Figure Of Merit ZT Research Articles

Demonstration of efficient Thomson cooler by electronic phase transition.

In the 1850s, Lord Kelvin predicted the existence of a thermoelectric cooling effect inside a whole material (the Thomson effect) according to thermodynamics1, in addition to the Peltier effect that enables cooling at the junction between dissimilar materials. However, the Thomson effect is usually negligible (ΔT/T < 2%) in conventional thermoelectric materials because the entropy change in charge carriers is fairly small2, leading to the guiding principles for advancing thermoelectric cooling to be based on the framework of the Peltier effect and that the figure of merit ZT should be maximized to optimize performance. Here, we demonstrate a Thomson-effect-enhanced thermoelectric cooler using a large Thomson coefficient (τ) induced by the direct manipulation of charge entropy through an electronic phase transition in YbInCu4. The devices achieve a steady temperature span (ΔT) of >5 K from T = 38 K. Our findings suggest not only another approach to advance thermoelectric coolers in addition to improving ZT but also technologically opens opportunities for solid-state cryogenic cooling applications.

Effect of Pressure on Thermoelectric Performance of Half Heusler Compounds

In this work, the density functional theory (DFT) combined with Boltzmann transport equations are used to explore the effect of pressure on the structural, mechanical, thermal, and electronic properties of two half Heusler compounds PdXSn (X = Zr,Hf). The electronic band structure is tuned under pressure which enhances the thermoelectric properties of both compounds. The band gap opens with pressure and its value is 0.91 eV (1.26) for PdZrSn and 0.82 eV (1.16 eV) for PdHfSn at 0GPa (30GPa). The indirect nature does not alter at any pressure. The seebeck coefficient enhances from 900 μV/K to 1161 μV/K for PdZrSn and from 763 μV/K to 1012 μV/K for PdHfSn at pressure 0GPa to 30GPa. Similar trends observed in case of electrical conductivity and electronic part of thermal conductivity. But the lattice thermal conductivity is reduced from15.16 W/mKto 12.51 W/mK for PdZrSn and from 9.53 W/mK to 8.34 W/mK for PdHfSn at 30GPa respectively. Overall, the figure of merit ZT is elevated and attained maximum value of 0.45(0.55) for PdZrSn (PdHfSn) under pressure up to 30GPa.

Thermoelectric Performance Enhancement in SnS Polycrystals Owing to Hole Doping Combined with Textured Microstructures.

Recently, the earth-abundant tin sulfide (SnS) has emerged as a promising thermoelectric material due to its phonon and electron structure similar to that of tin selenide (SnSe). However, compared with SnSe, limited progress has been achieved in the thermoelectric property enhancement of SnS. Textured SnS polycrystals with an enhanced thermoelectric performance have been developed in this work. The high carrier mobility benefited from the enhanced texture through the repressing strategy of spark plasma sintering, improving the electrical conductivity. In addition, Sn atom deficiencies in the texture sample led to an increased hole concentration, further boosting the electrical conductivity and power factor. The power factor exceeded 4.10 μW/cm·K2 at 423 K and 5.50 μW/cm·K2 at 850 K. The phonon scattering was strengthened by adjusting the multiscale microstructures including dislocations, defect clusters, etc., leading to an ultralow lattice thermal conductivity of 0.23 W/m·K at 850 K. A figure of merit zT > 1.3 at 850 K and an average zTave of 0.58 in the temperature range 373-850 K were achieved in the SnS polycrystal.

Effect of biaxial tensile strain on the thermoelectric properties of monolayer ZrTiCO2

Using ab-initio calculation principle calculations, we investigate the effect of biaxial tensile strain on the stability and electronic properties of the two-dimensional double transition metal MXenes ZrTiCO2. The monolayer ZrTiCO2 has stability and semiconducting properties under different strain conditions. The results of thermoelectric properties under other strain conditions calculated by Boltzmann transport theory and Slack model show that biaxial tensile strain can improve the electrical transport properties of monolayer ZrTiCO2 materials and lead to the decrease of lattice thermal conductivity. The thermoelectric efficiency of a material can be evaluated using the figure of merit ZT. The n-type ZrTiCO2 has a maximum ZT value of 1.49 at 900 K without adding biaxial strain and reaches a ZT value of 2.86 with 2% biaxial strain. The monolayer ZrTiCO2 material has the potential to become a thermoelectric material, and its thermoelectric properties can be improved by biaxial tensile strain.

