P-type Skutterudites Research Articles

Synthesis of P-Type Fe3CoSnxSb12-X Skutterudite Single Phase By Designing Double Filled Compositions

Double-filled Ce0.6Yb0.4Fe3CoSnxSb12-x (x=0.1 - 0.4) skutterudite compositions were designed for enhanced thermoelectric properties. We fabricated the skutterudite specimens by using a melt-spinning process with subsequent a spark plasma sintering (SPS). Effect of Sn doping amount on the thermoelectric characteristics of the Ce0.6Yb0.4Fe3CoSnxSb12-x system were investigated. Single phase formation was examined by a XRD (X-ray diffraction) method. The microstructure was investigated by SEM (scanning electron microscope). The carrier concentration and carrier mobility were also measured to analysis the Sn doping amount effects on formation of the single phase and their properties in detail.

High Temperature Transport Properties of Yb and In Double-Filled p-Type Skutterudites

Yb and In double-filled and Fe substituted polycrystalline p-type skutterudite antimonides were synthesized by direct reaction of high-purity elements, followed by solid-state annealing and densification by hot pressing. The stoichiometry and filling fraction were determined by both Rietveld refinement of the X-ray diffraction data and energy dispersive spectroscopic analyses. The transport properties were measured between 300 K and 830 K, and basically indicate that the resistivity and Seebeck coefficient both increase with increasing temperature. In both specimens, the thermal conductivity decreased with increasing temperature up to approximately 700 K, where the onset of bipolar conduction was observed. A maximum ZT value of 0.6 at 760 K was obtained for the Yb0.39In0.018Co2.4Fe1.6Sb12 specimen.

Super-fast preparation of Nd-filled p-type skutterudite compounds with enhanced thermoelectric properties

P-type filled skutterudite materials have been gained considerable research interest in recent years due to their promising thermoelectric power generation applications at intermediate temperature. Herein, we systematically investigated the influence of Nd filling on the thermoelectric properties of NdxFe2Co2Sb12 (x=0.4, 0.5 0.6, 0.7 and 0.8). Nd-filled skutterudites are synthesized using a simple and time-saving induction melt spinning technique followed by spark plasma sintering. The results show that Nd-filling leads to the significant reduction in the lattice thermal conductivity and enhancement of power factor over the entire temperature range. The most marked reduction in the lattice thermal conductivity is achieved with the value of 0.76W/mK for x=0.7 sample, due to strengthened phonon scattering. Meanwhile, the highest ZT=0.98 is attained at 740K for Nd0.7Fe2Co2Sb12. The rapid synthesis procedure provides an effective pathway for the fabrication of thermoelectric materials with high performance.

Dispersion of Multi-Walled Carbon Nanotubes in Skutterudites and Its Effect on Thermoelectric and Mechanical Properties.

Filled cobalt-antimony based skutterudites have proven themselves as very promising thermoelectric materials for generator applications in an intermediate temperature range between 400 and 800 K due to their high figure of merit. Besides the functional thermoelectric properties also the skutterudites’ mechanical properties play an important role to withstand external mechanical and internal thermomechanical loads during operation. Properties of interest are hardness as well as fracture toughness and resistance to fatigue. Carbon nano tubes are well known for their high tensile strength and may therefore be used to increase the mechanical strength of composite materials. Additionally, the thermoelectric properties of the composite material might benefit from the high electrical conductivity of carbon nano tubes and increased phonon scattering at interfaces between matrix and carbon nano tube. A main precondition for benefiting from embedded nano-tubes is to achieve a homogeneous distribution of the CNTs and good adhesion between carbon nano tube and matrix material. In this work we present the influence of the introduction of multi-walled carbon nano tubes on the thermoelectric and mechanical properties of p-type skutterudites Ce(0.14)La(0.06)Co(2)Fe(2)Sb(12). The influence of different carbon nano tube concentrations and preparation routes on the resulting composite material’s thermoelectric, mechanical and microstructural properties is studied. A reduction of electrical and thermal conductivity as well as fracture strength is observed with increasing carbon nano tube content which is attributed to strong agglomeration of the nano tubes. The results underline the pivotal role of a homogeneous distribution of the carbon nano tubes for improving the mechanical properties of skutterudites.

