Dimensionless Figure Of Merit Research Articles

Enhanced thermoelectric properties of (SnS)1−x(SnO)x: x ≤ 10.79% nanocomposites prepared by polyol method

(SnS)1−x(SnO)x: x = 0, 3.57%, 7.16%, and 10.79% nanocomposites (NCs) have been successfully synthesized using a simple polyol method. XRD data analysis shows formation of SnS and SnO NCs with an average crystallite size in the range from 30 to 35 nm. The Rietveld refinement analysis indicates layered orthorhombic crystal structure. FESEM images display the morphology of nanoflakes and nanoplates. Their positive Seebeck coefficients (α) suggest the p-type conduction. This is consistent with the deficiency of Sn found. (SnS)0.9284(SnO)0.0716 NC exhibits a thermal conductivity (κ) of 0.75 Wm−1 K−1 at 300 K. Electrical (σ) and thermal conductivities are decreased, but α enhances with increasing x. Consequently, the dimensionless figure of merit (ZT) increases with increasing x up to 7.16% for which ZT at 325 K is 0.0036. This is 55% enhancement compared to the pure SnS that opens a new window for improvement of thermoelectric performance near room temperature.

Role of grain alignment and oxide impurity in thermoelectric properties of textured n-type Bi–Te–Se alloy

A nanostructured n-type Bi2Te2.7Se0.3 (BTS) alloy with a unique microstructure was prepared using a facile melting-rotation-quenching process followed by ball-milling and uniaxial hot-press sintering at 623 K. Anisotropy in the resulting microstructure showed anisotropic electrical and thermal transport properties in two directions normal to the pressing axis. The texture of the nanostructured BTS alloy was analyzed by x-ray diffraction and scanning electron microscopy. Based on the geometric phase analysis of a high resolution transmission electron microscopy images, abundant dislocations, high grain boundary density, and oxide impurity were identified, which act as phonon scattering centers. Higher anisotropy in thermal conductivity combined with oxide impurity resulted in an ultra-low phonon thermal conductivity of ∼0.305 W mK−1 at 423 K in the nanostructured n-type BTS in the direction parallel to the pressing axis. Laser power- and temperature-dependent Raman spectra analyses provided a deeper insight into the anisotropy in thermal transport properties. Optimum power factor and low thermal conductivity, due to the combination of grain alignment and oxide impurity, resulted in a dimensionless figure of merit (zT ) value of ∼0.75 at 423 K. In comparison, the high and opposite temperature dependences of electrical conductivity and thermal conductivity led to a better average zT value of ∼0.68 and a thermoelectric energy conversion efficiency percentage of ∼4.4% in the operating temperature range (300–423 K) in the direction parallel to the pressing axis.

Top-down method to fabricate TiNi1+xSn half-Heusler alloy with high thermoelectric performance

The top-down method was used to fabricate the TiNi1+xSn half-Heusler alloy. Several knotty problems have been solved using the top-down method, including the generation of impurity phases, the visible discrepancy in the grain size of the TiNi2Sn second phase, and irregular variations in the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) with increasing Ni content. The generation of a large number of incoherent interfaces between the half-Heusler (HH) and full-Heusler (FH) phases fabricated by the top-down method drastically improved σ and retained κ and S, thereby leading to an enhancement in the dimensionless figure of merit (ZT) by ∼19 % for the TiNi1.05Sn sample as compared to the ZT of the samples prepared by the conventional method.

Thermoelectric properties of barium doped calcium cobaltite obtained by simplified chemical route

Calcium cobaltite (CCO) thermoelectric ceramics in Ca3Co4O9 and Ca2.95Ba0.05Co4O9 compositions were produced by a simplified chemical route developed in this study. Undoped ceramics sintered at 1193 K for 12 h presented a bulk density equivalent to 77% of the theoretical density, one of the highest values ever found for conventionally sintered undoped calcium cobaltite. The presence of barium improved densification, reaching 80% of the theoretical density. Thermoelectric properties were discussed based on X-ray diffraction patterns, scanning electron micrographs, Seebeck coefficient, electrical and thermal conductivity. Electrical characterizations presented values of about 6200 and 7000 S/m at 873 K for undoped and doped ceramics, respectively. However, all ceramics presented almost the same range of activation energy values, indicating the possibility of non-effective substitution through this methodology. The maximum global PF (power factor) was 0.10 mW.m-1.K-2 for the densest undoped sample and an overall ZT (dimensionless figure of merit) of 0.07 for the undoped ceramic at 873 K.

