Thermoelectric properties of indium-filled InxRh4Sb12 skutterudites

This study reports the synthesis and characterization of polycrystalline indium-filled In_xRh₄Sb₁₂ (0 ≤ x ≤ 0.2) skutterudites. The structural response to indium filling was monitored by whole pattern fitting of the powder X-ray diffraction data. Indium occupation of the oversized void-sites was verified by its unusually large thermal displacement parameter. The indium solubility limit approached 0.15. The principal thermoelectric properties were measured from 300 to 600 K. All samples are semiconducting. Indium void-site occupation reduced the lattice thermal conductivity of In_0.15Rh₄Sb₁₂ 30% at 300 K; however, the effect was subverted at elevated temperatures due to a coincident increase in bipolar thermal diffusion. The high-temperature thermoelectric figure of merits (ZT's) are low compared to the isostructural indium-filled In_xCo₄Sb₁₂ skutterudites due to a striking sign change in the Seebeck coefficients at 400 K and relatively high thermal conductivities.     

Thermoelectric properties of Cu1−xPtxFeO2 (0.0 ≤ x ≤ 0.05) delafossite-type transition oxide

The samples of Cu_1−xPt_xFeO₂ (0 ≤ x ≤ 0.05) delafossite were synthesized by solid state reaction method for studying thermoelectric properties. The properties of Seebeck coefficient, electrical conductivity and thermal conductivity were measured in the high temperature ranging from 300 to 960 K. The results of Seebeck coefficient, electrical conductivity and power factor were increased with increasing Pt substitution and temperature. The thermal conductivity was decreased from 5.8 to 3.5 W/mK with increasing the temperature from 300 to 960 K. An important results, the highest value of power factor and ZT is 2.0 × 10⁻⁴ W/mK² and 0.05, respectively, for x = 0.05 at 960 K.     

Synthesis and thermoelectric properties of double-filled skutterudites CeyYb0.5−yFe1.5Co2.5Sb12

Ce–Yb double-filled skutterudites Ce_0.5?yYb_yFe_1.5Co_2.5Sb₁₂ (y = 0.1, 0.2, 0.4 and 0.5) were synthesized by a melting method with subsequent annealing. The thermal conductivity, electrical conductivity and Seebeck coefficient were measured from room temperature up to 773 K. The thermal conductivities of all the double-filled skutterudites were found to be lower than the Yb single-filled skutterudites. An enhancement in the dimensionless thermoelectric figure of merit ZT was also observed in all the double-filled skutterudites as compared to the Yb single-filled skutterudite. Ce_0.3Yb_0.2Fe_1.5Co_2.5Sb₁₂ has the highest dimensionless figure of merit ZT of 0.32 at 723 K, which is 55% higher than the Yb single-filled skutterudite at the same temperature.     

High-temperature thermoelectric properties of late rare earth-doped Ca3Co4O9+δ

Misfit-layered oxides Ca_3−xLn_xCo₄O_9+δ with Ln = Dy, Er, Ho, and Lu were synthesized using solid state reactions. The resulting samples were hot-pressed (HP) at 1123 K in air for 2 h under a uniaxial pressure of 60 MPa. Thermoelectric properties of Ca_3−xLn_xCo₄O_9+δ were investigated up to 1200 K. Both the Seebeck coefficient and electrical resistivity increase upon Ln substitution for Ca. Among the Ln-doped samples, the magnitude of Seebeck coefficient tends to increase with decreasing ionic radius of Ln³⁺. The Ln-doped samples exhibit a lower thermal conductivity than the non-doped one due to a decrease of their lattice thermal conductivity. The dimensionless figure of merit, ZT, reaches 0.36 at 1073 K for the Ca_2.8Lu_0.2Co₄O_9+δ sample, which is about 1.6 times larger than that for the non-doped counterpart.     

