Effect of Ti Substitution on Thermoelectric Properties of W-Doped Heusler Fe2VAl Alloy

Effects of element substitutions on thermoelectric properties of Heusler Fe₂VAl alloys were evaluated. By W substitution at the V site, the thermal conductivity is reduced effectively because of the enhancement of phonon scattering resulting from the introduction of W atoms, which have much greater atomic mass and volume than the constituent elements of Fe₂VAl alloy. W substitution is also effective to obtain a large negative Seebeck coefficient and high electrical conductivity through an electron injection effect. To change the conduction type from n-type to p-type, additional Ti substitution at the V site, which reduces the valence electron density, was examined. A positive Seebeck coefficient as high as that of conventional p-type Fe₂VAl alloy was obtained using a sufficient amount of Ti substitution. Electrical resistivity was reduced by the hole doping effect of the Ti substitution while maintaining low thermal conductivity. Compared with the conventional solo-Ti-substituted p-type Fe₂VAl alloy, the ZT value was improved, reaching 0.13 at 450 K.     

Evaluation of the Thermoelectric Module Consisting of W-Doped Heusler Fe2VAl Alloy

Power generation performance of a thermoelectric module consisting of the Heusler Fe₂VAl alloy was evaluated. For construction of the module, W-doped Fe₂VAl alloys were prepared using powder metallurgy process. Power generation tests of the module consisting of 18 pairs of p–n junctions were conducted on a heat source of 373–673 K in vacuum. The reduction of thermal conductivity and improvement of thermoelectric figure of merit by W-doping enhanced the conversion efficiency and the output power. High output power density of 0.7 W/cm² was obtained by virtue of the high thermoelectric power factor of the Heusler alloy. The module exhibited good durability, and the relatively high output power was maintained after temperature cycling test in air.     

Thermoelectric Properties of Heavy Rare Earth Filled Skutterudites Dy y Fe x Co4?x Sb12

Heavy rare earth element Dy-filled skutterudites (Dy _y Fe _x Co_4?x Sb₁₂) have been synthesized by a melting–quenching–annealing method and sintered by the spark plasma sintering technique. Our results suggest that single-phased Dy _y Fe _x Co_4?x Sb₁₂ compounds could be obtained when the Fe content is less than 1.5. The maximum filling fraction of Dy in skutterudites increases with increasing Fe content. We also found significant lattice expansion induced by Fe substitution at Co sites and Dy filling in the voids. The electrical conductivity, Seebeck coefficient, and thermal conductivity have been measured in the temperature range from 300?K to 800?K. The low-temperature Hall coefficient and carrier mobility are reported in the temperature range from 2.5?K to 300?K. The power factor for Dy _y Fe _x Co_4?x Sb₁₂ increases with increasing Fe content. A significant reduction in lattice thermal conductivity is observed in heavy rare earth element Dy-filled skutterudites due to the low localized vibrational frequency of Dy that effectively scatters low-frequency lattice phonons. The sample with composition Dy_0.41Fe_1.45Co_2.55Sb_12.28 has lattice thermal conductivity as low as 1.05?W?m^?1?K^?1 at room temperature. The thermoelectric figure of merit (ZT) reaches a maximum value of 0.67 at 750?K.     

Effects of Heavy Element Substitution on Electronic Structure and Lattice Thermal Conductivity of Fe2VAl Thermoelectric Material

By using first-principles cluster calculations, we identified that Ta or W substitution for V is useful for decreasing the lattice thermal conductivity of the Fe₂VAl Heusler alloy without greatly affecting the electron transport properties. It was clearly confirmed that the Fe₂(V_1?x Ta _x )Al_0.95Si_0.05 (x?=?0, 0.025, 0.05), Fe₂(V_0.9?x Ta _x Ti_0.1)Al (x?=?0, 0.10, 0.20), and Fe₂(V_0.9?2x W _x Ti_0.1+x )Al (x?=?0, 0.05, 0.10) alloys indeed possessed large Seebeck coefficient regardless of the amounts of substituted elements, while their lattice thermal conductivity was effectively reduced. As a result of partial substitution of Ta for V, we succeeded in increasing the magnitude of the dimensionless figure of merit of the Heusler phase up to 0.2, which is five times as large as the Ta-free compound.     

