Thermoelectric Properties Of Materials Research Articles

Simultaneous realization of high power factor and low thermal conductivity in medium-entropy La-doped (Sr1/3Ba1/3Ca1/3)1-xLaxTiO3 oxides with porous structure

Some studies have proven the effectiveness of high-entropy alloying in reducing lattice thermal conductivity; however, the synchronous degradation in carrier mobility limits the thermoelectric properties of high-entropy materials. Herein, we present a series of medium-entropy oxides (Sr1/3Ba1/3Ca1/3)1-xLaxTiO3 (x = 0.05, 0.1, 0.15) with different levels of La doping, breaking the trade-off between carrier and phonon transport. The medium-entropy design helps keep the carrier mobility at a moderate level while modulating the carrier concentration through La3+ doping, thereby yielding a peak power factor value of 1842 μW/(m⋅K2) at 773 K. Meanwhile, the incorporation of nano-micro-porous structure and medium entropy effect leads to a significant reduction in lattice thermal conductivity, reaching 2.3 W/(m⋅K) at 1073 K. Consequently, a ZTmax value of 0.67 is obtained for the MEP-10La sample at 1073 K, which is one of the state-of-the-art ZT values reported for the SrTiO3-based materials. The present work provides a viable approach for the development of thermoelectric materials with enhanced performance.

Monolayer 1T-Ag6S2 with Excellent Thermoelectric Properties.

We obtained a new material called monolayer 1T-Ag6S2 by replacing metal atoms in 1T phase transition-metal dichalcogenide sulfides (TMDs) with octahedral Ag6 clusters. Subsequently, the thermoelectric transport properties of monolayer 1T-Ag6S2 were systematically investigated using first-principles calculations and the generalized gradient approximation (GGA-PBE) exchange correlation functional. The findings demonstrate that monolayer 1T-Ag6S2 displays characteristics of a wide-bandgap semiconductor, with a bandgap of 2.48 eV. Notably, the incorporation of Ag6 clusters disrupts the structural symmetry, effectively enhancing the electronic structure and phonon properties of the material. Due to the flat valence band near the Fermi level, the extended relaxation time of the hole results in a greater effective mass compared to the electron, leading to a significant increase in the Seebeck coefficient. Under optimal doping conditions, the power factor of monolayer 1T-Ag6S2 can achieve 14.9 mW/mK2 at 500 K. The intricate crystal structure induces phonon path bending, reduces the overall frequency of phonon vibrations (<10 THz), and causes hybridization of low-frequency optical and acoustic branches, resulting in remarkably low lattice thermal conductivity (0.20 and 0.17 W/mK along the x and y axes at 500 K, respectively). The monolayer 1T-Ag6S2 demonstrates a remarkably high figure of merit ZT of 3.14 (3.15) on the x (y) axis at 500 K, significantly higher than those of conventional TMD materials. Such excellent thermoelectric properties suggest that monolayer 1T-Ag6S2 is a promising thermoelectric (TE) material. Our work reveals the deep mechanism of cluster substitution to optimize the thermoelectric properties of materials and provides a useful reference for subsequent research.

Anion/Cation Co-Doping to Improve the Thermoelectric Performance of Sn-Enriched n-Type SnSe Polycrystals with Suppressed Lattice Thermal Conductivity.

Enhancing the thermoelectric performance of n-type polycrystalline SnSe is essential, addressing challenges posed by elevated thermal conductivity and compromised power factor inherent in its intrinsic p-type characteristics. This investigation utilized solid-state reactions and spark plasma sintering techniques for the synthesis of n-type SnSe. A significant improvement in the figure of merit (ZT) is achieved through strategic reduction in Se concentration and optimization of crystal orientation. The co-doping with Br and Ge further improves the material; Br amplifies carrier concentration, enhancing electrical conductivity, while Ge introduces effective phonon scattering centers. In the Br/Ge co-doped SnSe sample, thermal conductivity dropped to 0.38Wm⁻¹K⁻¹, yielding a remarkable power factor of 662µWmK- 2 at 773K, culminating in a ZT of 1.34. This signifies a noteworthy 605% improvement over the pristine sample, underscoring the pivotal role of Ge doping in enhancing n-type material thermoelectric properties. The enhancement is attributed to Br doping introducing additional electronic states near the valence band, and Ge doping modifying the band structure, fostering resonant states near the conduction band. The Br/Ge co-doping further transforms the band structure, influencing electrical conductivity, Seebeck coefficient, and thermal conductivity, advancing the understanding and application of n-type SnSe materials for superior thermoelectric performance.