Enhanced thermoelectric and mechanical properties of p-type skutterudites with in situ formed Fe3Si nanoprecipitate

Hardness and indentation fracture toughness of La0.8Ti0.1Ga0.1Fe3CoSb12can be improved byin situformed Fe3Si, without sacrificing thermoelectric properties.

Enhancing the thermoelectric properties of single and double filled p-type skutterudites synthesized by an up-scaled ball-milling process

The single and double filled p-type skutterudites Ce0.8Fe3CoSb12 and Ce0.5Yb0.5Fe3.25Co0.75Sb12 have been prepared by mechanical alloying. This offers a rapid method for the preparation of skutterudites that could be scaled up for adoption at industrial level. The large-scale samples prepared by ball-milling exhibit enhanced figures of merit ZT, compared with materials prepared by conventional solid-state reaction. At room temperature ZT is increased by ca. 19% for both single and double filled skutterudites. Maximum figures of merit, ZT = 0.68 and ZT = 0.93 are attained for Ce0.8Fe3CoSb12 at 773 K and Ce0.5Yb0.5Fe3.25Co0.75Sb12 at 823 K respectively. The improvement in thermoelectric values at room temperature may be traced to a reduction in thermal conductivity in the ball-milled samples arising from the reduced grain size. The influence of the microstructure on the thermoelectric properties, together with the stability in air and the performance of the materials after several heating and cooling cycles has been studied and are detailed in this work. The densified samples prepared by ball-milling also show a higher resistance to oxidation, which starts at 694 K for Ce0.8Fe3CoSb12 and at 783 K for Ce0.5Yb0.5Fe3.25Co0.75Sb12.

High-Temperature Thermoelectric Properties of Ge-Substituted p-Type Nd-Filled Skutterudites

To clarify the influence of Ge substitution on the electrical transport properties of Nd-filled p-type skutterudites, a series of Nd0.9Fe2Co2Sb12−xGex compounds with compositions x = 0, 0.1, 0.2, 0.3, and 0.4 were synthesized by a solid-state reaction method followed by a spark plasma sintering process. The crystal structure of the prepared samples was characterized by x-ray diffraction analysis, revealing successful formation of skutterudites as main pure phase. The electrical and thermal transport properties were investigated as a function of Ge doping content for fixed Nd filler content. All samples possessed positive Seebeck coefficient, indicating effective p-type behavior. The lightly doped samples (x = 0.1 and 0.2) showed higher figure␣of merit over the entire temperature range, with the x = 0.2 sample showing the highest ZT value of 0.56 at 674 K, 40% higher than that of the Ge-free sample. The enhancement in ZT can be ascribed to the optimized carrier concentration and reduced thermal conductivity. However, further increase of the Ge content resulted in second phase in the matrix, lowering the overall ZT. The large enhancement of ZT through Ge doping indicates that these compounds may have great potential for application as p-type segments of thermoelectric devices.