Thermoelectric Properties of Mn-Doped Cu2SnSe3

We report the thermoelectric properties of Mn-doped Cu₂Mn _x Sn_1?x Se₃ compound, with x ranging from 0.005 to 0.1 at temperature ranging from 80?K to 723?K. All samples maintain cubic zincblende-like structure, and no impurity phase was detected. The electrical resistivity decreases rapidly when Mn⁴⁺ replaces Sn²⁺ in the matrix. The excess Mn impurities in the x?=?0.05 and x?=?0.1 samples also affect the Seebeck coefficient. The total thermal conductivity is increased for Mn-doped samples except for the x?=?0.005 sample. In all, both power factor and figure of merit are improved by Mn doping over the entire temperature range. The ZT value of the x?=?0.02 sample reaches 0.035 at 300?K, and for x?=?0.01 reaches 0.41 at 716?K, which are comparable to the best thermoelectric performance for ternary Cu-based compounds.     

Synthesis and Thermoelectric Properties of LAST System Bulk Materials: Substitution of Sulfur for Tellurium

Nonstoichiometric lead-antimony-silver-tellurium (LAST) system thermoelectric bulk materials Ag_0.8Pb_22.5SbTe_20?x S _x (x?=?0 to 8.0) were fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS). The electrical and thermal transport properties were investigated in the temperature range of 300?K to 700?K. The x-ray diffraction (XRD) results indicated that sulfur entered the PbTe during MA process, but gradually precipitated in the sintering process in the form of PbS from the PbTe matrix when the sulfur content was changed from x?=?2.0 to x?=?8.0. It was confirmed that the addition of sulfur effectively reduced the lattice thermal conductivity. A low thermal conductivity of 0.83?W?m^?1?K^?1 at 673?K was obtained for the Ag_0.8Pb_22.5SbTe₁₂S₈ sample. Benefiting from the high electrical conductivity, the Ag_0.8Pb_22.5SbTe₁₈S₂ sample reached a maximum ZT value of ??0.97 at 673?K, which is 32% higher than its counterpart without sulfur substitution.     

Thermoelectric Performance of Sb- and La-Doped Mg2Si0.5Ge0.5

La _x Mg_2?x Si_0.49Ge_0.5Sb_0.01 compounds (x?=?0, 0.005, 0.01, 0.02) were synthesized by solid-state reaction followed by spark plasma sintering. The thermoelectric properties, such as the Seebeck coefficient, the electrical and thermal conductivities, and ZT, of these compounds have been studied in the temperature range of 300?K to 823?K. The figure of merit of this n-type compound has been raised above unity at 823?K for the sample with x?=?0.01, a value 60% higher than that of Mg₂Si_0.49Ge_0.5Sb_0.01. The reduction of the thermal conductivity via increasing phonon scattering is considered as the main reason for the enhanced ZT. These observations demonstrate an opportunity to improve the thermoelectric performance of Mg₂Si_1?x Ge _x solid solutions.     

Thermoelectric Properties of CdTe1−x Cl x Material Prepared by Spark Plasma Sintering Method

CdTe compound is a prospective thermoelectric material due to its high Seebeck coefficient and low thermal conductivity. In the present study, we optimized its carrier concentration by substituting Cl on the Te site in order to improve the electrical conductivity and decrease the lattice thermal conductivity. The polycrystalline CdTe_1?x Cl _x (x = 0.005, 0.01, 0.03, 0.05) samples were fabricated by solid state reaction followed with spark plasma sintering, and the relative densities of the sintered samples were higher than 98%. Thermoelectric properties, including Seebeck coefficient (α), electrical conductivity (σ). and thermal conductivity (κ), were measured in the temperature range of 300–700 K. The increase of Cl content (x) caused an increase of σ, and the maximum ZT value of 0.2 was obtained at about 630 K for the CdTe_0.97Cl_0.03 sample.     

