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.     

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.     

Preparation and Enhanced Thermoelectric Properties of p-Type BaFe12O19/CeFe3CoSb12 Magnetic Nanocomposite Materials

A series of p-type xBaFe₁₂O₁₉/CeFe₃CoSb₁₂ (x = 0, 0.05%, 0.10%, 0.20%, 0.40%) magnetic nanocomposite thermoelectric (TE) materials have been prepared by the combination of ultrasonic dispersion and spark plasma sintering (SPS). The effects of BaFe₁₂O₁₉ magnetic nanoparticles on the phase composition, microstructure, and TE properties of the nanocomposite materials were investigated in this work. x-Ray diffraction analysis shows that all the SPSed bulk samples are composed of main phase skutterudite besides a small amount of FeSb₂ and Sb. The TE transport measurements demonstrated that remarkable enhancements in electrical conductivity and Seebeck coefficient can be simultaneously realized by optimizing the doping content of BaFe₁₂O₁₉ magnetic nanoparticles. The lattice thermal conductivity was significantly reduced because of enhanced phonon scattering induced by BaFe₁₂O₁₉ nanoparticles. The highest ZT value reached 0.75 at 800 K for the sample with x = 0.05%, increased by 41.5% as compared with that of p-type CeFe₃CoSb₁₂ bulk material without BaFe₁₂O₁₉ magnetic nanoparticles. This work confirms that doping a small amount of BaFe₁₂O₁₉ magnetic nanoparticles can significantly improve the ZT value of p-type xBaFe₁₂O₁₉/CeFe₃CoSb₁₂ magnetic nanocomposite TE materials.     

Thermoelectric properties of Ce x Nd y Co4Sb12 skutterudites

Experimental results of studying the thermoelectric properties of Co₄Sb₁₂, Ce_0.1Nd_0.5Co₄Sb₁₂, and Ce_0.5Nd_0.1Co₄Sb₁₂ prepared by induction melting are presented. The thermoelectric figure of merit ZT of the studied Co₄Sb₁₂ is approximately two times higher than ZT of unfilled skutterudites prepared by the conventional solid-phase synthesis method. The figure of merit of Ce_0.1Nd_0.5Co₄Sb₁₂ and Ce_0.5Nd_0.1Co₄Sb₁₂ appears lower than ZT of Co₄Sb₁₂ due to the presence of an impurity phase of metal antimony in the first two samples. It is assumed that the thermoelectric properties of filled skutterudites can be significantly improved by optimizing the induction melting method.

     

Effects of Ge Dopant on Thermoelectric Properties of Barium and Indium Double-Filled p-Type Skutterudites

A series of Ge-doped and (Ba,In) double-filled p-type skutterudite materials with nominal composition Ba_0.3In_0.2FeCo₃Sb_12?x Ge _x (x = 0 to 0.4, Δx = 0.1) have been prepared by melting, quenching, annealing, and spark plasma sintering methods. The effects of Ge dopant on the phase composition, microstructure, and thermoelectric properties of these materials were investigated in this work. A single-phase skutterudite material was obtained in the samples with 0 < x ≤ 0.2, and trace Fe₃Ge₂ was detected in the samples with x ≥ 0.3. The electrical conductivity increased and Seebeck coefficient decreased with increasing x in the range of 0 to 0.2, while the inverse behaviors of electrical conductivity and Seebeck coefficient were observed in the samples with x ≥ 0.3. The variations of electrical conductivity and Seebeck coefficient are attributed to the significant increase in the carrier concentration in the x range of 0 to 0.2 and the intensive impact of Fe₃Ge₂ when x ≥ 0.3. The lattice thermal conductivity of all the Ge-doped samples was considerably reduced as compared with the undoped Ba_0.3In_0.2FeCo₃Sb₁₂ sample, and the lowest value of lattice thermal conductivity of the Ba_0.3In_0.2FeCo₃Sb_11.8Ge_0.2 sample reached 1.0 W m^?1 K^?1 at 700 K. The highest ZT value of 0.54 was obtained at 800 K for the Ba_0.3In_0.2FeCo₃Sb_11.7Ge_0.3 sample, increased by 10% as compared with that of Ba_0.3In_0.2FeCo₃Sb₁₂.     

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 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.     

High-Temperature Oxidation Behavior of Filled Skutterudites Yb y Co4Sb12

The oxidation behavior of filled skutterudites Yb _y Co₄Sb₁₂ was investigated. The overall oxidation of Yb _y Co₄Sb₁₂ consists of two stages. In the first stage, densified oxide layers form on the surface gradually due to the reaction between oxygen and skutterudite at high temperature. In the second stage, microcracks evolve in the oxide layers because of mismatch of coefficient of thermal expansion between the oxide layer and skutterudite matrix, which accelerates the oxidation by providing transport paths for both outside oxygen and inside Sb. The overall oxidation process can be described through the repetitive cycle: dense layer formation → stress release → microcrack formation → self-repair → dense layer formation. The oxidation activation energy of filled skutterudites determined using thermogravimetry method with multi-heating rates is lower than that of unfilled CoSb₃. Moreover, it was found that, with increasing Yb filling fraction, the oxidation activation energy decreases monotonically. Our results suggest that protection against oxidation is necessary for application of filled skutterudites.     

