Thermoelectric Behavior Research Articles

Planar phonon anisotropy, and a way to detect local equilibrium temperature in graphene

The effect of inclusion of the planar phonon anisotropy on thermo-electrical behavior of graphene is analyzed. Charge transport is simulated by means of Direct Simulation Monte Carlo technique coupled with numerical solution of the phonon Boltzmann equations based on deterministic methods.The definition of the crystal lattice local equilibrium temperature is investigated as well and the results furnish possible alternative approaches to identify it starting from measurements of electric current density, with relevant experimental advantages, which could help to overcome the present difficulties regarding thermal investigation of graphene.Positive implications are expected for many applications, as the field of electronic devices, which needs a coherent tool for simulation of charge and hot phonon transport; the correct definition of the local equilibrium temperature is in turn fundamental for the study, design and prototyping of cooling mechanisms for graphene-based devices.

Mechanical behaviour of thermoelectric materials – a perspective

ABSTRACT Research on thermoelectric materials – with their vast potential for applications in solid-state cooling or energy-conversion devices – has so far mainly focused on enhancing their conversion efficiency. However, understanding and tailoring the mechanical performance of thermoelectric modules and devices is crucial for their long-term use, as they are subjected to spatially-complex and time-varying thermomechanical stresses – both internal and external – which may lead to plastic, fatigue and/or creep deformation. This leads to changes in thermoelectric performance, dimensions (via strain accumulation) and mechanical integrity (via crack and pore formation, leading to failure). This review addresses the current understanding of various modes of stress-induced deformation that can take place during extended operation of thermoelectric materials and their impact on the strain (elastic, plastic, and creep), and the associated damage (bloating, fatigue, and fracture). Finally, some new areas of research straddling mechanical and thermoelectric behaviour are identified.

Controlling the carrier and phonon transport behavior of SnSe via stoichiometric adjustment

In this paper, we designed and fabricated SnSe1+x by vacuum melting with spark plasma sintering, and explored effect of stoichiometric change on energy band structure, bond anharmonicity and thermoelectric transport behavior under same process condition. Se overdose results in increase of electronical conductivity due to narrowing bandgap. Sn overdose causes obviously attenuated thermoelectric properties since the Fermi level moves up into the conduction band. The maximum powder factor value of 528 μWm−1K−2 for the SnSe1.01 sample at 773K results from energy gap reduction and steep density of states on valence band top. Non-stoichiometric defect causes enhancement of thermal conductivity due to evolution of bond characteristics between interlayers. The maximum ZT of the SnSe1.01 sample achieves 0.87 at 773K along “⊥” direction via stoichiometric adjustment.

Simulation study on the interaction between the battery module and busbar under typical driving conditions of electric vehicles

Accurate simulation of the battery thermoelectric coupling characteristics is the key to the thermal design and thermal management. As the electrically connected component between the battery electrodes, the heat production and heat transfer of the busbars have a significant impact on the battery thermoelectric behavior. A small number of battery thermal management studies have begun to model busbars recently, but the thickness, length, and material of the busbars are diverse. There is a lack of perfect simulation analysis and optimization methods. There is an urgent need to conduct in-depth research on the influence of characteristic parameters of the busbars on the battery thermoelectric behavior. Based on the MSMD-NTGK model, this paper investigates the influence of the thickness, length, and material of the busbars on the battery thermoelectric behavior without considering the economic cost and differences in the battery structure design. In this paper, a simulation analysis method and an ideal improvement solution for the design of the busbars are proposed. This paper compares the changes in the battery thermal behavior before and after the improvement of the busbars under the constant-rate discharge process, FTP75 condition, NEDC condition and WLTC condition respectively. This study can improve the realism of the battery thermal behavior simulation, provide a reliable simulation analysis method and reference basis for the industrial design and optimization of the busbars, and further improve the reliability of the subsequent battery thermal management simulation.

Quantum Interference and Contact Effects in the Thermoelectric Performance of Anthracene-Based Molecules.

