Temperature Dependence Of Thermoelectric Properties Research Articles

Conduction band engineering of Mg2Si by isotropic strain for enhancement of thermoelectric performance: A first-principles study

Mg2Si has attracted considerable attention as an environmentally friendly thermoelectric material. Previous studies have revealed that the thermoelectric performance of Mg2Si is strongly affected by tensile strain. The present study aims at determining the origin of the structural effect on the thermoelectric properties from the perspective of the effective mass, which allows the quantitative analysis of the strain dependence of the electronic states near the Fermi level. We performed first-principles calculations for Mg2Si with assumed different lattice constants and calculated the Seebeck coefficients and the electrical conductivities based on the obtained electronic band structures within the Boltzmann transport theory. As a result, a lattice constant assumed to be larger than that of the equilibrium state improved the thermoelectric power factor, which is consistent with the findings of previous works. We found that this enhancement of thermoelectric performance originates from a larger effective mass of the bottom conduction band as well as an effective convergence of two conduction bands. The present calculation successfully reproduces the value of an experimental effective mass. We also find the temperature dependences of thermoelectric properties using the relaxation time obtained using experimental data.

Phonon phase stability, structural, mechanical, electronic, and thermoelectric properties of two new semiconducting quaternary Heusler alloys CoCuZrZ (Z = Ge and Sn)

For meeting the energy demand, the development of new and novel thermoelectric (TE) materials for power generation is very vital. In this draft, we have theoretically investigated two new quaternary CoCuZrZ (Z = Ge and Sn) Heusler alloys for their structural, mechanical, electronic, and TE properties. In the energy minimization process, the alloys are found to be non-magnetic in the ground state. Based on calculated phonon dispersion curves, formation energy, and elastic constants, we propose that both CoCuZrGe and CoCuZrSn are stable. Furthermore, the mechanical properties indicate that CoCuZrGe (CoCuZrSn) has a brittle (ductile) nature. The electronic properties examined in Perdew-Burke-Ernzerhof (PBE), PBEsol, and modified Becke-Johnson (mBJ) potential, all predict that reported systems are narrow-gap semiconductors (SCs). In addition, the temperature dependent TE properties have been studied by calculating the electronic thermal conductivity (κ), Seebeck coefficient (S), power factor (PF) and electrical conductivity (σ/τ). The obtained positive value of S conveys the materials as p-type SCs, with a maximum value of 26.2 μV/K for CoCuZrGe and 28 μV/K for CoCuZrSn. The σ/τ, κ, and PF show increasing trends with rising temperature. The PF is found to be 1.55 × 1012 WK−2m−1s−1 for CoCuZrGe and 1.38 × 1012 WK−2m−1s−1 for CoCuZrSn. The proposed semiconducting Heusler alloys may receive attention for a range of TE and spintronic applications.

Thermoelectric Properties of Graphene Like Nanotube Structure Due to Next-to-nearest Neighbor Hopping Amplitude

We have addressed the density of states and thermoelectric properties of doped graphene like nanotubes for both zigzag and armchair types in the context of tight binding model hamiltonian. The effects of next nearest neighbor hopping amplitude for electrons and gap parameter on thermal conductivity, Seebeck coefficient and figure of merit have been investigated. Green’s function approach has been implemented to find the behavior of transport coefficients of nanotube within linear response theory. We have found the temperature dependence of thermoelectric properties for different values of the gap parameter and tube diameter in the presence of next nearest neighbor hopping integral. The results of electrical conductivity show the increase of diameter leads to increase thermal conductivity of armchair nanotubes. However the variation of tube diameter has no remarkable effect on thermal conductivity of zigzag nanotube. Also the increase of chemical potential reduces the conductivity of zigzag nanotube. Moreover the temperature behavior of the other thermoelectric properties, i.e. figure of merit and Seebeck coefficient, have been addressed for both types of nanotubes in details.