A Resistance Ratio Analysis for CoSb3-Based Thermoelectric Unicouples

In the search for desirable materials for use in thermoelectric generators, CoSb₃-based skutterudites have stimulated much scientific interest due to their high performance capabilities even at high temperatures. In this work, we tested the electrical power-generation characteristics of CoSb₃-based unicouples. We manufactured power-generation unicouples using n-type In_0.25Co_3.95Ni_0.05Sb₁₂ and p-type In_0.25Co_3.0Fe_1.0Sb₁₂ legs. The dimensions of the thermoelectric legs were 10?mm in diameter and 10?mm in height, with Cu sheets and Cu/Mo alloy as the electrode materials. For our unicouples, we evaluated the resistance ratio m?? (=R _o/R), which represents the ratio of the load resistance to the internal resistance of the unicouple. From this analysis of the resistance ratio m??, we obtained a considerable amount of information about the loss factors that caused the difference between the measured power output and the theoretical value. Through these analyses of two types of loss factors, we sought to improve the open-circuit voltage and internal resistance of a unicouple with CoSb₃/Ti/electrode interfaces. In addition, a long-term durability test of the unicouple at high temperature was performed to test the stability of the thermoelectric materials and of the interface between the electrodes and the thermoelectric legs at the same time.     

Design and Optimization of Gradient Interface of p-Type Ba0.3In0.3FeCo3Sb12/Bi0.48Sb1.52Te3 Thermoelectric Materials

A series of p-type Ba_0.3In_0.3FeCo₃Sb₁₂/Bi_0.48Sb_1.52Te₃ (FS/BT) thermoelectric (TE) materials containing one gradient layer (1GL) with FS/BT volume ratio of 5:5, 3GLs-I with 7:3–5:5–3:7, 3GLs-II with 3:7–5:5–7:3, and 3GLs-III with 3:7–5:5–3:7 from FS to BT were fabricated by a two-step spark plasma sintering method. The interface structure and mechanical properties of the p-type FS/BT TE materials were investigated in this work. Designing the GLs at the interface of FS and BT bulk TE materials can effectively relax the thermal stress induced by the large difference in thermal expansion coefficient and eliminate the macroscopic cracks that occur in FS/BT TE materials with no GL, hence resulting in a remarkable enhancement in the interface mechanical properties of the FS/BT TE materials with the GLs. The optimized gradient interface of the FS/BT TE materials is 3GLs-II with FS/BT volume ratio of 3:7–5:5–7:3. The highest flexural strength of the 3GLs-II sample reached 13.68 MPa, increased by 116%.     

Influence of ZnO Inclusions on the Low-Temperature Thermoelectric Properties of CoSb3

CoSb₃ composites with different amounts of ZnO nanoparticles (2?wt.% to 12?wt.%) were prepared from nanosized ZnO (commercial) and micron-sized CoSb₃ (obtained via solid-state reaction) particles mixed in solution and freeze dried. The resulting powders were densified by spark plasma sintering. The samples were characterized by x-ray diffraction and scanning electron microscopy. It was found that ZnO forms micron-sized clusters at the grain boundaries of the matrix material. The thermoelectric properties (electrical resistivity, thermopower, and thermal conductivity) were measured in the 2?K to 300?K temperature range. Both the electrical and thermal conductivities were observed to decrease with increasing ZnO content. The dimensionless figure of merit ZT was improved by up to 30% at 300?K for the sample containing 2?wt.% ZnO.     

Effects of Annealing on Microstructure and Thermoelectric Properties of Nanostructured CoSb3

Nanocrystallization of thermoelectric materials is an effective way to reduce their thermal conductivity, but so far the thermal stability of nanostructured thermoelectric materials has been little studied. Effects of annealing treatment on the microstructure and the thermoelectric properties of nanostructured CoSb₃ were investigated in this work. Samples with average grain size of 300 nm were prepared by spark plasma sintering of high-energy ball-milled nanosized CoSb₃ powders. The study shows that annealing has a very significant impact on the grain size of the samples. The grain size of the sample with 100 h annealing is three times greater than before annealing. The major phase in the 150-h-annealed sample is still skutterudite, except for a trace amount of Sb phase. With increasing annealing time, the density reduces slightly. In addition, the power factor of the sample decreases, thus resulting in a decrease of the thermoelectric figure of merit.