Thermoelectric Conductivity Research Articles

Carbon fiber and carbon fiber composites—creating defects for superior material properties

AbstractRecent years have seen a rise in the use of carbon fiber (CF) and its composite applications in several high-tech industries, such as the design of biomedical sensor components, 3D virtual process networks in automotive and aerospace parts, and artificial materials or electrodes for energy storage batteries. Since pristine CF have limited properties, their properties are often modified through a range of technologies, such as laser surface treatment, electron-beam irradiation grafting, plasma or chemical treatments, electrophoretic deposition, carbonization, spinning-solution or melt, electrospinning, and sol–gel, to greatly improve their properties and performance. These procedures cause faulty structures to emerge in CF. The characteristics and performances of CF (thermo-electric conductivity, resistivity, stress tolerance, stiffness and elasticity, chemical resistivity, functionality, electrochemical properties, etc.) vary greatly depending on the modification technique used. Thus, the purpose of this review is to demonstrate how the insertion of faults can result in the production of superior CF. The characteristics of CF defects were examined using a variety of analytical techniques, such as defect-forming chemistry, molecular organization, and ground-level chemistries like their crystallinities. Finally, some future work is also included. Graphical abstract

Spectra of correlators in the relaxation time approximation of kinetic theory

The relaxation time approximation (RTA) of the kinetic Boltzmann equation is likely the simplest window into the microscopic properties of collective real-time transport. Within this framework, we analytically compute all retarded two-point Green’s functions of the energy-momentum tensor and a conserved U(1) current in thermal states with classical massless particles (a ‘CFT’) at non-zero density, and in the absence and presence of broken translational symmetry. This is done in 2 + 1 and 3 + 1 dimensions. RTA allows a full explicit analysis of the analytic structure of different correlators (poles versus branch cuts) and the transport properties that they imply (the thermoelectric conductivities, and the hydrodynamic, quasihydrodynamic and gapped mode dispersion relations). Our inherently weakly coupled analysis thereby also enables a direct comparison with previously known strongly coupled results in holographic CFTs dual to the Einstein-Maxwell-axion theories.

Bose–Einstein distribution temperature features of quasiparticles around magnetopolaron in Gaussian quantum wells of alkali halogen ions

Abstract We have applied strong coupling unitary transformation method combined with Bose–Einstein statistical law to investigate magnetopolaron energy level temperature effects in halogen ion crystal quantum wells. The obtained results showed that under magnetic field effect, magnetopolaron quasiparticle was formed through the interaction of electrons and surrounding phonons. At the same time, magnetopolaron was influenced by phonon temperature statistical law and important energy level shifts down and binding energy increases. This revealed that lattice temperature and magnetic field could easily affect magnetopolaron and the above results could play key roles in exploring thermoelectric conversion and conductivity of crystal materials.

DFT calculations of new Li-based halides Li2AgAlY6 (Y = Cl, Br, I) for energy conversion applications

Recently, lead-free halide double perovskite (DP) materials have got scholarly attention owing to their environmentally sustainable characteristic, excellent stability and potential applications in solar cells and renewable energy. In this present article, we computationally examined the mechanical, optoelectronic and thermoelectric characteristics of a discernible compound Li2AgAlY6 (Y = Cl, Br, I) using the density functional theory (DFT). The evaluation of tolerance factor (tf), lattice constant (a0), and formation enthalpy (Hf) confirms the structural and thermodynamic stability of the investigated double DPs, also elastic constants further elaborate the mechanically stable nature. The studied compounds have direct band gap of 3.6 eV, 2.6 eV and 1.2 eV for Li2AgAlCl6, Li2AgAlBr6 and Li2AgAlI6 respectively, indicating the diverse energy absorption applications from ultraviolet to visible region. We used semi-classical Boltzman theory to determine the figure of merit (ZT) and corresponding Seebeck Coefficient (S), which validates the electrical and thermoelectric conductivity for the compounds under investigation. The current investigation establishes a theoretical foundation for the examined DPs, essential for comprehending and comparing forthcoming experimental inquiries aimed at exploring diverse optoelectronic and thermoelectric applications.