Thermoelectric properties of YSb: A first-principles approach

The electronic and thermoelectric properties of YSb have been investigated using Full Potential Linearized Augmented Plane wave method (FP-LAPW) based on density functional theory (DFT) in combination with semi-classical Boltzmann transport equation. The exchange and correlation effects have been taken into account by generalized gradient approximation (GGA) within Perdew-Burke-Ernzerhof (PBE) parameterization. The calculated elastic constants such as Bulk modulus, Young modulus, Poisson’s ratio and shear anisotropy factor confirm the mechanical stability of this compound. Boltzmann transport equations were used for the calculation of electronic part of thermoelectric transport parameters such as Seebeck coefficient, Power factor, electrical conductivity and electronic part of thermal conductivity as a function of chemical potential. The temperature dependence of thermoelectric properties of this material at pressure ∼12 GPa is studied over wide range of temperature (200–1200 K). The calculated value of power factor, electrical conductivity and thermal conductivity at T = 1200 K are found to be 65.92 × 1010 W K−2 cm−1 sec−1, 2.90 × 1020 S cm−1sec−1 and 0.97 × 1016 W m−1K−1 sec−1, respectively. The large value of power factor which is due to its non-trivial topological nature strongly suggests that this material can act as a good thermoelectric material in high temperature region.

Hydride assisted synthesis of the high temperature thermoelectric phase: Yb14MgSb11

Yb14MnSb11 is a p-type high temperature thermoelectric material that has been shown to have a peak zT of 1.3 at 1273 K and stable lifetime testing at that temperature for over 1500 h by NASA. Yb14MgSb11 is a structural analog, but the highest temperature thermoelectric properties have not yet been reported. Yb14MgSb11 has been prepared in an environmentally friendly route employing metal hydrides to provide phase pure samples with excellent control of stoichiometry. We present a comparative study employing either MgH2 or YbH2 as a reactive precursor that also facilitates milling of the elements. High purity compositions are synthesized, and their high temperature thermoelectric properties were measured on dense pellets. Temperature-dependent thermoelectric properties were measured from 300 to 1273 K. Yb14MgSb11 exhibited a peak zT = 1.2 at 1273 K due to an appreciable power factor and low-lattice thermal conductivity. Carrier concentration and hall mobility were also measured from 300 to 1275 K and ranged from 5.3 × 1020 to 1.3 × 1021 cm−3 and from 4.7 to 0.7 cm2 V−1 S−1, respectively.

Performance Analysis of a Thermoelectric Generator with a Segmented Leg

With the development of thermoelectric (TE) materials with high thermoelectric figure of merit (ZT) values, improved thermoelectric devices have been fabricated recently. Although the performance of thermoelectric generators (TEGs) with segmented legs has been evaluated experimentally, detailed numerical prediction of TEG performance is rarely considered. In this study, a new numerical solution method is devised to analyze a TE couple. We employ the finite volume method, which allows the conservation of thermal energy and electric charge. A computer program based on the numerical method is developed to evaluate the performance of a TEG with a segmented p-leg. Temperature-dependent thermoelectric properties are employed, without simplifying or neglecting Joule heating, Thomson heating, or Peltier heating. The numerical method is discussed in detail, including the derivation of algebraic equations from the integral forms of the governing equations for thermal energy and electric potential. The numerical results obtained with the computer program based on the method are compared with a mathematical solution for validation. Compared with the performance without segmentation, the results show an improvement for the TEG with a segmented leg.

Comparison and parametric study of two theoretical modeling approaches based on an air-to-water thermoelectric generator system

Numerical model and thermal resistance model are two commonly used modeling approaches to analyze the performance of thermoelectric devices. However, a great number of assumptions and simplifications are made in previous researches. This work presents a steady state and fluid-thermal-electric multi-physical numerical model and improves the conventional thermal resistance model to investigate the performance of an air-to-water thermoelectric generator system. In considering the multi-physical field coupling effects and temperature dependent thermoelectric material properties, the comparison of these two theoretical models and parametric investigation on the thermoelectric generator system are conducted. The results indicate that the thermal resistance model can be used to preliminarily evaluate the performance of thermoelectric generator system with acceptable accuracy at appropriate working conditions, but more precisely, the numerical model should be adopted. Besides, the heat losses should not be omitted in the theoretical analysis, and an optimal load resistance must be chosen between the maximum power one and the maximum conversion efficiency one. The findings of this work can guide the future theoretical analysis of the thermoelectric generator system by numerical model or thermal resistance model.

