Typical Thermoelectric Materials Research Articles

Significant enhancement in thermoelectric performance of Mg3Sb2 from bulk to two-dimensional mono layer

Thermoelectric materials can effectively convert waste heat into electrical energy and receive more and more attention. Zintl phase Mg3Sb2 is a typical layered thermoelectric material and the anisotropy in crystalline structure induces strong anisotropy in its thermoelectric properties. As a result, polycrystalline Mg3Sb2 usually can not achieve the maximum thermoelectric performance. Our research proves that the dimensionality reduction in material structure can not only avoid certain crystalline direction's poor thermoelectric properties but also effectively reduce the lattice thermal conductivity. Within the framework of the density functional theory, the thermoelectric performance of two-dimensional Mg3Sb2 mono layer is accurately investigated. The ZT value of the Mg3Sb2 nano-thin film can reach 2.5 at 900 K, which is larger than that of the bulk. The theoretical results indicate that the process of nano-engineered structure can significantly enhance the energy conversion efficiency of thermoelectrics.

A promising thermoelectric response of fully compensated ferrimagnetic spin gapless semiconducting Heusler alloy Zr2MnAl at high temperature: DFT study

In recent times, Heusler compounds have emerged as potential candidates for spintronics and thermoelectric materials owing to their interesting properties such as gapless semiconducting nature with fully compensated ferrimagnetic behavior, 100% spin polarization with high mobility of charge carriers and better thermoelectric figure of merit. Motivated by this fact, we have carried out a systematic theoretical investigation of structural, electronic, magnetic, thermoelectric and lattice dynamical properties of spin gapless Heusler alloy Zr2MnAl within the frame work of density functional theory based first principle calculation. Our ground state total energy calculations confirms the structural stability of antiferromagnetic phase of Zr2MnAl over ferromagnetic phase, which is in agreement with previous study. The positive value for Cauchy’s pressure (C12–C44) predicts ductile nature of the material. The band structure calculation reveals indirect band gap in spin down channel and zero band gap in spin up channel of valence and conduction bands confirming the spin gapless semiconducting nature of the compound. The real frequency of phonon modes throughout the Brillouin zone confirms the structural and dynamical stability of antiferromagnetic phase of Zr2MnAl, which is reported for the first time as per our knowledge. Calculated Seebeck coefficient in spin up and spin down channel reveals that the Zr2MnAl behaves as both n and p type thermoelectric materials with better output efficiency. The calculated thermoelectric figure of merit of 1.4 at 800 K, favors use of an alloy in recovery of waste heat at higher temperature and thermoelectric spin generators. The comprehensive analysis of various chemical potential dependent thermoelectric coefficients for enhancing thermoelectric performance for the material has been carried out. The obtained results propose Zr2MnAl as a potential candidate for high temperature thermoelectric devices. Higher optical gap in the phonon dispersive curve is responsible for decrease in lattice thermal conductivity with temperature, resulting in enhancement in thermoelectric figure of merit.

Thermoelectric properties of topological insulator lanthanum phosphide via first-principles study

We systematically investigate the thermoelectric properties of lanthanum phosphide (LaP) using first-principles calculations together with the Boltzmann transport theory. The dynamic stability of LaP is confirmed based on phonon dispersion. The different atomic masses of La and P atoms lead to an acoustic-optical bandgap that prohibits scattering between acoustic and optical phonon modes. The lattice thermal conductivity κl is 3.19 W/mK at room temperature, which is less than that of LaAs because of the smaller group velocities and the larger Grüneisen parameter. The calculated phonon-mode contribution to κl, scattering rate, and mean free path may provide significant guidance to further theoretical and experimental research. The power factor reaches 0.012 W m−1 K−2 at a carrier concentration of 1.05×1020cm−3, which is comparable to the power factor of the typical thermoelectric material Bi2Te3. This reasonable power factor and thermal conductivity suggest that LaP is a promising thermoelectric material. The dependence of the figure of merit ZT on carrier concentration n and temperature T indicates that a high ZT can be obtained by increasing the temperature and tuning the carrier concentration.