The Effect of Adding Nano-Bi2Te3 on Properties of GeTe-Based Thermoelectric Material

The efficient thermoelectric materials (GeTe)_0.85?x (Mn_0.6Sn_0.4Te)_0.15(Bi₂Te₃) _x (0 ≤ x ≤ 0.05), in which Bi₂Te₃ is nanopowder, were prepared by hot pressing. The effect of adding neutral nano-Bi₂Te₃ content on the thermoelectric properties of germanium telluride was investigated. With increasing x, the thermal conductivity of the prepared samples decreased significantly and the Seebeck coefficient declined slightly, while there was no obvious change in electrical conductivity. In both electrical conductivity and Seebeck coefficient curves at different x values, there are inflection points around 600 K. The maximum dimensionless figure of merit ZT of the prepared materials is 1.54, attained in the temperature range from 700 K to 750 K for x = 0.03. The x-ray diffraction (XRD) pattern shows that Bi₂Te₃ has been alloyed into the GeTe-MnTe-SnTe alloy, which is consistent with the high-resolution scanning electron microscopy (HRSEM) images. Adding nano-Bi₂Te₃ to GeTe-based materials could also increase their performance stability at high temperature as a result of decreasing the phase-transition temperature T _c.     

Thermoelectric Properties of Dy-Doped SrTiO3 Ceramics

Sr_1?x Dy _x TiO₃ (x?=?0.02, 0.05, 0.10) ceramics were prepared by the reduced solid-state reaction method, and their thermoelectric properties were investigated from room temperature to 973?K. The resistivity increases with temperature, showing metallic behavior. The Seebeck coefficients tend to saturate at high temperatures, presenting narrow-band behavior, as proved by ab?initio calculations of the electronic structure. The magnitudes of the Seebeck coefficient and the electrical resistivity decrease with increasing Dy content. At the same time, the thermal conductivity decreases because the lattice thermal conductivity is reduced by Dy substitution. The maximum value of the figure of merit reaches 0.25 at 973?K for the Sr_0.9Dy_0.1TiO₃ sample.     

Possible Enhancement of Thermoelectric Properties by Use of a Magnetic Semiconductor: Carrier-Doped Chalcopyrite Cu1−x Fe1+x S2

We focus on the chalcopyrite CuFeS₂ to utilize the interaction between carriers and magnetic moments of Fe as a possible source to achieve high power factor. Polycrystalline samples of Cu_1?x Fe_1+x S₂ were synthesized, and their thermoelectric properties are reported. Electrical resistivity decreased by two orders of magnitude with increasing x, while the Seebeck coefficient showed large values of ?200 μV/K at room temperature. Thermal conductivity also decreased with the increase of x. As a result, the power factor and the figure of merit, zT, of the carrier-doped samples are about 10 times larger than those of CuFeS₂. These observations suggest that magnetic semiconductors can make good thermoelectric materials.     

Thermoelectric Properties of Indium-Selenium Nanocomposites Prepared by Mechanical Alloying and Spark Plasma Sintering

Indium-selenium-based compounds have received much attention as thermoelectric materials since a high thermoelectric figure of merit of 1.48 at 705?K was observed in In₄Se_2.35. In this study, four different compositions of indium-selenium compounds, In₂Se₃, InSe, In₄Se₃, and In₄Se_2.35, were prepared by mechanical alloying followed by spark plasma sintering. Their thermoelectric properties such as electrical resistivity, Seebeck coefficient, and thermal conductivity were measured in the temperature range of 300?K to 673?K. All the In-Se compounds comprised nanoscaled structures and exhibited n-type conductivity with Seebeck coefficients ranging from ?159???V?K^?1 to ?568???V?K^?1 at room temperature.     