An annular thermoelectric couple analytical model by considering temperature-dependent material properties and Thomson effect

In this paper, an annular thermoelectric couple (ATEC) analytical model is proposed with simultaneous consideration of temperature-dependent thermoelectric (TE) material properties and Thomson effect. To obtain a more precise analytical solution, this model is analyzed with two different approximate assumptions and then applied in skutterudites (SKU) and half-Heusler TE materials. The reliability and accuracy of this model are validated by comparing with numerical results. The results show that the proposed analytical model with a parabolic assumption is especially appropriate to be applied in the obvious temperature-dependent TE materials, such as SKU. When this analytical model is applied in SKU material, the calculated relative errors of temperature, derivative of temperature, output power, and conversion efficiency are less than 0.14%, 0.9%, 0.6% and 1.52%, respectively. Besides, it is emphasized that the Thomson effect and temperature-dependent material properties should be taken into account simultaneously for precise performance analyses of an ATEC. The proposed analytical model could be applied for convenient calculation of the output power and conversion efficiency of ATECs. It also could be an approach of investigation and determination for the best parameter estimation if this model is applied for optimal design of ATECs.

Thermoelectric Property in Orthorhombic-Domained SnSe Film.

Single-crystal SnSe exhibits extremely high thermoelectric properties, and fabrication of SnSe films is promising for practical application and basic research on properties. However, the high thermoelectric properties have not yet been reported in SnSe films and their thermoelectric properties and nanostructure have not yet been analyzed in detail. In the present study, a-axis-oriented epitaxial SnSe films were prepared to discuss the thermoelectric properties of the SnSe films. While the electrical conductivity of the films was orders of magnitude smaller than that in the single crystals at room temperature, surprisingly, the thermoelectric property (power factor) of the films was slightly higher than that in the single crystals at high temperatures (∼300 °C). The SnSe films contained orthorhombic domain boundaries with a spacing of several hundred nanometers. The orthorhombic domain boundaries caused carrier scattering and degraded the mobility of the films at room temperature, but their effect decreased with increasing temperature. Thus, the carrier scattering at domain boundaries results in characteristic temperature dependence of thermoelectric properties in the SnSe films. High thermoelectric properties at high temperatures were successfully achieved in the SnSe films in spite of the existence of domain boundaries, demonstrating the possibility of high-performance of SnSe thermoelectric films.

Enhanced TE properties of Cu@Ag/Bi2Te3 nanocomposites by decoupling electrical and thermal properties

Cu@Ag/Bi2Te3 nanocomposites were prepared for the first time by ultrasonic dispersion-rapid freeze-drying method combined with spark plasma sintering (SPS). By changing the content of Cu@Ag nanoparticle, we could modulate the temperature dependent thermoelectric properties. The highest ZT value can be obtained at 450 K for 1 vol% Cu@Ag/Bi2Te3, which is benefited from the decoupling of electrical and thermal properties. With the increase of electrical conductivity, the absolute value of Seebeck coefficient lifts while the thermal conductivity declines. Meanwhile, the average ZT value between 300 K and 475 K was 0.61 for 1 vol% Cu@Ag/Bi2Te3, which is much higher than that of pristine Bi2Te3. Therefore, the decoupling effect of Cu@Ag nanoparticles incorporation could be a promising method to broaden the application of Bi2Te3 based thermoelectric materials.

One-pot fabrication of Ag–SrTiO3 nanocomposite and its enhanced thermoelectric properties

Abstract Ag–SrTiO3 ceramic nanoparticles were fabricated by doping SrTiO3 with various contents (0.5, 1, 3, and 5%, in mass ratio) of Ag. Composite samples were prepared through a one-pot solvothermal method and sintering process. The temperature-dependent thermoelectric properties of these sample were measured from 300 K to 500 K. The maximum power factor (843.3 μ·W/m·K2) at 500 K, which is ∼3.96 times higher than that of the pristine SrTiO3 ceramics, was obtained for the Ag–SrTiO3 composite sample with 1% of Ag. In addition, the thermal conductivity of the composites decreased due to the phonon scattering effect. The maximum thermoelectric figure of merit (ZT), i.e., ∼0.09, which was achieved with 1% of Ag at 500 K, yielded an enhanced power factor and a reduced thermal conductivity. This ZT value was ∼4.27 times larger than that of pristine SrTiO3 at the same temperature.