Thermoelectric transport properties of the van der Waals-type layered rhombohedral SnAs-based compound, EuSn2As2

The thermoelectric transport properties of the van der Waals-type layered rhombohedral SnAs-based compound, EuSn2As2, have been investigated. A densified polycrystalline sample of EuSn2As2 with porosity (ϕ) of 2.4(9) vol.% exhibited a weak orientation to the c-axis for hexagonal coordination system; the weak orientation is parallel (P ∥) to the pressing direction of hot pressing. Measurements of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) were conducted perpendicular (P ⊥) to the pressing direction. The experimental values of ρ and S exhibit metallic temperature dependence and p-type carrier polarity. The power factor (P) was 0.51(8) mW m−1 K−2 at 673(4) K. Using the Wiedemann–Franz–Lorenz law, the phonon thermal conductivity (κ ph) was estimated to be 0.4(6) W m−1 K−1 at 673(6) K. The dimensionless figure of merit, ZT, was 0.092(17) at 673(3) K.

Thermoelectric and Transport Properties of Permingeatite Cu3SbSe4 Prepared Using Mechanical Alloying and Hot Pressing.

Permingeatite (Cu3SbSe4) is a promising thermoelectric material because it has a narrow band gap, large carrier effective mass, and abundant and nontoxic components. Mechanical alloying (MA), which is a high-energy ball mill process, has various advantages, e.g., segregation/evaporation is not required and homogeneous powders can be prepared in a short time. In this study, the effects of MA and hot-pressing (HP) conditions on the synthesis of the Cu3SbSe4 phase and its thermoelectric properties were evaluated. The electrical conductivity decreased with increasing HP temperature, while the Seebeck coefficient increased. The power factor (PF) was 0.38–0.50 mW m−1 K−2 and the thermal conductivity was 0.76–0.78 W m−1 K−1 at 623 K. The dimensionless figure of merit, ZT, increased with increasing temperature, and a reliable and maximum ZT value of 0.39 was obtained at 623 K for Cu3SbSe4 prepared using MA at 350 rpm for 12 h and HP at 573 K for 2 h.

Carrier Concentration of CeFe<sub>2</sub>Al<sub>10</sub> as a Candidate of Thermoelectric Material

The study of Hall effect of Kondo semiconductor CeFe2Al10 is reported as a candidate of thermoelectric material used at low temperatures. Single crystals of CeFe2Al10 with orthorhombic crystal structure were grown by Al self-flux method. An anisotropy of the Hall effect is clarified by measuring Hall resistance by changing the direction of electrical current, magnetic field, and voltage respect to all the three crystal axes of orthorhombic crystal structure. The Hall effect of CeFe2Al10 has a strong anisotropy against the direction of magnetic field but weak anisotropy against the directions of current and voltage. The value of carrier concentration indicates that CeFe2Al10 is matallic, which causes a low performance as a thermoelectric material. In order to improve the value of dimensionless figure of merit, the electrons should be doped to CeFe2Al10.

Thickness-dependent thermoelectric properties of Si1−xGex films formed by Al-induced layer exchange

Highly reliable, thin-film thermoelectric generators are strongly desired for future sensor advancements. Al-induced layer exchange is a unique method for producing thermoelectric SiGe layers on a flexible plastic substrate at low temperatures. In this study, we investigated the thickening of the Si1−xGex (x = 0, 0.6, and 1) layers to improve the thermoelectric output power. The upper limit of the film thickness was approximately 1000 nm, while it influenced the crystal and electrical properties of the resulting Si1−xGex layers. The Si0.4Ge0.6 layer formed at 400 °C exhibited a high power factor of up to 850 μW m−1 K−2 at room temperature, which is the record-high value among p-type SiGe obtained at low temperature (<900 °C). The dimensionless figure of merit was determined to be 0.12 from the power factor and the thermal conductivity of 2.2 W m−1 K−1. The sequential layer exchange allowed us to form a 2000-nm-thick SiGe layer. Furthermore, the output power density was almost proportional to the film thickness and reached 37 nW cm−2 at room temperature under a temperature difference of 10 K. These achievements will present practical applications for next-generation thin-film thermoelectric generators based on highly reliable, human-friendly materials.