Attempts to further enhance ZT in skutterudites via nano-composites

Skutterudites are known as excellent thermoelectric p- and n-type materials and have already achieved good efficiencies for the conversion of heat to electricity, but nevertheless researchers try to further enhance the figure of merit, ZT. In the present work the aim was to mix p- and n-type skutterudite powders with additives in order to produce an evenly dispersed distribution of sub micron grains, preferable small nano-particles, which enhance the scattering of the heat carrying phonons of different wavelengths and reduce thermal conductivity without changing electrical resistivity and Seebeck coefficient. Various quantities of three groups of materials (a) nonmetallic oxides (Al2O3), (b) metallic oxides (Cu2O and La1.85Sr0.15CuO4) and (c) metallic borides (Fe2.25Co0.75B and Ta0.8Zr0.2B) were added to industrially produced p-type (DDyFe3CoSb12) and n-type ((Mm,Sm)yCo4Sb12) skutterudite powders. First the influence of pre-sieving and various ball milling conditions before hot pressing were studied, using Al2O3 as additive. As a consequence of these studies pre-sieved powders and high-energy ball milling were used for all following experiments. The goal, to enhance ZT was not reached with Al2O3 and Cu2O. La1.85Sr0.15CuO4 was successful for the n-type, Fe2.25Co0.75B for the p-type skutterudites, although ZT-enhancement was small, but with Fe2.25Co0.75B the maximum ZT could be shifted to lower temperatures, a valuable information for device production. Much better results in respect to ZT values were gained with adding 0.5, 1.0 and 1.5 wt.% Ta0.8Zr0.2B to p-type DDyFe3CoSb12. In this series it was possible to enhance ZT (from ZT ∼ 1.2 to ZT ∼ 1.3) as well as to significantly increase the thermal-electrical conversion efficiency η. In addition, we found that all boride additives enhanced the hardness, elastic moduli and fracture resistance.

Performance of Skutterudite-Based Modules

Due to their excellent thermoelectric (TE) performance, skutterudite materials have been selected by many laboratories and companies for development of TE modules to recover power from waste heat at high temperatures (300°C to 600°C). After years of effort, we have developed reliable n- and p-type skutterudite materials showing maximum figure of merit (ZT) of 1.0 at 550°C and 0.75 at 450°C, respectively. In this work, we systematically investigated the performance of a module made using these two kinds of skutterudite. We demonstrate ∼7.2% conversion efficiency for temperature of 600°C at the hot side of the module and 50°C at the cold side, and show that the module had excellent stability in the high-temperature environment. Further improving the TE performance of our skutterudites, the conversion efficiency reached ∼8.5% under the same condition.

Transport and mechanical properties of the double-filled p-type skutterudites La0.68Ce0.22Fe4−xCoxSb12

Till date, the dimensionless figure of merit (ZT) of p-type skutterudites lags its n-type counterpart. In this work, we report improved thermoelectric performance of La/Ce double filled p-type skutterudites. The samples were synthesized by hot pressing nano-powder made by ball milling annealed ingots of La0.68Ce0.22Fe4−xCoxSb12 with varying Co concentration. By tuning the Fe/Co ratio, we achieved a higher power factor (∼35 μW cm−1 K−2 at 475 °C), leading to a peak ZT of 1.15 at ∼475 °C in La0.68Ce0.22Fe3.5Co0.5Sb12. Nanoindentation experiment carried on fine surface of the best sample reveals hardness of 5.4 GPa and modulus of elasticity of 126 GPa, which is much higher than those of Bi2Te3 and PbTe based materials, indicating skutterudites are mechanically strong, good for some practical applications.

Raising the Thermoelectric Performance of Fe3CoSb12 Skutterudites via Nd Filling and In-Situ Nanostructuring.

p-type skutterudites NdxFe3CoSb12 with x equaling 0.8, 0.85, 0.9, 0.95, 1.0 have been synthesized by solid state reaction followed by spark plasma sintering. The influence of Nd filling on electrical and thermal transport properties has been investigated in the Nd-filled skutterudite compounds in the temperature range from room temperature to 800 K. It was found that the Seebeck coefficient is drastically enhanced via filling Nd in p-Type skutterudites as well as the corresponding power factor although electrical conductivity is reduced. In addition, a large reduction in thermal conductivity is achieved by Nd fillers through rattling effect along with the in-situ nanostructured precipitate through scattering phonons with much wider frequency. These concomitant effects result in an enhanced thermoelectric performance with the dimensionless figure of merit ZT. These observations demonstrate an exciting scientific opportunity to raise the figure-of-merit of p-type skutterudites.

Co x Ni4−x Sb12−y Sn y skutterudites: processing and thermoelectric properties

N-type and p-type skutterudite samples with the composition Co x Ni4−x Sb12−y Sn y were synthesized with composition range 0 0.8 and negative otherwise. Seebeck coefficients were low, ranging from −40 to 58 µV K−1. The combination of transmission electron microscopy with electron energy loss spectroscopy and powder X-ray diffraction established that Sn can substitute on 2a and 24g sites in the skutterudite structure. Due to the low Seebeck coefficients, the alloys exhibited low figure of merits (ZT) <0.05.