Exploration of the electronic structure and thermoelectric properties of the carbon-doped bulk GaN materials by first-principles calculations

As a common impurity, carbon was related to the physical properties of GaN-based materials closely. By using density functional theory and Boltzmann transport theory, the electronic structure and thermoelectric properties of carbon doped GaN were studied. The band structure, electronic density of states and thermoelectric properties were calculated. Comparing with the intrinsic bulk-GaN, (1) It was found that the Fermi level moves down, and the band gap becomes narrow for GaN with CN point defects. (2) Power factors become decrease, while thermal conductivity decreases more significantly. Therefore, electrical thermoelectric figure of merit (ZTe) increases, especially, the ZTe increase up to 1.150 at 500 K for GaN with CN point defects. The research could provide reference for experimental studies of GaN-based materials.

Fully Screen‐Printed, Flexible, and Scalable Organic Monolithic Thermoelectric Generators

AbstractEnergy‐harvesting technologies offer a sustainable, maintenance‐free alternative to conventional energy‐storage solutions in distributed low‐power applications. Flexible thermoelectric generators (TEGs) can generate electric power from a temperature gradient, even on complex surfaces. Organic materials are ideal candidates for flexible TEGs due to their good solution‐processability, natural abundance, low weight, and flexibility. Electronic and thermoelectric properties of organic materials have steadily progressed, while device architectures leveraging their advantages are largely missing. Here, a design and fabrication method are proposed for producing fully screen‐printed, flexible monolithic organic TEGs scalable up to m2, compatible with any screen‐printable ink. This approach is validated, along with its scalability, by printing TEGs composed of two different active inks, in three configurations, with up to 800 thermoelements, with performances well matching simulations based on materials parameters. It is demonstrated that by using an additive‐free graphene ink, a remarkable power density of 15 nW cm−2 at ΔT = 29.5 K can be achieved, with an estimated weight‐normalized power output of 1 µW g−1, highlighting a strong potential in portability. Owing to such power density, only limited areas are required to generate microwatts, sufficient for operating low‐power electronic devices such as sensors, and wearables.

Multi-scale parallel and texture interface to enhance thermoelectric performance of p-type Ca3Co4O9 semiconductor materials

Ca3Co4O9 is considered as an attractive material for high-temperature area application. The correlation between structure modulation and electric-thermal transport properties is an important prerequisite to optimize thermoelectric performance of Ca3Co4O9 materials. p-type Ca3Co4O9-based ceramics with texture interface were successfully prepared by tape casting method. The different content of template seeds was added into the Ca3Co4O9 semiconductor, successfully constructing the parallel interfaces. The “brick-wall” structure was formed due to the liquid film existed at grain boundaries. The sample with 15 wt% template seeds additives yielded ∼60 % rise in power factor with a peak value of 0.33 mW/(m·K2), and the low thermal conductivity of 0.97 W/(m·K) was obtained at 800 ℃. Additionally, the ZT value reached 0.34 due to the presence of multi-scale parallel interfaces among the stripe-like grains, which effectively scattered the long-wave, middle-wave and short-wave phonons and strengthened the charge carrier transporting. This texturing method offered a meaningful way for enhancing thermoelectric properties of oxides thermoelectric materials.