We report on the single-molecule electronic and thermoelectric properties of strategically chosen anthracene-based molecules with anchor groups capable of binding to noble metal substrates, such as gold and platinum. Specifically, we study the effect of different anchor groups, as well as quantum interference, on the electric conductance and the thermopower of gold/single-molecule/gold junctions and generally find good agreement between theory and experiments. All molecular junctions display transport characteristics consistent with coherent transport and a Fermi alignment approximately in the middle of the highest occupied molecular orbital/lowest unoccupied molecular orbital gap. Single-molecule results are in agreement with previously reported thin-film data, further supporting the notion that molecular design considerations may be translated from the single- to many-molecule devices. For combinations of anchor groups where one binds significantly more strongly to the electrodes than the other, the stronger anchor group appears to dominate the thermoelectric behavior of the molecular junction. For other combinations, the choice of electrode material can determine the sign and magnitude of the thermopower. This finding has important implications for the design of thermoelectric generator devices, where both n- and p-type conductors are required for thermoelectric current generation.

Tensile strain as an efficient way to tune transport properties of Graphdiyne/Borophene hetero-bilayers; a first principle investigation

Thermoelectric materials have attracted much attention due to their crucial role in heat conversion and cooling applications. Transport properties effectively conduct thermoelectricity efficiency, and therefore, many studies have particularly focused on enhancing these features. In the present study, through periodic density functional theory combined with semi-classical transport formulations, the influence of strain on thermoelectric characteristics of Graphdiyne/Borophene hetero-bilayers has been investigated. Based on mechanical properties data, we have shown that bilayer construction causes an increase in the ultimate stress of constituent monolayers. The maximum strain that each hetero-bilayer can withstand is determined by using stress-strain behavior. Among the studied hetero-bilayers, GBS0 has the most value of Young’s modulus [1,2] under equibiaxial strain. Temperature-dependent transport properties of strained hetero-bilayers suggest strain as an efficient way to tune the thermoelectric behavior of hetero-bilayers. The introduction of uni- and biaxial strains affects the electrical and thermal conductivity of GBS0 and GBS1. Furthermore, the power factor and figure of merit, as the key elements of thermoelectric devices, can be significantly engineered under external tensions. The impact of tensile strain on the Seebeck effect is disclosed. Surprisingly, the studied hetero-bilayers can possess either positive or negative Seebeck coefficients under different modes of external strains. The present research extends the strain field, particularly in the rational design of thermoelectric devices.

Thermo-electro-mechanical aging and degradation of conductive 3D printed PLA/CB composite

Conductive polymeric composites consist of a polymeric matrix filled with conductive particles, providing electrical properties to a naturally insulating material. Their use in material extrusion printers provides great flexibility to geometrical design functional components. However, the degradation of their mechanical behavior during and after the application of electric currents is still absent in the literature. The flow of electric currents induces material heating affecting the strength, and mechanical deformation alters the electric properties leading to bilateral dependences. These processes evolve during the application of electric fields and may degrade the material behavior permanently. In this work, we address these questions taking as baseline material a 3D printed conductive Polylactic Acid (PLA)/ Carbon Black (CB) composite. We performed multi-physical characterizations on samples with different printing orientations and lengths evaluating: i) electric behavior under direct current (DC) regime and a wideband analysis by a Frequency Response Analysis; ii) thermo-electrical behavior studying the temperature field evolution under applied electric fields; iii) thermo-mechanical behavior under uniaxial tension for different testing temperatures; iv) thermo-electrical aging/degradation effects on the mechanical behavior. The study considers a wide range of electric fields (from 180 V/m to 600 V/m) and temperatures (from 25° to 130 °C). The results show enhanced electrical conductivity and mechanical stiffness when using a printing orientation parallel to the electro-mechanical loading. This coupled response is highly influenced by thermal effects due to Joule heating, especially when surpassing the glass transition and cold crystallization temperatures of the composite. The occurrence of such phase transitions governs the material degradation by promoting the growth of mesostructural pores during heating cycles arising from the application of electric fields. The obtained results are essential to unravel changes in mechanical properties when standing for continuous electric conduction and provide an experimental background for the lifetime expectance for the devices and possible thermo-electro-mechanical failure.

