Skutterudite Matrix Research Articles

Giant Thermoelectric Efficiency of Single-Filled Skutterudite Nanocomposites: Role of Interface Carrier Filtering.

The advantage of secondary-phase induced carrier filtering on the thermoelectric properties has paved the way for developing cost-effective, highly efficient thermoelectric materials. Here, we report a very high thermoelectric figure-of-merit of skutterudite nanocomposites achieved by tailoring interface carrier filtering. The single-filled skutterudite nanocomposites are prepared by dispersing rare-earth oxides nanoparticles (Yb2O3, Sm2O3, La2O3) in the skutterudite (Dy0.4Co3.2Ni0.8Sb12) matrix. The nanoparticles/skutterudite interfaces act as efficient carrier filters, thereby significantly enhancing the Seebeck coefficient without compromising the electrical conductivity. As a result, the highest power factor of ∼6.5 W/mK2 is achieved in the skutterudite nanocomposites. The nonuniform strain distribution near the nanoparticles due to the local lattice misfit and concentration fluctuations affect the heat carriers and thereby reduce the lattice thermal conductivity. Moreover, the three-dimensional atom probe analysis reveals the formation of Ni-rich grain boundaries in the skutterudite matrix, which also facilitates the reduction of lattice thermal conductivity. Both the factors, i.e., the reduction in lattice thermal conductivity and the enhancement of the power factor, lead to an enormous increase in ZT up to ∼1.84 at 723 K and an average ZT of about 1.56 in the temperature range from 523 to 723 K, the highest among the single-filled skutterudites reported so far.

Reaction mechanism and thermoelectric properties of In0·22Co4Sb12 prepared by magnesiothermy

The magnesioreduction synthesis of In0·22Co4Sb12 with high In rattler concentration from Sb2O4 and In-doped Co3O4 precursors is reported. This process directly yields a submicronic powder in a single step of 96 h at 810 K. The reaction mechanism has been investigated by stopping the reaction every 12 h and quantifying the existing phases by X-ray diffraction and Rietveld refinements. The precursors are first reduced in CoO and Sb2O3 lower oxides, then form CoSb2O6 and CoSb2O4 intermediates which are finally reduced in InxCo4Sb12. A powder with 350 nm average size and mostly composed of In-filled skutterudite phase with composition close to In0·17Co4Sb12 is obtained. Upon spark plasma sintering, small residual amount of InSb reacts with the skutterudite matrix to form a single-phase densified pellet with composition close to In0·22Co4Sb12. The resulting densified material with 1.8 μm average grain size shows a figure of merit ZTmax of 0.95 at 750 K.

Nanostructuring as a tool to adjust thermal expansion in high ZT skutterudites

Although filled skutterudites have a high figure of merit, ZT, it is still an aim to further improve their thermoelectric behavior. One successful route is to homogeneously disperse nanoparticles into the skutterudite matrix and thereby to reduce the lattice thermal conductivity without disturbing the electrical conductivity and the Seebeck coefficient. In this work Ta0.8Zr0.2B was dispersed into n-type (Mm,Sm)yCo4Sb12 powder by ball-milling or high energy ball-milling prior to hot-pressing. Structural and physical properties, covering also the low temperature range, were studied and compared with the recently investigated impact of nano Ta0.8Zr0.2B on p-type skutterudites. ZT was enhanced by 16%.As for thermoelectric applications good mechanical properties are important, we investigated the influence of Ta0.8Zr0.2B nanoparticles, added to (Mm,Sm)yCo4Sb12, on the hardness, elastic moduli and fracture toughness and found an enhancement of all three mechanical properties. It is well known that p-type skutterudites generally have a higher thermal expansion coefficient than their n-type counterpart. We show that dispersed borides in both n-type (Mm,Sm)yCo4Sb12 and p-type DD0.7Fe3CoSb12 can level out this discrepancy.

High pressure synthesis and thermoelectric properties of micro/nano structures CoSb3

Nano-particle and defect have been proved to be effective in depressing the lattice thermal conductivity and improving the performance of thermoelectric materials. In this work, In0.5SnxCo4Sb12 were prepared using a high pressure and high temperature (HPHT) method, In and Sn was assumed that In plays the role of a filler and occupies the Sb-icosahedron voids. Doping of In and Sn results in the defects, nanostructuring, and mesoscale structuring were all combined into a single skutterudite matrix. The thermoelectric properties were measured in the temperature range of 321–710K. As expected, with the synthetic pressures increased，the thermal conductivity of samples improved significantly. The minimum value of In0.5Sn0.4Co4Sb12 synthesized at 3.8GPa is 1.79Wm−1K−1 and the highest Seebeck coefficient of sample In0.5Sn0.2Co4Sb12 synthesized at 2.8GPa reached 222μVK−1 finally.

