Solid-State Synthesis and Thermoelectric Properties of Al-Doped Mg2Si

The electronic transport and thermoelectric properties of Al-doped Mg₂Si (Mg₂Si:Al _m , m?=?0, 0.005, 0.01, 0.02, 0.03) compounds prepared by solid-state synthesis were examined. Mg₂Si was synthesized by solid-state reaction (SSR) at 773?K for 6?h, and Al-doped Mg₂Si powders were obtained by mechanical alloying (MA) for 24?h. Mg₂Si:Al _m were fully consolidated by hot pressing (HP) at 1073?K for 1?h, and all samples showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased significantly with increasing Al doping content, and the absolute value of the Seebeck coefficient decreased due to the significant increase in electron concentration from 10¹⁶ cm^?3 to 10¹⁹ cm^?3 by Al doping. The thermal conductivity was increased slightly by Al doping, but was not changed significantly by the Al doping content due to the much larger contribution of lattice thermal conductivity over electronic thermal conductivity. Mg₂Si:Al_0.02 showed a maximum thermoelectric figure of merit of 0.47 at 823?K.     

Nanoscale FeS2 (Pyrite) as a Sustainable Thermoelectric Material

We have synthesized undoped, Co-doped (up to 5%), and Se-doped (up to 4%) FeS₂ materials by mechanical alloying in a planetary ball mill and investigated their thermoelectric properties from room temperature (RT) to 600 K. With decreasing particle size, the undoped FeS₂ samples showed higher electrical conductivity, from 0.02 S cm^?1 for particles with 70 nm grain size up to 3.1 S cm^?1 for the sample with grain size of 16 nm. The Seebeck coefficient of the undoped samples showed a decrease with further grinding, from 128 μV K^?1 at RT for the sample with 70-nm grains down to 101 μV K^?1 for the sample with grain size of 16 nm. The thermal conductivity of the 16-nm undoped sample lay within the range from 1.3 W m^?1 K^?1 at RT to a minimal value of 1.2 W m^?1 K^?1 at 600 K. All doped samples showed improved thermoelectric behavior at 600 K compared with the undoped sample with 16 nm particle size. Cobalt doping modified the p-type semiconducting behavior to n-type and increased the thermal conductivity (2.1 W m^?1 K^?1) but improved the electrical conductivity (41 S cm^?1) and Seebeck coefficient (-129 μV K^?1). Isovalent selenium doping led to a slightly higher thermal conductivity (1.7 W m^?1 K^?1) as well as to an improved electrical conductivity (26 S cm^?1) and Seebeck coefficient (110 μV K^?1). The ZT value of FeS₂ was increased by a factor of five by Co doping and by a factor of three by Se doping.     

Enhancing the Thermoelectric Properties via Modulation of Defects in P-Type MNiSn-Based (M = Hf,Zr, Ti) Half-Heusler Materials

The thermoelectric figure-of-merit (zT) of p-type MNiSn (M = Ti, Zr, or Hf) half-Heusler compounds is lower than their n-type counterparts due to the presence of a donor in-gap state caused by Ni occupying tetrahedral interstitials. While ZrNiSn and TiNiSn, have been extensively studied, HfNiSn remains unexplored. Herein, this study reports an improved thermoelectric property in p-type HfNi_1−xCo_xSn. By doping 5 at% Co at the Ni sites, the Seebeck coefficient becomes reaching a peak value exceeding 200 µV K⁻¹ that breaks the record of previous reports. A maximum power factor of ≈2.2 mW m⁻¹ K⁻² at 973 K is achieved by optimizing the carrier concentration. The enhanced p-type transport is ascribed to the reduced content of Ni defects, supported by first principle calculations and diffraction pattern refinement. Concomitantly, Co doping also softens the lattice and scatters phonons, resulting in a minimum lattice thermal conductivity of ≈1.8 W m⁻¹ K⁻¹. This leads to a peak zT of 0.55 at 973 K is realized, surpassing the best performing p-type MNiSn by 100%. This approach offers a new method to manipulate the intrinsic atomic disorder in half-Heusler materials, facilitating further optimization of their electronic and thermal properties.     

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.     

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.     

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.     

Enhanced Thermoelectric Properties of Hole-Doped Bi2−x Ba x Sr2Co2O y Ceramics

The influence of Ba doping on the thermoelectric properties of Bi_2?x Ba _x Sr₂ Co₂O _y (x = 0.00, 0.025, 0.05, 0.075, 0.10, 0.125, and 0.15) samples prepared by the solid-state reaction method was investigated from 333 K to 973 K. For the samples with x ≤ 0.075, the electrical resistivity decreased with increase of the Ba doping amount due to p-type doping and they exhibited metallic electrical conductivity behavior, whereas the samples with x ≥ 0.10 exhibited semiconductor-like electrical conductivity behavior. The Seebeck coefficients of all the samples decreased with increase of the Ba doping amount. The thermal conductivity first decreased for x ≤ 0.075, then increased with higher Ba doping amounts. As an overall result, the dimensionless figure of merit (ZT) of Bi_1.925Ba_0.075Sr₂Co₂O _y reached the maximum value of 0.245 at 973 K, being 41% higher than that of the undoped sample.     