DFT studies on electronic and structure properties of PbSe1-xSx alloys for thermoelectric materials using VCA and RS

In this work, Density Functional Theory (DFT) was employed to investigate the crystal structure and electronic properties of lead selenide (PbSe) and lead sulfide (PbS) semiconductors and their pseudo binary alloys PbSe1-xSx. The virtual crystal approximation (VCA) and randomly substituted solid models were employed to study these materials. In addition, the thermoelectric properties as function of the alloy composition were studied. Lattice parameters obtained using the generalized gradient approximation (GGA) were compared with theoretical and experimental results. The electronic bands were calculated for different sulfur compositions (0≤x ≤ 1, Δx = 0.1) using the Generalized Gradient Approximation of TB09LDA (MGGA). It was observed that the transition from the valence band to the conduction band takes place at the L point, which agrees with previous theoretical investigations. Slight deviations from Vegard's law were observed in the lattice parameters and bandgap of the alloys, especially for the sulfur-rich alloys. Effective band diagrams obtained from the unfolding of supercell band diagrams, reported for the first time for this system, show that the impacts of alloy disorder are low in the vicinity of the L point. The influence of the alloy composition on the transport properties was studied within the framework of Boltzmann's semiclassical theory. The electrical conductivity shows a thermally activated increase which influences the total thermal conductivity above T > 600–700 K, and the figure of merit ZT. Seebeck coefficient dependence with the chemical potential suggests an ambipolar behavior of the alloys, which present both n and p character. The alloys have the ZT maxima above 900 K influenced by the PbSe1-xSx alloy composition. Remarkably, the alloy with x = 0.5 present a maximum ZT value comparable to that of PbSe, which indicates that is promising to substituting selenium by sulfur atoms to tailor the thermoelectrical properties. This work shows the suitability of the VCA approximation and the band unfolding method, to describe the composition-dependent properties of the PbSe1-xSx pseudo binary alloys.

Thermoelectric performance enhancement of Pb-doped α -MgAgSb near room temperature

α-MgAgSb is taken as the p-type leg material for recently focused Mg-based thermoelectric devices because of the high thermoelectric performance near room temperature. However, the thermoelectric performance of α-MgAgSb is inhibited by the existence of the Ag-rich second phase. The ordinary methods like carrier concentration optimization and minimizing lattice thermal conductivity were nearly invalid because of the extremely low doping level for heteroatoms and intrinsically low lattice thermal conductivity. The crystal structure of α-MgAgSb can be viewed as Ag atom filled in half distorted hexahedron in the distorted rock salt skeleton formed by the Mg–Sb sublattice. In this work, by replacing the smaller Mg in the sublattice with Pb, the volume of the distorted hexahedron is effectively expanded to accommodate Ag atoms and then lead to the re-dissolution of Ag-rich second phase in the matrix. In addition, as Ag is the main source of low-frequency phonons, the enhanced lattice anharmonicity by Pb doping leads to stronger scattering of phonons in the distorted hexahedron and results in 20% reduction of lattice thermal conductivity in the temperature range of 300–500 K. Finally, the figure of merit zT is enhanced by ∼40% in the whole temperature range, demonstrating that lattice management is a promising method for the optimization of α-MgAgSb materials.

A density functional theory study of the structural, mechanical, optoelectronics and thermoelectric properties of InGeX3 (X = F, Cl) perovskites

