Thermoelectric Behavior Research Articles

Ionic Dopant‐Induced Ordering Enhances the Thermoelectric Properties of a Polythiophene‐Based Block Copolymer

AbstractConjugated polymer‐based block copolymers (CP‐BCPs) are an unexplored class of materials for organic thermoelectrics. Herein, the authors report on the electronic conductivity (σ) and Seebeck coefficient (α) of a newly synthesized CP‐BCP, poly(3‐hexylthiophene)‐block‐poly (oligo‐oxyethylene methacrylate) (P3HT‐b‐POEM), upon solution co‐processing with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and subsequently vapor‐doping with a molecular dopant, 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ). It is found that the addition of the hydrophilic block POEM greatly enhances the processability of P3HT, enabling homogeneous solution‐mixing with LiTFSI. Notably, interactions between P3HT‐b‐POEM with ionic species significantly improve molecular order and unexpectedly cause electrical oxidizing doping of P3HT block both in solution and solid‐states, a phenomenon that has not been previously observed in Li‐salt containing P3HT. Vapor doping of P3HT‐b‐POEM‐LiTFSI thin films with F4TCNQ further enhances σ and yields a thermoelectric power factor PF = α2σ of 13.0 µW m−1 K−2, which is more than 20 times higher than salt‐free P3HT‐b‐POEM sample. Through modeling thermoelectric behaviors of P3HT‐b‐POEM with the Kang‐Snyder transport model, the improvement in PF is attributed to higher electronic charge mobility originating from the enhanced molecular ordering of P3HT. The results demonstrate that solution co‐processing CP‐BCPs with a salt is a powerful method to control structure and performance of organic thermoelectric materials.

Solution‐state doping‐assisted molecular ordering and enhanced thermoelectric properties of an amorphous polymer

Molecular doping of conjugated polymers is the performance-determining step in the fabrication of organic thermoelectric devices. Although strongly oxidizing salt-type dopants effectively produce charge carriers in polymer chains, their poor solubility in the processing solvents leads to undesired precipitation and complicates the film fabrication process. Thus, it is important to develop a conjugated polymer that can be readily doped in a mixed solution using organic dopants. In this study, we synthesized amorphous but highly electron-rich polymers (PCT1 and PCT2) by combining 4H-cyclopenta[2,1-b:3,4-b']dithiophene and thieno[3,4-b]thiophene moieties, whose full-names are poly[(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl)-alt-(2-ethylhexyl-3-fluorothieno[3,4-b]thiophene-2-carboxylate 4,6-diyl)] (PCTs). The PCT polymers were heavily doped with the organic dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), and the resulting PCT:F4TCNQ complex formed an integer charge transfer complex and promoted the intermolecular ordering of PCT polymers. Notably, the PCT:F4TCNQ complex exhibited unique thermoelectric behaviors, with significantly increased power factors even in the presence of large quantities of F4TCNQ, reaching values 3.2-fold higher than those achieved with FeCl3 sequential doping. Moreover, the higher-molecular-weight polymer, PCT2, exhibited more efficient molecular ordering and charge carrier transport than PCT1, which are crucial for enhancing the thermoelectric properties.

Half-metallic and thermoelectric properties of Sr2EuReO6

Structural parameters, electronic band structure, half-metallic and thermoelectric properties of Sr2EuReO6 compound materials are investigated. The computations are performed using density-functional calculations within the generalized gradient approximation. Electronic results show half-metallic nature with majority spin channels as metallic and minority spin channels as semi-conducting. Magnetic moment calculated within GGA + U was found equal to 8.00 μB. The half metallicity is attributed by the double-exchange interaction mechanism via the Re(5d)–O(2p)–Eu(4f) hybridization. Boltzmann theory is employed to check the thermoelectric response, where Seebeck coefficient of 297.97 μVK−1, electrical conductivity of 44.96 × 1019 Ω−1m−1s−1 and lattice thermal conductivity of 5.94 × 1015 W/mK−1s−1 is seen. The thermoelectric efficiency measured through ZT calculation with a value of 1 at 300 K, indicating that Sr2EuReO6 can work at low as well as high temperature thermoelectric devices operations. Based on our calculations, Sr2EuReO6 combine both good spintronic and thermoelectric behaviors that may be used in spin injection applications.

