Effects of Excess Sb on Thermoelectric Properties of Barium and Indium Double-Filled Iron-Based p-Type Skutterudite Materials

A series of Ba and In double-filled iron-based p-type skutterudite thermoelectric (TE) materials with nominal composition BaInFe_3.7Co_0.3Sb_12+m (0.72????m????2.4) have been prepared by melting, quenching, annealing, and spark plasma sintering (SPS) methods. The effects of excess Sb on the phase composition, microstructure, and TE transport properties of these materials were investigated in this work. All the SPS bulk materials are composed of the main skutterudite phase and trace InSb and FeSb₂. The content of FeSb₂ in the SPS bulk materials gradually decreased and that of InSb remained nearly invariable with increasing m. The impurities InSb and metallic Sb are found at grain boundaries. The amount of metallic Sb at grain boundaries gradually increased with increasing m. The excess Sb had no effect on the growth of grains. The dependence of the TE properties on m indicates that preventing the formation of FeSb₂ by adjusting the excess Sb value may significantly improve the TE properties of Ba and In double-filled iron-based p-type skutterudite materials. The significant increases in the carrier concentration and electrical conductivity as well as the remarkable reduction in the lattice thermal conductivity of the sample with m?=?0.96 are due to the significant reduction in the FeSb₂ content induced by the excess Sb. The gradual increase in ZT with increasing m from 0.72 to 1.44 is attributed to the gradual decrease of the FeSb₂ content, and the gradual decrease in ZT in the m range of 1.44 to 2.4 is due to the gradual increase of the Sb content in the Sb-In alloy impurity occurring at grain boundaries. The lowest lattice thermal conductivity of 0.31?W?m^?1?K^?1 and the highest ZT value of 0.63 were obtained at 800?K for the sample with m?=?1.44.     

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.     

Synthesis and Thermoelectric Properties of In and Pr Double-Filled Skutterudites In x Pr y Co4Sb12

Double-filled skutterudites In _x Pr _y Co₄Sb₁₂, which are currently being investigated for potential applications as thermoelectric materials, have been successfully prepared by inductive melting and annealing. Our results showed that In and Pr double filling effectively improves both electrical conductivity and Seebeck coefficient compared with pristine or single-filled CoSb₃, giving rise to a respectable power factor. The largest power factor, 2.33 m Wm^?1 K^?2, was achieved at 609 K for In_0.05Pr_0.05Co₄Sb₁₂; this value is approximately three times that for In _x Co₄Sb₁₂ (x ≤ 0.3) skutterudites. These results imply that In and Pr double filling are better than In single filling for efficient improvement of the thermoelectric properties of CoSb₃ skutterudite.     

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.     

Effects of Synthesis Temperature on the Microstructure and Thermoelectric Properties of Te-Se Codoped Skutterudites

Skutterudite compounds Co₄Sb_11.3Te_0.5Se_0.2 were synthesized by solid-state reaction at different temperatures (853 K, 903 K, 953 K, and 1003 K) with subsequent spark plasma sintering. x-Ray diffraction, field-emission scanning electron microscopy, and electron probe microanalysis were utilized to analyze the phase structure, microstructure, and actual compositions of the samples. The results showed that the actual composition and the grain size vary with the synthesis temperature. The thermoelectric properties of all samples were measured in the temperature range of 300 K to 800 K. As the synthesis temperature increases, the electrical conductivity increases rapidly, the absolute Seebeck coefficient falls, and the thermal conductivity first decreases and then increases. The highest dimensionless figure of merit ZT was achieved for the sample synthesized at 953 K, exceeding 1.0 at high temperature.     

Thermoelectric Properties of RuSb2Te Ternary Skutterudites

Polycrystalline samples of the RuSb₂Te ternary skutterudite compound were prepared by the powder metallurgy method, and the influence of various types of doping on its thermoelectric properties was studied. The phase purity of the prepared samples was checked by means of powder x-ray diffraction, and their compositions were checked by electron probe x-ray microanalysis. Hot-pressed p-type samples were characterized by measurements of electrical conductivity, Hall coefficient, Seebeck coefficient, and thermal conductivity. Various doping strategies, i.e., cation substitution (Ru_0.95Fe_0.05Sb₂Te), anion substitution (RuSb₂Sn_0.1Te_0.9) or partial filling of voids of the ternary skutterudite structure (Yb_0.05RuSb₂Te), were investigated, and the influence of the dopants on the changes of the resulting transport, thermoelectric, and thermal properties is described.     

