Maximum Thermoelectric Figure Of Merit Research Articles

A Colossal Enhancement of Thermoelectric Performance of Monolayer SbAs Using Strain Engineering

The effect of uniaxial strain on the thermoelectric (TE) conversion efficiency of puckered monolayer arsenic antimonide (SbAs) is explored based on the density function and Boltzmann transport equation (BTE) theories. The lattice thermal conductivity (κl) can be sharply lowered through tensile strain. Taking a + 3.25% tensile strain as an example, κl along the armchair (AC) direction at 300 K decreases almost three times than that without a strain, which is ascribed to the low phonon group velocity, large Grüneisen parameter, and high three‐phonon scattering phase space. The carrier effective mass decreases in imposing tensile strain monolayer SbAs and slightly suppresses the Seebeck coefficient; however, the electrical conductivity is improved due to the small effective mass and long relaxation time. Thus, power factor increases by 3.4 times under a + 3.25% tensile at cm−2 compared with a SbAs monolayer without strain. Thereafter, the figure of merit in an n‐type SbAs monolayer with a tensile strain of +3.25% is eight times as large as that in SbAs without strain at 300 K along the AC direction. Furthermore, the maximum TE figure of merit is 1.85 for an n‐type SbAs monolayer with +3.25% strain at 800 K. This study paves a way toward improving the TE performance through strain engineering in the TE field.

Synthesis and Properties of Thermoelectric Nanomaterial AgInSe2 with a Chalcopyrite Structure

AgInSe2 nanopowders with a chalcopyrite structure and an average particle size of ~75 nm were synthesized from solutions by the polyol method of chemical synthesis. Spark plasma sintering was used to obtain bulk polycrystalline nanomaterials with an average grain size of ~110 nm. Electrons are the main current carriers in bulk materials. In the range 300–700 K, the features of the temperature dependences of the electrical resistivity, Seebeck coefficient, and total thermal conductivity of the nanomaterials are determined. It was found that the maximum thermoelectric figure of merit, equal to 0.1, is achieved at a temperature of 700 K.

Особенности получения пленок MnXSiY на кремнии твердофазной реакцией

Recently, in the development of new device structures, structures with various metals on silicon have been used. Numerous studies have shown that as a result of solid-phase reactions of Mn and Si, it is possible to create heterophase systems from various manganese silicide. Such films possessing a unique set of thermoelectric properties. At the present time, obtaining a film with high values of a sufficient thermo-EMF, the maximum thermoelectric figure of merit of these materials of the pack has not yet been achieved. In this regard, there is a need for a detailed study of the structure and perfection of manganese silicide – silicon films obtained on a silicon substrate. In this work, we study the solid-phase reaction in the Mn-Si diffusion system and the process of the formation of manganese silicide phases on a silicon substrate.

Origin of low thermal conductivity in Nb1-xTixFe1.02Sb half-Heusler thermoelectric materials

Nb1-xTixFe1.02Sb half-Heusler thermoelectric materials were synthesized trough arc melting and subsequent spark plasma sintering (SPS). Doping effect of Ti at Nb-site results in high power factor due to the optimization of hole carrier concentration. Further, the lattice thermal conductivity reduced as the Ti content increased, which mainly resulted from point defects and mass fluctuation by Ti doping as well as coherent nano-scale second phase, Ti-rich and Fe-deficient Ti1.18Fe0.57Sb. For the sample of Nb0.6Ti0.4Fe1.02Sb, a minimum lattice thermal conductivity of ∼2.08 W/mK was obtained. This is attributed to the enhanced phonon scattering at the alternating white and dark lamellar interphase boundaries of (Nb0.6,Ti0.4)FeSb half-Heusler and Nb-doped Ti1.18Fe0.57Sb. A maximum thermoelectric figure of merit, ZT, of ∼0.81 was obtained at 973 K, which is comparable with values previously reported for this system.

A flexible electromagnetic wave-electricity harvester

Developing an ultimate electromagnetic (EM)-absorbing material that can not only dissipate EM energy but also convert the generated heat into electricity is highly desired but remains a significant challenge. Here, we report a hybrid Sn@C composite with a biological cell-like splitting ability to address this challenge. The composite consisting of Sn nanoparticles embedded within porous carbon would split under a cycled annealing treatment, leading to more dispersed nanoparticles with an ultrasmall size. Benefiting from an electron-transmitting but a phonon-blocking structure created by the splitting behavior, an EM wave-electricity device constructed by the optimum Sn@C composite could achieve an efficiency of EM to heat at widely used frequency region and a maximum thermoelectric figure of merit of 0.62 at 473 K, as well as a constant output voltage and power under the condition of microwave radiation. This work provides a promising solution for solving EM interference with self-powered EM devices.