Structural, electronic, lattice dynamics, optical, elastic, and thermoelectric features of Indium based Perovskite InGeX3 (X = F, Cl) are studied by using WIEN2k code within density functional theory DFT. A level of the Boltzmann transport theory is used to derive electronic transport coefficients together with anticipated electronic relaxation times. These compounds exhibit stability and are cubic with the space group Pm-3m 221, according to the structural analysis. The band structure having the 2.15 eV and 1.29 eV band gap and density of state (DOS) are calculated for electronic characteristics, indicating that both compounds are semiconducting. For the analysis of elastic characteristics, elastic constants, bulk modulus (49.99, 22.69), Poisson’s ratio (0.343, 0.261), anisotropy factor (0.52, 0.59), and Pugh’s ratio (2.87, 1.75) are examined. The ductile nature of the compounds is described by Pugh’s ratio, and the ionic nature can be ascertained by evaluating the Cauchy pressure. In the energy range of 0–10 eV, optical features like reflectivity, optical conductivity, refractive index, extinction coefficient and absorption coefficient are computed. These compounds broad-range absorption in the UV spectrum suggests that they could be good optoelectronic device candidates. The transport feature like Seebeck coefficient, thermal and lattice conductivity, power factor, electrical conductivity, and figure of merit zT (0.99, 0.98) can be calculated against temperature. The chemical potential against an excellent thermoelectric property of InGeX3 (X = F, Cl) have high figure of merit due to high Seebeck and electrical conductivity. These predicted outcomes indicated that both compounds are suitable candidates for thermoelectric applications. Such compounds can replace with other renewable energy sources in thermoelectric devices because of their powerful thermoelectric response.

Unveiling the temperature-dependent thermoelectric properties of the undoped and Na-doped monolayer SnSe allotropes: a comparative study

Motivated by the excellent thermoelectric (TE) performance of bulk SnSe, extensive attention has been drawn to the TE properties of the monolayer SnSe. To uncover the fundamental mechanism of manipulating the TE performance of the SnSe monolayer, we perform a systematic study on the TE properties of five monolayer SnSe allotropes such as α-, β-, γ-, δ-, and ε-SnSe based on the density functional theory and the non-equilibrium Green’s functions. By comparing the TE properties of the Na-doped SnSe allotropes with the undoped ones, the influences of the Na doping and the temperature on the TE properties are deeply investigated. It is shown that the figure of merit ZT will increase as the temperature increases, which is the same for almost all the Na-doped and undoped cases. The Na doping can enhance or suppress the ZT in different SnSe allotropes at different temperatures, implying the presence of the anomalous suppression of the ZT. The Na doping induced ZT suppression may be caused basically by the sharp decrease of the power factor and the weak decrease of the electronic thermal conductance, rather than by the decrease of the phononic thermal conductance. We hope this work will be able to enrich the understanding of the manipulation of TE properties by means of dimensions, structurization, doping, and temperature.

Response of Mg2X (X = Si, Ge and Sn) compounds to extreme uniaxial compression: first-principles calculations

Abstract In this study, we perform first-principles calculations using density functional theory to examine the structural, electronic, thermodynamic, and thermoelectric properties of the Mg2X (X = Si, Ge and Sn) compounds under uniaxial compression within the generalized gradient and modified Becke–Johnson approximations. It is found that the band gap of Mg2Si, Mg2Ge and Mg2Sn decreases with applied uniaxial pressure and changes its direction from Γ-X to Γ-K. The results of phonon frequencies indicate that the studied compounds are dynamically stable at zero and higher uniaxial strains. Furthermore, the uniaxial compression and temperature dependence of the Gibbs free energy, heat capacity and thermal expansion coefficient are investigated in the frame of the quasi-harmonic approximation. The semiclassical-Boltzmann method is used to study the Seebeck coefficient, electrical conductivity, thermal conductivity and figure of merit ZT as a function of both temperature and uniaxial pressure. It is shown that the Seebeck coefficient decreases with increasing pressure whereas thermal conductivity increases, which leads to the lowering in the value of ZT and thus to a worse thermoelectric performance of the studied materials.

Intensified Phonon Scattering in ZrCoBi Half-Heusler by Noble Metals Doping.