Comparative analysis of thermoelectric behavior of molybdenum silicide (MoSi2) and bismuth telluride (Bi2Te3)

A comparative analysis of the thermoelectric behavior of Molybdenum Silicide and Bismuth Telluride using the COMSOL-Multiphysics platform is presented. Parameters like temperature distribution, potential distribution, figure of merit all the way through the length of the model and isothermal contours for these materials are reported. The outcomes of the modeling and simulation have revealed the potential use of Bismuth Telluride in thermoelectric applications at low temperature applications whereas Molybdenum Silicide is utilized as thermoelectric materials at high temperatures applications.

Ohmic Heating of Basil-based Sauces: Influence of the Electric Field Strength on the Electrical Conductivity

The Moderate Electric Field (MEF) processing of foods consists in the application of an electric potential gradient (????/??) ranging from 1 to 1000 V/cm on a food item (homogeneous or heterogeneous) placed between two electrodes, its main effect being the food heating due to the dissipation of a part of the electric energy into heat within the food item. The heating performances of such a system depend on several process and system parameters, including the applied ????/??, the food electrical conductivity, and its thermo-physical properties. In this study, the effects due to the salt composition and to the applied ????/?? to a heterogeneous food (constituted by a basil-based sauce, mainly fibers dispersed in a slightly salted water-oil emulsion) treated in a custom MEF system on the food heating rate are investigated. The samples were prepared at different salinities (3.25, 1.63, 0.86 and 0.43% w/w respectively). In the explored range of compositions, the heating rate increased linearly with the square power of applied ????/??. A slight linearity deviation above 55°C was observed for the basil-based sauce at 1.63% and 5.20 V/cm, associated with bubble formation within the ohmic system and the electrolytic reactions occurring at the electrode-solution interface during the MEF heating process.The salt content as well as the ratio between water and oil in the sample formulation played a crucial role in determining the thermo-electrical behavior of the basil-based sauce samples. Samples with salinity of 1.63%, compared to samples at 3.25%, exhibited a higher electrical conductivity, being due to a minor concentration of the non-conductive phase (namely the oil phase as well as the dispersed vegetable fibers into the solution) that exerts a major degree of electrical insulation. As the salinity decreases from 1.63% to 0.43%, samples were characterized by lower electrical conductivities, being due to a reduced ionic mobility when the salt contained into the sample is drastically reduced.

Numerical modeling of a new integrated PV-TE cooling system and support

In this article, an innovative cooling system for photovoltaic panels is presented. This system uses the Seebeck effect to generate electricity. The proposed device differs from existing photothermoelectric systems by means of a compact, efficient and space-saving apparatus.In fact, in the proposed device the thermoelectric generator is integrated in the heat exchange system since the thermoelectric effect takes place inside the heat exchanger and only a small part of the removed heat is used to create the required temperature difference. A preliminary numerical analysis of the thermoelectric behaviour of the proposed device under different geometrical and fluid-dynamic conditions is also presented. For a standard photovoltaic panel of 100 ​× ​125 cm the proposed cooling system allows an increase of almost 15% of the electrical power converted by the cells. Moreover, the exploited Seebeck effect provides an electrical power (ranging from 61.2 to 71.2 ​W in the studied cases) that is respectively 10.9 and 1.33 times the power required for forced ventilation. The maximum system electrical power reachable, using commercial inorganic thermoelectric materials, considering all electrical power gains and losses is next to 300–310 ​W/m2.