Thermoelectric properties of antimony films: roles of oxidation and topological quantum state

Motivated by interesting physical and chemical properties created by doping and topological quantum state, we perform the density functional theory and the Boltzmann transport equation to systematically investigate the geometric structures, stabilities, electronic structures, thermal conductivities and thermoelectric properties for Sb and its oxidations (Sb2O and SbO). The predicted lattice thermal conductivity (kL) of Sb is 11.6 nW K−1 at 300 K, but it would fall drastically when introducing O atoms. This is mainly attributed to the strong anharmonic interactions by adding O atoms, and few contributions are from the decreasing phonon group velocities caused by the compressed phonon spectrum. SbO has been proven as a topological insulator with a relatively large topological band gap (Eg) ∼ 0.156 eV, and meanwhile its carrier mobilities (345.78 cm2/Vs for electrons) and scattering time (44.27 × 10−14 s for electrons) are also rather high among all 2D materials, exhibiting the excellent thermoelectric performance. The calculated maximum thermoelectric figure of merit () of the three Sb films for optimum n-type doping are close to each other at 300 K, but with an increasing temperature, the of Sb for optimum n-type doping climbs quickly and can reach up to 0.73 at 700 K, which is far higher than others. More interestingly, the of SbO can be increased sharply at 300 K after considering spin–orbit coupling (SOC): 0.50 for optimum p-type doping and 0.41 for optimum n-type doping. However, only the tiny changes in the of Sb can be found before and after considering SOC. Our research reveals how the doping and the topological quantum state affect thermoelectric performances, providing reference to design and search high thermoelectric materials in future.

Thermoelectric Transport Properties of n-Type Sb-doped (Hf,Zr,Ti)NiSn Half-Heusler Alloys Prepared by Temperature-Regulated Melt Spinning and Spark Plasma Sintering

Herein we report a significantly reduced lattice thermal conductivity of Sb-doped Hf0.35Zr0.35Ti0.3NiSn half-Heusler alloys with sub-micron grains (grain size of ~300 nm). Polycrystalline bulks of Hf0.35Zr0.35Ti0.3NiSn1−xSbx (x = 0.01, 0.02, 0.03) with a complete single half-Heusler phase are prepared using temperature-regulated melt spinning and subsequent spark plasma sintering without a long annealing process. In these submicron-grained bulks, a very low lattice thermal conductivity value of ~2.4 W m−1 K−1 is obtained at 300 K due to the intensified phonon scatterings by highly dense grain boundaries and point-defects (Zr and Ti substituted at Hf-sites). A maximum thermoelectric figure of merit, zT, of 0.5 at 800 K is obtained in Hf0.35Zr0.35Ti0.3NiSn0.99Sb0.01.

Preparation and thermoelectric performance of nano rare-earth oxides dispersed p-type BiSbTe alloys by mechanical milling and spark plasma sintering

Simultaneously, the enhancement of power factor and reduction in thermal conductivity is a critical issue to improve the thermoelectric performance of a certain material. In the present work, the rare earth oxide, Pr6O11 was added to the Bi-Sb-Te material and the corresponding thermoelectric properties have been systematically investigated. The Seebeck coefficient was slightly improved due to the increase in effective mass by addition of rare earth oxides in BiSbTe matrix. The electrical conductivity was decreased with increasing rare earth oxide content due to the decreasing mobility owing to the carrier scattering at grain boundaries. The thermal conductivity was reduced of 8% for Pr6O11/BiSbTe composites in which the 4 wt% added Pr6O11 particles, act as a scattering center for middle to higher wavelength phonons and induce the phonon scattering. As a result, the maximum thermoelectric figure of merit, ZT of 0.71 was obtained for 2 wt% added BiSbTe/Pr6O11 composites at 350 K.

Effects of Co Doping on the Thermoelectric Properties of Cu<sub>3</sub>SbSe<sub>4</sub> at Moderate Temperature

Cu3SbSe4-based thermoelectric materials are a class of thermoelectric materials with diamond-like structure which exhibit high thermoelectric properties at moderate temperature region and have broad research prospects. In this study, the p-type Co-doped Cu3-xCoxSbSe4 (x=0-0.015) thermoelectric materials were fabricated by melting-annealing-ball milling-hot pressing process to investigate the effects of Co doping on the thermoelectric properties of Cu3SbSe4. It is found that the average power factor of Cu2.995Co0.005SbSe4 was increased by 30% compared with the pure sample, indicating that Co doping had a great effect on the electrical properties of Cu3SbSe4. The energy gap of ternary p-type semiconductor Cu3SbSe4 was around 0.27eV. As the Co content increasing, the lattice distortion enhanced the phonon scattering, which led to the decrease in lattice thermal conductivity. The maximum thermoelectric figure of merit, ZTmax, reached 0.46 at 600K for the Cu2.995Co0.005SbSe4.

