; however, they face drawbacks such as strong magnetic disturbances and low mobility due to high magnetization. Herein, we demonstrate that YbMnBi2, a canted antiferromagnet, has a large ANE conductivity of ~10 A m−1 K−1 that surpasses large values observed in other ferromagnets (3–5 A m−1 K−1). The canted spin structure of Mn guarantees a non-zero Berry curvature, but generates only a weak magnetization three orders of magnitude lower than that of general ferromagnets. The heavy Bi with a large spin–orbit coupling enables a large ANE and low thermal conductivity, whereas its highly dispersive px/y orbitals ensure low resistivity. The high anomalous transverse thermoelectric performance and extremely small magnetization make YbMnBi2 an excellent candidate for transverse thermoelectrics.

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.

Leveraging bipolar effect to enhance transverse thermoelectricity in semimetal Mg2Pb for cryogenic heat pumping

Toward high-performance thermoelectric energy conversion, the electrons and holes must work jointly like two wheels of a cart: if not longitudinally, then transversely. The bipolar effect — the main performance restriction in the traditional longitudinal thermoelectricity, can be manipulated to be a performance enhancer in the transverse thermoelectricity. Here, we demonstrate this idea in semimetal Mg2Pb. At 30 K, a giant transverse thermoelectric power factor as high as 400 μWcm−1K−2 is achieved, a 3 orders-of-magnitude enhancement than the longitudinal configuration. The resultant specific heat pumping power is ~ 1 Wg−1, higher than those of existing techniques at 10~100 K. A large number of semimetals and narrow-gap semiconductors making poor longitudinal thermoelectrics due to severe bipolar effect are thus revived to fill the conspicuous gap of thermoelectric materials for solid-state applications.

Large transverse thermoelectric figure of merit in a topological Dirac semimetal

Thermoelectric (TE) conversion in conducting materials is of eminent importance for providing renewable energy and solid-state cooling. Although traditionally, the Seebeck effect plays a key role for the TE figure of merit zST, it encounters fundamental constraints hindering its conversion efficiency. Most notably, there are the charge compensation of electrons and holes that diminishes this effect, and the intertwinement of the corresponding electrical and thermal conductivities through the Wiedemann-Franz (WF) law which makes their independent optimization in zST impossible. Here, we demonstrate that in the Dirac semimetal Cd3As2 the Nernst effect, i.e., the transverse counterpart of the Seebeck effect, can generate a large TE figure of merit zNT. At room temperature, zNT = 0.5 in a small field of 2 T; it significantly surmounts its longitudinal counterpart zST for any field and further increases upon warming. A large Nernst effect is generically expected in topological semimetals, benefiting from both the bipolar transport of compensated electrons and holes and their high mobilities. In this case, heat and charge transport are orthogonal, i.e., not intertwined by the WF law anymore. More importantly, further optimization of zNT by tuning the Fermi level to the Dirac node can be anticipated due to not only the enhanced bipolar transport, but also the anomalous Nernst effect arising from a pronounced Berry curvature. A combination of the former topologically trivial and the latter nontrivial advantages promises to open a new avenue towards high-efficient transverse thermoelectricity.

Design and power generation of tilted Cu/Fe2V(Al0.9Si0.1) multilayers via the transverse thermoelectric effect

We have conducted a computer simulation of thermoelectric (TE) properties in tilted Cu/Fe2V(Al0.9Si0.1) multilayers. Such a tilted configuration yields a transverse (off-diagonal) TE effect whereby an electric current can flow perpendicularly to the temperature gradient. Appropriately controlling the relative Cu thickness and the tilting angle realizes a higher power factor than that of the parent TE materials. In our multilayers, the estimated power factor exceeds 8mW/K2m, being approximately three times higher than that of the parent TE materials. On the basis of the simulation results, we have fabricated several modules and achieved a maximum power of 4.83 mW in the module with a relative Cu thickness of 0.7 and a tilting angle of 30°.

Seebeck Tensor Analysis of (p × n)-type Transverse Thermoelectric Materials

ABSTRACTSingle-leg (p × n)-type transverse thermoelectrics (TTE) are reviewed as an alternative to conventional or “longitudinal” double-leg thermoelectrics for applications at room temperature and below. As the name suggests, this unique behavior of (p × n)-type transverse thermoelectrics results from choosing ambipolar anisotropic materials that have a Seebeck tensor with orthogonal p- and n-type Seebeck coefficients, leading to transverse relation between net heat and net electrical current. One feature of such materials is that they can operate near intrinsic doping and, therefore will not suffer from dopant freeze-out, opening the possibility of new cryogenic operation for solid state cooling. In this work, a Seebeck tensor analysis of thermoelectric materials is presented. To compare the performance of transverse thermoelectric materials, a transverse power factor PF⊥ is introduced. Materials searches based on these simple criteria reveal that over 1/4 of the database of about 48,000 inorganic materials could potentially function as (p × n)-type TTE’s, demonstrating the underappreciated prevalence of this class of materials.

Effect of material anisotropy on the transverse thermoelectricity of layered composites

Thermoelectric devices can achieve conversion between thermal and electrical energies. In contrast to longitudinal thermoelectrics, transverse thermoelectrics can decouple the directions of heat flux and electric current, which in turn offers more flexibility in device design. While many studies have focused on composite structures constructed using materials with isotropic properties, this work investigated the effect of material anisotropy on the transverse thermoelectricity of layered composite materials. A simplified analytical model was derived based on the equivalent circuit principle to correlate the effective thermoelectric properties of the composite structure and the properties of the constituting phases. The effects of the geometrical design parameters of the composite structure and the material anisotropy were investigated. The transverse thermoelectric figure of merit increases with the increase in material anisotropy. The analytical model was then compared with the finite element analysis model with respect to the cooling capacity of transverse thermoelectric devices. The good agreement achieved between the two approaches indicated the effectiveness of the simplified analytical model in predicting the transverse thermoelectric performance of layered structures with anisotropic material properties.