Improved Thermoelectric Properties of Se-Doped n-Type PbTe1−x Se x (0 ≤ x ≤ 1)

Enhancement of the thermoelectric figure of merit is of prime importance for any thermoelectric material. Lead telluride has received attention as a potential thermoelectric material. In this work, the effect of Se substitution has been systematically investigated in PbTe_1?x Se _x . The thermoelectric properties of synthesized alloys were measured in the temperature range of 300 K to 873 K. For the particular composition of x = 0.5, α was highest at ~292 μV/K, while k was lowest at ~0.75 W/m-K, resulting in the highest dimensionless figure of merit of ZT ≈ 0.95 at 600 K. The increase in thermopower for x = 0.5 can be attributed to the high distortion in the crystal lattice which leads to the formation of defect states. These defect states scatter the majority charge carriers, leading to high thermopower and high electrical resistivity. The dramatic reduction of the thermal conductivity for x = 0.5 can be attributed to phonon scattering by defect states.     

On the thermoelectric properties and band gap of silicon–germanium alloys in the high-temperature region

For n- and p-type Si_0.85Ge_0.15 alloys, the thermoelectric properties (thermopower, resistivity, thermal conductivity, and thermoelectric efficiency) are determined in the range from room temperature to 1200°C. The measurements are carried out with an upgraded device [1] by absolute and steady-state methods with thermal screens. The device is upgraded to extend the working-temperature range to ~1500 K. On the basis of these data, the energy-related capabilities of the alloy are estimated; the thermal band-gap width is calculated in the temperature range of ~1300–1400 K.     

Preparation and Thermoelectric Properties of p-Type Yb-Filled Skutterudites

p-Type Yb _z Fe_4?x Co _x Sb₁₂ skutterudites were prepared by encapsulated melting and hot pressing, and the filling and doping (charge compensation) effects on the transport and thermoelectric properties were examined. The electrical conductivity of all specimens decreased slightly with increasing temperature, indicating that they were in a degenerate state due to high carrier concentrations of 10²⁰ cm^?3 to 10²¹ cm^?3. The Hall and Seebeck coefficients exhibited positive signs, indicating that the majority carriers are holes (p-type). The Seebeck coefficient increased with increasing temperature to maximum values of 100 μV/K to 150 μV/K at 823 K. The electrical and thermal conductivities were reduced by substitution of Co for Fe, which was responsible for the decreased carrier concentration. Overall, the Yb-filled Fe-rich skutterudites showed better thermoelectric performance than the Yb-filled Co-rich skutterudites.     

Lower Thermal Conductivity and Higher Thermoelectric Performance of Fe-Substituted and Ce, Yb Double-Filled p-Type Skutterudites

Substituting Fe on Co sites is an effective way to produce p-type skutterudite compounds as well as to reduce the thermal conductivity of skutterudites. In this work, we investigated thermoelectric properties of Fe-substituted and Ce + Yb double-filled Ce _x Yb _y Fe _z Co_4?z Sb₁₂ (x = y = 0.5, z = 2.0 to 3.25 nominal) skutterudite compounds by studying the Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient over a broad range of temperatures. All samples were prepared by using the traditional method of melting–annealing and spark plasma sintering. The signs of the Hall coefficient and Seebeck coefficient indicate that all samples are p-type conductors. Electrical conductivity increases with increasing Fe content. The temperature dependence of electrical conductivity indicates that a transition from the extrinsic to the intrinsic regime of conduction depends on the amount of Fe substituted for Co. The temperature dependence of mobility reflects the dominance of acoustic phonon scattering at temperatures above ambient. Except for Ce_0.5Yb_0.5Fe_3.25Co_0.75Sb₁₂, the thermal conductivity increases with increasing Fe content, reaching the maximum value of 2.23 W/m K at room temperature for Ce_0.5Yb_0.5Fe₃CoSb₁₂. A high power factor (27 μW/K² cm) combined with a rather low thermal conductivity for Ce_0.5Yb_0.5Fe_3.25Co_0.75Sb₁₂ (nominal) lead to a dimensionless figure of merit ZT = 1.0 at 750 K for this compound, one of the highest ZT values achieved in p-type skutterudite compounds prepared by the traditional method of melting–annealing and spark plasma sintering.     