N-Type Bismuth Telluride Nanocomposite Materials Optimization for Thermoelectric Generators in Wearable Applications.

Thermoelectric materials could play a crucial role in the future of wearable electronic devices. They can continuously generate electricity from body heat. For efficient operation in wearable systems, in addition to a high thermoelectric figure of merit, zT, the thermoelectric material must have low thermal conductivity and a high Seebeck coefficient. In this study, we successfully synthesized high-performance nanocomposites of n-type Bi2Te2.7Se0.3, optimized especially for body heat harvesting and power generation applications. Different techniques such as dopant optimization, glass inclusion, microwave radiation in a single mode microwave cavity, and sintering conditions were used to optimize the temperature-dependent thermoelectric properties of Bi2Te2.7Se0.3. The effects of these techniques were studied and compared with each other. A room temperature thermal conductivity as low as 0.65 W/mK and high Seebeck coefficient of −297 μV/K were obtained for a wearable application, while maintaining a high thermoelectric figure of merit, zT, of 0.87 and an average zT of 0.82 over the entire temperature range of 25 °C to 225 °C, which makes the material appropriate for a variety of power generation applications.

Optimized high performance thermoelectric generator with combined segmented and asymmetrical legs under pulsed heat input power

In this study, a segmented asymmetrical thermoelectric generator (SASTEG) is numerically investigated to optimize its electrical performance and mechanical reliability under transient and steady state conditions. The thermal and electrical performance of the SASTEG and TEG under transient and steady state heating conditions are studied and compared. A three-dimensional numerical model is developed and solved using finite element method in COMSOL 5.3 Multiphysics software. Temperature dependent thermoelectric material properties are considered, and the elastoplastic behaviour of copper and solder is accounted for in the thermal stress analysis. The initial SASTEG geometry used in this study is subsequently optimized to reduce the maximum von Mises stress developed in its legs while maintaining its enhanced power output. Results obtained show that the optimized SASTEG provided a power output enhancement of 117.11% compared to that of the conventional TEG under rectangular pulsed heat condition. Also, the asymmetrical leg geometry used in the SASTEG n-type leg provided a reduced thermal stress of 39.21% in the lower segment (cold side) compared to the symmetrical leg geometry used in the p-type leg lower segment. This study will provide vital guidance in the design of high-performance TEG with improved mechanical reliability.

Comparative study of a concentrated photovoltaic-thermoelectric system with and without flat plate heat pipe

Thermal management of photovoltaic cells is an essential research objective for increasing the conversion efficiency of the photovoltaic. Flat plate heat pipe is a passive cooling device capable of effectively reducing the solar cell temperature. Therefore, this study presents a numerical investigation of a hybrid photovoltaic-thermoelectric system with and without a flat plate heat pipe. A detailed comparative analysis of the electrical performance of the photovoltaic only, photovoltaic-thermoelectric and photovoltaic-thermoelectric-heat pipe systems is performed. The influence of solar concentration ratio, ambient temperature, wind speed and thermoelectric generator cold side temperature on the efficiency and power output of the photovoltaic only and hybrid photovoltaic-thermoelectric systems are studied using COMSOL 5.4 Multiphysics software. A three-dimensional finite element study is carried out and temperature dependent thermoelectric material properties are considered to increase the simulation accuracy. Results show that the photovoltaic-thermoelectric-heat pipe efficiency is 1.47% and 61.01% higher compared to that of the photovoltaic-thermoelectric and photovoltaic only systems respectively at a concentration ratio of 6. In addition, the photovoltaic-thermoelectric-heat pipe is recommended for highly concentrated systems because of its superior performance. Furthermore, the photovoltaic-thermoelectric system is a better alternative to the photovoltaic only system because of its enhanced performance which is second only to that of the photovoltaic-thermoelectric-heat pipe system. Results also show that ineffective cooling of the thermoelectric generator can adversely affect the performance of the hybrid systems. This study will proper valuable information on the feasibility of hybrid photovoltaic-thermoelectric systems with and without heat pipe. Finally, the three-dimensional nature of this study makes it very useful in understanding the actual temperature distribution in the hybrid systems.