Improved thermoelectric properties of Ge2Sb2Te5 films by Bi doping: Experiments and first-principles calculation

The high carrier concentration and low Seebeck coefficient limit the thermoelectric properties of Ge2Sb2Te5 materials. In this paper, Bi-doped Ge2Sb2Te5 thin films with different doping amounts were prepared by magnetron sputtering. The thermoelectric properties of Bi-doped Ge2Sb2Te5 were measured by experiments and predicted by density functional theory and Boltzmann transport theory. The results show that Bi atoms occupy Ge vacancies and bond with Te atoms. The replacement of Ge2+ vacancy with Bi3+ effectively modulates the carrier concentration and improves the Seebeck coefficient. Bi doping increases the degeneracy of the valence band and density of state near the valence band maximum (VBM), and thereby beneficial for the thermoelectric properties of p-type Ge2Sb2Te5, while not desirable for n-type. At 673 K, the power factor of the sample at 20 W doping power reaches 549.56 μWK−2m−1, which is nearly fifteen times that of the intrinsic Ge2Sb2Te5.

Electronic transport and the thermoelectric properties of donor-doped SrTiO3

Strontium titanate (SrTiO3) is widely recognised as an environmentally-benign perovskite material with potential for thermoelectric applications. In this work we employ a systematic modelling approach to study the electronic structure and thermoelectric power factor (PF) of pure SrTiO3 and donor-doped Sr(Ti0.875M0.125)O3 (M = Cr, Mo, W, V, Nb, Ta). We find that the carrier concentration required to optimise the PF of SrTiO3 is on the order of 1021 cm−3, in line with experimental studies. Substitution at the Ti (B) site with 12.5 mol% Nb or Ta is predicted to yield the best PF among the six Group V/VI dopants examined, balancing the Seebeck coefficient and electrical conductivity, and doping with the more abundant Nb would likely give the best price/performance ratio. Although W doping can significantly improve the electrical conductivity, this is at the expense of a reduced Seebeck coefficient. The first-row elements V and Cr have a significantly different impact on the electrical properties compared to the other dopants, forming resonant levels or creating hole carriers and leading to poor thermoelectric performance compared to the second- and third-row dopants. However, the reduction in the bandgap due obtained with these dopants may make the materials suitable for other applications such as photovoltaics or photocatalysis. Our modelling reveals the critical carrier concentrations and best B-site dopants for optimising the electrical properties of SrTiO3, and our predictions are supported by good agreement with available experimental data. The work therefore highlights avenues for maximising the thermoelectric properties of this archetypal oxide material.

Comparative study on properties of n-PbS bulk thermoelectric materials prepared by chemical method

Comparative study on properties of n-PbS bulk thermoelectric materials prepared by chemical method

Investigation of AcXO3 (X = Al, Ga) perovskites for energy harvesting applications: a DFT approach

Full potential linearized augmented plane wave (FP-LAPW) based computational study is presented for AcAlO3 and AcGaO3. Tolerance factor (τG) is 0.93 for AcAlO3 and 0.90 for AcGaO3 which reveal the stability of proposed perovskite oxides in cubic phase. The calculated band structures for both materials reveal the nonmagnetic semiconductive nature with energy band gaps of 3.98 and 2.75 eV for respective Al and Ga based perovskites. Total, and partial density of states (DOS) are computed for meaningful understanding of semiconducting behavior of the proposed perovskites. The partially filled O-2p and Ac-6d states are observed to lie in the vicinity of Fermi level. Furthermore, various parameters of optical spectra have also been computed to study the light matter interaction. Moreover, thermoelectric (TE) properties of both materials have been investigated by using semiclassical Boltzmann theory with constant relaxation time approximation. Calculated figure of merit (ZT) values for AcAlO3 and AcGaO3 are 0.23 and 0.20 at 800 K, respectively. Overall, computational study for titled perovskites indicate that these materials have application in UV sensors, photovoltaic and thermoelectric devices.