Thermal Conductivity in Bi1−x Sb x Solid Solutions

Bi_1?x Sb _x solid solutions have attracted much attention as promising low-temperature thermoelectric materials. Previously, we observed distinct extrema in the isotherms of the transport and mechanical properties of polycrystalline Bi_1?x Sb _x and attributed their presence to the transition from diluted to concentrated solid solutions and to the reconstruction of the energy band structure under increasing Sb concentration. The goal of the present work is a detailed study of the concentration dependences of the thermal conductivity λ for Bi_1?x Sb _x polycrystalline solid solutions (x = 0 to 0.09) in the temperature range of 170 K to 300 K. It is established that the λ(x) dependences exhibit a nonmonotonic behavior: in certain concentration ranges an anomalous increase in λ with increasing x is observed. It is shown that the concentration dependences of the thermoelectric figure of merit calculated on the basis of the measured λ values are also nonmonotonic. The obtained data represent additional evidence in favor of our assumptions stated earlier about a significant effect of electronic phase transitions observed in Bi_1?x Sb _x solid solutions on the concentration dependences of their thermoelectric properties. These results should be taken into account when developing new Bi_1?x Sb _x -based materials.     

Thermoelectric Properties of Indium-Added Skutterudites In x Co4Sb12

The high-temperature thermoelectric properties of In _x Co₄Sb₁₂ (0.05 ≤ x ≤ 0.40) skutterudite compounds were investigated in this study. The phase states of the samples were identified by x-ray diffraction analysis and field-emission scanning electron microscopy at room temperature. InSb and CoSb₂ were found as secondary phases in samples with x = 0.10 to 0.40. The filling limit of In into the CoSb₃ cages of In _x Co₄Sb₁₂ was in the range 0.05 < x < 0.10. The electrical resistivity, Seebeck coefficient, and thermal conductivity of the In _x Co₄Sb₁₂ samples were measured from room temperature to 773 K. The Seebeck coefficient of all samples was negative. Reduction of the thermal conductivity by In addition resulted in a high thermoelectric figure of merit (ZT) of 0.67 for In_0.35Co₄Sb₁₂ at 600 K.     

Low-Temperature Transport Properties of InxFeyCo4?ySb12

Interesting results for cobalt triantimonide partially filled with indium have encouraged us to explore skutterudites filled with higher indium fractions. For pure In _x Co₄Sb₁₂, the fraction of voids filled is limited to about x = 0.25. To enable the insertion of more indium atoms, charge compensation is necessary. In this work, we studied the skutterudite compound In _x Fe _y Co_4−y Sb₁₂ partially filled with indium, where iron substitution for cobalt was employed for charge compensation. Polycrystalline samples were prepared by direct reaction of constituents. Structural and chemical characterization were accomplished by x-ray diffraction and energy-dispersive x-ray spectroscopy. Electrical resistivity, thermoelectric power, and thermal conductivity were measured between 2 K and 350 K. The influence of indium and iron on the charge-carrier transport properties and thermal conductivity in In _x Fe _y Co_4−y Sb₁₂ compounds is presented and discussed.     

Effect of Nano-ZrW2O8 on the Thermoelectric Properties of Bi85Sb15/ZrW2O8 Composites

In this study, Bi₈₅Sb₁₅/x wt.% ZrW₂O₈ (x?=?0, 0.1, 0.5, 1) thermoelectric nanocomposites were prepared successfully by ball milling and spark plasma sintering. The effect of ZrW₂O₈ nanoparticles on the thermoelectric properties of the Bi₈₅Sb₁₅/ZrW₂O₈ composite was investigated. Thermal conductivity, Seebeck coefficient, and electrical conductivity were measured between 77?K and 300?K. x-Ray diffraction and scanning electron microscopy were adopted for microstructure characterization of the composites. The electrical transport properties are mainly discussed with regard to the microstructures. The results show that nanoinclusions did not grow during sintering. It is found that the thermal conductivity decreases with the addition of a small amount of ZrW₂O₈ nanoparticles, which serve as additional phonon-scattering centers. The obtained thermal conductivity is 0.5?W/m?K for the Bi₈₅Sb₁₅/1?wt.% ZrW₂O₈ composite at 80?K, which is just half of the value for the Bi₈₅Sb₁₅ matrix. However, the electrical transport properties are degraded with increasing content of ZrW₂O₈. The calculated ZT is also degraded due to the poor electrical properties.