Transverse Thermoelectric Power Research Articles

Cooperative Nernst effect of multilayer systems: Parallel circuit model study

Transverse thermoelectric power generation has emerged as a topic of immense interest in recent years owing to the orthogonal geometry which enables better scalability and fabrication of devices. Here, we investigate the thickness dependence of longitudinal and transverse responses in film-substrate systems, i.e., the Seebeck coefficient, the Hall coefficient, the Nernst coefficient, and the anomalous Nernst coefficient in a unified and general manner based on the circuit model, which describes the system as the parallel setup. By solving the parallel circuit model, we show that the transverse responses exhibit a significant peak, indicating the importance of a cooperative effect between the film and the substrate, arising from circulating currents that occur in these multilayer systems in the presence of a temperature gradient. Finally, on the basis of realistic material parameters, we predict that the Nernst effect in bismuth thin films on doped silicon substrates is boosted to unprecedented values if the thickness ratio is tuned accordingly, motivating experimental validation. Published by the American Physical Society 2024

Permanent-magnet-based transverse thermoelectric generator with high fill factor driven by anomalous Nernst effect

A transverse thermoelectric generator for magnetic-field-free and high-density power generation utilizing the anomalous Nernst effect is constructed and its performance is characterized. By alternately stacking two different permanent magnets with the large coercivity and anomalous Nernst coefficients of opposite sign, transverse thermoelectric voltage and power can be generated in the absence of external magnetic fields and enhanced owing to a thermopile structure without useless electrode layers. In the permanent-magnet-based stack, the magnetic attractive force enables easy construction of the thermopile structure with a high fill factor. In this study, we construct a bulk module consisting of 12 pairs of SmCo5- and Nd2Fe14B-type permanent magnets having positive and negative anomalous Nernst coefficients, respectively, whose fill factor reaches ∼80%, whereas that of conventional thermoelectric modules based on the Seebeck effect is typically 30%–60%. We demonstrate magnetic-field-free anomalous Nernst power generation up to 177 µW at a temperature difference of 75 K around room temperature, which corresponds to the largest anomalous Nernst power density of 65 µW/cm2. The presented module structure concept will provide a design guideline for high-performance transverse thermoelectric power generation.

THE INFLUENCE OF EDDY CURRENTS ON PARAMETERS OF ANISOTROPIC UNIPOLAR THERMOELEMENT

The purpose of this study is to evaluate the influence of eddy thermoelectric and electric currents and eddy heat flows with a laminar flow character on the value of transverse thermoEMF E⊥ and efficiency η of an anisotropic unipolar thermocouple. In the case of transverse thermoEMF of an anisotropic unipolar thermoelement, the assessment of such influence was carried out taking into account the eddy current caused by the anisotropy of the thermoEMF, and for the evaluation of the efficiency η – taking into account the eddy currents caused by the anisotropy of the coefficients of thermoEMF αi, electrical conductivity σik and thermal conductivity κik (i, k = 1..3). The research results showed that the influence of the studied current leads to a change in the value of the optimal angle of inclination γ between one of the selected crystallographic axes and the length a of the thermoelement and to some decrease in the value of the transverse thermoelectric power. Efficiency η under the action of eddy thermoelectric and electric currents and eddy heat flow is more significantly affected. Numerical analysis shows that in the case of using such a material as cadmium antimonide, the effect on the transverse thermoEMF is 36% (γ = 45⁰), and the effect on the efficiency is more significant.

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.

Anomalous Hall and Nernst effects in ferrimagnetic Mn4N films: Possible interpretations and prospects for enhancement

Ferrimagnetic Mn4N is a promising material for heat flux sensors, based on the anomalous Nernst effect (ANE), because of its sizeable uniaxial magnetic anisotropy (Ku) and low saturation magnetization (Ms). We experimentally and theoretically investigated the ANE and anomalous Hall effect in sputter-deposited Mn4N films. It was revealed that the observed negative anomalous Hall conductivity (σxy) could be explained by two different coexisting magnetic structures, that is, a dominant magnetic structure with high Ku, contaminated by another structure with negligible Ku, owing to an imperfect degree of order of N. The observed transverse thermoelectric power (SANE) of +0.5 μV/K at 300 K yielded a transverse thermoelectric coefficient (αxy) of +0.34 A/(m · K), which was smaller than the value predicted from the first-principles calculation. The interpretation for αxy based on the first-principles calculations led us to conclude that the realization of single magnetic structure with high Ku and optimal adjustment of the Fermi level are promising approaches to enhance SANE in Mn4N through the sign reversal of σxy and the enlargement of αxy up to a theoretical value of 1.77 A/(m · K).