Study of optoelectronic and thermoelectric properties of double perovskites X2HfI6 (X = Ga, In, Tl) for renewable energy

The double perovskites (DPs) are outstanding materials for renewable energy and other optoelectronic applications. Here in this paper, the thermoelectric and optical behavior of X2HfI6 (X = Ga, In, Tl) have been explained systematically by DFT. The structural stability has been certified using ionic radii in terms of tolerance factor, and dynamic stability has been confirmed by positive phonon dispersion frequencies in acoustic modes. The band structure findings show the direct band gaps for (Ga, In, Tl) based DPs. The optical properties have been investigated by dispersion, absorption, refection, and related other optical characteristics. The absorption bands are existing in ultraviolet region and shifted to higher energy (blue shift). The reflection and optical loss of light are noted less than 0.3 which increases their potential for optoelectronic devices. The high Seebeck coefficient and electrical conductivity along with ultralow lattice thermal conductivity confirm the high figure of merit at and below the room temperature.

Effect of (Sm, In) Doping on the Electrical and Thermal Properties of Sb2Te3 Microstructures.

Doped Sb2Te3 narrow-band-gap semiconductors have been attracting considerable attention for different electronic and thermoelectric applications. Trivalent samarium (Sm)- and indium (In)-doped Sb2Te3 microstructures have been synthesized by the economical solvothermal method. Powder X-ray diffraction (PXRD) was used to verify the synthesis of single-phase doped and undoped Sb2Te3 and doping of Sm and In within the crystal lattice of Sb2Te3. Further, the morphology, structure elucidation, and stability have been investigated systematically by scanning electron microscopy (SEM), Raman analysis, and thermogravimetric analysis (TGA). These analyses verified the successful synthesis of hexagonal undoped Sb2Te3 (AT) and (Sm, In)-doped Sb2Te3 (SAT, IAT) microstructures. Moreover, the comparison of dielectric parameters, including dielectric constant, dielectric loss, and tan loss of AT, SAT, and IAT, was done in detail. An increment in the electrical conductivities, both AC and DC, from 1.92 × 10-4 to 4.9 × 10-3 Ω-1 m-1 and a decrease in thermal conductivity (0.68-0.60 W m-1 K-1) were observed due to the doping by trivalent (Sm, In) dopants. According to our best knowledge, the synthesis and dielectric properties of (Sm, In)-doped and undoped Sb2Te3 in comparison with their electrical properties and thermal conductivity have not been reported earlier. This implies that appropriate doping with (Sm, In) in Sb2Te3 is promising to enhance the electronic and thermoelectric behavior.

Predicting thermoelectric transport properties from composition with attention-based deep learning

Thermoelectric materials can be used to construct devices which recycle waste heat into electricity. However, the best known thermoelectrics are based on rare, expensive or even toxic elements, which limits their widespread adoption. To enable deployment on global scales, new classes of effective thermoelectrics are thus required. Ab initio models of transport properties can help in the design of new thermoelectrics, but they are still too computationally expensive to be solely relied upon for high-throughput screening in the vast chemical space of all possible candidates. Here, we use models constructed with modern machine learning techniques to scan very large areas of inorganic materials space for novel thermoelectrics, using composition as an input. We employ an attention-based deep learning model, trained on data derived from ab initio calculations, to predict a material’s Seebeck coefficient, electrical conductivity, and power factor over a range of temperatures and n- or p-type doping levels, with surprisingly good performance given the simplicity of the input, and with significantly lower computational cost. The results of applying the model to a space of known and hypothetical binary and ternary selenides reveal several materials that may represent promising thermoelectrics. Our study establishes a protocol for composition-based prediction of thermoelectric behaviour that can be easily enhanced as more accurate theoretical or experimental databases become available.

A simple Pb-doping to achieve bonding evolution, VSn and resonant level shifting for regulating thermoelectric transport behavior of SnTe

The intensification of energy crises and environmental pollution inspire researchers' attention to environment-friendly SnTe thermoelectric materials. In this work, we achieved a lower lattice thermal conductivity and optimized the power factor via the synergistic optimization of bonding characteristic, VSn, and resonant level for the SnTe system, respectively. Pb-introduction produces weak bonding strength, mass fluctuation, and stress distortion, which result in lower thermal conductivity. The lowest lattice thermal conductivity achieves 0.66 W m–1 K–1 at 773 K. Further introduced VSn relieves loss of electrical conductivity caused by Pb-introduction, and it also makes the bigger g(E) and up-shift of resonance level. The VSn, enhanced g(E), and resonant level make electrical conductivity and Seebeck coefficient enhance simultaneously. Finally, the further optimization of thermal and electronic transport performance contributes to a higher ZT value of ∼0.86 at 773 K in the Sn0.685Pb0.285In0.015Te0.7Se0.3 sample. The strategy of bonding characteristic, VSn, and resonant level synergistic engineering will be widely applicable to various TE systems for achieving better thermoelectric performance.