Evaluation of Thermoelectric Performance of Bi2Te3 Films as a Function of Temperature Increase Rate during Heat Treatment

Thin film thermoelectric generators are expected to be applied as power supplies for various Internet of Thing devices owing to their small size and flexible structure. However, the primary challenges of thin film thermoelectric generators are to improve their thermoelectric performance and reduce their manufacturing cost. Hence, Bi2Te3 thin films were deposited using direct current magnetron sputtering, followed by heat treatment at 573 K with different temperature increase rates ranging from 4 to 16 K/min. The in-plane Seebeck coefficient and electrical conductivity were measured at approximately 293 K. The in-plane thermal conductivity was calculated using the models to determine the power factor (PF) and dimensionless figure of merit (ZT). The temperature increase rate clearly affected the atomic composition, crystal orientation, and lattice strains, but not the crystallite size. The PF and dimensionless ZT increased as the temperature increase rate increased. The highest PF of 17.5 µW/(cm·K2) and ZT of 0.48 were achieved at a temperature increase rate of 16 K/min, while the unannealed thin film exhibited the lowest PF of 0.7 µW/(cm·K2) and ZT of 0.05. Therefore, this study demonstrated a method to enhance the thermoelectric performance of Bi2Te3 thin films by heat treatment at the appropriate temperature increase rate.

Thermoelectric performance of BaBiNa and SrBiNa: A first-principle study

The high thermoelectric performance is expected to develop practical thermoelectric materials. The relationship between the dimensionless figure of merit (ZT) and the carrier concentration and the temperature is constructed to identify the best thermoelectric performance of XBiNa (X = Ba and Sr). The electronic structure and transport properties of the two semiconductors are calculated by combining the first-principle DFT with the semi-classical Boltzmann transport theory to obtain the Seebeck coefficient, electronic thermal conductivity, and electrical conductivity. The relaxation time is determined by the deformation potential theory, and the lattice thermal conductivity is calculated by the Slack model. The maximum ZTs of 1.14 and 1.16 can be achieved for the n-type BaBiNa and SrBiNa at a carrier concentration of 5.62 × 1018/3.60 × 1018 cm−3 at 800 K, respectively. The maximum thermoelectric conversion efficiency of 21.25 % is attributed to the n-type SrBiNa, implying that the compound is potential thermoelectric material. The different thermoelectric mechanisms for the n- and p-type XBiNa are elucidated through evaluating the tendency of the transport coefficients and ZT.

First-principles investigation on the thermoelectric performance of half-Heusler compound CuLiX(X = Se, Te)

The remarkable thermoelectric performance is predicted for half-Heusler (HH) compounds of CuLiX (X = Se, Te) based on the first-principles calculation, the deformation potential (DP) theory, and semi-classical Boltzmann theory. The Slack model is employed to evaluate the lattice thermal conductivity and the result is in good agreement with the previously reported data. The results of mechanical properties demonstrate that CuLiSe is ductile but CuLiTe is brittle. The relaxation time and the carrier mobility are calculated with DP theory. The electrical and thermal conductivities are obtained by using the semi-classical Boltzmann theory based on the relaxation approximation. The Seebeck coefficient and power factor are obtained and their characters are analyzed. The dimensionless figure of merits (ZT) is obtained for the p- and n-type CuLiX. The maximum ZT of 2.65 can be achieved for n-type CuLiTe at the carrier concentration of 3.19 × 1019 cm−3 and 900 K, which indicates that this compound is a very promising candidate for the highly efficient thermoelectric materials.

Strain-tunable electronic, optical and thermoelectric properties of BP monolayer investigated by FP-LAPW calculations

In present work, the strain effect on the electronic, optical and thermoelectric properties of the Boron phosphide (BP) monolayer have been systematically investigated using first-principles calculations. Simulations show that the single layer at hand is a direct semiconductor possessing a K−K band gap of 0.910 eV. This parameter increases when switching the strain nature from compressive to tensile. Results indicate that the optical absorption in the visible and near ultraviolet regimes may be enhanced by the lattice compression. Finally, the thermoelectric properties are considered by determining the Seebeck coefficient, electrical conductivity, electronic thermal conductivity and dimensionless figure of merit. Compressive strains lead to the improvement of the thermoelectric performance with the figure of merit up to 0.665 at room temperature. However, chemical potential variations may favor the figure of merit increasing up to values close to unity. The findings may provide important information to estimate the prospective applicability of the BP monolayer in the optoelectronic and thermoelectric technologies.