Prediction of giant anomalous Nernst effect in Sm(Co,Ni)5

Sm-Co bulk alloys are well-known permanent magnets having large remanent magnetizations and coercive forces and are widely used in many industrial products. Recently, a large transverse thermoelectric conversion was observed for SmCo5 over a wide temperature range in the absence of magnetic fields. The large thermoelectric conductivity tensors (αxy) was also confirmed by the first-principles density functional theory (DFT) calculations. In this study, we predicted further enhancement of the αxy by including Ni in Co site of SmCo5. We showed that the αxy of Sm(Co1−xNix)5 increases with increasing the Ni ratio and takes the maximum value αxy = 11.3 A K−1 m−1 around x = 0.08 at 300 K, which is about 77% enhancement of αxy = 6.4 A K−1 m−1 in SmCo5. We clarified that the band proximity points near the nodal line of Sm(Co0.92Ni0.08)5 are the main contributing factor to the large Berry curvature, providing the steep slope of the energy dependence in the anomalous Hall conductivity around the Fermi energy.

Composition dependence of anomalous Nernst effect in amorphous TbFeCo thin films with perpendicular magnetic anisotropy

In this study, we systematically investigated the anomalous Nernst effect in perpendicularly magnetized amorphous TbFeCo thin films with various compositions. It was found that the magnitude of the off diagonal thermopower (ODT), which corresponds to the anomalous Nernst effect, can be uniformly explained with respect to the Tb content regardless of the concentration above or below the compensation composition. The maximum ODT of 1.3 μV/K and the thermoelectric conductivity of 1.59 A/mK at room temperature were obtained, which is more significant than other perpendicular magnetic anisotropy thin films to achieve a large Nernst voltage for roll-type thermoelectric devices. By considering the thermoelectric tensor, Mott’s equation, and the scaling law, it was shown both experimentally and theoretically that the magnitudes of the first and second terms contributing to the anomalous Nernst effect are comparable. It was also found that the ODT of TbFeCo thin films is twice or more significant than the product of the Seebeck coefficient and the Hall angle. Furthermore, amorphous metals and Mn-alloys with a large Berry curvature are located above the relation that the ODT is twice the product of the Seebeck coefficient and the Hall angle, which means that amorphous metals are expected to enhance the ANE.

Nonlinear Landauer formula for thermal transport of the electrons

The Landauer formula, originally formulated in the context of linear transport, has been a powerful tool for studying quantum devices. However, recent research has shown that extending its application to nonlinear and nonreciprocal transport is crucial for a more comprehensive understanding. In this work, we develop a nonlinear Landauer formula for thermal transport of the electrons and apply it to investigate thermal transport in graphene. Our study reveals intriguing phenomena especially in the presence of large temperature gradients and at low system temperatures. At these conditions, higher-order nonlinear currents emerge, indicating the significance of nonlinear effects in thermal transport. Unlike thermoelectric conductivity, thermal conductivity can be decomposed into intrinsic and extrinsic terms. This decomposition is based on whether the contributions rely on the derivative of the transmission coefficient with respect to energy. This nonlinear Landauer formula presented here serves as a valuable tool for future investigations into the intricate interplay between temperature gradients, system temperatures, and thermal transport in quantum devices.

Thermoelectric properties of cement mortar doped with steel fiber under heating conditions

In order to supply the multifunctional demand for building materials in modern construction industry, research on the thermoelectric properties of cement mortar mixed with steel fibers (diameter 0.22 mm) for engineering purposes has been carried out in this paper. The self-built thermoelectric response test system is used to collect the thermoelectric power, surface temperature and conductivity of the cement-based composite specimens in real time. Based on the comparative measurement method, the thermal conductivity of large-sized specimens was obtained. The effect of different SF content on thermal conductivity, Seebeck coefficient and ZT of the specimens were investigated. The results show that the specimens doped with steel fiber is p-type thermoelectric material, and the maximum value of thermoelectric figure of merit ZT of the cement mortar doped with 7.5 wt% steel fiber at 50℃ is 0.175. The ZT of cement mortar specimens mixed with steel fibers is relatively low, but it has good application prospects as a low-cost and widely used building material.

The Anomalous Photo‐Nernst Effect of Massive Dirac Fermions In HfTe5

AbstractThe quantum geometric Berry curvature results in an anomalous correction to the band velocity of crystal electrons with a corresponding transverse (thermo)electric conductivity. However, time‐reversal symmetry typically constrains the direct observation and exploitation of anomalous transport to magnetic compounds. Here, it is demonstrated the anomalous Hall and Nernst conductivities are essential for describing the optoelectronic transport in thin films of the non‐magnetic, weakly gapped semimetal HfTe5 subject to an external magnetic field. A focused photoexcitation adresses the symmetries of the local Nernst conductivity, which unveils a hitherto hidden, anomalous photo‐Nernst effect of three‐dimensional (3D) massive Dirac fermions. The experimental temperature and density dependencies are compared with a semiclassical Boltzmann transport model. For HfTe5 thin films with the Fermi level close to the gap, the model suggests that the anomalous photo‐Nernst currents originate from an intrinsic Berry curvature mechanism, where the Zeeman interaction effectively breaks time‐reversal symmetry of the massive Dirac fermions already at moderate external magnetic fields.