An insight into the electronic, optical and transport properties of promising Zintl-phase BaMg2P2

Electronic structure, optical and transport properties of Zintl phase BaMg2P2 compound are investigated theoretically using density functional theory as implemented in WIEN2k code. Comprehensive study on the aforementioned properties is carried out using local density (LDA) and generalized gradient approximations (GGA) along with the recently developed modified Backe-Johnson (mBJ) exchange potential. Zintl-Phase BaMg2P2 shows a semiconducting character having band-gap value of 0.54, 0.68 and 1.07 eV as calculated with LDA, GGA and mBJ, respectively. Optical properties are also calculated with the respective potentials. Thermoelectric response of the material is also assessed with the help of BoltzTrap computational code. The calculated power factor shows an increase with the temperature with mBJ potential for the considered Zintl phase BaMg2P2. Moreover, the figure of merit (ZT) shows maximum value of 0.8 at high temperatures with mBJ potential, which shows a strong thermoelectric behavior of the compound recommending it as a potential candidate for renewable energy devices.

Anomalous thermopower oscillations in graphene-nanowire vertical heterostructures

Thermoelectric measurements have the potential to uncover the density of states (DOSs) of low-dimensional materials. Here, we present the anomalous thermoelectric behavior of monolayer graphene-nanowire (NW) heterostructures, showing large oscillations as a function of the doping concentration. Our devices consist of InAs NW and graphene vertical heterostructures, which are electrically isolated by thin (∼10 nm) hexagonal boron nitride (hBN) layers. In contrast to conventional thermoelectric measurements, where a heater is placed on one side of a sample, we use the InAs NW (diameter ∼50 nm) as a local heater placed in the middle of the graphene channel. We measure the thermoelectric voltage induced in graphene due to Joule heating in the NW as a function of temperature (1.5–50 K) and carrier concentration. The thermoelectric voltage in bilayer graphene (BLG)-NW heterostructures shows sign change around the Dirac point, as predicted by Mott’s formula. In contrast, the thermoelectric voltage measured across monolayer graphene (MLG)-NW heterostructures shows anomalous large-amplitude oscillations around the Dirac point, not seen in the Mott response derived from the electrical conductivity measured on the same device. The anomalous oscillations are a signature of the modified DOSs in MLG by the electrostatic potential of the NW, which is much weaker in the NW-BLG devices. Thermal calculations of the heterostructure stack show that the temperature gradient is dominant in the graphene region underneath the NW, and thus sensitive to the modified DOSs resulting in anomalous oscillations in the thermoelectric voltage. Furthermore, with the application of a magnetic field, we detect modifications in the DOSs due to the formation of Landau levels in both MLG and BLG.

Understanding the Temperature Dependence of the Seebeck Coefficient from First-Principles Band Structure Calculations for Organic Thermoelectric Materials

Understanding the Temperature Dependence of the Seebeck Coefficient from First-Principles Band Structure Calculations for Organic Thermoelectric Materials

Disassembly Study of Ultrasonically Welded Thermoplastic Composite Joints via Resistance Heating.

This manuscript explores the disassembly potential of ultrasonically welded thermoplastic composite joints for reuse or recycling through resistance heating via a nanocomposite film located at the welded interface. Nanocomposite films containing multi-walled carbon nanotubes (MWCNTs) were characterized for thermo-electrical behavior to assess self-heating. It was generally observed that maximum temperature increased with MWCNT and film thickness. To demonstrate potential for disassembly, glass fiber/polypropylene adherends were welded with nanocomposite films. Shear stress during disassembly was measured for three initial adherend’s surface temperatures. It was found that the required tensile load decreased by over 90% at the highest temperatures, effectively demonstrating the potential for disassembly via electrically conductive films. Fracture surfaces suggested that disassembly was facilitated through a combination of nanocomposite and matrix melting and weakened fiber–matrix interface. Limitations, such as slow heating rates and the loss of contact at the interface, imply that the method could be more suited for recycling, instead of repair and reuse, as the heat-affected zone extended through the adherends’ thickness at the overlap during heating.