Grain boundary engineering with nano-scale InSb producing high performance InxCeyCo4Sb12+z skutterudite thermoelectrics

Thermoelectric semiconductors based on CoSb3 hold the best promise for recovering industrial or automotive waste heat because of their high efficiency and relatively abundant, lead-free constituent elements. However, higher efficiency is needed before thermoelectrics reach economic viability for widespread use. In this study, n-type InxCeyCo4Sb12+z skutterudites with high thermoelectric performance are produced by combining several phonon scattering mechanisms in a panoscopic synthesis. Using melt spinning followed by spark plasma sintering (MS-SPS), bulk InxCeyCo4Sb12+z alloys are formed with grain boundaries decorated with nano-phase of InSb. The skutterudite matrix has grains on a scale of 100–200 nm and the InSb nano-phase with a typical size of 5–15 nm is evenly dispersed at the grain boundaries of the skutterudite matrix. Coupled with the presence of defects on the Sb sublattice, this multi-scale nanometer structure is exceptionally effective in scattering phonons and, therefore, InxCeyCo4Sb12/InSb nano-composites have very low lattice thermal conductivity and high zT values reaching in excess of 1.5 at 800 K.

Mesostructured thermoelectric Co1−yMySb3 (M = Ni, Pd) skutterudites

n-type skutterudites Co0.97Pd0.03Sb3 + nano-CeO2 and Co0.94Ni0.06Sb3 + nano-CeO2 nanocomposites were synthesized by ball milling and spark plasma sintering in order to promote phonon scattering at the grain boundaries. Addition of nano-CeO2 slows down the grain size growth of the skutterudite matrix which occurs during sintering, leading to grain sizes as small as respectively 220 and 170 nm in Co0.97Pd0.03Sb3 + 2% CeO2 and Co0.94Ni0.06Sb3 + 3% CeO2. Subsequently, the lattice thermal conductivity is reduced by up to 47% at 300 K. An experimental value of ∼110 nm can be estimated for the bulk phonon mean free path in both compounds. In Co0.94Ni0.06Sb3 + 3% CeO2, an improved ZT (∼0.7) is obtained at 700 K. Annealing Co0.97Pd0.03Sb3 + 2% CeO2 at 825 K leads to a slightly increased grain size (240 nm), which remains stable on a long term (95 days).

The Influence of Sn Additions on the Thermoelectric and Transport Properties of FeSb2Te-based Ternary Skutterudites

The influence of Sn additions was studied in a series of samples of a nominal composition FeSb2Te1−xSnx (x = 0, 0.05, 0.1, 0.15, 0.2). SnTe compound was primarily identified in the matrix compound of the ternary skutterudite structure in the multiphase composite samples. It was determined that Sn atoms preferentially react with Te atoms which are present in order to form SnTe compound instead of entering the skutterudite structure. A detailed analysis of the composition of the ternary skutterudite matrix compound evoked by the striking similarities of the observed changes between the samples and another two published systems (FeSb2Te1−xGex and FeSb2+xTe1−x) revealed the crucial role of the Sb/Te ratio as the dominant factor driving the observed changes of the measured properties. The anomalous changes of the measured transport properties values were explained in terms of an effective medium theory for two-phase FeSb2Te-SnTe composites. A maximum value of thermoelectric figure-of-merit, ZT = 0.47 at 673 K, was attained for the sample of a nominal composition FeSb2Te0.85Sn0.15.