Bayesian optimization-driven enhancement of the thermoelectric properties of polycrystalline III-V semiconductor thin films

Studying the properties of thermoelectric materials needs substantial effort owing to the interplay of the trade-off relationships among the influential parameters. In view of this issue, artificial intelligence has recently been used to investigate and optimize thermoelectric materials. Here, we used Bayesian optimization to improve the thermoelectric properties of multicomponent III–V materials; this domain warrants comprehensive investigation due to the need to simultaneously control multiple parameters. We designated the figure of merit ZT as the objective function to improve and search for a five-dimensional space comprising the composition of InGaAsSb thin films, dopant concentration, and film-deposition temperatures. After six Bayesian optimization cycles, ZT exhibited an approximately threefold improvement compared to its values obtained in the random initial experimental trials. Additional analysis employing Gaussian process regression elucidated that a high In composition and low substrate temperature were particularly effective at increasing ZT. The optimal substrate temperature (205 °C) demonstrated the potential for depositing InGaAsSb thermoelectric thin films onto plastic substrates. These findings not only promote the development of thermoelectric devices based on III–V semiconductors but also highlight the effectiveness of using Bayesian optimization for multicomponent materials.

DFT Study of Electronic, Optical, Thermoelectric, and Thermodynamic Properties of the HfO2 Material

DFT Study of Electronic, Optical, Thermoelectric, and Thermodynamic Properties of the HfO2 Material

Recent Developments on Flexible Materials for Thermoelectrics

In recent decades, thermoelectric materials have gained a significant attention due to their ability to directly convert waste heat into electricity. While extensive research has been focused on enhancing the thermoelectric properties of inorganic bulk materials, a new frontier has emerged in this area as flexible thermoelectric materials. These flexible thermoelectrics are driven by the limitations posed by the rigidity, less functional and toxicity of conventional inorganic thermoelectric materials. Despite notable achievements in the development of high-performance flexible thermoelectric materials, they still pose inferior figure of merit (zT) in comparison to their inorganic counterparts. This review encompasses various aspects of flexible thermoelectric materials including theoretical foundations which highlight the principles that explain the concepts and parameters of thermoelectricity to enhance the overall performance, serving as a foundation for understanding the subsequent developments. Further it is overviewed in detail the corresponding state of the art in development of conducting thermoelectric polymers, inorganic-polymer composites, carbon-based thermoelectric and inorganic thermoelectrics on flexible substrates to enhance their thermoelectric performances. Finally it includes the overlook to the future of flexible thermoelectric for wearable thermoelectrics.

Synergistic strategy for enhancing the thermoelectric properties of Bi0.5Sb1.5Te3 with excess Te through low-temperature liquid phase sintering method

The thermoelectric properties of Bi0.5Sb1.5Te3 synthesized through the low-temperature liquid phase sintering (LPS) method can be significantly improved by employing a synergistic approach. During the heat treatment, excess Te undergoes self-swelling under optimal sintering pressure, facilitating the slip of matrix grains perpendicular to the applied pressure and subsequent recrystallization, leading to enhanced electrical properties. Building upon this basis, the heightened Seebeck coefficient and power factor of the thermoelectric matrix stem from the incorporation of 0.6 vol% B4C nanoparticles. Simultaneously, lattice thermal conductivity experiences notable suppression due to enhanced phonon scattering induced by additional interfaces, defects, and second-phase. Lastly, a markedly improved maximum ZT value of 1.30 at 333 K is achieved when the volume percentage of B4C nanoparticles is 0.6 vol%, representing a remarkable 23% enhancement compared to the untreated sample. These results indicate that the LPS method, coupled with the enhanced second nanophase scattering mechanism, is beneficial to enhancing the thermoelectric properties of Bi2Te3-based materials.