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.     

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.     

Low-Temperature Thermoelectric Properties of Fe2VAl with Partial Cobalt Doping

Ternary metallic alloy Fe₂VAl with a pseudogap in its energy band structure has received intensive scrutiny for potential thermoelectric applications. Due to the sharp change in the density of states profile near the Fermi level, interesting transport properties can be triggered to render possible enhancement in the overall thermoelectric performance. Previously, this full-Heusler-type alloy was partially doped with cobalt at the iron sites to produce a series of compounds with n-type conductivity. Their thermoelectric properties in the temperature range of 300?K to 850?K were reported. In this research, efforts were made to extend the investigation on (Fe_1?x Co _x )₂VAl to the low-temperature range. Alloy samples were prepared by arc-melting and annealing. Seebeck coefficient, electrical resistivity, and thermal conductivity measurements were performed from 80?K to room temperature. The effects of cobalt doping on the material??s electronic and thermal properties are discussed.     

Highly efficient n-(Bi, Sb)2Te3 thermoelectric materials for temperatures below 200 K

The possibility of using an n-type Bi_2?x Sb_xTe₃ solid solution in thermoelectric refrigerators at T<200 K is considered. It is shown that, if the material under consideration is optimized for the above temperature region, the temperature dependence of the Seebeck coefficient α becomes less pronounced, and the crystal-lattice thermal conductivity κ_L decreases as compared to what is observed in a conventional n-Bi₂Te_3?y Se_y solid solution. These factors and a high mobility of charge carriers μ₀ bring about an increase in the parameter β ～ ZT, where Z is the thermoelectric efficiency.     

Thermoelectric Performance of p-Type (Bi85Sb15)1?x Sn x Materials Prepared by a Pressureless Sintering Technique

In this study, a series of Sn-doped (Bi₈₅Sb₁₅)_1?x Sn _x (x?=?0, 0.025, 0.05, 0.1, 0.2, 0.3) thermoelectric materials was fabricated through mechanical alloying followed by pressureless sintering. The crystal structure was characterized by x-ray diffraction. The electrical transport properties and thermal properties were measured in the temperature range from 77?K to 300?K. The electrical transport as a function of temperature appeared to be characteristic of a semimetal. The Seebeck coefficient gradually changed from negative to positive with increasing Sn doping, showing p-type electrical transport properties. It is found that the Seebeck coefficients of the p-type Bi-Sb alloys decrease with increasing dopant concentration of Sn, which may be due to increasing carrier concentration. Among the p-type alloys, the power factor of (Bi₈₅Sb₁₅)_0.975Sn_0.025 reached a maximum value of 1.3?×?10^?3?W/mK² at 265?K, and the optimum figure of merit value of 0.13 was obtained at 240?K. The results indicate that good p-type Bi-Sb alloys can be prepared by this synthesis procedure.     

Thermoelectric Properties of Ca-Filled CoSb<Subscript>3</Subscript>-Based Skutterudites Synthesized by Mechanical Alloying

Ca _z Co_4−x (Fe/Mn) _x Sb₁₂ skutterudites were prepared by mechanical alloying and hot pressing. The phases of mechanically alloyed powders were identified as γ-CoSb₂ and Sb, but they were transformed to δ-CoSb₃ by annealing at 873 K for 100 h. All specimens had a positive Hall coefficient and Seebeck coefficient, indicating p-type conduction by holes as majority carriers. For the binary CoSb₃, the electrical conductivity behaved like a nondegenerate semiconductor, but Ca-filled and Fe/Mn-doped CoSb₃ showed a temperature dependence of a degenerate semiconductor. While the Seebeck coefficient of intrinsic CoSb₃ increased with temperature and reached a maximum at 623 K, the Seebeck coefficient increased with increasing temperature for the Ca-filled and Fe/Mn-doped specimens. Relatively low thermal conductivity was obtained because fine particles prepared by mechanical alloying lead to phonon scattering. The thermal conductivity was reduced by Ca filling and Fe/Mn doping. The electronic thermal conductivity was increased by Fe/Mn doping, but the lattice thermal conductivity was decreased by Ca filling. Reasonable thermoelectric figure-of-merit values were obtained for Ca-filled Co-rich p-type skutterudites.     