Thermoelectric Properties of Sintered n-Type and p-Type Tellurides

Characterization of powder-metallurgically manufactured (Bi _x Sb_1?x )₂(Te _y Se_1?y )₃ thermoelectric materials is presented. The manufacturing methods were spark plasma sintering (SPS) and hot isostatic pressing (HIP). x-Ray diffraction (XRD) and density measurements as well as transport characterization and scanning electron microscopy were performed on the materials. It is shown that both sintering techniques yield reasonable thermoelectric characteristics for p-type (x = 0.2, y = 1) as well as n-type (x = 0.95, y = 0.95) materials. Insight into the underlying reasons such as the scattering processes limiting the characteristics is gained by fitting experimental transport data using a theoretical model. The limitations and further optimization issues of our approach in thermoelectric material preparation are discussed.     

Microstructure and Thermoelectric Properties of Yb-Filled Skutterudites Prepared by Rapid Solidification

Single-phase nanostructured bulk Yb_0.2Co₄Sb₁₂ skutterudites have been prepared by combining a melt spinning technique with spark plasma sintering. The effects of a pre-annealing process on the microstructure and phase composition of ribbon samples and bulk materials are investigated. After the pre-annealing process, average grain size increases from 200 nm to 300 nm for ribbon samples and from 250 nm to 350 nm for bulk materials, and nearly single-phase skutterudites have formed. Because of the nanostructure, the thermal conductivity of bulk skutterudites notably decreases 25% at 800 K. As␣a result, the ZT values are improved compared with starting material prepared by the traditional method.     

Effect of In Impurity on Thermoelectric Properties of Ba and In Double-Filled n-Type Skutterudite Materials

A series of (Ba,In) double-filled n-type skutterudite materials with nominal composition Ba_0.4In _m Co₄Sb₁₂ (m?=?0 to 0.4, ??m?=?0.1) has been prepared by melt quenching, annealing, and spark plasma sintering (SPS). The presence of In impurity and its effect on the thermoelectric properties of the filled skutterudite materials have been precisely investigated in this work. All samples consisted of skutterudite phase, while traces of In-containing impurity were detected in samples with m????0.3. The electrical conductivity and thermal conductivity decreased, and the absolute value of the Seebeck coefficient increased with increasing m in the range 0 to 0.2; however, the inverse behavior of the electrical conductivity, thermal conductivity, and Seebeck coefficient was observed in the samples with m????0.3. The thermoelectric properties of Ba_0.4In _m Co₄Sb₁₂ in the m range of 0 to 0.2 were changed because of carrier concentration degradation and strong lattice scattering induced by the In filler, while they were intensively affected by the In-containing impurity for m????0.3. Compared with the Ba single-filled skutterudite material, the power factors of all (Ba,In) double-filled skutterudite materials significantly increased and the lattice thermal conductivity dramatically decreased. As a result, two large ZT values for the samples with m?=?0.2 and 0.4 reached 1.19 and 1.25 at 800?K, which is an enhancement of 52% and 60%, respectively.     

Thermoelectric Properties of Multifilled Skutterudites with La as the Main Filler

Bulk multifilled n- and p-type skutterudites with La as the main filler were fabricated using the spark plasma sintering (SPS) method. The thermoelectric properties and thermal stability of these skutterudites were investigated. It was found that the interactions among the filling atoms also play a vital role in reducing the lattice thermal conductivity of the multifilled skutterudites. ZT = 0.76 for p-type La_0.8Ba_0.01Ga_0.1Ti_0.1Fe₃CoSb₁₂ and ZT = 1.0 for n-type La_0.3Ca_0.1Al_0.1Ga_0.1In_0.2Co_3.75Fe_0.25Sb₁₂ skutterudites have been achieved. Furthermore, the differential scanning calorimetry (DSC) results show that there is no skutterudite phase decomposition till 750°C for the La_0.8Ba_0.01Ga_0.1Ti_0.1Fe₃CoSb₁₂ sample. The thermal stability of the La_0.8Ba_0.01Ga_0.1Ti_0.1Fe₃CoSb₁₂ skutterudite is greatly improved. Using the developed multifilled skutterudites, the fabricated module with size of 50 mm × 50 mm × 7.6 mm possesses maximum output power of 32 W under the condition of hot/cold sides = 600°C/50°C.     

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.     

Thermoelectric Properties of Two-Phase PbTe with Indium Inclusions

The thermoelectric figure of merit (zT) can be increased by introduction of additional interfaces in the bulk to reduce the thermal conductivity. In this work, PbTe with a dispersed indium (In) phase was synthesized by a matrix encapsulation technique for different In concentrations. x-Ray diffraction analysis showed single-phase PbTe with In secondary phase. Rietveld analysis did not show In substitution at either the Pb or Te site, and this was further confirmed by room-temperature Raman data. Low-magnification (~1500×) scanning electron microscopy images showed micrometer-sized In dispersed throughout the PbTe matrix, while at high magnification (150,000×) an agglomeration of PbTe particles in the hot-pressed samples could be seen. The electrical resistivity (ρ) and Seebeck coefficient (S) were measured from 300 K to 723 K. Negative Seebeck values showed all the samples to be n-type. A systematic increase in resistivity and higher Seebeck coefficient values with increasing In content indicated the role of PbTe-In interfaces in the scattering of electrons. This was further confirmed by the thermal conductivity (κ), measured from 423 K to 723 K, where a greater reduction in the electronic as compared with the lattice contribution was found for In-added samples. It was found that, despite the high lattice mismatch at the PbTe-In interface, phonons were not scattered as effectively as electrons. The highest zT obtained was 0.78 at 723 K for the sample with the lowest In content.     