Enhanced thermoelectric properties of lightly Nb doped SrTiO3 thin films.

Novel thermoelectric materials developed for operation at room temperature must have similar or better performance along with being as ecofriendly as those commercially used, e.g., Bi2Te3, in terms of their toxicity and cost. In this work, we present an in-depth study of the thermoelectric properties of epitaxial Nb-doped strontium titanate (SrTi1−xNbxO3) thin films as a function of (i) doping concentration, (ii) film thickness and (iii) substrate type. The excellent crystal quality was confirmed by high resolution transmission electron microscopy and X-ray diffraction analysis. The thermoelectric properties were measured by the three-omega method (thermal conductivity) and van der Pauw method (electrical resistivity), complemented by Seebeck coefficient measurements. A maximum power factor of 8.9 × 10−3 W m−1 K−2 and a thermoelectric figure of merit of 0.49 were measured at room temperature in 50 nm-thick films grown on lanthanum strontium aluminate. The mechanisms behind this high figure of merit are discussed in terms of a possible two-dimensional electron gas, increase of the effective mass of the electrons, electron filtering and change in strain due to different substrates. The overall enhancement of the thermoelectric properties suggests that SrTi1−xNbxO3 is a very promising n-type candidate for room- to high-temperature applications.

Flash-sintering of antimony telluride and its thermoelectric properties

This study set out to examine the flash sintering of thermoelectric materials. A finite element method analysis of electric current activated/assisted sintering with an insulating mold suggested that the use of a sufficiently high Joule-heating rate, such as >10 000 K/s, relative to the thermal diffusion rate of the mold material, can realize uniform sample heating. Based on the analysis results, the sintering of Sb2Te3, a typical thermoelectric material, was conducted with a current feed duration of 1 s, using a sintering apparatus that had been specially devised for flash-sintering. Although a slight reduction in the density and degree of grain alignment was observed, the crystallographic phase was unaffected by the rapid heating, with the thermoelectric performance of the Sb2Te3 sample prepared by the 1-s sintering being comparable with that of a sample that had been conventionally sintered at 773 K for 3 min. This significant reduction in the sintering time can result in the energy consumption for the sintering process being decreased to less than 1% of that required for conventional current sintering. This significant energy-conservation effect of the flash sintering should be appealing from a practical point of view, especially for the fabrication of those thermoelectric materials used for power generation and aimed at energy recovery.

Effect of the interface on the mechanical properties and thermal conductivity of bismuth telluride films

Bismuth telluride (Bi2Te3) is a type of thermoelectric material used for energy generation that does not cause pollution. Increasing the thermoelectric conversion efficiency (ZT) is one of the most important steps in the development of thermoelectric components. In this study, we use molecular dynamics to investigate the mechanical properties and thermal conductivity of quintuple layers of Bi2Te3 nanofilms with different atomic arrangements at the interface and study the effects of varying layers, angles, and grain boundaries. The results indicate that the Bi2Te3 nanofilm perfect substrate has the ideal Young’s modulus and thermal conductivity, and the maximum yield stress is observed for a thickness of ∼90 Å. As the interface changed, the structural disorder of atomic arrangement affected the mechanical properties; moreover, the phonons encounter lattice disordered atomic region will produce scattering reduce heat conduction. The results of this investigation are helpful for the application of Bi2Te3 nanofilms as thermoelectric materials.

Thermoelectric properties of Cu2Se1−xTex solid solutions

We demonstrate that the quality factor of Cu2Se1−xTex solid solutions is higher than those of most typical thermoelectric materials.

