Session N64: Advances in Thermal Energy Conversion

Predicting Phonon Transport in Disordered Alloys from a Highly Accurate Machine Learning Interatomic Potential

The Ⅱ₂Ⅳ family of materials, such as Mg₂Si, Mg₂Sn, Sr₂Si, and Sr₂Ge, among others, are highly regarded as promising high-performance thermoelectric materials. In our previous research, we calculated the maximum figure of merit ZT for the promising II-IV family thermoelectric compounds Sr₂Si and Sr₂Ge, yielding values of 1.15 and 1.44 at 900 K through first-principles calculations. To improve thermoelectric performance, the common practice involves alloying to reduce lattice thermal conductivity and enhance the Seebeck coefficient. Nevertheless, determining the optimal alloy ratios through first-principles calculations can be quite challenging because disordered effects require a large supercell in the computations. Here, we introduce a highly accurate machine learning interatomic potential (MLIP) for Sr₂Si_1-xGe_x disordered alloys. This MLIP is created through a machine learning technique trained on first-principles density functional theory (DFT) data, and it attains accuracy levels comparable to those achieved with DFT. This approach empowers us to carry out efficient molecular dynamics simulations for entire alloy concentration 0 ≤ x ≤ 1 in Sr₂Si_1-xGe_x and make accurate thermal property predictions. Our work provides a solution to explore compositions that offer the most potential for high-performance thermoelectric disordered alloys while assessing the contributions of phonon modes to phonon transport.   

Electron-hole dichotomy and enhancement of thermoelectric power factor by electron-hole-asymmetric relaxation time: a model study on a two-valley system with strong intervalley scattering

The role of electron-phonon scattering in thermoelectric transport has been paid much attention, especially in multivalley systems. In this study [1], we analyze a minimal model for a two-electron-valley system with intravalley and intervalley electron-phonon scattering; accordingly, find three electron transport regimes under strong intervalley scattering. In addition to the usual power factor (PF) peak near the band edge, strong electron-hole asymmetry of electron relaxation time due to the intervalley scattering enhances the Seebeck coefficient and thus PF for larger doping concentration. For example, when electron carriers are strongly affected by the intervalley scattering while hole carriers are not, strongly electron-hole-asymmetric relaxation time results in a large Seebeck coefficient. This situation can also cause an anomalous sign change of the Seebeck coefficient as pointed out in Ref. [2]. Our finding sheds light on unexplored thermoelectric transport under the strong electron-phonon scattering.

 

[1] M. Ochi, arXiv:2306.04075 (2023). [2] N. S. Fedorova, A. Cepellotti, and B. Kozinsky, Adv. Funct. Mater. 32, 2111354 (2022).   

Band Engineering and Synergistic Modulation Doping for Excellent Thermoelectric Performance in Composites Ti1–xNbxCoSb–Nb0.8+δCoSb

We observed the dramatic improvement in the thermoelectric performance of TiCoSb by introducing three-dimensional (3D) modulation doping and synergistic band engineering in the composites of the form (1 – f)A + fB, where A and B refer to the phases Ti_1–xNb_xCoSb and Nb_0.8+δCoSb, respectively, and f is the volume fraction of phase B. We show that the electrical conductivity and Seebeck coefficient of these composites increase simultaneously due to modulation doping, giving rise to colossal power factor (PF) enhancement from 0.3 μW cm^–1 K^–2 (TiCoSb) to 18 μW cm^–1 K^–2 (x = f ≈ 0.05) at 300 K and exceeding 25 μW cm^–1 K^–2 over a broad temperature range (T > 600 K). Due to the Ti–Nb point mass fluctuation in phase A, high concentration of defects in phase B, and interfacial phonon scattering between A and B, these composites also exhibit very low lattice thermal conductivity (κ_L), resulting in a high zT of 0.81 near 970 K. The simulation of κ_L using the Klemens model successfully describes the significant reduction of κ_L for these composites, observed experimentally. Our ab initio DFT calculations show that Ti_1–xNb_xCoSb exhibits band convergence as x increases, which contributes to improving the charge transport. Thus, benefiting from the synergistic effect of band convergence and 3D modulation doping, a high zT is obtained. Using multiple concepts togather can be a new approach to enhance heat to electricity conversion efficiency.   

