Nanostructured Thermoelectrics Research Articles

Physical and chemical properties of low-temperature nanostructured thermoelectric materials on the basis of Bi2Te2.8Se0.2 and Bi0.5Sb1.5Te3

Nanostructured thermoelectric (TE) materials Bi2Te2.8Se0.2 (0.16 wt% CdCl2) n-type and Bi0.5Sb1.5Te3 (2.2 wt% Te and 0.16 wt% TeI4) p-type were fabricated, and complex investigations of these materials were carried out. The thermoelectric materials were obtained by spark plasma sintering (SPS) of nanodispersed powder ground in planetary ball mill. It was found that the average particle sizes in thermoelectric materials after milling for 60 min were about 30 nm. After SPS of the powders, the average crystallite sizes were from 86 to 96 nm. Thermal stability of materials up to 550 K was observed. The relationship between the composition, structure and properties of TE materials was established. The maximum ZT obtained for nanostructured TE materials were ZT = 1.16 at 380 K for Bi2Te2.8Se0.2 and ZT = 1.35 at 370 K for Bi0.5Sb1.5Te3. Increase in ZT for nanostructured thermoelectric materials is due to the decrease of the lattice thermal conductivity.

Enhanced thermoelectric figure of merit in graphene nanoribbons by creating a distortion and transition-metal doping

Thermoelectric (TE) materials have the exceptional ability to directly convert waste heat into electricity and theoretical studies-based guidance is an effective way to investigate high-performance nanostructured TE materials throughout a range of temperatures. Herein, we studied the influence of distortion and transition metal (TM) atoms (V, Cr, Co and Pt) on the electronic and TE properties of armchair graphene nanoribbons (AGNRs) at various temperatures. The results showed that TM-doping and distortion might significantly alter the electronic structure of AGNRs. The band gap and valence band convergence are reduced as a result of the synergistic effect. A higher TE figure of merit results from a combination of decreased heat conductivity and increased electrical conductivity brought about by the electronic structural change. This study proposes that by doping and distortion-induced TE characteristics engineering of AGNR, high-performance nanostructured TE materials may be produced that have a wide range of TE applications.

Empirical test of the Kelvin relation in thermoelectric nanostructures

Thermoelectric (TE) nanostructures with dimensions of ∼100 nm can show substantially better TE properties compared to the same material in the bulk form due to charge and heat transport effects specific to the nanometer scale. However, TE physics in nanostructures is still described using the Kelvin relation (KR) Π = αT, where Π is the Peltier coefficient, α the thermopower, and T the absolute temperature, even though derivation of the KR uses a local equilibrium assumption (LEA) applicable to macroscopic systems. It is unclear whether nanostructures with nanostructures with dimensions on the order of an inelastic mean free path satisfy a LEA under any nonzero temperature gradient. Here, we present an experimental test of the KR on a TE system consisting of doped silicon-based nanostructures with dimensions comparable to the phonon–phonon and electron–phonon mean-free-paths. Such nanostructures are small enough that true local thermodynamic equilibrium may not exist when a thermal gradient is applied. The KR is tested by measuring the ratio Π/α under various applied temperature differences and comparing it to the average T. Results show relative deviations from the KR of |(Π/α)/T − 1| ≤ 2.2%, within measurement uncertainty. This suggests that a complete local equilibrium among all degrees of freedom may be unnecessary for the KR to be valid but could be replaced by a weaker condition of local equilibrium among only charge carriers.