Thermal transport and microscopic dynamics in filled skutterudite YbFe4Sb12 studied by ab initio molecular dynamics simulation

Filled skutterudite is a typical thermoelectric material with high thermoelectric figure of merit at intermediate temperatures. One of the important features is the low lattice thermal conductivity (L) caused by the low frequency vibrations of filler atoms in the oversized void cages. In the past decades, it has been still under debate whether the underlying phonon scattering mechanism should be considered to be resonant scattering or enhanced three-phonon process. To clarify the role played by the filler atoms in reducing the lattice thermal conductivity, we study the microscopic dynamical process of filler and related interactions by means of ab initio molecular dynamics (AIMD) and temperature dependent effective potential (TDEP) technique. Firstly, we simulate the dynamical trajectories of fully filled skutterudite YbFe4Sb12 at different temperatures through AIMD. In this approach, the nonlinear guest-host interactions at finite temperatures are taken into consideration naturally from dynamical trajectories. Then, we extract the effective temperature-dependent harmonic and anharmonic interatomic force constants (IFCs) by TDEP method through the statistical analyses of both trajectories and forces. The atomic participation ratios and lifetimes of phonon modes are calculated based on the effective IFCs. The results demonstrate that the local vibration modes of Yb couple with acoustic branches and reduce the lifetimes of the lattice phonons significantly. However, the calculated L, which is on the assumption that the filler interacts with lattice phonons through three-phonon collision, still deviates from the experimental result. In order to rationalize the discrepancy, we analyze the correlation properties between different Yb atoms by velocity coherence in atomic dynamical motions. The localized and independent vibration characteristic of Yb is found in this analysis. This implies that the motions of Yb atoms deviate from the periodic and collective vibration excitation paradigm of phonon. Therefore, the mechanism for how filler atoms scatter lattice phonon and enhance thermal resistance is beyond three-phonon scattering process. We thus introduce resonant scattering into the lifetimes of Yb-dominant localized vibration modes, and so-calculated L is in a good agreement with the experimental data. Overall, we come to a conclusion that both the phonon-phonon interaction and the resonant scattering due to the localized oscillators cause the low lattice thermal conductivity of YbFe4Sb12.

AgKTe: An intrinsic semiconductor material with high thermoelectric properties at room temperature

The good thermoelectric properties of AgKTe at room temperature are identified by using the first-principles density functional theory. Based on the optimized geometrical structure, electronic properties, we calculate the relaxation time with deform potential theory. The transport coefficients including electronic thermal conductivity, Seebeck coefficient, and electrical conductivity are obtained with semiclassical Boltzmann transport theory and relaxation time approximation. The lattice thermal conductivity at room temperature is calculated by using the Debye-Callaway model. The thermoelectric figure of merit of AgKTe is determined and compared with other typical thermoelectric materials. The results demonstrate that for both the n-type and the p-type AgKTe, the maximum thermoelectric figure of merit of 1.7 can be achieved at room temperature by means of tuning the carrier concentration. Therefore, AgKTe could be a promising candidate of thermoelectric materials.

Low lattice thermal conductivity and good thermoelectric performance of cinnabar

Based on the combination of first-principles calculations, Boltzmann transport equation, and electron-phonon interaction (EPI), we investigate the thermal and electronic transport properties of crystalline cinnabar $(\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{HgS})$. The calculated lattice thermal conductivity ${\ensuremath{\kappa}}_{L}$ is remarkably low, e.g., $0.60\phantom{\rule{4pt}{0ex}}{\mathrm{Wm}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at $300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is about $30%$ of the value for the typical thermoelectric material PbTe. Via taking fully into account the k dependence of the electron relaxation time computed from the EPI matrix, the accurate numerical results of thermopower $S$, electrical conductivity $\ensuremath{\sigma}$, and electronic thermal conductivity ${\ensuremath{\kappa}}_{E}$ are obtained. The calculated power factor ${S}^{2}\ensuremath{\sigma}$ is relatively high while the value of ${\ensuremath{\kappa}}_{E}$ is negligible, which, together with the fairly low ${\ensuremath{\kappa}}_{L}$, leads to a good thermoelectric performance in the $n$-type doped $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{HgS}$, with the figure of merit $zT$ even exceeding 1.4. Our analyses reveal that (i) the large weighted phase space and the quite low phonon group velocity result in the low ${\ensuremath{\kappa}}_{L}$, (ii) the presence of flat band around the Fermi level combined with the large band gap causes the high $S$, and (iii) the small electron linewidths of the conduction band lead to a large relaxation time and thus a relatively high $\ensuremath{\sigma}$. These results support that $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{HgS}$ is a potential candidate for thermoelectric applications.