Measurement of Thermal Conductivity Using Steady-State Isothermal Conditions and Validation by Comparison with Thermoelectric Device Performance

A new technique for measuring thermal conductivity with significantly improved accuracy is presented. By using the Peltier effect to counterbalance an imposed temperature difference, a completely isothermal, steady-state condition can be obtained across a sample. In this condition, extraneous parasitic heat flows that would otherwise cause error can be eliminated entirely. The technique is used to determine the thermal conductivity of p-type and n-type samples of (Bi,Sb)₂(Te,Se)₃ materials, and thermal conductivity values of 1.47?W/m?K and 1.48?W/m?K are obtained respectively. To validate this technique, those samples were assembled into a Peltier cooling device. The agreement between the Seebeck coefficient measured individually and from the assembled device were within 0.5%, and the corresponding thermal conductivity was consistent with the individual measurements with less than 2% error.     

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.     

Stability of Skutterudite Thermoelectric Materials

By multifilling with La, Ba, Ga, Ti, Yb, Ca, Al, and In, the dimensionless figure of merit ZT of filled skutterudites has been improved in this work. ZT reached 0.75 for p-type (La,Ba,Ga,Ti) _x (Fe,Co)₄Sb₁₂ (x = 0.8 to 1.0) and 1.0 for n-type (Yb,Ca,Al,Ga,In) _y (Co,Fe)₄Sb₁₂ (y = 0.7 to 0.9). After annealing at 873 K for 180 h, 300 h, 710 h, 1000 h, and 5000 h in vacuum, the Seebeck coefficient S and the electrical resistivity ρ of the samples increased while the thermal conductivity λ decreased with increasing annealing time. As a result, the ZT values of both p- and n-type skutterudites remained unchanged or were slightly improved, demonstrating the excellent thermal stability of these skutterudites.     

Thermoelectric Performance Optimization in p-Type Ce y Fe3CoSb12 Skutterudites

In this study, we investigated the impact of the Ce filling fraction on the thermoelectric properties of p-type filled skutterudites Ce _y Fe₃CoSb₁₂ (y = 0.6 to 1.0). The electrical conductivity decreases gradually with increasing y, while the Seebeck coefficient displays an opposite variation tendency, consistent with the expected electron donor role of the Ce filler in this compound. The overall power factors are invariable among all the samples. Alteration of the Ce filling fraction exerts little influence on the phonon transport, but the total thermal conductivity markedly declined with increasing y due to the reduced contribution to heat transfer from carriers. As a consequence, the maximum thermoelectric figure of merit ZT reaches ～0.8 for the sample with y = 0.9, comparable to that of pure Fe-based skutterudite CeFe₄Sb₁₂; more importantly, the former possesses a much larger average ZT between 300 K and 800 K than the latter, showing superior potential for use in intermediate-temperature thermoelectric power generation applications. Further enhancement of ZT in p-type Fe₃CoSb₁₂-based skutterudites could be realized via nanostructuring or a multiple-filling approach.     

Solid-State Synthesis of Te-Doped Mg<Subscript>2</Subscript>Si

Te-doped Mg₂Si (Mg₂Si:Te _m , m = 0, 0.01, 0.02, 0.03, 0.05) alloys were synthesized by a solid-state reaction and mechanical alloying. The electronic transport properties (Hall coefficient, carrier concentration, and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, thermal conductivity, and figure of merit) were examined. Mg₂Si was synthesized successfully by a solid-state reaction at 673 K for 6 h, and Te-doped Mg₂Si powders were obtained by mechanical alloying for 24 h. The alloys were fully consolidated by hot-pressing at 1073 K for 1 h. All the Mg₂Si:Te _m samples showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased and the absolute value of the Seebeck coefficient decreased with increasing Te content, because Te doping increased the electron concentration considerably from 10¹⁶ cm⁻³ to 10¹⁸ cm⁻³. The thermal conductivity did not change significantly on Te doping, due to the much larger contribution of lattice thermal conductivity over the electronic thermal conductivity. Thermal conduction in Te-doped Mg₂Si was due primarily to lattice vibrations (phonons). The thermoelectric figure of merit of intrinsic Mg₂Si was improved by Te doping.     

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.     

A Comparison Between the Effects of Sb and Bi Doping on the Thermoelectric Properties of the Ti0.3Zr0.35Hf0.35NiSn Half-Heusler Alloy

We deal here with Sb and Bi doping effects of the n-type half-Heusler (HH) Ti_0.3Zr_0.35Hf_0.35NiSn alloy on the measured thermoelectric properties. To date, the thermoelectric effects upon Bi doping on the Sn site of HH alloys have rarely been reported, while Sb has been widely used as a donor dopant. A comparison between the measured transport properties following arc melting and spark plasma sintering of both Bi- and Sb-doped samples indicates a much stronger doping effect upon Sb doping, an effect which was explained thermodynamically. Due to similar lattice thermal conductivity values obtained for the various doped samples, synthesized in a similar experimental route, no practical variations in the thermoelectric figure of merit values were observed between the various investigated samples, an effect which was attributed to compensation between the power factor and electrical thermal conductivity values regardless of the various investigated dopants and doping levels.