Onsager Reciprocal Relation between Anomalous Transverse Coefficients of an Anisotropic Antiferromagnet.

Whenever two irreversible processes occur simultaneously, time-reversal symmetry of microscopic dynamics gives rise, on a macroscopic level, to Onsager's reciprocal relations, which impose constraints on the number of independent components of any transport coefficient tensor. Here, we show that in the antiferromagnetic YbMnBi_{2}, which displays a strong temperature-dependent anisotropy, Onsager's reciprocal relations are strictly satisfied for anomalous electric (σ_{ij}^{A}) and anomalous thermoelectric (α_{ij}^{A}) conductivity tensors. In contradiction with what was recently reported by Pan etal. [Nat. Mater. 21, 203 (2022)NMAACR1476-112210.1038/s41563-021-01149-2], we find that σ_{ij}^{A}(H)=σ_{ji}^{A}(-H) and α_{ij}^{A}(H)=α_{ji}^{A}(-H). This equality holds in the whole temperature window irrespective of the relative weights of the intrinsic or extrinsic mechanisms. The α_{ij}^{A}/σ_{ij}^{A} ratio is close to k_{B}/e at room temperature but peaks to an unprecedented magnitude of 2.9k_{B}/e at ∼150 K, which may involve nondegenerate carriers of small Fermi surface pockets.

Thermoelectric conductivity of monolayer FeCl2 under LDA+U

We investigated the influence of Coulomb repulsion via the LDA+U method on the thermoelectric conductivity of monolayer 1T-FeCl2. As the spin-orbit coupling is included, the state transition from metallic state to insulating state appears as the Coulomb repulsion increases, in good agreement with previous results. This is also followed by reduction of thermoelectric conductivity. Nevertheless, the magnetic moment remains unchanged as the Coulomb parameter increases, thus keeping the magnetism in monolayer FeCl2. This indicates that the Coulomb repulsion reduces the thermoelectric conductivity but keeps the magnetism in FeCl2.

Large Anomalous Nernst Effects at Room Temperature in Fe3 Pt Thin Films.

Heat current in ferromagnets can generate a transverse electric voltage perpendicular to magnetization, known as anomalous Nernst effect (ANE). ANE originates intrinsically from the combination of large Berry curvature and density of states near the Fermi energy. It shows technical advantages over the conventional longitudinal Seebeck effect in converting waste heat to electricity due to its unique transverse geometry. However, materials showing giant ANE remain to be explored. Herein,a large ANE thermopower of Syx ≈ 2 µV K-1 at room temperature in ferromagnetic Fe3 Pt epitaxial films is reported, which also show a giant transverse thermoelectric conductivity of αyx ≈ 4 A K-1 m-1 and a remarkable coercive field of 1300Oe. The theoretical analysis reveals that the strong spin-orbit interaction in addition to the hybridization between Pt 5d and Fe 3d electrons leads to a series of distinct energy gaps and large Berry curvature in the Brillouin zone, which is the key for the large ANE. These results highlight the important roles of both Berry curvature and spin-orbit coupling in achieving large ANE at zero magnetic field, providing pathways to explore materials with giant transverse thermoelectric effect without an external magnetic field.

Designing composition ratio of magnetic alloy multilayer for transverse thermoelectric conversion by Bayesian optimization

We demonstrated the effectiveness of the machine learning method combined with first-principles calculations for the enhancement of the anomalous Nernst effect (ANE) of multilayers. The composition ratio of CoNi homogeneous alloy superlattices was optimized by Bayesian optimization so as to maximize the transverse thermoelectric conductivity (αxy). The nonintuitive optimal composition with a large αxy of ∼10 A K−1 m−1 was identified through the two-step Bayesian optimization using rough and fine candidate pools. The Berry curvature and band dispersion analyses revealed that αxy is enhanced by the appearance of the flat band near the Fermi level due to the multilayer formation. The magnitude of the energy derivative of the anomalous Hall conductivity increases owing to the large Berry curvature near the flat band along the R-M high symmetry line, which emerges only in the optimized superlattice, leading to the αxy enhancement. The effective method verified here will broaden the choices of ANE materials to more complex systems and, therefore, lead to the development of transverse thermoelectric conversion technologies.