Effect of hot pressing on the microstructure and thermoelectric properties of TGZM-grown YbFe-doped CoSb3 skutterudite

Skutterudites based on CoSb3 are considered potential candidates in thermoelectric applications in the field of mid-temperature power generation. The annealing process necessitates more than a week to complete the peritectic phase transition for traditional melting-quenching-annealing-sintering methods. Herein, a new strategy combining the temperature gradient zone melting process with hot pressing (TGZM-HP) was successfully employed to synthesize YbFe-doped p-type CoSb3 in a shorter time. The resulting samples show enhanced thermoelectric properties compared to TGZM-grown ones, especially in the case of 1 at.% YbFe-doped CoSb3. This compound exhibits a maximal Seebeck coefficient of 166.3 μVK−1 and a power factor of 1.8 mWm−1K−2 at 723 K, showing an increase in these values of 25% and 51%, respectively. Moreover, phonon scattering appears to be enhanced due to the significant reduction in the grain size of the TGZM-HP samples, resulting in a minimum thermal conductivity of 2.71 Wm−1K−1. A dimensionless figure of merit (ZT) of 0.48 was obtained for 1 at.% YbFe-doped CoSb3. This value is 1.6 times larger than that measured for TGZM-grown CoSb3. This work provides an alternative strategy to fabricate skutterudite materials and foster their development.

Thermodynamic phase diagram and thermoelectric properties of LiMgZ (Z = P, As, Bi): ab initio method study

ABSTRACT Using first principle calculations based on the density functional theory (DFT), the structural, elastic, electronic, and thermoelectric properties, and thermodynamic stability of the LiMgZ (Z = P, As, Bi) half-Heusler ternary compounds were studied. The results of structural calculations and elastic constants and investigation of the thermodynamic phase diagram represent stability for these compounds in the half-Heusler cubic structure with F4-3 m symmetry. Studying the electronic structure using generalised gradient approximation (GGA) as well as spin-orbit calculations (SOC) reveal that all three LiMgZ (Z = P, As, Bi) structures are non-magnetic semiconductors with an indirect band gap along Г-X direction with values of 1.63, 1.37 and 0.5 eV, respectively. The electron transport behaviours of these materials at various temperatures show that their dimensionless figure of merit (ZT) at the temperature of 200 K reaches its maximum value of 0.813, 0.81 and 0.78 for LiMgP, LiMgAs and LiMgBi, respectively.

Utilizing of high-pressure high-temperature synthesis to enhance the thermoelectric properties of Zn0.98Al0.02O with excellent electrical properties* *Project supported by the National Natural Science Foundation of China (Grant No. 51171070) and the Project of Jilin Science and Technology Development Plan, China (Grant No. 20170101045JC).

The temperature in the high-pressure high-temperature (HPHT) synthesis is optimized to enhance the thermoelectric properties of high-density ZnO ceramic, Zn0.98Al0.02O. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy show that HPHT can be utilized to control the crystal structure and relative density of the material. High pressure can be utilized to change the energy band structure of the samples via changing the lattice constant of samples, which decreases the thermal conductivity due to the formation of a multi-scale hierarchical structure and defects. The electrical conductivity of the material reaches 6 × 104 S/m at 373 K, and all doped samples behave as n-type semiconductors. The highest power factor (6.42 μW ⋅ cm−1⋅K−2) and dimensionless figure of merit (zT = 0.09) are obtained when Zn0.98Al0.02O is produced at 973 K using HPHT, which is superior to previously reported power factors for similar materials at the same temperature. Hall measurements indicate a high carrier concentration, which is the reason for the enhanced electrical performance.

Thin-film thermoelectric generator based on polycrystalline SiGe formed by Ag-induced layer exchange

SiGe alloys are a promising material for highly reliable, human-friendly thin-film thermoelectric generators for micro-energy harvesting. However, it is difficult to obtain high performances at low thermal budgets in SiGe layers, especially in n-type materials. Ag-induced layer exchange enables the synthesis of Si1−xGex (x: 0–0.3) layers at 500 °C and dynamically controls the Fermi level owing to the self-organizing manner of impurity doping during the layer exchange. Intrinsic, p-type (hole concentration >1019 cm−3), and highly n-type (electron concentration >1020 cm−3) SiGe layers are obtained using pure Ag, B-doped Ag, and As-doped Ag, respectively. Owing to the high carrier concentrations, the thermoelectric power factor at room temperature exhibits high values: 230 μW m−1 K−2 for the p-type and 1000 μW m−1 K−2 for the n-type. The latter value is the highest reported power factor at room temperature for SiGe formed below 1000 °C. The dimensionless figure of merit is determined to be 0.19 from the power factor and the thermal conductivity of 1.6 W m−1 K−1. A thermoelectric generator fabricated with the low-temperature SiGe layers demonstrates a relatively large output for thin films (50 nm): 1.4 nW at room temperature with a temperature difference of 15 °C.