High pressure synthesis of p-type Fe-substituted CoSb3 skutterudites

Fe-substituted CoSb3 skutterudites were successfully synthesized with high pressure synthesis method. The composition, structure, and thermoelectric properties were investigated. Co4−xFexSb12 samples have the skutterudite structure of \(\text{Im} \bar{3}\) symmetry. The hole concentration increases with elevating Fe substitution level. Compared with those ambient-pressure synthesized Fe-substituted CoSb3 samples, our samples exhibit enhanced power factor and suppressed thermal conductivity which benefitted from high pressure. The optimal Co3.2Fe0.8Sb12 shows a peak ZT of 0.53 at 823 K, the highest value for Co-rich p-type skutterudites with single-element substitution. Current study on Co4−xFexSb12 provides a basis for further thermoelectric performance enhancement of Co-rich p-type skutterudites.

Melt spinning synthesis of p-type skutterudites: Drastically speed up the process of high performance thermoelectrics

In this study, via a home-made induction melting process combined with spark plasma sintering technique, we successfully synthesize the nanostructured skutterudites NdxYbyFe3CoSb12 (x+y=1.0, x=0.3, 0.4, 0.5, 0.6, 0.7) in less than 3h, which is significantly faster than that of the solid state reaction (168h) and the traditional melt spinning method (40h). Systematical microstructures analysis of the sintered bulk materials reveals the largely refined matrix grains (100–300nm) and ubiquitous YbSb precipitates (~30nm), resulting in a very low lattice thermal conductivity. A peak ZT up to 1.02 at 760K is obtained for Nd0.6Yb0.4Fe3CoSb12 compound.

Enhancement of thermoelectric properties of Ce0.9Fe3.75Ni0.25Sb12p-type skutterudite by tellurium addition

Te addition in p-type skutterudites induces multiple effects: augmentation of effective mass, distortion of Sb4rings, formation of hierarchical microstructure.

The Influence of Synthesis Procedure on the Microstructure and Thermoelectric Properties of p-Type Skutterudite Ce0.6Fe2Co2Sb12

We have investigated p-type skutterudite samples with the nominal composition Ce0.6Co2Fe2Sb12 synthesized from elementary constituents by gas atomization and conventional melting, and also those synthesized from ternary and binary phases such as FexCo1−xSb2 and CeSb2, respectively, which were mixed and subsequently ball-milled. We conducted measurements of the temperature-dependent transport properties (Seebeck coefficient, thermal/electrical conductivity) and carried out scanning electron microscope analysis, electron probe micro-analysis and powder x-ray diffraction to obtain information about microstructure and elementary distribution of the phases. We show that the presented synthesis methods each possess particular strengths but ultimately, however, lead to different final compositions of the skutterudite phase and secondary phases, which significantly influence the thermoelectric properties of the material. Material prepared using an educt method gave the best thermoelectric properties with a peak ZT of 0.7. Furthermore, we show that even an apparent homogeneous skutterudite area within the material exhibits varying stoichiometry in each grain even though they conform to the solubility range of cerium in this p-type skutterudite. Moreover, we show that marcasite is preferred as an educt over the arsenopyrite phase and discuss the formation of the p-type skutterudite phase with these synthesis techniques.

Synthesis, characterization and thermoelectric properties of Sm filled Fe4−xNixSb12 skutterudites

High figure of merit of n-type skutterudites have encouraged the researchers to focus on its p-type counterpart. In the following study, we report a rather simple synthesis route for Sm filled and Ni-doped p-type skutterudites (SmyFe4−xNixSb12.8, y = 0.7–0.8, x = 0.4–1.2, nominal composition), along with high temperature thermoelectric properties. Sample were prepared by melting, quenching, followed by annealing and the resulted chunks were cut into disks form to measure the thermoelectric properties without applying high pressure sintering techniques. Ni addition helped in optimizing the carrier concentration and resulted in a final ZT of 0.55 at 560 K.