Seebeck coefficient of two-dimensional Dirac electrons in an organic conductor under pressure

The Seebeck coefficient, which is proportional to a ratio of the thermoelectric conductivity to electrical conductivity has been examined for Dirac electrons in the organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ [BEDT-TTF denotes a molecule given by bis(ethylenedithio)tetrathiafulvalene] under a uniaxial pressure using a two-dimensional tight-binding model with both impurity and electron--phonon (e--p) scatterings. We calculate an anomalous temperature ($T$) dependence of the Seebeck coefficient $S_{\nu}$ with $\nu = x$ (perpendicular to the molecular stacking axis) and $y$, which shows $S_\nu > 0$ with a maximum at high temperatures and $S_{\nu} < 0$ with a minimum at low temperatures.The microscopic mechanism of such a sign change of $S_\nu$ is clarified in terms of the spectral conductivity. The result is compared with experiments on $\alpha$-(BEDT-TTF)$_2$I$_3$.

Anisotropic Behavior of the Thermoelectric Power and the Thermal Conductivity in a Unidirectional Lateral Superlattice: A Typical Anisotropic System Exhibiting Two Distinct Nernst Coefficients

We have calculated the thermoelectric conductivity tensor εij and the thermal conductivity tensor λij of a unidirectional lateral superlattice (ULSL) (\(i,j = x,y\), with the x-axis aligned to the principal axis of the ULSL), based on the asymptotic analytic formulas of the electrical conductivity tensor σij in the literature valid at low magnetic fields where large numbers of Landau levels are occupied. With the resulting analytic expressions, we clarify the conditions for the Mott formula (Wiedemann–Franz law) to be applicable with high precision to εij (λij). We further present plots of the commensurability oscillations δεij, δλij, δκij, and δSij in εij, λij, (an alternative, more standard definition of) the thermal conductivity tensor κij, and the thermopower tensor Sij, calculated using typical parameters for a ULSL fabricated from a GaAs/AlGaAs two-dimensional electron gas (2DEG). Notable features of the δSij are (i) anisotropic behavior (δSxx ≠ δSyy) and (ii) the dominance of the xy component over the other components (\(|\delta S_{xy}| \gg |\delta S_{yx}|,|\delta S_{xx}|,|\delta S_{yy}|\)). The latter clearly indicates that the two Nernst coefficients, Sxy and Syx, can be totally different from each other in an anisotropic system. Both (i) and (ii) are at variance with the previous theory and are attributable to the inclusion of a damping factor due to the small-angle scattering characteristic of GaAs/AlGaAs 2DEGs, which have not been taken into consideration in δSij thus far.

Anomalous Nernst effect in epitaxially grown Fe4−xNixN films

Anomalous Nernst effect of epitaxial Fe4−xNixN films on MgAl2O4(001) substrates was characterized. The Ni/Fe ratio (x) in Fe4−xNixN films was changed in the range of 0 ≤ x ≤ 2.8, and the Fe4−xNixN phase started to decompose into FeNi at about x = 2.2. The anomalous Nernst coefficient (SANE) decreased with x from 1.7 to 0.6 μV/K and the Seebeck coefficient increased with x from −2.3 to 1.2 μV/K. The transverse thermoelectric conductivity (αxy) evaluated from the experimental data showed a decrease with x and the change of αxy dominated the change of SANE. The anomalous Hall conductivity and αxy of Fe4N and Fe2.8Ni1.2N were obtained by first-principles calculations, showing a significant difference from the experimental results, which might be caused by the contribution from the extrinsic mechanisms.