(Invited) Manipulating Electrical and Thermal Transport in Nanowire Based Nanocomposite through Doping for Thermoelectric Applications

In this talk, we will discuss our work on how to use doping and porosity effects to manipulate the electrical and thermal properties in nanowire based bulk nanocomposites. Our success in increasing the efficiency of p-type BixSb2-xTe3 at higher temperatures can be largely explained by the increase in the optical band gap, according to the Burstein-Moss shift, due to doping, thus delaying the onset of bipolar conduction, which can shift the operation temperature to over 500K with a much improved ZT over 1. In addition, we will discuss the scalable synthesis of hollow nanostructures and the subsequent sintering of them into a highly porous thermoelectric nanocomposite. The as-sintered material has high porosity and holds a record low thermal conductivity, however, its zT is comparable to or even better than the state-of-the-art. The relative density leads to the less cost in raw material and better portability. We foresee that this large-scale approach can be extended to other types of thermoelectric materials and will inspire the utilization of hollow nanostructure in other fields.

Detection of large thermal vibration for Cu atoms in tetrahedrite by high-angle annular dark-field imaging

Tetrahedrite (Cu12Sb4S13) is a new type of thermoelectric material with an extremely low thermal conductivity attributed to the anomalous large thermal vibration of specific Cu sites. The tetrahedrite crystal was observed from the [111] direction by high-angle annular dark-field (HAADF) imaging and the image intensity was found to be 64% lower at specific sites. This could be explained by the blurring of the intensity distribution owing to a large atomic displacement, suggesting that anomalous large thermal vibrations at specific sites in the crystal can be distinguished in HAADF images.

Investigation of microstructure and thermoelectric properties at different positions of large diameter pellets of Bi0.5Sb1.5Te3 compound

The conversion efficiency of typical thermoelectric materials is still below 10% which leads the cost of the device was high. Recently, considerable attention has been paid to reduce the production cost and reliability of the performance. In this work, a low cost and an efficient method for large-scale production of high-quality thermoelectric powders via high-pressure gas atomization process and it have been sintered to a large diameter of pellets (50 mm) by spark plasma sintering process. The current and power densities of large diameter SPS processed specimens varied from center to edge portion. In order to verify the reliability of large diameter pellets, we investigated microstructure, thermoelectric and mechanical properties at different positions of pellets. The grain size variations could be clearly identified using SEM and EBSD techniques from center to edge part of pellets. Enhanced Seebeck coefficient and reduction of thermal conductivity for edge part of pellet were due to microstructure refinement which mainly occurs from temperature gradient during the sintering process. As a result, mechanical and thermoelectric properties were significantly enhanced. Ultimately, the thermoelectric performance of the large diameter pellets exhibits almost nearly equal from center to the edge which reaches 1.25 at 348 K.

Phase diagram of ternary Cu-Ga-Te system and thermoelectric properties of chalcopyrite CuGaTe2 materials