A Study of the atypical Heusler Alloy Fe2 (V1-xWx) Al

The Heusler system permits great flexibility in electronic, thermal, morphological, and structural attributes. A typical (full) Heusler compound has a composition XY₂Z; here X and Y are transition metals and Z is a p-block element. Due to nonstoichiometric composition the atypical alloy offers additional tunability.  Interestingly random atomic scale substitution concomitant with off-stoichiometric composition affects transport properties. In addition, both thermal and electronic properties can be selectively targeted. For instance, electrical response is sensitive to defects in valency whereas phonon properties are affected by atomic weight disorder. Consequently, thermal conductivity in the quaternary alloy Fe₂ (V_1-xW_x) Al, is likely to be  impacted by the large variation in the atomic weights of Vanadium (51) and Tungsten (184).

Thermoelectric materials offer a great potential for directly converting heat into electricity and are essential for wide-scale renewable energy application. For efficient conversion high electrical conductivity and low thermal conductivity is desirable. In this work we investigate the influence of synthesis and annealing on transport properties and the thermoelectric figure of merit (ZT) of this material. We will describe experimental details and numerical results on thermal and electrical properties of bulk Fe₂V_0.8W_0.2Al. Briefly, both electrical resistivity and thermal conductivity, and Seebeck coefficient are measured. The implication of our findings will be discussed.   

Ferromagnetic fluctuations in thermoelectric Heusler alloy Fe2VAl: A weak-coupling approach

To study the enhanced thermopower by ferromagnetic fluctuations in Heusler compound Fe₂VAl, its low-energy effective model is numerically investigated by a weak-coupling theory. Recently, it is experimentally observed in doped Fe₂VAl that the thermoelectric properties such as the Seebeck coefficient and power factor are enhanced around the Curie temperature. In this study, first-principles calculations based on the density functional theory (DFT) are performed and then a 19-orbital model of Fe₂VAl is constructed by the maximally localized Wannier functions. The obtained tight-binding model includes the Fe-3d, V-3d, and Al-3sp3 orbitals and the energy band structure reproduces the original DFT band structure around the Fermi level. Furthermore, the 19-orbital Hubbard model is considered by adding interaction terms to the tight-binding model and investigated by the random phase approximation. The spin susceptibility shows the ferromagnetic instability at the low temperatures around the original band filling. In our presentation on the day, we also plan to report on the numerical evaluation of the thermoelectric performance.   

Enhancing Thermoelectric Properties of Some Defect Pyrochlores by Lowering Thermal Conductivity

Lowering thermal conductivity (κ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>κ) is vital in designing thermoelectrics as it can significantly increase their figure of merit. This study aims to lower κ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>κ by changing site occupancies in metal-like defect pyrochlores (A₂B₂O₆O´_x, 0≤x<1), specifically Pb₂Ru₂O₆O´_0.5, and derivatives with electron lone pairs on the A site and vacancies on the O´ site. Partial B site substitutions (Pb²⁺₂(Ru⁴⁺_2-yPb⁴⁺_y)O_6.5-x, y = 0, 0.3, 0.5, 0.7, 0.9) increased the lattice parameter (^VIPb⁴⁺ > ^VIRu⁴⁺), affected electrical conductivity (σ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>σ) and replaced Pb–O–Ru by Pb–O–Pb bonds. The number of lone pair electrons (^VIIIPb²⁺ on A site) remained constant. Removing some Pb²⁺ from the A site (Pb_1.8Ru₂O₆O´_x, 0.5≤x<1) reduces lone pair electrons and introduces O´ vacancies. ‘x’ indicates potential changes in the number of O´ vacancies (not measured). In all cases, lattice defects and resulting anharmonicity increase phonon scattering κ<!--[if gte msEquation 12]>κ and σ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>σ were measured and the underlying scattering processes were explained by existing quantum-physical models. Relative to Pb₂Ru₂O_6.5, derivatives showed up to 2.3× increase of σ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>σ and significant decreases in κ_L, the lattice component of κ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>κ, (κ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>κ = κ_e + κ​​​​​​​_L). Lowering Pb²⁺ on the A site yielded the electronic component κ​​​​​​​_e to be greater than σ<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>σ and the lattice component κ​​​​​​​_L to be negative, suggesting failure of the Wiedemann-Franz law when using the Sommerfeld value for the Lorenz number (L) to calculate κ​​​​​​​_e. This phenomenon will be discussed along with an attempt to calculate a correct L using Seebeck coefficient.   