On the electrophoretic deposition of Bi2Te3 nanoparticles through electrolyte optimization and substrate design

Assembly of thermoelectric nanostructures with pre-defined morphology and surface chemistry on solid substrates has been one of the challenges for in-plane TE devices. Electrophoretic deposition (EPD) has the potential to be used for this purpose, where the use of non-conductive substrates is required to enable a reliable evaluation of the transport property of electrically active films. Bi2Te3 nanoparticles, which were synthesized using microwave-assisted hydrothermal route, were used for the EPD of thermoelectric films on glass substrates. A special substrate was fabricated using maskless photolithography, to evaluate the electronic transport properties of the TE films without the interference of the substrate. Electrolyte composition was optimized for high mobility of the suspended nanoparticles, and Bi2Te3 EPD films were fabricated with a high deposition rate, reaching 10 μm/min. Initial EPD films showed high resistivity, ascribed to the surface oxide layer and capping ligands. The resistance was significantly reduced by the addition of a dithiol molecular linker, capable of interconnecting the Bi2Te3 nanoparticles through ligand-exchange. Seebeck coefficient in the range −150 to −180 μV/K was measured, revealing the transport through the deposited films. Finally, a power factor of 169 nW/K2.m was estimated, revealing the potential for the application of this technology to large area TE films as active coatings using the developed EPD process.

Higher-order efficiency bound and its application to nonlinear nanothermoelectrics.

Power and efficiency of heat engines are two conflicting objectives. A tight efficiency bound is expected to give insights on the fundamental properties of such a power-efficiency tradeoff. Here, we derive an upper bound on the efficiency of steady-state heat engines, which incorporates higher-order fluctuations of power. In a prototypical model of nonlinear nanostructured thermoelectrics, we show that the obtained bound is tighter than a well-established efficiency bound derived from the thermodynamic uncertainty relation, demonstrating that the higher-order terms have rich information about the thermodynamic efficiency in the nonlinear regime. In particular, we find that the higher-order bound is exactly achieved if the tight coupling condition is satisfied. The obtained bound gives a consistent prediction with an observation that nonlinearity enhances the power-efficiency tradeoff, and would also be useful in a variety of nanoscale engines.

High thermoelectric figure of merit of porous Si nanowires from 300 to 700\u2009K

Thermoelectrics operating at high temperature can cost-effectively convert waste heat and compete with other zero-carbon technologies. Among different high-temperature thermoelectrics materials, silicon nanowires possess the combined attributes of cost effectiveness and mature manufacturing infrastructures. Despite significant breakthroughs in silicon nanowires based thermoelectrics for waste heat conversion, the figure of merit (ZT) or operating temperature has remained low. Here, we report the synthesis of large-area, wafer-scale arrays of porous silicon nanowires with ultra-thin Si crystallite size of ~4 nm. Concurrent measurements of thermal conductivity (κ), electrical conductivity (σ), and Seebeck coefficient (S) on the same nanowire show a ZT of 0.71 at 700 K, which is more than ~18 times higher than bulk Si. This ZT value is more than two times higher than any nanostructured Si-based thermoelectrics reported in the literature at 700 K. Experimental data and theoretical modeling demonstrate that this work has the potential to achieve a ZT of ~1 at 1000 K.

Tunable thermoelectric properties of free-standing PEDOT nanofiber film through adjusting its nanostructure

Recently, organic thermoelectric (TE) materials have attracted increasing attention in green energy conversion. However, researches on polymer nanostructured TE materials are still scarce, especially for the 1 D nanostructured polymer materials. In this work, 1D poly(3,4-ethylenedioxythiophene) (PEDOT) nanofiber was synthesized by a self-assembled micellar soft-template approach using anionic surfactant sodium dodecyl sulfate (SDS). Then, a flexible and free-standing PEDOT nanofiber film was prepared through a simple vacuum-assisted filtration method. FE-SEM and TEM characteristics real 1D configuration for all the PEDOT nanostructures. Through adjusting the molar ratio of SDS, oxidizing agent FeCl3 and EDOT monomer (nSDS: nFeCl3: nEDOT), nanofiber with different diameter (10–200 nm) and microtopography can be obtained. Electrical conductivity and Seebeck coefficient measurement indicates that the TE properties of PEDOT can be effectively tuned by adjusting its nanostructure, and a variation range as high as three orders of magnitude for the power factors (10-2–101 μW/mK2) can be realized. Due to a more ordered structure caused by the edge-on packing mode of PEDOT chain, the sample prepared with nSDS: nFeCl3: nEDOT = 30:15:7 shows the highest power factor of 7.2 μW/mK2. This value can be further enhanced to 14.4 μW/mK2 by constructing PEDOT nanofiber/SWCNTs composite.