Among all types of thermoelectric materials, chalcopyrite CuGaTe2 has been viewed as a promising candidate for use of thermoelectric generator due to its high figure-of-merit (zT) at the high temperatures. Herein the 923 K isothermal section of ternary Cu-Ga-Te system is determined, using various thermally-equilibrated Cu-Ga-Te alloys, and ternary CuGaTe2 phase is stabilized within the compositional region of 48.0–53.0 at%Te and 25.0 at%-30.0 at%Cu. Moreover, the solubility of Cu in binary Ga2Te3 and Ga3Te4 compounds at 923 K is negligible, while that in GaTe phase reaches 7.9 at%Cu. The as-determined isothermal section, depicting the phase stability regime of CuGaTe2, provides options for precisely locating the compositions of CuGaTe2-based materials that lead to promising and reproducible thermoelectric properties. A zT peak of 0.6 has been achieved in the Bridgman-grown Cu25Ga26Te49 alloy at 750 K, which is nearly eight times higher than the neighboring Cu28Ga25Te47 alloy, presumably due to the fact that the Cu25Ga26Te49 alloy, which exhibits high phase purity of CuGaTe2, has lower lattice thermal conductivity (κL∼1.3 (W/mK) and higher power factor (PF∼11.2 (μW/mK2)), comparing with that of Cu28Ga25Te47 alloy (κL∼1.8 (W/mK) and PF∼1.9 (μW/mK2)), which locates in a three-phased CuGaTe2+Cu2Te + Cu9Ga4 region, with only a slight deviation in the starting composition.

Role of chemical doping on the enhancement of thermoelectric performance in metal-based thermoelectric system SnCCo3

We report the effects of chemical doping on thermoelectric properties of metal-based thermoelectric materials Sn1−xMxCCo3 (M = Ag, In, Ge, Pb, Sb, Te, and Bi) with antiperovskite structure. Compared to parent compound SnCCo3, both Seebeck coefficient (S) and figure of merit (ZT) can be effectively enhanced by hole doping, while a decreased S and ZT appear in equivalent- and electron-doped SnCCo3. Among all the doped samples, the maximum ZT value of 0.045(1) is obtained in Sn0.95Ag0.05CCo3, which is about 50% larger than that of parent compound SnCCo3 (ZT ∼ 0.03). This enhanced ZT is mainly attributed to the increased S induced by locally modified density of states near Fermi level. In addition, our optimized ZT value (∼0.045, at 235 K) is relatively large compared with other typical low-temperature thermoelectric materials.

Enhanced thermoelectric properties of carbon fiber reinforced cement composites

Thermoelectric properties of carbon fiber reinforced cement composites (CFRCs) have attracted relevant interest in recent years, due to their fascinating ability for harvesting ambient energy in urban areas and roads, and to the widespread use of cement-based materials in modern society. The enhanced effect of the thin pyrolytic carbon layer (formed at the carbon fiber/cement interface) on transport and thermoelectric properties of CFRCs has been studied. It has been demonstrated that it can enhance the electrical conduction and Seebeck coefficient of CFRCs greatly, resulting in higher power factor 2.08µWm−1K−2 and higher thermoelectric figure of merit 3.11×10−3, compared to those reported in the literature and comparable to oxide thermoelectric materials. All CFRCs with pyrolytic carbon layer, exhibit typical semiconductor behavior with activation energy of electrical conduction of 0.228-0.407eV together with a high Seebeck coefficient. The calculation through Mott’s formula indicates the charge carrier density of CFRCs (1014–1016cm−3) to be much smaller than that of typical thermoelectric materials and to increase with the carbon layer thickness. CFRCs thermal conductivity is dominated by phonon thermal conductivity, which is kept at a low level by high density of micro/nano-sized defects in the cement matrix that scatter phonons and shorten their mean free path. The appropriate carrier density and mobility induced by the amorphous structure of pyrolytic carbon is primarily responsible for the high thermoelectric figure of merit.

Characterization of thermoelectric properties of multifunctional multiscale composites and fiber-reinforced composites for thermal energy harvesting

Abstract We have fabricated two types of multifunctional composites, carbon nanotube (CNT)/glass fiber (GF)/epoxy composites and carbon fiber (CF)/epoxy composites, and evaluated thermoelectric properties of the composites for applications as n/p type thermoelectric materials as well as load carrying structural composites. Several test samples of CNT/GF/epoxy composites with various CNT concentrations were fabricated using a three-roll mill and hand-layup process on a hot plate, while CF/epoxy composite samples were manufactured using a hand-layup process. Experimental results demonstrated that the electrical resistivity of the CNT/GF/epoxy composite (multiscale composite) samples decreased as the CNT concentration increased. In-plane samples showed higher electrical and thermal conductivities due to partial alignment of CNTs in the multiscale composites and continuity of carbon fibers in CF/epoxy composites. Generally, CF/epoxy composites had better electrical and thermal conductivities than those of multiscale composites. In the Seebeck coefficient test, the multiscale composites showed n-type thermoelectric behavior, whereas the CF/epoxy composites showed p-type behavior. When temperature gradients were applied to closed circuits comprised of multiscale composites and CF/epoxy composites as n-type and p-type materials, respectively, an electric current was successfully generated.