Self-contained calibration samples and measurements of the thermoelectric figure of merit: A method to improve accuracy

Despite more than seven decades of active research and development in thermoelectricity, the accurate measurement of the thermoelectric (TE) properties of bulk materials has remained a challenge, mainly because of the strong interrelation between thermal and electrical phenomena. This work highlights practical advancements in methods and instrumentation dedicated to the simultaneous measurements of TE properties such as the Seebeck coefficient (S), the thermal (κ), and electrical (σ) conductivities and the dimensionless TE figure of merit ZT = S2σT/κ. The accuracy of a Harman based approach, as implemented by the ZT-Scanner (TEMTE Inc.), applicable to the simultaneous measurement of the above TE properties, has been made possible by a self-contained calibration procedure, which is based on the availability of two samples of the same homogeneous material having different shape factors. It is of practical importance that this approach provides a simple procedure to obtain the calibration for the figure of merit ZT and the thermal conductivity in the temperature interval from 300 to 720 K. In addition, we show that a simplified Harman setup with no thermocouples attached to the sample can also be used for self-contained calibrated ZT measurements. It is concluded that the implemented steady-state approach decreases the relative error down to 1%–2% for ZT measurements and can be recommended for most applications not involving dynamical behavior. In particular, it is proposed that self-generated calibration samples can critically increase the quality and ease of comparison of TE measurements if they are adopted by the TE community.

Boosting Thermoelectric Performance in Nanocrystalline Ternary Skutterudite Thin Films through Metallic CoTe2 Integration.

Metal-semiconductor nanocomposites have emerged as a viable strategy for concurrently tailoring both thermal and electronic transport properties of established thermoelectric materials, ultimately achieving synergistic performance. In this investigation, a series of nanocomposite thin films were synthesized, embedding metallic cobalt telluride (CoTe2) nanophase within the nanocrystalline ternary skutterudite (Co(Ge1.22Sb0.22)Te1.58 or CGST) matrix. Our approach harnessed composition fluctuation-induced phase separation and in situ growth during thermal annealing to seamlessly integrate the metallic phase. The distinctive band structures of both materials have developed an ohmic-type contact characteristic at the interface, which raised carrier density considerably yet negligibly affected the mobility counterpart, leading to a substantial improvement in electrical conductivity. The intricate balance in transport properties is further influenced by the metallic CoTe2 phase's role in diminishing lattice thermal conductivity. The presence of the metallic phase instigates enhanced phonon scattering at the interface boundaries. Consequently, a 2-fold enhancement in the thermoelectric figure of merit (zT ∼ 1.30) is attained with CGST-7 wt. % CoTe2 nanocomposite film at 655 K compared to that of pristine CGST.

Unphysical grain size dependence of lattice thermal conductivity in Mg3(Sb, Bi)2: An atomistic view of concentration dependent segregation effects

Grain boundary (GB) engineering is one of the most common strategies to reduce the lattice thermal conductivity and improve thermoelectric materials. However, in the case of the promising thermoelectric material Mg3(Sb1−xBix)2, an abnormal dependence of lattice thermal conductivity on grain size was observed at the concentration of x = 0.25, where smaller grain polycrystalline materials exhibited higher lattice thermal conductivity compared to larger grain materials. The proposed theory about the overestimation of lattice thermal conductivity in inhomogeneous materials with GBs cannot clarify the concentration dependence of this anomaly. Here we elucidate the interplay between segregation, concentration, and lattice thermal conductivity in Mg3(Sb1−xBix)2 alloys through atomistic simulations with a highly accurate machine learning interatomic potential. We find the largest segregation of Bi atoms to GBs in Mg3(Sb1−xBix)2 at the concentration of x = 0.25 for both twist and tilt GB structures due to the combination effects of site spectrality and solute interactions. Our molecular dynamic simulations demonstrate that the pronounced segregation of heavier Bi atoms, particularly at x = 0.25, leads to a substantial increase in the lattice thermal resistance at GBs, thus contributing to the degree of inhomogeneity. The concentration dependent segregation reveals the atomic origin of the observed unphysical inverse relationship between grain size and lattice thermal conductivity at the specific concentration of x = 0.25. These results highlight the need to design the alloy concentration to tune the atomic segregation and tailor the thermal properties of thermoelectric materials with GBs.