Study of thermal stability of n-type skutterudites Sr0.07Ba0.07Yb0.07Co4Sb12 by differential thermal analysis and Knudsen effusion method

With a high figure of merit, ZT, described in detail in the introduction, multi-filled CoSb3 skutterudites attract attention for their use as thermoelectric materials. In this work, the thermal and phase stability of n-type skutterudites, Sr0.07Ba0.07Yb0.07Co4Sb12, prepared by ball milling, hot pressing and high-pressure torsion or by combinations thereof, with ZT values of about 1.4, have been studied via differential thermal analysis and Knudsen effusion method. The results from evaporation of antimony, strontium, barium and ytterbium as the volatile elements and those from phase transformation measurements are presented. The information, supported by microstructure investigation and measurement of diffusion profiles, is summarized and the long-term operation stability of the studied bulk and nano-structured thermoelectrics is evaluated. Almost no strontium, barium and ytterbium evaporation and only a slight evaporation of antimony demonstrate a long-term operation stability of the bulk and nano-structured thermoelectrics investigated.

Performance Evaluation of a Nanomaterial-Based Thermoelectric Generator with Tapered Legs

A thermoelectric generator (TEG) converts thermal energy to electricity using thermoelectric effects. The amount of electrical energy produced is dependent on the thermoelectric material properties. Researchers have applied nanomaterials to TEG systems to further improve the device’s efficiency. Furthermore, the geometry of the thermoelectric legs has been varied from rectangular to trapezoidal and even X-cross sections to improve TEG’s performance further. However, up to date, a nanomaterial TEG that uses tapered thermoelectric legs has not been developed before. The most efficient nanomaterial TEGs still make use of the conventional rectangular leg geometry. Hence, for the first time since the conception of nanostructured thermoelectrics, we introduce a trapezoidal shape configuration in the device design. The leg geometries were simulated using ANSYS software and the results were post-processed in the MATLAB environment. The results show that the power density of the nanoparticle X-leg TEG was 10 times greater than that of the traditional bulk material semiconductor X-leg TEG. In addition, the optimum leg geometry configuration in a nanomaterial-based TEG is dependent on the operating solar radiation intensity.

Highly Sensitive Self‐Powered H2 Sensor Based on Nanostructured Thermoelectric Silicon Fabrics

AbstractSelf‐powered sensors running on small differences of temperature are considered promising candidates to cover the increasing demand of sustainable and maintenance‐free wireless sensor networks for the Internet of Things (IoT). Under this context, a cost‐effective and self‐powered hydrogen thermoelectric sensor is presented here as a proof of concept for battery‐less IoT nodes. The device is based on low‐density paper‐like fabrics made of functionalized thermoelectric silicon nanotubes able to harvest energy from the heat released by exothermic reactions, such as the hydrogen catalytic oxidation. This gives an accurate value of the reacting gas concentration without any external power requirement. Experimental results confirm that this self‐powered sensor can autonomously measure concentrations as low as 250 ppm of hydrogen in air while generating power densities up to 0.5 μW cm−2 for 3% H2 at room temperature that can be eventually used to store or send the reading. Due to the universality of the concept, this new class of devices will positively contribute toward the development of other advanced self‐powered sensor nodes in the advent of the Internet of things to be used in different safety scenarios.

Tailoring nanostructured NbCoSn-based thermoelectric materials via crystallization of an amorphous precursor