Thermoelectric properties of Bi2SexTe3−x prepared by Bridgman method

Bi₂Se_xTe_3−x crystals with various x values were grown by Bridgman method. The electrical conductivity, σ, was found to decrease with increasing Se content. The highest σ of 1.6 × 10⁵ S m⁻¹ at room temperature was reached at x = 0.12 with a growth rate of 0.8 mm h⁻¹. The Seebeck coefficient, S, was less dependent on Se content, all with positive values showing p-type characteristics, and the highest S was measured to be 240 μV K⁻¹ at x = 0.24. The lowest thermal conductivity, κ, was 0.7 W m⁻¹ K⁻¹ at x = 0.36. The electronic part of κ, κ_el, showed a decrease with increasing Se content, which implies that the hole concentration as the main carriers was reduced by the addition of Se. The highest dimensionless figure of merit, ZT, at room temperature was 1.2 at x = 0.36, which is attributed to the combination of a rather high electrical conductivity and Seebeck coefficient and low thermal conductivity.     

Thermoelectric properties of p-type skutterudites YbxFe3.5Ni0.5Sb12 (0.8 ? x ? 1)

p-Type skutterudites, with nominal compositions Yb_xFe_3.5Ni_0.5Sb₁₂ (0.8 ? x ? 1), have been synthesized by induction melting with subsequent annealing, and their thermoelectric properties evaluated from 3.5 to 745 K to assess their suitability for thermoelectric-based waste heat recovery applications. We report results for the synthesis and measurements of Seebeck coefficient (S), electrical resistivity (ρ), thermal conductivity (κ), Hall coefficient (R_H) and effective mass (m^*/m₀) of Yb_xFe_3.5Ni_0.5Sb₁₂ (0.8 ? x ? 1). Powder X-ray diffraction and electron probe microanalysis show that this system has a narrow filling fraction range of x ∼ 0.84-0.86 for Yb in the crystallographic voids. All samples show positive R_H for the entire temperature range studied, with carrier concentrations ranging from 9.6 × 10²⁰ to 2.8 × 10²¹ cm⁻³ at room temperature. Relatively high values of S result in high power factors up to 17 μW cm⁻¹ K⁻² at room temperature. However, large values of κ and a sharp reduction in the S at high temperature due to bipolar conduction prevent the attainment of high thermoelectric figure of merit.     

Growth and transport properties of oriented bismuth telluride films

Oriented n-type bismuth telluride thin films with various layered nanostructures have been fabricated by radio-frequency (RF) magnetron sputtering. The crystal structures and microstructures of the films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The transport properties including carrier concentration, mobility, Seebeck coefficient and in-plane electrical conductivity were measured, which showed strong microstructure-dependent behaviors. The relationship between morphologies and transport properties of the films was explored. The optimal morphology and transport properties of films were obtained at the substrate temperature of 350 °C under the pressure of 1.0 Pa with oriented layered structure. Based on these results, a formation mechanism of these nanostructures is proposed and discussed. The interfaces and grain boundaries formed in these layered structures are beneficial to the reduction in thermal conductivity, which could result in potential TE films with high ZT value.     

High-pressure synthesis of phonon-glass electron-crystal featured thermoelectric LixCo4Sb12

Li-filled CoSb₃, which is inaccessible under ambient pressure, was successfully synthesized with a high-pressure synthesis technique, demonstrating a fast and effective way to broaden elemental species that can be filled into voids of skutterudites. The optimized Li_0.36Co₄Sb₁₂, with a greatly enhanced thermal power factor and much reduced thermal conductivity, has a ZT value of 1.3 at 700 K, the highest among all single elemental filled CoSb₃ materials at this temperature. In addition, an instructive linear relationship between the Einstein temperatures of the distinct rattling fillers and their ionic radii is revealed, which as a reference can easily be applied to the multiple elemental filling strategy for selecting suitable filling elemental species to reduce the lattice thermal conductivity more effectively.     