Effect of porosity and crack on the thermoelectric properties of expanded graphite/carbon fiber reinforced cement‐based composites

Cement-based composites is a promising type of structural material, which has prospective applications in relieving the urban heat island effect in summer and melted snow with low energy consumption. However, the major drawbacks of cement-based composites are heterogeneity, porosity, and brittleness. Porosity and microcrack have considerable influence on the thermoelectric of cement-based composites applied in large-scale concrete structures in future. This paper studied in detail the effect of porosity and crack on thermoelectric properties of the cement-based composite. The proper pores and cracks in the cement matrix are advantageous to enhance the Seebeck effect, but meanwhile it also reduces the electrical conductivity. So combined with Seebeck effect, electrical conductivity and other factors, it can obtain a comparatively low electrical conductivity (0.063S cm−1) of expanded graphite/carbon fiber reinforced cement-based composites (EG-CFRC), but EG-CFRC manifests the maximum thermoelectric figure of merit (ZT) has reached 2.22 × 10−7 when the porosity is 3.90%. With different porosity, the Seebeck effect of prepared EG-CFRC was strengthened when the crack existed. The effect is most pronounced by a factor of 2 when the porosity is 28.90%. Therefore, based on stabilizing the conductivity, the crack is fittingly made to have a good effect on the Seebeck coefficient.

Is LiI a Potential Dopant Candidate to Enhance the Thermoelectric Performance in Sb-Free GeTe Systems? A Prelusive Study

As a workable substitute for toxic PbTe-based thermoelectrics, GeTe-based materials are emanating as reliable alternatives. To assess the suitability of LiI as a dopant in thermoelectric GeTe, a prelusive study of thermoelectric properties of GeTe1−xLiIx (x = 0–0.02) alloys processed by Spark Plasma Sintering (SPS) are presented in this short communication. A maximum thermoelectric figure of merit, zT ~ 1.2, was attained at 773 K for 2 mol% LiI-doped GeTe composition, thanks to the combined benefits of a noted reduction in the thermal conductivity and a marginally improved power factor. The scattering of heat carrying phonons due to the presumable formation of Li-induced “pseudo-vacancies” and nano-precipitates contributed to the conspicuous suppression of lattice thermal conductivity, and consequently boosted the zT of the Sb-free (GeTe)0.98(LiI)0.02 sample when compared to that of pristine GeTe and Sb-rich (GeTe)x(LiSbTe2)2 compounds that were reported earlier.

Improvement in the thermoelectric performance of highly reproducible n-type (Bi,Sb)2Se3 alloys by Cl-doping.

(Bi,Sb)2Se3 alloys are promising alternatives to commercial n-type Bi2(Te,Se)3 ingots for low-mid temperature thermoelectric power generation due to their high thermoelectric conversion efficiency at elevated temperatures. Herein, we report the enhanced high-temperature thermoelectric performance of the polycrystalline Cl-doped Bi2−xSbxSe3 (x = 0.8, 1.0) bulks and their sustainable thermal stability. Significant role of Cl substitution, characterized to enhance the power factor and reduce the thermal conductivity synergetically, is clearly elucidated. Cl-doping at Se-site of both Bi1.2Sb0.8Se3 and BiSbSe3 results in a high power factor by carrier generation and Hall mobility improvement while maintaining converged electronic band valleys. Furthermore, point defect phonon scattering originated from mass fluctuations formed at Cl-substituted Se-sites reduces the lattice thermal conductivity. Most importantly, spark plasma sintered Cl-doped Bi2−xSbxSe3 bulks are thermally stable up to 700 K, and show a reproducible maximum thermoelectric figure of merit, zT, of 0.68 at 700 K.

Realization of Both n- and p-Type GeTe Thermoelectrics: Electronic Structure Modulation by AgBiSe2 Alloying.