Thermoelectric Properties of Group 13 Elements-Triple Filled Skutterudites: Nominal In<i><sub>x</sub></i>Ga<sub>0.02</sub>Tl<sub>0.20</sub>Co<sub>4</sub>Sb<sub>12</sub>

The high-temperature thermoelectric (TE) properties of group 13 elements (Ga, In, and Tl) triple-filled skutterudites in the nominal compositions of InxGa0.02Tl0.20Co4Sb12 (0 x 0.30) were investigated. All samples contained impurity phases in addition to the skutterudite matrix phase, i.e. some of the filler elements added to CoSb3 precipitated as the metal state and antimonides. With increase the starting amount of In, the actual content of In increases while that of Tl decreases. Owing to phonon scattering by rattling of the group 13 elements, low lattice thermal conductivity was achieved, leading to relatively high TE figure of merit (ZT) for all samples. The maximum ZT value was 0.72 at 773K obtained for the sample in the nominal composition of In0.30Ga0.02Tl0.20Co4Sb12. [doi:10.2320/matertrans.E-M2014826]

Intensive suppression of thermal conductivity in Nd0.6Fe2Co2Sb12-xGex through spontaneous precipitates

The best p-type skutterudites are all Fe-rich and show larger coefficient of thermal expansion than CoSb3-based n-type skutterudites, which deteriorate the reliability of thermoelectric couples constituted by p- and n-skutterudites. In this study, we synthesize Nd0.6Fe2Co2Sb12-xGex (0≤x≤1) with equal amounts of Fe and Co. Microstructure analysis reveals a unique microstructure of precipitate phases, including nanoprecipitates in skutterudite matrix, structurally imperfect nanoparticles (less than 100 nm), and unprecedented thin-layer coverings about 10 nm thick. Such rich microstructures dominate the scattering of phonons with full-length scale and remarkably suppress the lattice thermal conductivity. As a result, ZT for Nd0.6Fe2Co2Sb11.7Ge0.3 is 50% improved compared with that of a Ge free sample even with a moderately decreased power factor.

AgSbTe2 nanoinclusion in Yb0.2Co4Sb12 for high performance thermoelectrics

Yb0.2Co4Sb12 based composites with AgSbTe2 nanoinclusion have been successfully prepared by a combined process of vacuum melting, pulverization and hot press sintering. XRD, FESEM, TEM and EDS were performed to characterize the microstructure and composition of the nanocomposites. It shows AgSbTe2 inclusions are about 50 nm in size and they distribute in the grain boundaries of Yb0.2Co4Sb12 filled skutterudite matrix. When the content of AgSbTe2 is less than 8wt%, Seebeck coefficient and electrical conductivity of the composites increase with increasing content of AgSbTe2, while the lattice thermal conductivity reduces with the increase of AgSbTe2 nanoinclusions. As a result, thermoelectric performance is improved with addition of AgSbTe2 nanoinclusion and a maximum ZT of 1.27 has been obtained for the sample of Yb0.2Co4Sb12/4wt%AgSbTe2 at about 300 °C.

Effect of Nanostructuring on the Thermoelectric Properties of Co0.97Pd0.03Sb3

We synthesized n-type CeO2/Co0.97Pd0.03Sb3 composites with nanometric grain sizes (200 nm to 300 nm) by spark plasma sintering in order to promote phonon scattering at grain boundaries. Powdered samples were initially obtained by ball milling Co0.97Pd0.03Sb3 together with x vol.% (x = 0, 0.5, 1, 2) of CeO2 nanoparticles. This additive slows down the grain size growth of the skutterudite matrix which occurs during sintering, thereby contributing to phonon scattering. The nanostructured samples display reduced Hall electron concentration compared with that of the reference Co0.97Pd0.03Sb3 because of Fe contamination by the steel balls and vials. However, the electronic transport properties are nearly identical to those of Co0.98Pd0.02Sb3, which allows for comparison with this latter compound. The lattice thermal conductivity is strongly decreased in nano-Co0.97Pd0.03Sb12 (−40% at 300 K). This results in an enhanced (+32%) ZT value peaking at 0.65 at 650 K in nano-Co0.97Pd0.03Sb12 + 2% CeO2 when compared with micro-Co0.98Pd0.02Sb3.

Microstructure and thermoelectric properties of CoSb2.75Ge0.25−xTex prepared by rapid solidification

Since the vibration modes of the pnicogen rings in CoSb3-based skutterudites fall within the range of frequencies of heat-carrying phonons, disruption of the rings by doping should have a strong influence on heat transport in this material. To test the premise, single-phase double-doped CoSb2.75Ge0.25−xTex (x=0.125–0.20) compounds were synthesized by combining melt spinning with a spark plasma sintering method. Following the melt-spinning process, the side of the ribbons contacting the copper drum is featureless and reflects its amorphous nature while the free surface of the ribbons is composed of 30–80nm grains. After spark plasma processing the average grain size of the bulk samples is about 200nm. High-resolution transmission electron microscopy images show an in situ nanostructure consisting of circular, 15nm diameter dots of Te- and Ge-enriched skutterudite phase embedded in the skutterudite matrix. Transport properties were measured from 2 to 800K as a function of Te and Ge content on the pnicogen (Sb) rings and the results were correlated with the structural data. Double-doping on pnicogen rings with Ge and Te, and using melt-spinning processing, results in binary skutterudite compounds that possess an impressive figure of merit of ZT∼1.1 at 750K.