Enhanced thermoelectric performance of dual-element-filled skutterudites BaxCeyCo4Sb12

Single-phase polycrystalline dual-element-filled skutterudites Ba_xCe_yCo₄Sb₁₂ (0 < x < 0.4, 0 < y < 0.1) are synthesized by the melting–quenching–annealing and spark plasma sintering methods. The electrical conductivity, Seebeck coefficient, thermal conductivity and low-temperature Hall data of these compounds are reported. Our results suggest that there is essentially no difference in electrical transport properties between the dual-element-filled Ba_xCe_yCo₄Sb₁₂ and single-element-filled Ba_yCo₄Sb₁₂ systems. The Ba–Ce co-filling is more effective in lattice thermal conductivity reduction than Ba single filling in the temperature range of 300–850 K. Very low lattice thermal conductivity values less than 2.0 W m^?1 K^?1 are obtained at room temperature. Consequently, enhanced thermoelectric figure of merits (ZT) for these dual-element-filled CoSb₃ skutterudites are achieved at elevated temperatures, in particular ZT = 1.26 at 850 K for Ba_0.18Ce_0.05Co₄Sb_12.02.     

Enhanced thermoelectric properties of n-type Bi2Te3-based nanocomposite fabricated by spark plasma sintering

Highly dense n-type Bi₂Te₃-based thermoelectric materials dispersed with x vol.% γ-Al₂O₃ nanoparticles (x = 0, 0.4, 1.0, 1.5) were fabricated by spark plasma sintering method. The effects of γ-Al₂O₃ addition on microstructure and the thermoelectric properties were studied. It was found that γ-Al₂O₃ nanoparticles locate both at grain boundaries and inside Bi₂Se_0.3Te_2.7 grains. The nanoparticles induce both potential barrier scattering effect and additional phonon scattering effect, which simultaneously enhance the Seebeck coefficient and reduce the lattice thermal conductivity of the nanocomposites in the measured temperature range of 300-500 K, respectively. The maximum dimensionless figure of merit (ZT_max) reaches up to 0.99 for the sample with x = 1.0 at 400 K, which is 35% improvement over the Bi₂Te₃-based matrix. More importantly, the average ZT value of the sample increases from 0.65 to 0.91 in the temperature range 300-500 K, making the nanocomposites much more applicable in cooling and power generation.     

Realization of high thermoelectric performance in p-type unfilled ternary skutterudites FeSb2+xTe1−x via band structure modification and significant point defect scattering

FeSb₂Te, a ternary derivative of binary CoSb₃, displays anomalous electrical and thermal transport properties because of considerable modifications in the band structure induced by Fe and significant mixed valence state (namely Fe²⁺ and Fe³⁺) scattering of phonons. The substitution of Te for Sb generates more holes without notably affecting the band structure, while markedly improving the electrical conductivity and retaining a high Seebeck coefficient due to the enhanced density of states, thereby leading to dramatically increased power factors. Furthermore, the heat carrying phonons are strongly scattered with increasing x value because of the formation of solid solutions between two end members: □FeSb₂Te and □FeSb₃ (where □ can be viewed as a vacancy). Consequently, high thermoelectric figures of merit were achieved in the FeSb_2+xTe_1?x compounds, with the largest ZT value reaching ～0.65 for the sample with x = 0.2. This is the highest value among all p-type unfilled skutterudites and is comparable with some filled compositions. Prospects for further improving the performance of p-type FeSb₂Te-based skutterudites are discussed.     

Enhanced power factor of Indium co-doped ZnO:Al thin films deposited by RF sputtering for high temperature thermoelectrics

An improvement in the thermoelectric power factor of Al doped ZnO has been achieved by means of co-doping with indium using a dual magnetron sputtering system. The concentration of indium in the film was varied from 0 to 10 atomic % by varying the RF power of the In target, with the ZnO:Al target fixed at 100 W. It has been found that the films with In concentrations at or below 5 at.% have no significant change in microstructure, and yet a marked improvement in thermopower. At higher doping levels, the Seebeck coefficient continues to increase, however poly-crystallinity is induced in the ZnO matrix which results in a considerable decrease in electrical conductivity. This factor ultimately has a negative impact on the materials power factor. Taking into account the films studied, (ZnO)Al_.03In_.02 exhibited the best thermoelectric properties with an electrical conductivity of 5.88 × 10² S/cm and a Seebeck coefficient of −220 μV/K at 975 K, resulting in a power factor is 22.1 × 10⁻⁴ Wm⁻¹ K⁻², which is three times greater than for the film with no In doping. Film microstructure, composition, and thermal stability were investigated using X-ray diffraction, scanning electron microscopy, and Auger electron spectroscopy.     