Low Thermal Conductivity and Enhanced Thermoelectric Performance in In and Lu Double-Filled CoSb<Subscript>3</Subscript> Skutterudites

The thermoelectric properties of indium (In) and lutetium (Lu) double-filled skutterudites In _x Lu _y Co₄Sb₁₂ prepared by high-pressure synthesis were investigated in detail from 4 K to 365 K. Our results indicate that In and Lu double filling can remarkably reduce the thermal conductivity, and substantially improve the thermoelectric performance. A thermoelectric figure of merit of ZT = 0.27 for In_0.13Lu_0.05Co_4.02Sb₁₂ was achieved at 365 K, being larger by one order of magnitude than that for CoSb₃. It is thought that the large difference in resonance frequencies of the In and Lu elements broadens the range of normal phonon scattering in the multifilled skutterudites, helping to achieve an even lower lattice thermal conductivity. This investigation suggests that an effective way to improve the thermoelectric performance of skutterudite materials is to use In and Lu double filling.     

Thermoelectric Properties and n- to p-Type Conversion of Co-Doped ZrNiSn-Based Half-Heusler Alloys

Half-Heulser thermoelectric materials ZrNi_1?y Co _y Sn (y?=?0, 0.02, 0.04, 0.08, 0.12) were prepared by a time-efficient levitation melting and spark plasma sintering procedure. X-ray diffraction analysis and electron probe microanalysis showed that single-phase half-Heusler compounds without compositional segregation have been obtained. The effects of Co doping on the electrical conductivity, Seebeck coefficient, and thermal conductivity of ZrNiSn-based half-Heusler alloys have been investigated from 300?K to 900?K. The Seebeck coefficient displayed a change from negative to positive values above nominal Co doping content of y?=?0.02, indicating a transition in the conduction behavior from n-type to p-type. The maximum dimensionless figure of merit ZT of undoped ZrNiSn sample reached 0.5 at 870?K.     

Effects of Double Substitution with Ge and Te on Thermoelectric Properties of a Skutterudite Compound

Studies have shown that the thermoelectric properties of CoSb₃ could be improved by the substitution of group IVB or VIB elements for Sb. However, the substitution volume is limited. To get a better picture of the substitution volume in view of thermoelectric properties, Ge and Te double-substituted skutterudite materials were prepared with the nominal composition of Co₄Sb _x Ge_5.9−0.5x Te_6.1−0.5x (x = 11, 10, 9, 8) by the traditional solid-state reaction method and spark plasma sintering, and Rietveld analysis was employed to refine the crystal structure. The results showed that the lattice parameter decreased linearly and the solubility limitations of group IVB and VIB elements were greatly alleviated by the Ge and Te codoping. Besides, the thermoelectric properties were analyzed through measurements of electrical and thermal conductivities as well as room-temperature electrical transport properties. The results showed that the substitution volume of Ge and Te could play an important role in the thermoelectric properties, and a minimum lattice thermal conductivity value of 1.56 W m⁻¹ K⁻¹ was obtained at around 673 K for Co₄Sb₈Ge_1.9Te_2.1. Co₄Sb₁₁Ge_0.4Te_0.6 achieved the best figure of merit of 0.89 at around 773 K, which was remarkably improved over that of untreated CoSb₃.     

High-Temperature Thermoelectric Properties of p-Type Yb-Filled Skutterudite Nanocomposites with FeSb2 Nanoinclusions

Bulk thermoelectric (TE) nanocomposite materials have attracted considerable attention due to their great potential to exhibit higher dimensionless figure of merit ZT. Filled skutterudites of both n-type and p-type have already demonstrated their excellent high-temperature TE performance, good mechanical properties, and thermal stability. Herein, we extend this work to Yb-filled p-type skutterudite nanocomposites with in?situ precipitated FeSb₂ nanoinclusions. Such a nanocomposite material can be easily synthesized by fine control of starting stoichiometry and the subsequent heat treatment process. By taking advantage of these naturally occurring FeSb₂ nanoparticles, we achieve ZT _max?=?0.74 in Yb_0.6Fe₂Co₂Sb₁₂/0.05FeSb₂ at 780?K. We apply the method of four coefficients to calculate the density-of-states effective mass and the carrier scattering parameter. We find that a larger effective mass induced by the presence of nanoparticles is the origin of the enhanced Seebeck coefficient.