High-Temperature Oxidation Behavior of Filled Skutterudites Yb y Co4Sb12

The oxidation behavior of filled skutterudites YbyCo4Sb12 was investigated. The overall oxidation of YbyCo4Sb12 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.

Fabrication and thermal aging behavior of skutterudites with silica-based composite protective coatings

Silica-based composite coatings with dispersion of glass or alumina granular particles were fabricated on n-type Yb0.3Co4Sb12 and p-type CeFe3CoSb12 skutterudite (SKD) thermoelectric (TE) materials using hybrid silica sol as raw material which was synthesized through hydrolysis and condensation under acid condition process. The dispersion of either glass particles or alumina particles restrained the formation of micro-cracks during solidification due to relaxing tremendous cohesive stress in the gel. The high temperature stability for both the coating materials and the coated skutterudite matrix were investigated through thermal aging at 873K in vacuum. Inter-diffusion of Sn from glass toward SKDs and Sb from SKDs toward coating layer was observed in the glass particle dispersed silica composite coating, which resulted in the failure of protecting function of the coating layer. In the alumina particle dispersed silica coating, neither diffusion nor reaction between the coating and SKD materials was observed after thermal aging at 873K. The sublimation of antimony is suppressed in the SKDs overlaid by the alumina dispersed silica composite coating.

Transport properties and microstructure of indium-added cobalt–antimony-based skutterudites

Abstract

Thermoelectric properties of Co0.9Fe0.1Sb3-based skutterudite nanocomposites with FeSb2 nanoinclusions

Bulk thermoelectric nanocomposite materials have great potential to exhibit higher figure of merit due to effects arising from the nanostructure. In this paper, we report thermoelectric properties from 80 K to 800 K of Co0.9Fe0.1Sb3 based skutterudite nanocomposites containing FeSb2 nanoinclusions. The nanoscale FeSb2 precipitates are well dispersed in the skutterudite matrix and reduce the lattice thermal conductivity due to additional phonon scattering from the nanoscopic interfaces. Moreover, the nanocomposite samples also exhibit enhanced Seebeck coefficients relative to the regular iron substituted skutterudite samples. As a result, our best nanocomposite sample reached a dimensionless figure of merit of 0.59 at 788 K, a factor of two higher than that of the control sample Co0.9Fe0.1Sb3.

Low-Temperature Thermoelectric Properties of Co0.9Fe0.1Sb3-Based Skutterudite Nanocomposites with FeSb2 Nanoinclusions

Bulk thermoelectric nanocomposite materials have great potential to exhibit higher ZT due to effects arising from their nanostructure. Herein, we report low-temperature thermoelectric properties of Co0.9Fe0.1Sb3-based skutterudite nanocomposites containing FeSb2 nanoinclusions. These nanocomposites can be easily synthesized by melting and rapid water quenching. The nanoscale FeSb2 precipitates are well dispersed in the skutterudite matrix and reduce the lattice thermal conductivity due to additional phonon scattering from nanoscopic interfaces. Moreover, the nanocomposite samples also exhibit enhanced Seebeck coefficients relative to regular iron-substituted skutterudite samples. As a result, our best nanocomposite sample boasts a ZT = 0.041 at 300 K, which is nearly three times as large as that for Co0.9Fe0.1Sb3 previously reported.

High performance InxCeyCo4Sb12 thermoelectric materials with in situ forming nanostructured InSb phase

n -Type skutterudites InxCeyCo4Sb12 with in situ forming nanostructured InSb phase have been prepared by a melt-quench-anneal-spark plasma sintering method. Doping of In results in a nanostructured InSb phase with the grain size of 10–80 nm that is evenly distributed on the boundaries of the skutterudite matrix. The nanostructured InSb phase has a strong influence on phonon scattering and leads to a notable suppression of the lattice thermal conductivity of InxCeyCo4Sb12. The combined effect of In and Ce doping results in high performance skutterudite materials. The highest thermoelectric figure of merit ZT=1.43 is achieved at 800 K in the In0.2Ce0.15Co4Sb12 compound.