Field-free switching model of spin–orbit torque (SOT)-MTJ device with thermal effect based on voltage-controlled magnetic anisotropy (VCMA)

Electrically driven magnetization switch has attracted much attention in the new spintronic memory, especially for spin–orbit torque (SOT)-based magnetic random access memory (MRAM). However, the published models are facing limitations with the continuous shrinkage of the feature size down to nanoscale. Also, the thermal effect caused by switching operation is non-negligible. Therefore, an effective model is needed to represent the switching dynamic of the device concerning the influences of the nanoscale and the thermal effect. In the paper, a compact model of three-terminal SOT-driven switching is established. The influence of the voltage-controlled magnetic anisotropy (VCMA) and spin transfer torque (STT) effect induced by bias voltage on the field-free SOT-driven switching is considered by numerically solving the LLG equations. Furthermore, a 3D model of the SOT-MTJ device is established by finite element method to trace the thermoelectric behavior inside the device. The thermoelectric behavior is integrated into the compact model to show the influence of the temperature on the switching behavior, highlighting the importance of the thermal effect for the realistic modelling of SOT-driven switching. Finally, a novel voltage pulse scheme is proposed, which can effectively shorten the switching time and improve the reliability of the device. The established model could provide strategies and guidelines for next-generation memory design and application.

First principles investigation on half metallic ferromagnetism properties of cubic VLaO3 compound

In this investigation, we thoroughly examined the equilibrium structural, electrical, magnetic, elastic, thermodynamic, as well as thermoelectric behavior of the vanadium lanthanum oxide perovskite VLaO3. The computations were executed with the FP-LAPW method, as in cooperated in the WIEN2k package that rely on the first principles method, using generalized gradient approximation (PBE-GGA). The explored compound has been observed to be ferromagnetically stable i.e., in FM phase as it has lowest energy in FM state than in paramagnetic phase i.e., in NM phase. The thermodynamical and mechanical stability of the studied VLaO3 compound is also assured by evaluating its formation energy, cohesive energy and second order elastic constants. Furthermore, our computed findings reveal that the studied compound belongs to half metallic perovskites with an energy gap of 2.56 eV in the majority spin and possesses a total magnetic moment of 3.99µB/unit cell. Additionally, the thermal as well as pressure dependence of volume, specific heat capacity, and bulk modulus are also examined with the aid of GIBBS which rely on quasi harmonic Debye theory. Finally, the thermoelectric transport properties are also evaluated as a function of chemical potential in the range extending from −2.0 eV to 2.0 eV for different temperatures at 300 K, 700 K and 1200 K to explore budding possibility of this compound for energy harvesting applications.

Spin thermoelectric effects of skyrmions in ferromagnetic topological insulators

The thermoelectric effects of ferromagnetic topological insulators with either two-dimensional circular or one-dimensional domain wall skyrmions are studied theoretically. It is found that the topological spin-textures play a significant role in the manipulation of spin-dependent thermoelectric properties. In the vicinity of the charge neutrality point, spin Seebeck coefficients possess finite values whose sign and magnitude can be tuned by temperature in spite of vanishing charge Seebeck coefficients. The majority of the effects of circular skyrmions occurs in the edge-state transport regime by generating Fano antiresonances. While the domain wall skyrmion primarily influences the thermoelectric behaviors near the boundary between the edge-state and bulk-state transport regimes with the resonant tunneling mechanism. Both types of skyrmions which function effectively in distinct transport regimes have potential applications in thermoelectrics.