Tailoring nanostructures is nowadays a common approach for enhancing the performance of thermoelectric Heusler compounds by decreasing the thermal conductivity without significantly affecting the electrical conductivity. However, the most widely reported method for obtaining nanostructured thermoelectrics, an approach based on crushing as-cast alloy ingots followed by sintering of the debris, only gives limited control of the final nanostructure due to residual elemental segregation after casting. Here, a novel approach for fabricating nanostructured Heusler compounds is presented, which is based on crystallizing an amorphous precursor of NbCo1.1Sn composition. This method yields two distinct nanostructures, namely one comprising only half-Heusler grains and another one comprising half-Heusler grains and full-Heusler nano-precipitates. The latter sample exhibits enhanced negative Seebeck coefficients as compared to the former over a wide temperature range. Using advanced characterization techniques, such as high-resolution transmission electron microscopy and atom probe tomography, in conjunction with ab initio density functional theory, detailed insights into the nanostructure and electrical properties of the specimens are provided. Filtering of low energy and mobility electrons at the half-Heusler and full-Heusler interface along with the formation of Co interstitial defects in the half-Heusler matrix are proposed to be the possible causes for the enhanced Seebeck coefficient of the nano-precipitate containing specimen.

First-principles study of phonon thermal transport in II–VI group graphenelike materials

Using the first-principles calculations and combining with the phonon Boltzmann transport equation, we systematically investigated the phonon thermal transport properties of II–VI group graphenelike materials (BeO, MgO, CaO, SrS, and SrSe). The lattice thermal conductivities κL’s of 118.68, 42.48, 11.13, 3.33, and 3.09 W/mK are obtained at 300 K in BeO, MgO, CaO, SrS, and SrSe, respectively, which is much lower than that of graphene. Strikingly, the κL of SrS and SrSe is lower than most two-dimensional materials. Further analyses revealed that (i) the lower phonon group velocity of SrS and SrSe caused by a larger atomic mass reduces the κL and (ii) strong anharmonicity and more available phonon scattering channels enhance phonon anharmonic scattering, resulting in lower κL. By means of the relationship between κL and the phonon mean-free path and the width of nanowires, the size dependence of lattice thermal conductivity is investigated as well, which provides a theoretical basis for the design of thermoelectric nanostructures.

Spatially resolving heterogeneous thermal conductivity of BiCuSeO based thermoelectric nanostructures via scanning thermal microscopy

Oxyselenide BiCuSeO is a promising thermoelectric material in the moderate temperature range, and nanostructuring is often adopted to enhance its thermoelectric properties. In these processes, secondary phases can be either induced inadvertently via doping or intentionally processed through nanocomposites, and their effects on local thermal transport remain unresolved from the experimental point of view. In this Letter, we take Pb-doped BiCuSeO and BiCuSeO embedded with rGO (BiCuSeO-rGO) as examples, and we reveal the effect of the secondary phase on local thermal conduction via quantitative scanning thermal microscopy in combination with finite element simulation. It is found that Pb-doping effectively reduces the thermal conductivity of BiCuSeO, while the rGO secondary phase slightly increases its thermal conductivity. Good quantitative agreement is observed between our nanoscale measurement and bulk values reported in the literature. Our work thus not only offers guidance for tuning the thermal transport properties of BiCuSeO but also provides an effective method to spatially resolve heterogenous thermal conductivity at the nanoscale.

Thermoelectric properties of strontium sulfide via first-principles calculations

Thermoelectric materials are attracting increasing attention for their ability to convert heat into cleaner electricity. In this work, we have systematically studied the thermoelectric properties of strontium sulfide (SrS) in the rock-salt structure based on first-principle calculations together with Boltzmann transport theory. The phonon spectra of the rock-salt structure SrS have no imaginary frequency modes, indicating the dynamic stability of SrS. The room-temperature lattice thermal conductivity κι of SrS is calculated to be 10.70 W/mK. We also calculated the phonon-mode contribution to κι, scattering rate, and mean free path (MFP). It is found that the acoustic branches have obvious contribution to the total lattice thermal conductivity, and the MFP can provide guidance for designing thermoelectric nanostructures. The ZT values of SrS with respect to carrier concentration n and temperature T are also obtained. It is found that, when temperature is from 300 K to 700 K, for p-type, the highest ZT value calculated by PBE varies from 0.15 to 0.41, and the highest ZT value by HSE varies from 0.10 to 0.30; For n-type, the highest ZT value by PBE varies from 0.08 to 0.26, and the highest ZT value by HSE varies from 0.09 to 0.28. In general, the p-type SrS has the better thermoelectric property than the n-type SrS.