ZrCoBi-based half-Heuslers have great potential in power generation applications because of their high thermoelectric performance in both p- and n-type constituents. In this work, n-type ZrCoBi with improved thermoelectric performance has been realized by intensifying the phonon scattering via noble metal doping, e.g., Pd and Pt doping. The carrier concentration was effectively tuned to the optimal range, and the lattice thermal conductivity was greatly suppressed via the strong strain field and mass fluctuation scattering brought about by the large difference in atomic size and mass between Pd or Pt and Co. Consequently, the state-of-art figure of merit zT ∼1 was achieved in Pd- or Pt-doped ZrCoBi. In addition, the average zTavg values for ZrCo0.95Pd0.05Bi and ZrCo0.925Pt0.075Bi have reached 0.58 and 0.51, respectively, which are higher than those of most of the reported n-type ZrCoBi-based and ZrCoSb-based half-Heusler alloys.

The evolution of penta-graphene thermoelectrics: External fields as a key enabler for High-Performance devices

The tight-binding model is used in this study to examine the electronic and thermal properties of penta-Graphene (PG). To analyze these properties, approaches including the Kubo formula, linear response theory, and Green's function method are applied. The obtained numerical results show that by introducing bias voltage U, external magnetic field Π, and electron doping μ, significant modifications in electrical conductivity, magnetic susceptibility, and thermoelectric properties including power factor PF(T), Seebeck coefficients S(T), and figure of merit ZT(T) are observed. The band structure and density of states (DOS) of PG structure can be significantly altered by applying a bias voltage or an external magnetic field. The wide band gap of PG exhibited a marked reduction with the application of these external fields, displaying greater sensitivity to Π compared to U. Thermal properties of PG, including electrical conductivity σ(T), exhibit a peak as a function of temperature, which its peak position depends on the external fields and μ. Below the peak, σ(T) increases significantly with increasing external fields and μ, due to the reduced band gap and increased number of excited charge carriers, respectively. σ(T) demonstrates greater sensitivity to μ compared to Π, and both parameters have a more pronounced effect than bias voltage U. The magnetic field and bias voltage shift the peak position of the power factor PF(T) in opposite directions. Through tuning of the external fields, which alters the electronic structure of PG, the ZT(T) can be enhanced and its rate of increase is more dependent on bias voltage than on magnetic field.

Electronic structure, optical properties and high thermoelectric figure of merit in iron phthalocyanine polymer: FP-LAPW comparative studies

The comparative study of optoelectronic and thermoelectric properties of Iron phthalocyanine polymer, FePc, has been performed using the full-potential linearized augmented plane wave (FP-LAPW) method. The results show that the band gap of FePc is 2.039 eV with two deep trap bands. Optical spectra such as the real and imaginary parts of the dielectric function, reflectivity, electron energy loss spectroscopy, refractive index, extinction, and absorption coefficients are calculated and the main peaks are compared with the available experimental data. Calculated optical spectra are in good agreement with the experimental data of diethylamine and triethylamine polymers. The maximum values of 2190.2 μVK−1 and 1.02 are obtained for the Seebeck coefficient and the figure of merit ZT at 300 K for the chemical potentials near the Fermi level, respectively. The results show that FePc polymer is a good candidate for optoelectronic device applications.

Prospective lead-free fluorine double perovskites Rb2B'BiF6 (B' = Ag, Cu, In): Unveiling solar cell and optoelectronic promise through ab-initio insights

This study takes a comprehensive approach to investigate the optical and transport attributes of the fourth stable double perovskites—Rb2AgBiF6, Rb2CuBiF6 and Rb2InBiF6with a focus on their potential applications in photovoltaic, optoelectronic devices and thermoelectric systems. Calculated indirect band gap values are unveiled, measuring 2.07/1.90, 1.10/1.03 eV for Rb2AgBiF6 and Rb2CuBiF6 using PBE-GGA/LDA respectively. While for Rb2InBiF6 a direct band gap of 1.0/0.7 eV was found. We meticulously discuss optical features, encompassing dielectric constants, absorption coefficients, refractive indices, and reflectivity. Notably, prominent absorption peaks occur within the visible and ultraviolet spectrum across the three compounds, solidifying their suitability for solar cells and optoelectronic endeavors. Electron transport properties are scrutinized, encompassing electrical conductivity, thermal conductivity, Seebeck coefficient, and the figure of merit ZT. Utilizing the BoltzTraP code, these calculations span temperatures from 50 K to 1200 K. Our analysis of transport properties foresees the p-type semiconducting nature of the inorganic double perovskites Rb2B'BiF6 (B' = Ag, Cu, In), adding depth to the characterization of their transport behavior. These findings hold significance for their potential deployment in various photovoltaic, optoelectronic and thermoelectric applications, underlining their potential for advancing diverse technological landscapes.