Successful applications of a thermoelectric material require simultaneous development of compatible n- and p-type counterparts. While the thermoelectric performance of p-type GeTe has been improved tremendously in recent years, it has been a challenge to find a compatible n-type GeTe counterpart due to the prevalence of intrinsic Ge vacancies. Herein, we have shown that alloying of AgBiSe2 with GeTe results in an intriguing evolution in its crystal and electronic structures, resulting in n-type thermoelectric properties. We have demonstrated that the ambient rhombohedral structure of pristine GeTe transforms into cubic phase in (GeTe)100-x(AgBiSe2)x for x ≥ 25, with concurrent change from its p-type electronic character to n-type character in electronic transport properties. Such change in structural and electronic properties is confirmed from the nonmonotonic variation of band gap, unit cell volume, electrical conductivity, and Seebeck coefficient, all of which show an inflection point around x ∼ 20, as well as from the temperature variations of synchrotron powder X-ray diffractions and differential scanning calorimetry. First-principles density functional theoretical (DFT) calculations explain that the shift toward n-type electronic character with increasing AgBiSe2 concentration arises due to increasing contribution of Bi p orbitals in the conduction band edge of (GeTe)100-x(AgBiSe2)x. This cubic n-type phase has promising thermoelectric properties with a band gap of ∼0.25 eV and ultralow lattice thermal conductivity that ranges between 0.3 and 0.6 W/mK. Further, we have shown that (GeTe)100-x(AgBiSe2)x has promising thermoelectric performance in the mid-temperature range (400-500 K) with maximum thermoelectric figure of merit, zT, reaching ∼1.3 in p-type (GeTe)80(AgBiSe2)20 at 467 K and ∼0.6 in n-type (GeTe)50(AgBiSe2)50 at 500 K.

The thermoelectric properties of monolayer SiP and GeP from first-principles calculations

Monolayer silicon phosphide (SiP) and germanium phosphide (GeP) are predicted to exhibit fascinating electronic characters with highly stable structures, which indicate their potential applications in future electronic technologies. By using first-principles calculations combined with the semiclassical Boltzmann transport theory, we systematically investigate the thermoelectric properties of monolayer SiP and GeP. High anisotropy is observed in both phonon and electron transport of monolayer SiP and GeP where the thermal and electrical conductivity along the xx crystal direction are smaller than those along the yy crystal direction. The lattice thermal conductivity (room temperature) along the xx crystal direction is about 11.05 W/mK for monolayer SiP and 9.48 W/mK for monolayer GeP. However, monolayer SiP and GeP possess almost isotropic Seebeck coefficient, and the room temperature values with both n- and p-type doping approach 2.9 mV/K and 2.5 mV/K, respectively. Based on the electron relaxation time estimated from the deformation potential theory, the maximum thermoelectric figure of merit of monolayer SiP and GeP with n-type doping approach 0.76 and 0.78 at 700 K, respectively. The results presented in this work shed light upon the thermoelectric performance of monolayer SiP and GeP and foreshow their potential applications in thermoelectric devices.

Investigation of Effect of Pore Shape, Position and Passivation on the Thermoelectric Properties of Porous Armchair Silicene Nanoribbons

In this paper, thermoelectric properties of porous armchair silicene nanoribbons (ASiNRs) have been investigated as a function of pore shape, position and passivation using the Non-equilibrium Green’s function (NEGF) method and Extended Huckel Technique (EHT). Here, nanopores of circular, rectangular, rhombus and triangular nature at different positions have been incorporated with an intention to optimize the structure for maximum thermoelectric figure of merit. In addition, the effect of passivation of the pore edges on the thermoelectric performance has been studied for all the shapes. Further, the effect of temperature variation on the thermoelectric efficiency has been studied. Ballistic transport regime and semi-empirical method using Huckel basis set are used to obtain the electrical properties, while the Brenner potential is used for the phononic system.

Enhancing thermoelectric transport properties of n-type PbS through introducing CaS/SrS

N-type PbS have been studied widely for purpose of substituting PbTe attributed to the low price and high earth abundance of sulfur element and it is significant to research the strategy of boosting the thermoelectric performance of PbS through adjusting carrier and phonon transports synergistically. Here, the thermoelectric performance of n-type PbS is enhanced effectively by introducing CaS/SrS. CaS/SrS alloying could simultaneously enhance Seebeck coefficients and reduce lattice thermal transport properties over broad temperature range. As a consequence, the maximum thermoelectric figure of merit (ZT) ~ 1.2 at 930 K and high average ZT (ZTave) ~0.8 at 423 K–930 K are obtained for n-type PbS+3%CaS/SrS+1%PbCl2 based materials. These results indicate that the methods of CaS/SrS alloying have great prospect in improving thermoelectric performance of n-type PbS.