Spinodal decomposition in (Ti, Zr)CoSb half-Heusler: A nanostructuring route toward high efficiency thermoelectric materials

Half-Heuslers (HH) represent an emerging family of thermoelectric (TE) materials, wherein intrinsic doping enables a wide range of electronic functionalities. In recent years, the solid-state transformation phenomenonon of spinodal decomposition has been actively explored as an effective paradigm to attain bulk nanostructured TE materials via induced phase separation. In the present work, the implication of intrinsic doping and spinodal decomposition on the thermal and electrical transport parameters of nonstoichiometric (Ti, Zr)CoSb HH systems is examined and corroborated with the help of microstructural characteristics. The synthesized HH nanocomposites were found to contain coherent nanoscale heterogeneities along with nanoscale grain, which severely depress the lattice thermal conductivity, while the intrinsic doping due to interstitial Co and defects induced by excess Co off-stoichiometry favorably tunes the electrical transport. A maximum ZT of ∼0.7 at 873 K was obtained for optimized p-type ZrCo1.03Sb0.8Sn0.2 HH nanocomposites, which is among the highest obtained in p-type HH alloys. The present work thus provides a fundamental basis to the understanding of defect engineering and to achieve highly efficient and cost effective HH compositions.

Critical role of atomic-scale defect disorders for high-performance nanostructured half-Heusler thermoelectric alloys and their thermal stability

Atomic-scale defects are essential for improving thermoelectric (TE) performance of most state-of-the-art materials by simultaneously tuning the electronic and thermal properties. However, because the plural atomic-scale defects are generally inherent and disordered in nanostructured TE materials, their complexity and ambiguity on determining TE performance remain a challenge to be solved. Furthermore, the thermal stability of atomic-scale defects in nanostructured TE materials has not been studied much so far. Herein, we report that the atomic-scale defect disorders are indispensable for high TE performance of nanostructured Ti1–xHfxNiSn1–ySby half-Heusler alloys, but gradually degraded at over 773 K, deteriorating the TE performance. It is found from the thermal annealing of nanostructured Ti0.5Hf0.5NiSn0.98Sb0.02 alloys that the annihilation of Ti,Hf/Sn antisite defects primarily reduces atomic-scale defect disorders and largely contributes to the increase of lattice thermal conductivity. Moreover, it is verified that the Ni interstitial defects mainly dominate the electronic transport properties, leading to the enhancement of power factor. Direct atomic structure observations clearly demonstrate the inherent Ni interstitial defects and the thermal vulnerability of Ti,Hf/Sn antisite defects. These results provide an important guide for the application of half-Heusler alloys with highly disordered atomic-scale defects.

Lattice thermal conductivity of Bi2Te3 and SnSe using Debye-Callaway and Monte Carlo phonon transport modeling: Application to nanofilms and nanowires

The present work addresses the problem of thermal conductivity simulation in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and SnSe thermoelectric nanostructures. It first details phonon lifetime calculation in both thermoelectric compounds assuming polynomial dispersion properties and the Debye-Callaway model for the relaxation-time approximation. For both materials, distinct crystallographic directions are considered, i.e., $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}Z$ (trigonal axis) and $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}X$ (basal plane) for ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}X$ ($a$ axis), $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}Y$ ($b$ axis), and $\mathrm{\ensuremath{\Gamma}}\ensuremath{-}Z$ ($c$ axis) for SnSe. On this basis, the lifetime model is parametrized and bulk thermal conductivity is computed through the resolution of the phonon Boltzmann transport equation with a Monte Carlo method. Gr\"uneisen parameter and mass-disorder lifetime are adjusted to fit experimental temperature dependence. The second part of the study addresses the calculation of thermal conductivity for these two thermoelectric materials in the case of thin films (cross-plane case) and nanowires. The main goals of this work are to provide a fully parametric description of heat transport in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and SnSe nanofilms and nanowires showing reliable behavior on an extended size range, from 20 nm to $2\phantom{\rule{0.0pt}{0ex}}\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$, and large temperature range (100--500 K for ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$; 200--800 K for SnSe). Comparisons to bulk thermal conductivity calculations and measurements as well as to recent investigations on nanowires, demonstrate the effectiveness of the proposed methodology to deal with nanostructured ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and SnSe.