Enhanced figure of merit for famatinite Cu3SbSe4via band structure tuning and hierarchical architecture

The p-type Te-free Cu3SbSe4 with famatinite structure is a potential candidate for thermoelectric materials due to the low cost and eco-friendly constituent elements. However, its strong bipolar effect and high lattice thermal conductivity (κlat) are the main challenges for its performance enhancement. Herein, we report a new strategy to enhance its figure of merit zT ∼0.86 at 673 K for Cu3Sb0.95Fe0.05Se2.8S1.2via band structure tuning and hierarchical architecture. Firstly, S substituted Se atoms in lattice can widen the band gap to alleviate the bipolar effect. Secondly, Fe doping in Sb site significantly increases the density of states, thus increasing the carrier effective mass, and obtaining a remarkably high Seebeck coefficient of ∼560 μV/K at 300 K. Moreover, the induced hierarchical architecture defects resulting in a minimum κlat of ∼0.48 W·m−1·K−1 at 673 K. Consequently, the improved Seebeck coefficient combined with low thermal conductivity leads to an enhanced zT.

Enhancing thermoelectric properties of AgSbTe2 thin films by modulating vacancies and microstructures

Thin-film thermoelectrics (TE) are of significance for micro-TE coolers and generators. AgSbTe2 is attracting extensive attention due to the ultra-low thermal conductivity and large Seebeck coefficient, but the TE performance of AgSbTe2 film is still not satisfactory. Herein, oriented Cd-doped AgSbTe2 films have been prepared by magnetron sputtering. Tuning the annealing temperature can change the concentration of vacancies and grain size of the AgSbTe2 film, leading to enhanced electrical conductivity and power factor. Due to the effective scattering induced by the nanostructure and grain boundaries, the thermal conductivity of the AgSbTe2-based thin films is also significantly reduced. As a result, the figure of merit zT can reach 1.38 at 523 K, and the average zT reaches 0.91 in the temperature range from 300 to 523 K. Our work demonstrates that the modulation of vacancies and microstructures is a feasible way to enhance the TE performance of the AgSbTe2-based films.

Impact of the nanostructuring on the thermal and thermoelectric properties of α-SrSi2

α-SrSi2 is a promising material for thermoelectric application around room temperature, especially if we can decrease the lattice contribution to the thermal conductivity. We report in this article the effect of the nanostructuring and the effect of the purity on the thermal and thermoelectric properties. Combining ball milling and spark plasma sintering we show that it is possible to obtain nanostructured pellets with grain size about 200 nm which permit to decrease the lattice components to the thermal conductivity close to that expected to highly disordered or amorphous materials. The results obtained in this study also show that the figure of merit ZT is improved after nanostructuring to a value of 0.20 around room temperature. The effect of the Sr purity on the thermoelectric properties is also presented.

Large Improvement of Thermoelectric Performance by Magnetism in Co-Based Full-Heusler Alloys.

Full-Heusler alloys (fHAs) exhibit high mechanical strength with earth-abundant elements, but their metallic properties tend to display small electron diffusion thermopower, limiting potential applications as excellent thermoelectric (TE) materials. Here, it is demonstrated that the Co-based fHAs Co2 XAl (X = Ti, V, Nb) exhibit relatively high thermoelectric performance due to spin and charge coupling. Thermopower contributions from different magnetic mechanisms, including spin fluctuation and magnon drag are extracted. A significant contribution to thermopower from magnetism compared to that from electron diffusion is demonstrated. In Co2 TiAl, the contribution to thermopower from spin fluctuation is higher than that from electron diffusion, resulting in an increment of 280µWm-1 K-2 in the power factor value. Interestingly, the thermopower contribution from magnon drag can reach up to -47µVK-1 , which is over 2400% larger than the electron diffusion thermopower. The power factor of Co2 TiAl can reach 4000µWm-1 K-2 which is comparable to that of conventional semiconducting TE materials. Moreover, the corresponding figure of merit zT can reach ≈0.1 at room temperature, which is significantly larger than that of traditional metallic materials. The work shows a promising unconventional way to create and optimize TE materials by introducing magnetism.