Decoupling Seebeck coefficient and resistivity, and simultaneously optimizing thermoelectric and mechanical performances for n-type BiTeSe alloy by multi-pass equal channel angular extrusion

Equal channel angular extrusion (ECAE) has become a new process for both preparing ultrafine grained uniform microstructure and minimizing contamination. In the present work, the BiTeSe polycrystalline bulk alloys with uniform microstructure was obtained by applying ECAE method to extruding the cast ingot directly. The lamellae coarse grains (hundreds microns to several millimeters) of initial cast ingot were efficiently and uniformly refined to about 18 ~ 24.7 μm. Experiments combining with numerical simulation confirmed that the uniformity of the grain size distribution was remarkably improved by increasing the number of extrusion pass owing to the continuous refinement of grains in the unrefined region and the retainment of grains in the refined region. The Vickers Hardness was doubled at 4th pass in this connection. On the other hand, it is inspiring that applying ECAE to the initial cast ingot not only decoupled the Seebeck coefficient and resistivity, but also simultaneously optimized the electrical and thermal transport performances. For this reason, the dimensionless figure of merit (ZT) increased rapidly with the increase of the number of extrusion pass. Finally, BiTeSe billet extruded for 4 passes achieved the maximum ZT, which was far higher than that of the initial state.

Preparation and thermoelectric properties of CuAlO2 compacts by tape casting followed by SPS

We applied a novel method of tape casting followed by spark plasma sintering (SPS) to prepare CuAlO2 compacts. During the tape casting process, plate-like Al2O3 particles distributed along the flow direction of slurry under the action of the shear-thinning effect. It greatly elevated the orientation degree of the CuAlO2 compacts formed during the following SPS. Meanwhile, the relative density of these compacts was also considerably increased due to the reduction in porosity. The green compacts and as-sintered compacts were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The thermoelectric properties including electrical resistivity, Seebeck coefficient, thermal conductivity, power factor, and the dimensionless figure of merit were measured. The results show that sample CuO-45-TC exhibited the highest thermoelectric performance with a ZT value of 0.042 at 973 K. The value should be the highest performance reported in the literature as far as we know. The remarkable increase in orientation degree and relative density should be responsible for the improvement in the thermoelectric performance of CuAlO2.

Dimensionless figure of merit of constantan estimated using impedance spectroscopy

Commercialized bulk constantan with a low dimensionless figure of merit and its film fabricated by electroplating were prepared to demonstrate the feasibility of impedance spectroscopy for estimating thermoelectric parameters. From the frequency dependence of the impedance, the resistivity and dimensionless figure of merit were directly estimated as 5.3 × 10−7 Ωm and 0.026 for the bulk and 3.8 × 10−7 Ωm and 0.0098 for the film at room temperature (302 K), respectively. We concluded that impedance spectroscopy is a suitable technique for estimating the thermoelectric parameters of any material with an apparent Seebeck coefficient.

Low lattice thermal conductivity and high figure of merit in p-type doped K3IO* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11974302, 11774396, and 11704322) and the Shandong Natural Science Funds for Doctoral Program, China (Grant No. ZR2017BA017).

Based on first-principles calculations, Boltzmann transport equation and semiclassical analysis, we conduct a detailed study on the lattice thermal conductivity κ L, Seebeck coefficient S, electrical conductivity σ, power factor S 2 σ and dimensionless figure of merit, zT, for K3IO. It is found that K3IO exhibits relatively low lattice thermal conductivity of 0.93 W⋅m−1⋅K−1 at 300 K, which is lower than the value 1.26 W⋅m−1⋅K−1 of the classical TE material PbTe. This is due to the smaller phonon group velocity ν g and smaller relaxation time τλ . The low lattice thermal conductivity can lead to excellent thermoelectric properties. Thus maximum zT of 2.87 is obtained at 700 K, and the zT = 0.41 at 300 K indicate that K3IO is a potential excellent room temperature TE material. Our research on K3IO shows that it has excellent thermoelectric properties, and it is a promising candidate for applications in fields in terms of thermoelectricity.