ElecTra code: Full-band electronic transport properties of materials

This paper introduces ElecTra, an open-source code which solves the linearized Boltzmann transport equation in the relaxation time approximation for charge carriers in a full-band electronic structure of arbitrary complexity, including their energy, momentum, and band-index dependence. ElecTra stands for ‘ELECtronic TRAnsport’ and computes the electronic and thermoelectric transport coefficients electrical conductivity, Seebeck coefficient, electronic thermal conductivity, and mobility, for semiconductor materials, for both unipolar and bipolar (small bandgap) materials. The code uses computed full-bands and relevant scattering parameters as inputs and considers single crystal materials in 3D and 2D. The present version of the code (v1) considers: i) elastic scattering with acoustic phonons and inelastic scattering with non-polar optical phonons in the deformation potential approximation, ii) inelastic scattering with polar phonons, iii) scattering with ionized dopants, and iv) alloy scattering. The user is given the option of intra- and inter-band scattering considerations. The simulation output also includes relevant relaxation times and mean-free-paths. The transport quantities are computed as a function of Fermi level position, doping density, and temperature. ElecTra can interface with any DFT code which saves the electronic structure in the ‘.bxsf’ format. In this paper ElecTra is validated against ideal electronic transport situations of known analytical solutions, existing codes employing the constant relaxation time approximation, as well as experimentally well-assessed materials such as Si, Ge, SiGe, and GaAs. Program summaryProgram title: ElecTra – Electronic Transport simulation labCPC Library link to program files:https://doi.org/10.17632/ycgx2fjzb6.1Licensing provisions: GPLv3Programming Language: MATLAB®Nature of the problem: computing the electronic and thermoelectric charge transport coefficients of materials with arbitrary complex full-band electronic structures, considering the carrier energy-, momentum-, and band-dependence of the scattering rates.Solution method: Semiclassical Linearized Boltzmann transport equation, with electronic structures (DFT or analytical) as input, formed into constant-energy surfaces, with scattering rates evaluated using Fermi's Golden Rule.Additional comments including restrictions and unusual features:•Programming interface: any DFT code which saves data in the ‘.bxsf’ format.•RAM: a case study for a half-Heusler bandstructure on a 51×51×51k-mesh, 2 Gb per processor is needed•Running time: for the example above, depending on the number and complexity of the scattering mechanisms and the number of simulated Fermi levels and temperatures considered, the time needed varies from ∼ 1 hour on a desktop PC or laptop (light simulations), to 5-10 hours on an HPC with 30-45 cores (heavy simulations). Using the constant relaxation time and constant mean-free-path approximations on a desktop PC or laptop, the running time is of the order of minutes.

Unconventional thermoelectric transport in tilted Weyl semimetals

We analyze the effect of the tilt on the transverse thermoelectric coefficient of Weyl semimetals in the \emph{conformal} limit, i.e., zero temperature and zero chemical potential. Using the Kubo formalism, we find a nonmonotonic behavior of the thermoelectric conductivity as a function of the tilt perpendicular to the magnetic field, and a linear behavior when the tilt is aligned to the magnetic field. An ``axial Nernst" current (chiral currents counter propagating perpendicularly to the magnetic field and the temperature gradient) is generated in inversion symmetric Weyl materials when the tilt vector has a projection in the direction of the magnetic field. This analysis will help in the design and interpretation of thermoelectric transport experiments in recently discovered topological quantum materials.

Thermoelectric transport in holographic quantum matter under shear strain

We study thermoelectric transport under shear strain in two spatial dimensional quantum matter using the holographic duality. General analytic formulae for the DC thermoelectric conductivities subjected to finite shear strain are obtained in terms of black hole horizon data. Off-diagonal terms in the conductivity matrix also appear at zero magnetic field, resembling an emergent electronic nematicity, which cannot nevertheless be identified with the presence of an anomalous Hall effect. For an explicit model study, we numerically construct a family of strained black holes and obtain the corresponding nonlinear stress–strain curves. We then compute all electric, thermoelectric, and thermal conductivities and discuss the effects of strain. While the shear elastic deformation does not affect the temperature dependence of thermoelectric and thermal conductivities quantitatively, it can strongly change the behavior of the electric conductivity. For both shear hardening and softening cases, we find a clear metal-insulator transition driven by the shear deformation. Moreover, the violation of the previously conjectured thermal conductivity bound is observed for large shear deformation.

An Ab Initio Analysis of Structural, optical, Electronic, and Thermal Properties of Cubic SrSnO3 using Wein2k

This paper investigated the structural, optical, electronic and thermal characteristics of SrSnO3 perovskites that were calculated using the density functional theory. Software called WEIN2K is used to perform the calculation. According to our calculations, the band gap energy of the SrSnO3 is roughly 4.00 eV and it adopts a distorted cubic shape in the space group Pm3-m. The band structure and partial density of state reflects the major contribution of O 2p in the valence band while 5s orbital from Sn in the conduction band. The electron density plot significantly shows the contribution different clusters SrO12 and SnO6 that plays crucial role in electronic and optical properties. The creation of covalent bonds between the atoms of Sn and O as well as the ionic interaction between the atoms of Sr and O are both demonstrated by the electron density graphs and SCF calculation. The refractive index and extinction coefficient directly correlated with the real and imaginary part of complex dielectric function. Real part of dielectric function shows higher values at two major point of energy 3.54 eV and 9.78 eV associated with the absorption and optical activity of SrSnO3. Negative part of imaginary dielectric function part suggests metallic behavior also supported by -grep lapw method. Thermoelectric and thermal conductivity properties suggest the power factor need to be improved for the device application.