Electronic transport computation in thermoelectric materials: from ab initio scattering rates to nanostructures

Over the last two decades a plethora of new thermoelectric materials, their alloys, and their nanostructures were synfthesized. The ZT figure of merit, which quantifies the thermoelectric efficiency of these materials increased from values of unity to values consistently beyond two across material families. At the same time, the ability to identify and optimize such materials, has stressed the need for advanced numerical tools for computing electronic transport in materials with arbitrary bandstructure complexity, multiple scattering mechanisms, and a large degree of nanostructuring. Many computational methods have been developed, the majority of which utilize the Boltzmann transport equation (BTE) formalism, spanning from fully ab initio to empirical treatment, with varying degree of computational expense and accuracy. In this paper we describe a suitable computational process that we have recently developed specifically for thermoelectric materials. The method consists of three independent software packages that we have developed and: (1) begins from ab initio calculation of the electron–phonon scattering rates, (2) to then be used within a Boltzmann transport simulator, and (3) calculated quantities from the BTE are then passed on to a Monte Carlo simulator to examine electronic transport in highly nanostructured material configurations. The method we describe is computationally significantly advantageous compared to current fully ab initio and existing Monte Carlo methods, but with a similar degree of accuracy, thus making it truly enabling in understanding and assessing thermoelectric transport in complex band, nanostructured materials.

Ab initio predictions of thermoelectric, mechanical, and phonon characteristics of FeTiSe half-Heusler compound

In order to meet the increasing global energy demand, half-Heusler (H–H) alloys are found to be a cheap and efficient choice for energy generation applications. The electrical, structural, thermoelectric, dynamical, and mechanical properties of the novel FeTiSe half-Heusler alloy were predicted using the density functional theory (DFT). The equilibrium lattice constant was obtained as 5.62 Å. The electronic band-structure (B–S) and projected-density of states (P-DOS) were evaluated, and the band gap was obtained as 0.813 eV. Furthermore, applying the density functional perturbation theory (DFPT), we predicted the alloy's dynamical stability. Our findings from elastic constants revealed that the alloy is stiff, brittle, elastically anisotropic, and mechanically stable. For the n-type and p-type compositions, the Seebeck coefficients were found to be −273.53 and 190.13 μV/K at 1300 K, and figure of merit zT = 1.02 and 0.46 and zT = 2.92 and 2.8 at 300 and 1300 K, respectively. Thus, FeTiSe exhibits promising thermoelectric characteristics.

High-Performance Industrial-Grade p-Type (Bi,Sb)2 Te3 Thermoelectric Enabled by a Stepwise Optimization Strategy.

As the sole dominator of the commercial thermoelectric (TE) market, Bi2 Te3 -based alloys play an irreplaceable role in Peltier cooling and low-grade waste heat recovery. Herein, to improve the relative low TE efficiency determined by the figure of merit ZT, an effective approach is reported for improving the TE performance of p-type (Bi,Sb)2 Te3 by incorporating Ag8 GeTe6 and Se. Specifically, the diffused Ag and Ge atoms into the matrix conduce to optimized carrier concentration and enlarge the density-of-states effective mass while the Sb-rich nanoprecipitates generate coherent interfaces with little loss of carrier mobility. The subsequent Se dopants introduce multiple phonon scattering sources and significantly suppress the lattice thermal conductivity while maintaining a decent power factor. Consequently, a high peak ZT of 1.53 at 350 K and a remarkable average ZT of 1.31 (300-500 K) are attained in the Bi0.4 Sb1.6 Te0.95 Se0.05 + 0.10 wt% Ag8 GeTe6 sample. Most noteworthily, the size and mass of the optimal sample are enlarged to Ø40 mm-200g and the constructed 17-couple TE module exhibits an extraordinary conversion efficiency of 6.3% at ΔT= 245 K. This work demonstrates a facile method to develop high-performance and industrial-grade (Bi,Sb)2 Te3 -based alloys, which paves a strong way for further practical applications.