Role of Impurity Defects in Thermoelectric Material Property Degradation of Mg2(Si,Sn) Modules

Mg₂(Si,Sn) materials are prominent in mid-temperature range thermoelectric applications. Despite their potential, challenges arise due to material degradation during the joining process with metallic electrodes. In this study, we used hybrid-density functional calculations to understand the influence of point impurities in Mg₂Si and Mg₂Sn thermoelectric materials. Charged defect formation energy calculations revealed that several elemental impurities, such as Ag, serve as electron trap centers, especially in n-type Bi-doped Mg₂(Si,Sn). Interestingly, both the formation energy and diffusion barrier for these impurities are remarkably low. This suggests that point impurities can easily diffuse from the metal electrode deep into the thermoelectric materials. The Seebeck microprobe measurements further validate our "defect diffusion and charge compensation model". In particular, the Seebeck coefficient near the contact shows a significant increase, due to charge compensation resulting from the diffusion of these impurities.   

Layered metal-semiconductor composites for high-performance transverse thermoelectric devices

Devices tailored for efficient thermal energy harvesting may be enabled via the transverse Seebeck effect (TSE) in anisotropic layered metal-semiconductor composites (MSC). As an alternative to single crystal materials, MSCs offer a highly tunable platform to optimize performance characteristics of TSE-based devices and can draw from a broad pool of thermoelectric constituent materials. MSC performance characteristics in applications such as energy harvesting, cooling, and heat flux sensing, were investigated as a function of composite material and geometric parameters (i.e., local layer thickness, global dimensions, and tilt angle). Using a hybrid theoretical and data-driven approach, we propose selection rules that optimize device performance. For highly efficient devices, the two constituent materials must exhibit a large difference in their Seebeck coefficient values (S), in their thermal conductivity values (k), and in the ratio S/k. The relative impact of each selection rule on device performance was investigated using real-world materials parameters.   

Development of thermoelectric materials based on 143-Zintl phase

Various high-performance thermoelectric materials have been found in Zintl phase compounds. In particular, 122-Zintl phase has been intensively explored with successful findings of new thermoelectric materials that the dimensionless figure-of-merit (ZT) exceeds the value of 1. Recently, we found that Rb(Zn,Cu)₄As₃ show ZT = 0.53 at T = 797 K [1,2], demonstrating that 143-Zintl phase compounds can be a new class of high-performance thermoelectric materials. 

 To prove the potential of 143-Zintl phase, we further explored the performance of RbCd₄As₃. We synthesized samples by solid-state reaction. We optimized the synthesis condition to obtain single phase with appropriate carrier doping. The obtained maximum power factor was about 0.5mW/mK² at T = 500 K. Details will be discussed on the conference.    

Magnetothermopower of Nodal Line Semimetals

The search for materials with large thermopower is of great practical interest, e.g. for the design of thermocouples and thermoelectric generators. Dirac/Weyl semimetals under magnetic field have recently proven to exhibit superior thermoelectric properties, as compared to conventional metals and semiconductors. Unfortunately, in these materials, the thermopower is suppressed at low temperatures. Here, we study the thermoelectric properties of straight and circular nodal line semimetals, computing the Seebeck and Nernst coefficients for arbitrary temperature and magnetic field. Strikingly, when a sufficiently strong magnetic field is applied along the direction of the nodal line, the large degeneracy of states leads to a large, temperature-independent thermopower that grows in a non-saturating way with applied field. This result suggests a potential advantage of nodal line semimetals for low-temperature thermopower applications.   