ANALYTICAL AND NUMERICAL ANALYSES OF THE HEAT CONDUCTION OF CARBON NANOTUBES UNDER APPLIED VOLTAGES

Electron field emission experiments of carbon nanotubes (CNT) show that due to heat CNTs breakdown faster under applied voltage than expected. Therefore, different systems with varying temperature distributions are considered analytically and numerically by use of the heat conduction equation. This methodology yields a phenomenological understanding and description of the thermoelectric behavior of CNTs under applied voltage

Numerical investigations of heat transfer and skin effect characteristics within rods located in a 48-rod Siemens reactor heated by AC

Abstract In the polysilicon reduction furnace, all the energy is supplied by the heat generation on silicon rods which are heated up by the passage of electric current. The alternating current (AC) produces the alternating magnetic field, and the oscillating magnetic field induces the corresponding eddy current in the conductor, which causes the phenomenon of skin effect. In the present paper, the coupling model of thermal and electromagnetic field for the silicon rods has been developed by coupling Joule heating equation controlled by frequency and heat dissipation (radiation, convection and reaction energy). Comparing the calculated results with the industrial data, it is found that the average relative errors of the current intensities of the inner, middle and outer ring silicon rods are 7.55%, 5.55% and 7.43%, respectively, indicating that the model calculation results are accurate and can be used to accurately analyze the thermoelectric behavior of the silicon rods. Based on the Joule heating process of silicon rods in a 48-rod polysilicon reduction furnace, the influences of AC frequency on temperature and current density profile within rod arranged in different rings have been studied. The interesting results show that the high frequency current sources can generate an even temperature profile and an isothermal region inside the rod can be obtained when the RF increases to 10. Therefore, utilizing the high frequency current sources can decrease the temperature gradient within silicon rod effectively, which provides a method for producing larger diameter silicon rod. Furthermore, the voltage-current curves of low frequency (50 Hz) and high frequency (10 kHz and 50 kHz) can be predicted by the present Joule heating model, at the same time, a hybrid heating process of low frequency and high frequency is proposed.

Inelastic Tunnel Transport and Nanoscale Junction Thermoelectricity with Varying Electrode Topology

AbstractIntricacies in the charge transport behavior of single‐molecule junctions often revolve around the complex nature of orbital hybridization stemming from diversified metal‐molecule coupling at interfaces. It will hence be pertinent to fathom the myriad aspect of inelastic charge transport and nanoscale thermoelectricity in single‐molecule junctions. It is examined from first‐principles, achiral and chiral quasi‐1D electrodes to understand the role of junction heterogeneity in overall thermoelectric behavior during charge transport. This calculated inelastic electron tunneling spectra exhibit intense peaks in the low‐frequency regime (< 25 meV) with characteristic bending and rocking modes, which auger well with the available measured data. It is subsequently found that the thermopower can be as high as 212 µV K−1 for σ‐saturated molecular moieties, depending strongly on the configuration of electrodes. Besides, a majority of such single‐molecule junctions turn out to deviate from the Wiedemann–Franz law, leading to anomalies in the electronic thermal conductance. The authors' analysis suggests that as nanoscale electrodes, Au(111) nanowires are likely to render a relatively high thermoelectric figure of merit (ZT > 1) for a small molecular junction at room temperature. Discerning the diversity as driven exigently by the electrode topology may thus augment the device performance in terms of thermal stability at molecular scale.

Versatile Vanadium Doping Induces High Thermoelectric Performance in GeTe via Band Alignment and Structural Modulation

AbstractOwing to the moderate energy offset between light and heavy band edges of the rock‐salt structured GeTe, its figure‐of‐merit (ZT) can be enhanced by the rational manipulation of electronic band structures. In this study, density functional theory calculations are implemented to predict that V is an effective dopant for GeTe to enlarge the bandgap and converge the energy offset, which suppresses the bipolar conduction and increases the effective mass. Experimentally, V‐doped Ge1−xVxTe samples are demonstrated to have an enhanced Seebeck coefficient from ≈163 to ≈191 µV K−1. Extra alloying with Bi in Ge1−x−yVxBiyTe can optimize the carrier concentration to further enhance the Seebeck coefficient up to ≈252 µV K−1, plus an outstanding power factor of ≈43 µW cm−1 K−2. Comprehensive structural characterization results also verify the refinement of grain size by V‐doping, associated with highly dense grain boundaries, stacking faults, nanoprecipitates, and point defects, reinforcing the wide‐frequency phonon scattering and in turn, securing an ultralow thermal conductivity of ≈0.59 W m−1 K−1. As a result, the Ge0.9V0.02Bi0.08Te sample shows a peak ZT of >2.1 at 773 K, with an average plateaued average ZT of >2.0 from 623 and 773 K, which extends better thermoelectric behavior for GeTe over a wider temperature range. This study clarifies the multiple benefits of V‐doping in GeTe‐based derivatives and provides a framework for a new‐type of high‐performance middle‐temperature thermoelectric material.