Transverse thermoelectric power in a semiconductor quantum well in the presence of the Rashba effect

We have studied the Rashba spin-orbit effect on a quasi-two dimensional semiconductor quantum well with a parabolic potential placed in a magnetic field parallel to the plane of the quantum well, taking into account the Zeeman coupling. The expression for the density of states and the transverse thermoelectric power of the electron gas in a quasi-two-dimensional quantum well with a parabolic potential have been obtained for the case of strong degeneracy statistics. We show that the transverse thermoelectric power depends on the strength of the spin-orbital coupling parameter, the frequency ω0, which is a characteristic parameter of the electron gas thickness, and the strength of the magnetic field.

Power generation by the transverse Seebeck effect in Pb–Bi2Te3 multilayers

The transverse Seebeck effect in tilted metal-semiconductor multilayer structures has been exploited for electric power generation. In tilted Pb–Bi2Te3 multilayer samples, coefficients of performance for transverse thermoelectric power generation have been determined, in dependence of temperature difference between hot and cold sample surfaces. The results can be described well by modeling calculations of transverse thermoelectric power generation.

Nernst effect of Dirac fermions in graphene under a weak magnetic field

The derivation for the transport coefficients of an electron system in the presence of temperature gradient and the electric and magnetic fields are presented. The Nernst conductivity and the transverse thermoelectric power of the Dirac fermions in graphene under charged impurity scatterings and weak magnetic field are calculated on basis of the self-consistent Born approximation. The result is compared with so far the available experimental data.

Enhancement of transverse thermoelectric power factor in tilted Bi/Cu multilayer

We investigated transverse thermoelectric response in tilted Bi/Cu multilayer. Estimation through an equivalent circuit model showed that transverse thermoelectric power factor (PF) reaching 155 μW/cm K2 is possible with optimum device parameters. Finite element analysis confirmed the validity of the estimation using tensorial representation, which implicitly assumes homogeneous anisotropy. The Bi/Cu multilayer was fabricated by pressure injection of molten Bi into a periodically slitted Cu block. The measurement of the fabricated sample verified an enhancement in transverse PF up to 50.1 μW/cm K2, which is approximately 1.5 times greater than the conventional PF of constituent Bi, 34 μW/cm K2.

Transverse thermoelectric power in the molybdenum blue bronze K 0.3MoO 3

Transverse thermoelectric power (TEP) along the [102] direction ( S [102]) was measured in a series of pure or doped potassium blue bronzes from 80 to 300 K. With the temperature increases, unlike that along the b axis ( S b ), S [102] gradually deviates from typical semiconductor behavior and metallic behavior below and above the Peierls transition ( T p), respectively. These departures were qualitatively understood in the scenario of the transverse tunnelling among the 1D chains, which can be consolidated with the significant influence of the alkali doping in the samples.

Thermoelectric power of semiconductors in the extreme quantum limit—I: The ‘electron-diffusion’ contribution

A study is made of the effect of the quantization of the electron energy levels in a strong magnetic field on the ‘electron-diffusion’ contribution, S e , of the transverse thermoelectric power of a high-purity isotropic semiconductor ( n-type gallium arsenide GaAs) in the extreme quantum limit, when all the carriers in the conduction band are in the lowest Landau level. A theoretical expression for S e is derived, taking into account of spin splitting. The sign of S e is either positive or negative, depending on the sign of the conducting carriers. The transition to nondegeneracy, which takes place when the lowest Landau level is driven through the Fermi level, has a large effect on the variation of S e with magnetic field. This effect is characterized by a large and rapid increase in | S e |. Spin splitting effects, even with a small effective mass ( m∗ = 0·07 m 0 , where m 0 is the free-electron mass) and a very small g-factor ( g ≈ 0·32), greatly modify and reduce | S e | a a function of field. For large fields, | S e | increases monotonically with H. Calculations are carried out for n-type GaAs at T = 0·5°K with N = 1·2 × 10 16cm −3 carriers.