Innovating in-situ characterization: a comprehensive measurement system for measuring the ZT and the contact resistance of vertical thermolegs exploiting the vertical transfer length method

In order to optimize their system design and manufacturing processes, it is crucial to undertake a thorough electrical and thermal characterization of micro thermoelectric generators (µTEGs). To address this need, a highly advanced and fully integrated in-situ measurement system has been developed. The main objectives of this system are to (1) enable the measurement of ZT and thereby of all thermoelectric (TE) properties of thermolegs made from powder-based TE materials and (2) at the same time accurately measure the contact resistance between the TE material and the electrical contacts. The µTEG fabrication concept used in this study is based on copper-cladded printed circuit board (PCB) material as a substrate, using the Cu layers for easy contact formation. In a first step, an innovative measurement concept, based on a distinctive vertical rendition of the well-established transfer length method, has been realized, allowing for the in-situ measurement of contact resistance between the TE material and the copper conductors on the PCB substrate. This enables a comprehensive assessment of the impact exerted by the applied force and temperature during e.g. a hot-pressing step for compacting the powder-based thermolegs during the manufacturing process. In a second step, a comprehensive measurement platform, referred to as the ZT-Card, has been devised to facilitate the evaluation of all relevant TE material properties—Seebeck voltage, electrical conductivity and thermal conductivity (all measured in vertical cross-plane orientation)—inherent to a highly miniaturized thermoleg. Additionally, the ZT-Card also allows for the assessment of contact resistance between the copper contacts and the TE material. Successful testing of this measurement system inspires confidence in the capabilities of the platform and will aid in future µTEG development.

Optimization of the average figure-of-merit zT in medium-entropy GeTe-based materials via entropy engineering

Entropy engineering has emerged as an effective strategy for improving the figure-of-merit zT by decelerating the phonon transport while maintaining good electrical transport properties of thermoelectric materials. Herein, a high average zT of 1.54 and a maximum zT of 2.1 are achieved in the mid-entropy GeTe constructed by Ag, Sb, and Pb alloying. At room temperature, the mid-entropy GeTe tends to be a cubic structure. And the power factor is improved from 7.7 μW·cm−1·K−2 to 16.2 μW·cm−1·K−2 due to the large increase in effective mass and the optimized carrier concentration. The increasing disorder created by heavy and off-centering Ag, Sb, and Pb atoms induces strong mass/strain fluctuations and phonon scattering to decelerate the phonon transport in GeTe. A low lattice thermal conductivity is obtained in the medium-entropy GeTe-based material. Moreover, a GeTe-based thermoelectric cooler is fabricated with the cooling temperature difference of 66.6 K with the hot end fixed at 363 K. This work reveals the effectiveness of entropy engineering in improving the average zT in GeTe and shows potential application of GeTe as a thermoelectric cooler.

Phonon stability boundary and deep elastic strain engineering of lattice thermal conductivity

Recent studies have reported the experimental discovery that nanoscale specimens of even a natural material, such as diamond, can be deformed elastically to as much as 10% tensile elastic strain at room temperature without the onset of permanent damage or fracture. Computational work combining ab initio calculations and machine learning (ML) algorithms has further demonstrated that the bandgap of diamond can be altered significantly purely by reversible elastic straining. These findings open up unprecedented possibilities for designing materials and devices with extreme physical properties and performance characteristics for a variety of technological applications. However, a general scientific framework to guide the design of engineering materials through such elastic strain engineering (ESE) has not yet been developed. By combining first-principles calculations with ML, we present here a general approach to map out the entire phonon stability boundary in six-dimensional strain space, which can guide the ESE of a material without phase transitions. We focus on ESE of vibrational properties, including harmonic phonon dispersions, nonlinear phonon scattering, and thermal conductivity. While the framework presented here can be applied to any material, we show as an example demonstration that the room-temperature lattice thermal conductivity of diamond can be increased by more than 100% or reduced by more than 95% purely by ESE, without triggering phonon instabilities. Such a framework opens the door for tailoring of thermal-barrier, thermoelectric, and electro-optical properties of materials and devices through the purposeful design of homogeneous or inhomogeneous strains.