Dopant-Assisted Matrix Stabilization Enables Thermoelectric Performance Enhancement in n-Type Quantum Dot Films.

Efficient thermoelectric generators require further progress in developing n-type semiconductors that combine low thermal conductivity with high electrical conductivity. By embedding colloidal quantum dots (CQDs) in a metal halide matrix (QDMH), the metal halide matrix can enhance phonon scattering, thus suppressing thermal transport; however, simultaneously achieving high electrical conductivity in such systems has previously been limited by the deleterious impact of a large density of interfaces on charge transport. Therefore, new strategies are needed to improve charge carrier transport without sacrificing matrix-enabled low thermal transport. Here, we report the use of chemical doping in the solution state to improve electron transport while maintaining low thermal transport in QDMH films. By incorporating cesium carbonate (Cs2CO3) salts as a dopant prior to matrix formation, we find that the dopant stabilizes the matrix in colloidal inks and enables efficient n-type doping in QDMH films. As a result, this strategy leads to an enhanced n-type thermoelectric behavior in solution-processed QDMH films near room temperature, with a thermal conductivity of 0.25 W m-1 K-1-significantly lower than in prior films based on organic-ligand-cross-linked CQD films (>0.6 W m-1 K-1) and spark-plasma-sintered CQD systems (>1 W m-1 K-1). This study provides a pathway to developing efficient n-type thermoelectric materials with low thermal conductivity using single-step deposition and low-temperature processing.

Influence of cation size on the thermoelectric behavior of salt-doped organic nanocomposite thin films

Developing and understanding novel doping strategies for thermoelectric materials is imperative to efficiently convert waste into a useful voltage. One such method for improving the power factor of polymer nanocomposites is through salt doping. The cation size of a monovalent salt dopant was varied in a layer-by-layer (LbL)-assembled film composed of poly(diallyldimethylammonium chloride) (PDDA) and double-walled carbon nanotubes (DWNTs) stabilized by poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) [PEDOT:PSS]. Doping a 20 bilayer PDDA/DWNT-PEDOT:PSS film doped with 3 mmol CsCl yields the maximum power factor of 485 ± 29 μW m−1 K−2. This value was obtained through a five times increase in the electrical conductivity with a minimal decrease in the Seebeck coefficient relative to the undoped film. Cs+ is believed to more heavily dope the carbon nanotube network due to its relatively larger hydrophobicity, while also separating PEDOT from PSS due to charge screening. This study demonstrates the significance of the salt dopant identity, and the insight herein can likely be applied more broadly to improve a variety of organic thermoelectric materials.

Essential Material Knowledge and Recent Model Developments for REBCO-Coated Conductors in Electric Power Systems.

The manufacturing of commercial REBCO tapes, REBCO referring to Rare-earth barium copper oxide, has matured enough to lead to a variety of applications ranging from scientific instruments to electric power systems. In particular, its large current density with a high n index and low hysteresis losses make it a strong candidate for specific applications relying on the dependence of its resistance on current. Despite its advantages, there are still issues that remain to be addressed, such as the scarcity of experimental data for the basic characteristics of the superconductor over a wide range of temperature and applied magnetic field, the inhomogeneity of these characteristics along the conductor length, as well as the anisotropy of the critical current and n index with respect to the direction of the applied magnetic field. To better utilize the technology, it is therefore sensible to understand the relevancy of these issues so that one could simulate as accurately as possible the physics of the superconductor, at least the dynamics that may impact the correct operation of the superconducting device. There are different levels of modelling to achieve such a goal that can either focus on the performance of the superconductor itself, or on the whole device. The present work addresses some of the latest developments in the modelling of commercial REBCO tapes in power systems with a particular focus on the thermoelectric behavior of superconducting devices connected to external circuits. Two very different approaches corresponding to two different scales in the modelling of superconducting devices are presented: (1) analysis using equivalent models and lumped parameters to study the thermoelectric response of superconducting devices as a whole, (2) Finite Element Analysis (FEA) to compute distributed fields such as current density, magnetic flux density and local losses in tapes. In this context, this paper reviews both approaches and gives a broad variety of examples to show their practical applications in electric power systems. Firstly, they show the relevance of the technology in power systems engineering. Secondly, they allow inferring the necessary level of model details to optimize the operation of superconducting power devices in power grids. This level of details relies completely on the knowledge of some basic measurable properties of superconducting tapes (critical current and n index) and their cooling conditions.