Microstructure design in Bi-Ga-Te system using a combination of thermodynamic calculations and experiments for potential thermoelectric material

Tellurium-based alloys are emerging as attractive candidates for designing new thermoelectric materials. Various phases (e.g., Bi2Te3 and Ga2Te3) in the Bi-Ga-Te system exhibit good thermoelectric behavior. Microstructural features especially phase chemistry and fraction in alloy play a critical role in improving thermoelectric properties like the Seebeck coefficient and thermal conductivity. Although pure stoichiometric phases in Bi–Te and Ga–Te systems such as Bi2Te3 and Ga2Te3 are well investigated in the literature, there is limited study on the microstructure design in the ternary Bi-Ga-Te system that will be best suited for thermoelectric behavior. Accordingly, various alloys were developed using the recently developed thermodynamic database of the Bi-Ga-Te system. Alloys were melted in a vacuum induction furnace followed by microstructure characterization. The thermodynamic calculations and microstructural investigations of the selected compositions reveal varying phase fractions of Bi, β, Bi2Te3, Ga2Te3 and Te as both primary and eutectic phase. In addition, differential thermal analysis (DTA) was used to ascertain the phase transitions in alloys, which will assist in determining their thermoelectric operational temperature.

Computational Exploration of Ultralow Lattice Thermal Conductivity and High Figure of Merit in p-Type Bulk RbX2Sb (X = K, Na)

Using density functional theory and the Boltzmann transport equation, we have herein studied the electronic structure and thermoelectric behavior of two bulk bialkali antimonides RbK2Sb and RbNa2Sb. Our calculation reveals that both the antimonides exhibit ultralow lattice thermal conductivities resulting from the intrinsic phonon scattering. The lattice thermal conductivities of RbK2Sb and RbNa2Sb at 700 K (300 K) are found to be 0.150 (0.350) and 0.228 (0.532) W m–1 K–1, respectively. Thermodynamically, mechanically, dynamically, and thermally stable p-type RbK2Sb proclaims high values of the Seebeck coefficient due to the large band effective masses and thus the DOS effective masses of the holes, while p-type RbNa2Sb exhibits a moderate Seebeck coefficient and optimum electrical conductivity due to the relatively lower band effective mass and the high relaxation time (corresponding to high mobility) of the holes. Estimated maximum values of the figure of merit at 700 K (300 K) are 4.40 (1.91) and 5.84 (2.76) for the p-type RbK2Sb and RbNa2Sb, respectively.

Atomic and electronic structures of correlated SrRuO3/SrTiO3 superlattices

Atomic-scale precision epitaxy of perovskite oxide superlattices provides unique opportunities for controlling the correlated electronic structures, activating effective control knobs for intriguing functionalities including electromagnetic, thermoelectric, and electrocatalytic behaviors. In this study, we investigated the close interplay between the atomic and electronic structures of correlated superlattices synthesized by atomic-scale precision epitaxy. In particular, we employ superlattices composed of correlated magnetic SrRuO3 (SRO) and quantum paraelectric SrTiO3 (STO) layers. In those superlattices, RuO6 octahedral distortion is systematically controlled from 167 to 175 degrees depending on the thickness of the STO layers, also affecting the TiO6 octahedral distortion within the STO layer. Customized octahedral distortion within SRO/STO superlattices in turn modifies the electronic structures of both the Ti and Ru compounds, observed by X-ray absorption spectroscopy. Our results identify the close correlation between atomic lattice and electronic structures enabled by the facile controllability of atomic-scale epitaxy, which would be useful for designing future correlated oxide devices.

Theoretical insights into the structural, optoelectronic, thermoelectric, and thermodynamic behavior of novel quaternary LiZrCoX (X = Ge, Sn) compounds based on first-principles study.