Enhanced thermoelectric performance via randomly arranged nanopores: Excellent transport properties of YbZn2Sb2 nanoporous materials

Fabricating nanoporous bulk thermoelectric (TE) materials with periodically arranged nanopores is highly challenging and expensive, although TE materials exhibit high power factors (α²σ) and low thermal conductivities (κ). Enhanced TE performance via randomly arranged nanopores is demonstrated with a YbZn₂Sb₂ nanoporous material (nPM) fabricated by a combination of melt quenching and two stage spark plasma sintering in less than 10 h. Measurement of the electrical conductivity, Hall mobility, Seebeck coefficient, and thermal conductivity show that simultaneously enhancing α²σ and reducing κ can realize in the YbZn₂Sb₂ nPM with randomly arranged nanopores about 50-200 nm in diameter. Compared with YbZn₂Sb₂ dense bulk materials (dBM) fabricated by a conventional method taking more than 180 h, α²σ at 300 K increases by 122%, κ at 300 K decreases by 29%, and the maximum ZT value at 775 K reaches 0.67, increasing by 46% for the nPM725 sample. This work shows that a periodic arrangement of nanopores is not essential for the fabrication of attractive TE materials, which offers a wider approach to nanostructure engineering to improve TE performance.     

Electronic transport properties of Te-doped CoSb<Subscript>3</Subscript> skutterudites prepared by hot pressing

Te-doped CoSb₃ (CoSb_3−yTe_y) skutterudites were prepared by hot pressing and their electronic transport properties examined. A single δ-phase was successfully obtained. The Seebeck and Hall coefficients confirmed that all the Te-doped CoSb₃ showed n-type conduction. The Te atoms successfully acted as electron donors by substitution of the Sb atoms. The carrier concentration increased an order of 10²⁰ cm⁻³ by Te doping, whereas the carrier mobility decreased as the doping content increased. The Seebeck coefficient and electrical resistivity decreased with an increase in the Te content. The doping considerably reduced the thermal conductivity due to electron-phonon scattering. The lattice contribution was dominant over the electronic contribution.     

Thermoelectric properties and electronic structure of p-type Mg2Si and Mg2Si0.6Ge0.4 compounds doped with Ga

Mg₂Si:Ga_x and Mg₂Si_0.6Ge_0.4:Ga_x (x = 0.4% and 0.8%) solid solutions have been synthesized by direct melting in tantalum crucibles and hot pressing. The effect of Ga doping on the thermoelectric properties has also been investigated by measurements of thermopower, electrical resistivity, Hall coefficient and thermal conductivity in temperature range from 300 to 850 K. All samples exhibit a p-type conductivity evidenced by positive sign of both thermopower and Hall coefficient in the investigated temperatures. The maximum value of the dimensionless figure of merit ZT was reached for the Mg₂Si_0.6Ge_0.4:Ga(0.8%) compound at 625 K (ZT ∼ 0.36). The p-type character of thermoelectric behaviours of Ga-doped Mg₂Si and Mg₂Si_0.6Ge_0.4 compounds well corroborates with the results of electronic structure calculations performed by the Korringa-Kohn-Rostoker method and the coherent potential approximation (KKR-CPA), since Ga diluted in Mg₂Si and Mg₂Si_0.6Ge_0.4 (on Si/Ge site) behaves as hole donor due to the Fermi level shifted to the valence band edge. The onset of large peak of DOS from Ga impurity at the valence band edge, well corroborates with high Seebeck coefficient measured in Ga-doped samples.     

Phase range and physical properties of the thallium tin tellurides Tl10−xSnxTe6 (x ≤ 2.2)

Crystal structures and physical property measurements were determined for Tl_10−xSn_xTe₆ with a phase range of 0 ≤ x ≤ 2.2. These tellurides are substitution variants of Tl₅Te₃. Electronic structure calculations indicate that Tl₈Sn₂Te₆ should be an intrinsic semiconductor, and the Sn-poor variants, extrinsic ones with p-type conduction. The positive Seebeck values increase with increasing Sn content, while the electrical and thermal conductivity values decrease. Low thermal conductivity values, well below 1 W m⁻¹ K⁻¹, are the best asset of these materials with respect to thermoelectric performance. At x = 2.2, the best thermoelectric properties were obtained, with a figure-of-merit ZT = 0.60 at 617 K as determined on sintered cold-pressed pellets.     