Thermomigration and Soret effect in NaxCoO2 as thermoelectric material: Preparation and characterization of sodium cobaltate thin films

Sodium cobaltate (NaxCoO2, NCO) is a model type thermoelectric material for high temperature applications, which is representative for the class of non‐stoichiometric and mixed ion‐electron conducting thermoelectrics (e.g., Cu2 – xSe). The present study deals with the kinetic instability of the originally homogeneous chemical composition when a temperature gradient is applied – a common situation in thermoelectric materials with a mobile component (element), but rarely considered in thermoelectric materials research so far. In order to investigate a well reproducible system, highly epitaxial thin films of NaxCoO2 with an atmosphere protective capping layer of alumina are prepared via pulsed laser deposition on sapphire (001) and lanthanum aluminate (111). A self‐designed non‐isothermal set‐up allows the precise determination of the heat of transport for mobile sodium as 8.3 kJ mol−1 and, therefore, the quantification of thermodiffusion (i.e., the Ludwig‐Soret effect in the stationary state). The experiments also allow to estimate the chemical diffusion coefficient of sodium at 422 K as about = 5 × 10−4 cm2 s−1.

Nanostructured TTT(TCNQ)2 Organic Crystals as Promising Thermoelectric n-Type Materials: 3D Modeling

Thermoelectric properties of quasi-one-dimensional TTT(TCNQ)2 organic crystals are investigated in order to appreciate the prospect of using this compound as n–type thermoelectric material. A more complete three-dimensional (3D) physical model is elaborated. It takes into account two the most important interactions of conduction electrons with longitudinal acoustic phonons, the electrons’ scattering on neighbor molecular chains, as well as the scattering by impurities and defects. The electrical conductivity, thermopower, the power factor, electronic thermal conductivity and the thermoelectric figure of merit in the direction along conductive molecular chains are calculated numerically for different degrees of crystal purity. It is shown that in stoichiometric compounds the thermoelectric figure of merit ZT remains small even after the increase of crystal perfection degree. The thermoelectric properties may be significantly enhanced by simultaneous increase of crystal perfection and of electron concentration. The latter can be achieved by additional doping with donors. For less pure crystals, the interaction with impurities predominates over the weak interchain interaction and the simpler one-dimensional (1D) physical model is applicable. When the impurity scattering is reduced, the interchain interaction begins to limit the carrier mobility and the application of the 3D physical model is required. The optimal parameters permitting to predict ZT~ 1 are determined.

Traveling heater method에 의해 성장된 Bi2Te2.7Se0.3의 열전특성

Bi Te -Bi Se alloy which is typical n-type thermoelectric material were grown by traveling heater method (THM) technique. We investigate the effect of the composition of 100-x(Bi Te ) - x(Bi Se ) and doping of n-type dopants such as SbI and CdCl . Maximum figure of merit of Bi Te -Bi Se alloy was observed with CdCl 0.1 wt% (Z: 2.73 × 10 / K) and SbI 0.05 wt% (Z: 2.29 × 10 /K). Deviation along the length of Bi Te -Bi Se ingot grown by THM method is low, which indicates that the ingot is very homogenized. Also we observed the close relationship of between anisotropy ratio and dopant in the 90(Bi Te ) - 10(Bi Se ) alloys. And we confirmed the fact that anisotropy ratio exerts thermoelectric performance in Bi Te based n-type thermoelectric material.