Comparing half-metallic, MOKE, and thermoelectric behavior of the CrTiZ (Z = As, P) half-Heuslers: a DFT study

Structural, half-metallic, magneto-optic, and thermoelectric properties of CrTiZ (Z = As, P) half-Heusleres compounds are investigated based on density functional theory. These compounds have mechanical stability in the ferromagnetic state with a high bulk modulus. They are often half-metallic with a large and integer magnetic moment and are very attractive in spintronics, magneto-optics applications. The magnetic moments of CrTiAs and CrTiP were 2.9865 μ B and 3.00 μ B, respectively, which were attributed to their ferromagnetic phase. Additionally, the positive sign of the phonon branches indicates the dynamic stability of these compounds. Applying both GGA and mBJ approximations, CrTiAs and CrTiP compounds exhibited a half-metallic nature by 100% spin polarization. The Kerr angle obtained from magneto-optic results demonstrated a high-intense peak for these compounds in the visible edge with a negative sign. Eventually, a figure of merit with a value above the room temperature was found for both compounds in which the holes are charge carriers.

Pressure dependence of electronic, optical and thermoelectric properties of RbTaO3 perovskite

The effect of pressure (0-75GPa) applied on RbTaO3 perovskite is investigated using density functional theory (DFT) and an indirect to direct band gap transformation is observed. The influence of band gap transformation on the corresponding electronic, optical and thermoelectric behaviors is analyzed. The application of modified Becke and Johnson (mBJ) potential confirms that most precise electronic properties are reported, because various other functionals compute properties not agreeing to the experiments. The increasing pressure suppresses the cubic unit cell dimensions, resultantly, the lattice constant decays. The reducing lattice constant results in strongly interacting states shifted in energy, due to which, the indirect band gap increases in the visible energy range and becomes direct at 75GPa illustrating potential optoelectronic applications. The thermoelectric characteristics investigated for RbTaO3 at 0GPa and 75GPa also reveal thermoelectric device applications.

2D Nb2SiTe4 and Nb2GeTe4: promising thermoelectric figure of merit and gate-tunable thermoelectric performance

Recently, the experimentally synthesized Nb2SiTe4 was found to be a stable layered narrow-gap semiconductor, and the fabricated field-effect transistors (FETs) based on few-layers Nb2SiTe4 are good candidates for ambipolar devices and mid-infrared detection (Zhao et al 2019 ACS Nano 13 10705–10). Here, we use first-principles combined with Boltzmann transport theory and non-equilibrium Green’s function method to investigate the thermoelectric transport coefficients of monolayer Nb2XTe4 (X = Si, Ge) and the gate voltage effect on the thermoelectric performance of the FET based on monolayer Nb2SiTe4. It is found that both monolayers have large p-type Seebeck coefficients due to the ‘pudding-mold-type’ valence band structure, and they both exhibit anisotropic thermoelectric behavior with optimal thermoelectric figure of merit of 1.4 (2.2) at 300 K and 2.8 (2.5) at 500 K for Nb2SiTe4 (Nb2GeTe4). The gate voltage can effectively increase the thermoelectric performance for the Nb2SiTe4-based FET. The high thermoelectric figure of merit can be maintained in a wide temperature range under a negative gate voltage. Furthermore, the FET exhibits a good gate-tunable Seebeck diode effect. The present work suggests that Nb2XTe4 monolayers are promising candidates for 2D thermoelectric materials and thermoelectric devices.