Optical and transport properties of few quintuple-layers of Bi2-xSbxSe3 nanoflakes synthesized by hydrothermal method

Hydrothermal synthesis as a commonly bottom-up growth method has considerable advantages for manufacturing thermoelectric nanomaterials with advanced thermoelectric properties. However, the hydrothermally synthesized thermoelectric nanostructures often show a low thermoelectric performance due to their low power factor. In this work, we report on using a hydrothermal method for the growth of n-type Bi2-xSbxSe3 nanoflakes with a fixed thickness of ∼16 quintuple-layers. The controlling of the stoichiometric composition, phase purity and crystallinity of the Bi2-xSbxSe3 nanoflakes are demonstrated by the X-ray diffraction, Raman spectroscopy, and high resolution transmission electron microscopy. We further prove that adding of antimony into Bi2Se3 compound mostly influences the in-plane vibration mode. The optical energy gap is sharply increased as the Sb content increases. The effect of the antimony incorporation on the electrical conductivity, Seebeck coefficient and power factor of Bi2-xSbxSe3 nanoflakes is systematically investigated. The Bi1.92Sb0.08Se3 sample is found to have the highest power factor ∼13.17 μW/cm.K2 at 470 K which is much higher than those published for other various nanostructured or bulk Bi2Se3 compounds. The results propose a great prospect for further enhancing the thermoelectric power factor of the Bi2Se3 nanostructures synthesized by this hydrothermal method. Taking into consideration the progress in Bi–Se compounds, the results of this work advocate the promise of Bi–Se nanostructures towards producing high performance thermoelectric devices.

Grain Boundary Engineering Nanostructured SrTiO3 for Thermoelectric Applications

AbstractNanostructuring to reduce thermal conductivity is among the most promising strategies for designing next‐generation, high‐performance thermoelectric materials. In practice, electrical grain boundary resistance can overwhelm the thermal conductivity reduction induced by nanostructuring, which results in worse overall performance. Since a large body of work has characterized the transport of both polycrystalline ceramics and single crystals of SrTiO3, it is an ideal material system for conducting a case study of electrical grain boundary resistance. An effective mass model is used to characterize the transport signatures of electrical grain boundary resistance and evaluate thermodynamic design principles for controlling that resistance. Treating the grain boundary as a secondary phase to the bulk crystallites explains the transport phenomena. Considering that the interface can be engineered by controlling oxygen partial pressure, temperature, and the addition of extrinsic elements into the grain boundary phase, the outlook for SrTiO3 as a nanostructured thermoelectric is promising, and the zT could be greater than 0.5 at room temperature.

Diffusive Phonons in Nongray Nanostructures

Nanostructured semiconducting materials are promising candidates for thermoelectrics (TEs) due to their potential to suppress phonon transport while preserving electrical properties. Modeling phonon-boundary scattering in complex geometries is crucial for predicting materials with high conversion efficiency. However, the simultaneous presence of ballistic and diffusive phonons challenges the development of models that are both accurate and computationally tractable. Using the recently developed first-principles Boltzmann transport equation (BTE) approach, we investigate diffusive phonons in nanomaterials with wide mean-free-path (MFP) distributions. First, we derive the short MFP limit of the suppression function, showing that it does not necessarily recover the value predicted by standard diffusive transport, challenging previous assumptions. Second, we identify a Robin type boundary condition describing diffuse surfaces within Fourier's law, extending the validity of diffusive heat transport in terms of Knudsen numbers. Finally, we use this result to develop a hybrid Fourier/BTE approach to model realistic materials, obtaining good agreement with experiments. These results provide insight on thermal transport in materials that are within experimental reach and open opportunities for large-scale screening of nanostructured TE materials.