Charge transport mechanism and thermoelectric behavior in Te:(PEDOT:PSS) polymer composites

Polymer thermoelectics are an emerging class of materials for realizing flexible thermoelectric devices due to its low cost, solution processability and bio-compatibility. We report a systematic investigation of the charge transport and thermoelectric properties of polymer composites comprising of tellurium (Te) nanorods and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) spanning over a temperature range from 10 K up to 320 K. The temperature dependence of electrical resistivity for Te:(PEDOT:PSS) composites is dominated by the thermal assisted hopping following Mott’s and Efros-Shklovskii (ES) variable range hopping (VRH) mechanism. A cross-over from ES to Mott VRH model is observed near 105 K. The Seebeck coefficient and the electrical resistivity values for Te:(PEDOT:PSS) composites are found to decrease with the increase in PEDOT:PSS concentration at 300 K. Moreover, the thermal conductivity of the polymer composites is found to decrease due to enhanced phonon-phonon scattering ascribed to the acoustical mismatch between the Te-nanorods and PEDOT:PSS interface. The maximum thermoelectric figure of merit, ZT, of the polymer composites is estimated to be 0.013 ± 0.001 for Te:(PEDOT:PSS) (1:5) weight percentage. The bending test done on the polymer composite film and mechanical properties investigated using nanoindentation technique suggest the flexible nature of the composite sample which is promising for room temperature flexible thermoelectric application.

Effect of In and Cd co-doping on the thermoelectric properties of Sn1−xPbxTe

Pristine tin telluride (SnTe) with a similar electronic structure to PbTe shows inferior thermoelectric performance owing to high p-type hole concentration (1021 cm−3), high lattice thermal conductivity, κlatt (∼2.8 W mK−1 at room temperature) and large energy gap between light and heavy hole valence bands. Interestingly, 30 mol% substitution of lead in SnTe decreases the excess hole carrier concentration and lattice thermal conductivity (∼0.67 W m−1K−1 at 300 K) significantly. Here, we report the promising thermoelectric performance in Sn0.70Pb0.30Te by enhancing the Seebeck coefficient via the co-adjuvant effect of resonant level formation and valence band convergence. We obtain a Seebeck coefficient value of ∼141 μV K−1 at 300 K, which further increases to ∼260 μV K−1 at 708 K for Sn0.70Pb0.30Te—3% Cd and 0.50% In sample. This is one of the highest S values for SnTe based system, to the best of our knowledge. In and Cd have discrete but complementary roles to augment the Seebeck coefficient value of Sn0.70Pb0.30Te where In acts as a resonant dopant and Cd serves as valence band convergent, respectively, as demonstrated by the well-known Pisarenko plot of SnTe. Finally, we have achieved a maximum thermoelectric figure of merit, zT, of ∼0.82 at 654 K for Sn0.70Pb0.30Te—3% Cd and 0.25% In sample.

Enhanced thermoelectric properties of Pb-doped Cu1.8S polycrystalline materials

Digenite (Cu1.8S) has attracted extensive attention as a candidate for use in thermoelectric applications owing to its low cost and low toxicity, but its thermoelectric (TE) properties are still poor. In this study, Pb doping was used to improve the TE properties of Cu1.8S polycrystalline materials. Pb-doped Cu1.8S was fabricated using mechanical alloying (MA) and the spark plasma sintering (SPS) technique. The effects of Pb doping on the TE properties of Cu1.8S were investigated in detail. The power factor of Cu1.8S was maintained upon the introduction of Pb owing to an improved Seebeck coefficient and slightly reduced electrical conductivity. A maximum TE figure of merit (ZT) value of 0.77 was attained at 773 K for a Pb0.01Cu1.79S sample, which was 1.57 times higher than that of a pristine Cu1.8S sample.

A strategy for low thermal conductivity and enhanced thermoelectric power factor in one-dimensional Bi2Te3/Cu2Te nanowire composites

One-dimensional (1D) nanostructured composites consisting of Bi2Te3 and Cu2Te nanowires (NWs) were fabricated with various Cu2Te NW content, and their thermoelectric (TE) properties were examined. Bi2Te3/Cu2Te NW composites were produced from two NWs, which were fabricated using a Te NW. The maximum power factor of the Bi2Te3/Cu2Te NW composite at room temperature was 20 wt% of Cu2Te (595.4 μ W/m K2), which was was ~ 1.2 times higher than the pristine Bi2Te3 NW’s maximum power factor. In addition, low thermal conductivity was derived from their 1D nanostructure. The maximum thermoelectric figure of merit (ZT) of 0.376 was achieved at room temperature with the 20 wt% of the Cu2Te composite due to its enhanced power factor and reduced thermal conductivity. This value was approximately 1.46 times larger than the pure Bi2Te3 NW sample’s thermoelectric figure of merit.