A study of the synthesis of bismuth tellurium selenide nanocompounds and procedures for improving their thermoelectric performance

A bismuth tellurium selenide (Bi₂Te_ySe_3−y) nanocompound for thermoelectric applications was successfully prepared via a water-based chemical reaction in an atmospheric environment. The compound was less that ca. 100 nm in size, with a crystalline structure corresponding to the rhombohedral Bi₂Te_2.7Se_0.3. We sintered the compound via a spark plasma sintering process under the designated sintering conditions and measured the transport properties (i.e., thermal conductivity, resistivity, Seebeck coefficient). The resulting specimens consisted of nanosized grains exhibiting a remarkably low thermal conductivity. Subsequently, we endeavored to improve the other transport properties by adjusting the carrier density of the compound and derived the overall thermoelectric performance by the figure of merit (ZT).     

In situ structural investigation of amorphous and nanocrystalline Fe40Co38Mo4B18 microwires

Thermal evolution of the structure of glass-coated nanocrystalline FeCoMoB microwire during its devitrification has been studied. It is shown that annealing at the temperature above 411 °C leads to the formation of crystalline α-FeCo grains with diameter ∼12 nm. Annealing at higher temperature increases the crystalline weight fraction up to 40% at 565 °C. However, crystalline grains size increases very weakly to ∼13 nm. The thermal expansion coefficient of nanocrystalline microwire decreases by one half comparing to that of the amorphous precursor.     

Influence of the oxygen content and the preparation method on the power factor of PrBaCo2O5+δ samples (0.54 ≤ δ ≤ 0.84)

In this work, we focus on the influence of the oxygen content and the preparation method on the power factor of different PrBaCo₂O_5+δ samples (0.54 ≤ δ ≤ 0.84). The samples have been initially synthesized by the Pechini method. Their oxygen content has been subsequently modified by annealing under argon/oxygen flow or by electrochemical oxidation/reduction. The oxygen stoichiometry has a high impact on the electrical conductivity and Seebeck coefficient of the resulting materials. Moreover, by adequately reducing their oxygen content while increasing their intergrain conductivity (by increasing the particle size and degree of sintering) the power factor of these samples can be drastically improved. The best result is shown by the PrBaCo₂O_5.54 sample, that was annealed at high temperature under argon flow, whose power factor is as high as 6.5 μW/K² cm⁻¹ at ∼135 K, more than two orders of magnitude higher than that shown by the initial PrBaCo₂O_5.76 reference sample.     

Effect of Dy on the properties of Sm-doped ceria electrolyte for IT-SOFCs

Co-doped ceria-based electrolytes of Ce_0.8Sm_0.2−xDy_xO_2−δ (x = 0, 0.02, 0.06, 0.10, 0.14) were sintered from powders obtained by solid state reaction method. The phase identification, thermal expansion, ionic conductivities and microstructures of samples were studied by X-ray diffraction (XRD), dilatometry, AC impedance spectroscopy (IS) and scanning electron microscopy (SEM). The results showed that the addition of Dy led to higher ionic conductivity and lower activation energy in comparison with Sm singly doped ceria Ce_0.8Sm_0.2O_2−δ (SDC) in the temperature range of 300-800 °C. As the addition amount of Dy increased up to 2 mol% (Ce_0.8Sm_0.18Dy_0.02O_2−δ), the sample attained the highest ionic conductivity, about 50% higher than that of SDC at 500 °C. The effect of Dy on the grain boundary conductivity was more apparent than that of the bulk conductivity. XRD measurements indicated that all the samples were single phase. The thermal expansion was linear for all the samples. The addition of Dy did not change the thermal expansion coefficient (TEC) significantly.