Growth of Cu2InO4 thin films on Si substrate by thermal evaporation technique and enhancement of thermoelectric properties by post-growth annealing

Thin films of Cu2InO4 were grown by thermal evaporation on Si substrate using tube furnace. Cu and In metals were evaporated in the tube furnace having pressure 0.2 × 10−2 Torr and the oxygen gas flow rate was set 60 standard cubic centimeters (sccm). The post-growth annealing in oxygen environment was performed at different temperatures ranging from 600 to 800 °C for 1 h in programmable furnace. The X-Ray Diffraction (XRD) data confirmed that as grown sample showed the amorphous nature, but it was converted into crystalline at higher annealing temperatures (700–800 °C). Raman spectroscopy measurements further confirmed the XRD results which showed a peak at 305 cm−1 at higher annealing temperatures, which is related to the Cu2InO4 structure along with Si peak at 521 cm−1. Photoluminescence spectroscopy (PL) data was demonstrated a band to band emission of Cu2InO4 at 1.9 eV and the intensity of this band was enhanced with the annealing temperature. XRD, Raman spectroscopy and PL results suggested that the structure of grown thin films was improved with increasing annealing temperature. After structure verification, In metal contacts were fabricated on all samples to investigate the temperature dependent thermoelectric properties. The Seebeck coefficient of Cu2InO4 thin films was found to be increased with annealing as well as measurement temperature. The reported results were explained according to charge mobility engineering, which was reported earlier by our group. We have also performed Hall measurements to strengthen our argument.

Thermoelectric Properties of Scandium Sesquitelluride.

Rare-earth (RE) tellurides have been studied extensively for use in high-temperature thermoelectric applications. Specifically, lanthanum and praseodymium-based compounds with the Th3P4 structure type have demonstrated dimensionless thermoelectric figures of merit (zT) up to 1.7 at 1200 K. Scandium, while not part of the lanthanide series, is considered a RE element due to its chemical similarity. However, little is known about the thermoelectric properties of the tellurides of scandium. Here, we synthesized scandium sesquitelluride (Sc2Te3) using a mechanochemical approach and formed sintered compacts through spark plasma sintering (SPS). Temperature-dependent thermoelectric properties were measured from 300–1100 K. Sc2Te3 exhibited a peak zT = 0.3 over the broad range of 500–750 K due to an appreciable power factor and low-lattice thermal conductivity in the mid-temperature range.

High performance and thermal stress analysis of a segmented annular thermoelectric generator

Annular thermoelectric generators can eliminate the thermal contact resistance formed due to geometry mismatch when flat-plate thermoelectric generators are used with round shaped heat source or heat sink. Therefore, in this study, the numerical simulation of a segmented annular thermoelectric generator (SATEG) is investigated using three-dimensional finite element analysis. The thermoelectric and mechanical performance of the segmented annular thermoelectric generator is studied by considering temperature dependent thermoelectric material properties and elastoplastic behaviour of copper and the welding layer (solder). The influence of segmented pin geometry on the performance of the segmented annular thermoelectric generator is investigated and comparison is made with non-segmented annular thermoelectric generators. COMSOL 5.3 Multiphysics software is used to investigate the effects of heat source temperature, thermoelectric leg length and leg angle on the electrical and mechanical performance of the segmented and non-segmented annular thermoelectric generators. Results show that the segmented annular thermoelectric generator has a greater efficiency compared to the annular thermoelectric generator (ATEG) with Bismuth telluride material when the temperature difference is greater than 100 K. In addition, the efficiency of the SATEG is found to be 21.7% and 82.9% greater than that of the Bismuth telluride ATEG and Skutterudite ATEG respectively at 200 K temperature difference. Finally, the results show that increase in thermoelectric leg length can reduce the thermal stress and electrical performance of the segmented and non-segmented thermoelectric generators. Results obtained from this study would influence the design and optimization of segmented annular thermoelectric generators.