The structural, magnetic, electronic, elastic, vibrational, optical, thermodynamic as well as thermoelectric properties of newly predicted quaternary LiZrCoX (X = Ge, Sn) Heusler compounds are evaluated intricately with the aid of ab initio techniques developed under the framework of density functional theory. The computed structural properties are found to be in tandem with the existing analogous theoretical and experimental facts. Structural optimization has been carried out in three different structural arrangements, i.e., Type-1, Type-2, and Type-3. Further analysis of the optimization curves reveals that the Type-3 phase, which has the least amount of energy, is the most stable structure for the compounds under consideration. The tabulated cohesive energy and formation energy of these compounds depict their chemical as well as thermodynamic stability. The absence of negative phonon frequencies in the phonon band spectrum of the studied compounds depicts their dynamic stability. Similarly, the tabulated second-order elastic constants (Cij) and the linked elastic moduli show their stability in the cubic phase. The calculated value of Pugh's ratio and Cauchy pressure reveal that LiZrCoGe is brittle whereas LiZrCoSn is ductile. Additionally, the optical characteristics of the compounds are studied in terms of the dielectric function, refractive index, extinction coefficient, absorption coefficient, reflectivity, energy loss function, and optical conductivity. The obtained high value of power factor and figure of merit of the studied lithium-based quaternary compounds predict good thermoelectric behavior in these compounds. Thus, LiZrCoX (X = Ge, Sn) compounds can therefore be used to create innovative and intriguing thermoelectric materials as well as optoelectronic and energy-harvesting equipment.

One-step post-treatment boosts thermoelectric properties of PEDOT:PSS flexible thin films

• High power factor of 64.4 µW m −1 K −2 is achieved in PEDOT:PSS films at 360 K. • 80 vol.% DMAC solution boosts the electrical conductivity from 5 to 964 S cm −1 . • 0.5 mol L −1 LAA improves the Seebeck coefficient from 18.7 to 25 µV K −1 . • Mott variable range hopping model explains the thermoelectric transport behavior. Developing high-performance poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) significantly widens the practical applications of flexible organic thermoelectric devices, while the water-based co-solvent dopants and/or post-treatments are still rarely studied so far. Here, we develop a one-step post-treatment to improve the power factor of PEDOT:PSS films by using a water-based solution, which is composed of co-solvent (polar solvent dimethylacetamide (DMAC) and deionized water) and organic reducing agent L-ascorbic acid (LAA). The 80 vol.% DMAC solution significantly boosts the room-temperature electrical conductivity of the films from 5 to 964 S cm −1 , while the Seebeck coefficient can be further enhanced from 18.7 to 25 µV K −1 by treating with 0.5 mol L −1 LAA, contributing to a significantly improved power factor of 55.3 µW m −1 K −2 . The boosted electrical conductivity is ascribed to the separated PEDOT and PSS phases triggered by the high dielectric constant and polarity of DMAC; while the improved Seebeck coefficient is attributed to the reduced oxidation degree of PEDOT from the reducing agent LAA, both confirmed by the comprehensive structural and morphological characterizations. Furthermore, a maximum power factor of 64.4 µW m −1 K −2 can be achieved at 360 K and the observed temperature-dependent electrical transport behavior can be well explained by the Mott variable range hopping model. Besides, a flexible thermoelectric device, assembled by the as-fabricated PEDOT:PSS films, exhibits a maximum output power of ∼23 nW at a temperature difference of 25 K, indicating the potential for applying to low-grade wearable electronics.

Intrinsic and strain dependent ultralow thermal conductivity in novel AuX (X = Cu, Ag) monolayers for outstanding thermoelectric applications.

A large power factor and ultralow lattice thermal conductivity in 2D-monolayers of AuX (X = Cu and Ag) are achieved via first principles calculations. Low phonon frequency, small Debye temperature and high Gruneisen parameter limit the intrinsic thermal conductivity of both the studied materials. An ultra-low lattice thermal conductivity of 0.13 (0.30) W m-1 K-1 and 0.66 (1.59) W m-1 K-1 is obtained for unstrained AuCu and AuAg monolayers, respectively, at 700 (300) K, which further reduces to 0.04 (0.09) and 0.26 (0.63) W m-1 K-1 at 6% biaxial tensile strain. Such values of thermal conductivity are lower than the critical thermal conductivity for the state-of-art thermoelectric materials (kl < 2 W m-1 K-1). The peak values of ZT for unstrained monolayers are 2.20 and 1.40, which enhances to 3.61 and 2.91 at 6% strain for AuCu and AuAg monolayers, respectively. Interestingly pudding-mold band textures are found to be responsible for this unusual thermoelectric behaviour. The stability concerns (chemical/dynamic/mechanical) of these monolayers are ensured to stimulate experimental determinations for novel synthesis and possible applications.