Thermoelectric Performance Enhancement of CeFe4Sb12 p-Type Skutterudite by Disorder on the Sb4 Rings Induced by Te Doping and Nanopores

Skutterudites have attracted a lot of attention because of the intrinsic voids in their crystal structure. However, another important structural feature of p-type skutterudites seems to have been ignored, namely the nearly square four-membered antimony rings. To explore the influence of substitution of Te for Sb on the microstructure and thermoelectric properties, a series of p-type skutterudites with composition CeFe4Sb12−xTex with x = 0, 0.1, and 0.2 have been synthesized. The electrical resistivity decreases while the Seebeck coefficient increases with Te doping. In addition, due to disturbance of the Sb4 rings and extra phonon scattering by nanopores, the lattice thermal conductivity is reduced. The thermoelectric figure of merit for the CeFe4Sb11.9Te0.1 compound reaches 0.76 at 773 K, being about 61% higher than that of the CeFe4Sb12 sample.

Rare-earth free p-type filled skutterudites: Mechanisms for low thermal conductivity and effects of Fe/Co ratio on the band structure and charge transport

A key driver of cost for a thermoelectric generator is the thermoelectric materials themselves, and the heavy reliance on rare earth (RE) elements as fillers, particularly for p-type formulations, represents a strategic risk. In the present study, we have investigated fully filled p-type skutterudites with nominal compositions CaFexCo4−xSb12 (2⩽x⩽3.5) using both experimental and theoretical methods. High purity samples were successfully synthesized using a combined melt-spinning (MS) and spark plasma sintering (SPS) technique. Structural analysis confirmed phase-pure samples with high filling fraction (>90%), homogeneous filler and transition metal distribution, and exceptionally low levels of oxygen contamination. Electrical and thermal transport property measurements revealed large power factors and unexpectedly low thermal conductivities. Electronic band structure calculations found a rapid increase in the density of states at the Fermi level with increasing Fe content. The large power factors observed in this system are attributable to this effect. Maximum zT of 0.9 was achieved in CaFe3CoSb12 at 773K. To understand the low lattice thermal conductivity in these compounds, lattice dynamics calculations were performed. The small size of Ca atoms and their concomitantly weak interactions with the surrounding Sb atoms result in a small force constant between fillers and the host, giving rise to a heretofore unreported low frequency optical phonon mode. This low frequency mode at 7meV is in addition to the previously described mode at 17meV identified by modeling the Ca as a simple harmonic oscillator. The lower energy phonon mode imparted by Ca filling results in a comparable lattice thermal conductivity reduction to early lanthanide species such as La.

New bulk p-type skutterudites DD0.7Fe2.7Co1.3Sb12−xXx (X = Ge, Sn) reaching ZT > 1.3

The best p-type skutterudites so far are didymium filled, Fe/Co substituted, Sb-based skutterudites. Substitution at the Sb-sites influences the electronic structure, deforms the Sb4-rings, enhances the scattering of phonons on electrons and impurities and in this way reduces the lattice thermal conductivity.In this paper we study structural and transport properties of p-type skutterudites with the nominal composition DD0.7Fe2.7Co1.3Sb11.7{Ge/Sn}0.3, which were prepared by a rather fast reaction–annealing–melting technique. The Ge-doped sample showed impurities, which did not anneal out completely and even with ZT>1 the result was not satisfying. However, the single-phase Sn-doped sample, DD0.7Fe2.7Co1.3Sb11.8Sn0.2, showed a lower thermal and lattice thermal conductivity than the undoped skutterudite leading to a higher ZT=1.3, hitherto the highest ZT for a p-type skutterudite. Annealing at 570K for 3days proved the stability of the microstructure. After severe plastic deformation (SPD), due to additionally introduced defects, an enhancement of the electrical resistivity was compensated by a significantly lower thermal conductivity and the net effect led to a record high figure of merit: ZT=1.45 at 850K for DD0.7Fe2.7Co1.3Sb11.8Sn0.2.