Tailoring the thermoelectric properties of sol-gel grown CZTS/ITO thin films by controlling the secondary phases

In this work, we have demonstrated the effect of secondary phases on the thermoelectric properties of Cu2ZnSnS4 thin films (TF). The samples were grown on ITO substrate by chemical solution method and sulfurized in tube furnace having constant annealing temperature (500 °C) for different time duration (20, 40, 60 and 80 min). The temperature dependent (25–100 °C) thermoelectric properties (Seebeck coefficient, electrical conductivity and power factor) were measured using Seebeck effect and Hall effect. It was found that the value of Seebeck coefficient was increased from 70 to 753 μV/°C as the sulpurization time decreased from 80 to 20 min. The enhancement in Seebeck coefficient with decreasing sulpurized time duration is related with emergence of secondary phases which cause the filtering of slow carriers at the interface of secondary phases. The presence of secondary phases in the grown samples was confirmed by various characterization techniques. Raman spectroscopy data suggested CZTS/ITO sample sulpurized for 80 min has less secondary phase but as the sulpurized time duration decreased to 20 min, secondary phases such as Cu2S, SnS, SnS2 and ZnS were emerged and enhanced the thermoelectric properties. The XRD measurements were also verified that CZTS/ITO sample sulpurized for 20 min has poor crystal quality as compared to other samples due to the presence of secondary phases. SEM and UV/Visible spectroscopy measurements were also performed to support our argument.

Comprehensive modeling for geometric optimization of a thermoelectric generator module

This paper develops a one-dimensional simulation model of a thermoelectric module aiming at its characteristic exploration and output performance prediction. Almost all related effects are considered including Seebeck, Peltier, Thomson effects, the spatial- and temperature-dependent thermoelectric material properties, as well as the other irreversible factors. In order to ensure its high accuracy and computational efficiency, the differential equations built for components of non-linear physical properties, and analytical equations for the others of constant properties are jointly solved by numerical methods and implemented in MATLAB integrated development environment. A small deviation of 1.44% exists between its calculated output power and that by a previously reported three-dimensional model under a specific wide load range (0–250 Ω), validating its accuracy of this model. Besides, it indicates that both the proportions of heat leakage through the occupied air zone and its maximum output power to the input total heat flow have the same order of magnitude. A rising proportion for the former has to be paid attention in case of a higher temperature difference applied. The assumption of regarding its electrical contact resistance as an identical external resistance connected with the load in series is computationally proved to generate a negligible discrepancy in terms of TEM characteristics and its output performance. Furthermore, with respect to the impacts of TEM geometry, the pre-set fixed substrate area weakens the equivalence of regarding a thermoelectric module with larger cross-section area of thermoelectric legs as multiple identical modules with smaller leg areas connected in parallel. It accounts for the better linearity of rising output power by a long leg than that by a short leg as the leg area increases. A long thermoelectric leg means an increased induced electrical potential and inner resistance as well, thus leading to a single peak of the output power as the leg length increases. Finally, a thermoelectric geometric optimization approach abased on the Hill-climbing algorithm is introduced and first applied for the maximum output. Its accuracy and efficiency are validated and analyzed detailedly by supplying different variables as initial search point. All the methods and conclusions in this paper can assist thermoelectric generators’ performance estimation and guide the geometric optimizations regarding their large-scale applications in the future.

Enhancement of thermoelectric properties by lattice softening and energy band gap control in Te-deficient InTe1−δ

The InTe has intrinsically low lattice thermal conductivity κL originating from the anharmonic bonding of In1+ ion in the lattice, which scatters the phonons. Here we report the enhancement of thermoelectric properties in Te-deficient InTe1−δ (δ = 0, 0.01, 0.1, and 0.2) polycrystalline compounds by lattice softening and energy band gap opening. Te-deficiency gives rise to more weak chemical bonding between In1+ atoms and In3+Te2− clusters than those of pristine InTe, resulting in the reduction of κL near the room temperature. The weak ionic bonding is confirmed by the increase of lattice volume from the X-ray diffraction and lattice softening by the decrease of Debye temperature with increasing Te-deficiency. We observed the low lattice thermal conductivity κL of 0.53 W m−1 K−1 at 300 K for InTe0.99, which is about 25 % decreased value than those of InTe. The Te-deficiency also induces energy band gap so that the electrical resistivity and Seebeck coefficient are increased due to the decrease of carrier concentration. Temperature-dependent thermoelectric properties shows the high Seebeck coefficient at high temperature and high electrical conductivity near room temperature, resulting in the temperature-insensitive high power factor S2σ over a wide temperature range. Owing to the temperature-insensitive high power factor and intrinsic low lattice thermal conductivity by Te-deficiency, the thermoelectric performances of figure-of-merit ZT and engineering ZTeng are enhanced at mild temperature range (≤550 K).