Magnetocaloric, magnetooptic thermal devices: thermal switches and refrigerators

Magnetic materials possess unique thermal properties that can give rise to powerful applications in thermal engineering. The magnetocaloric effect is the change in temperature of a magnetic material as a result of a change in its magnetization. Its primary application has been magnetic refrigerators, which have the potential to cool down to cryogenic temperatures and achieve better coefficient of performance (COP) than vapor-compression refrigerators. On the other hand, magnetic materials can be used control radiative heat transfer via magnetooptic effects in the infrared spectrum. Over the past decade, degenerately doped semiconductors and magnetic Weyl semimetals have been extensively studied for use in both far-field and near-field radiative heat transfer. In our recent work, we combined magnetocalorics and near-field magnetooptics to model a thermal switch where both the temperature delta and thermal conductance between two semi-infinite slabs can be magnetically controlled. In this talk, we discuss our ongoing work on modeling radiative heat transfer in magnetic refrigerators. Since radiation can transfer more entropy than conduction, we hypothesize that using radiation instead of conduction to reject heat can achieve a higher, magnetically controllable COP.   

A thermally rechargeable electrochemical oscillator for harvesting near room temperature ultra-low grade waste heat

We present Soret effect-driven electrochemical devices that generate 1.02 V with a mere 10 K temperature difference with the cold end at room temperature, i.e., a Seebeck coefficient of 102 mV/K, which is almost four times the previously reported highest value (24 mV/K)¹ for cells with cellulose based separators. The Seebeck coefficient was found to depend not only on the electrolyte composition but also on the electrode porosity and microstructure (a neglected area of study). Interestingly, during galvanostatic discharge (with and without the temperature gradient), we observed novel voltage oscillations for both porous and non-porous electrodes. We conclusively prove (via models and experiments) that interplay between ionic diffusion and ionic migration within the electric double layer at the electrode-electrolyte interface is the root cause for voltage oscillations. We demonstrate that the oscillations can be controlled by changing the electrode porosity and the discharge current. Using up to 4 cells in series, we powered a calculator and performed basic calculations in real time. The presence of voltage oscillations indicates potential for novel applications. 

 

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Thermoelectric Transport in 3D Printed Polymers

In this presentation, we will delve into the thermoelectric performance of innovative nanomaterial-thermoplastic polymer composites crafted through fused deposition modeling. Our focus will be on examining the temperature-dependent electrical conductivity and Seebeck coefficient of both poly (lactic acid) (PLA) and graphene-PLA composites. We will discuss how factors such as the choice of nanofillers, the geometric design, and the macroscopic printing patterns influence the charge transport behavior, heat transfer, and overall thermoelectric efficiency of these materials. To provide insights into the observed experimental results, we will also employ finite element simulations of 3D printed composites based on semi-classical transport theory. Finally, we will present the possibility of using PLA and gPLA composites in battery-thermal management systems.   

Harvesting energy from freestanding graphene thermal fluctuations

Highly flexible, electrically conductive freestanding graphene membranes hold great promise for vibration energy harvesting applications. In this study, we present numerical results for a graphene ripple treated as a Brownian particle coupled to an energy harvesting circuit. When circuit and particle are at the same temperature, the second law forbids harvesting energy from the thermal motion of the Brownian particle, even if the circuit contains a rectifying diode. However, when the circuit contains a junction followed by two diodes wired in opposition, a special ultraslow convergence to equilibrium may be imposed. Detailed balance is temporarily broken as current flows between the two diodes and charges two storage capacitors. The harvested energy is taken from the surrounding thermal environment, and we show that the system obeys the first and second laws of thermodynamics.