CoSb3 Compounds Research Articles

Electronic and magnetic properties of CoSb3, Cr-doped CoSb3, and related compound thin films

The electronic, lattice, and magnetic properties of CoSb3, Cr-doped CoSb3, and related compound thin films were calculated by using the total energy pseudopotential method. A vacuum region and slab as a thin film were periodically repeated in the supercell. We considered three slab structures. One is symmetric, another is asymmetric, and the third is unusual. They are free-standing in the supercell. The magnetic moment of doped Cr atom is large in all Cr-doped cases. Magnetic moments of several Co atoms are enhanced in the asymmetric and unusual cases. The calculation results in this study indicate that electronic states of most thin films are ferromagnetic and they are energetically more favorable than nonmagnetic cases.

Solvothermal synthesis of nanosized cobalt antimonide for thermoelectrics

The present paper highlights the synthesis of cobalt antimonide (CoSb3) micro/nanostructures by following solvothermal technique with water as solvent. Recipe is optimized for preparation of refined CoSb3 compounds and demonstrated that a high processing temperature of 500 °C and long duration of 72 hr indicates presence of CoSb3 phase. The microstructures and composition of the as synthesized CoSb3 nanocomposites are characterized to achieve the optimized phase. The morphologies as imaged using field emission scanning electron microscope resemble granules for the as-synthesized CoSb3. The phase purity and crystallographic structure of the as-synthesized CoSb3 nanocomposites as determined by XRD indicates the formation of the cubic phase of CoSb3 and agrees well with the JCPDS data mentioned for the highly pure CoSb3. The EDX estimates the elemental composition of Co and Sb in 1:3 stoichiometric ratio for the as-synthesized CoSb3. The currently prepared nanosized skutterudite CoSb3 material synthesized by solvothermal method could be utilized as active material for the development of highly efficient thermoelectric devices. Copyright © 2018 VBRI Press.

Influence of the order of fabrication sequences on the thermoelectric properties of skutterudite CoSb3–Cu0.6Ni0.4 nanocomposites

Nanocomposite samples of CoSb3-xCu0.6Ni0.4 (x = 1, 3.5, 6, and 9 wt%) were fabricated through two different approaches to understand the influence of fabrication processes on the thermoelectric properties of CoSb3 skutterudite. CoSb3 matrix and Cu0.6Ni0.4 nanoalloy were synthesized separately via hydrothermal procedures. The mixtures of as-hydrothermally synthesized powders were prepared and annealed in the first method, while in the second fabrication route, CoSb3 powders were initially annealed, followed by mixing with Cu0.6Ni0.4 and annealing of the composite at the same conditions. The results showed improved electrical conductivity of CoSb3 compounds, which is consistent with the systematic increase of the carrier concentration upon increasing the Cu0.6Ni0.4 content. The grain boundary potential barrier mechanism was used to describe the influence of Cu0.6Ni0.4 nanoalloy on the height of the energy barrier of CoSb3-xCu0.6Ni0.4, as compared with that of pristine CoSb3 synthesized using the similar hydrothermal method. The onset of bipolar thermopower reduction is shifted to higher temperatures; however, the room temperature thermopower is reduced upon increasing the content of Cu0.6Ni0.4 nanoinclusions. The sample containing 1 wt% of Cu0.6Ni0.4 fabricated through the first fabrication procedure showed the highest zT value at ∼550 K.

Thermoelectric properties of co-sputtered CoSb3 thin films as a function of stoichiometry

The skutterudite compound CoSb3 exhibits excellent thermoelectric properties for thermoelectric application. The composition plays an important role in affecting the microstructural and thermoelectric behavior of CoSb3, especially in the form of thin film. In this study, the Co-excess and Sb-excess CoSb3 thin films were prepared by radio frequency and direct magnetron co-sputtering technique. It was found that the thin film with excess Co exhibits a significant n-type conduction behavior and transforms to p-type conduction behavior with Sb rich. The X-ray diffraction patterns show that either the Co-excess or Sb-excess samples contain skutterudite CoSb3 and a secondary phase CoSb2. Additionally, Sb can be found in the Sb-excess samples, yet no Co can be observed in the Co-excess samples.

Electronic structures of Si- and Te-doped CoSb3 compounds under high pressures

The electronic and lattice properties of various Si- and Te-doped CoSb3 compounds under hydrostatic compression conditions were calculated by using the total energy pseudopotential method. The calculated compositions of Si- and Te-doped CoSb3 compounds are Co8Sb22Si2, Co8Sb22Te2, Co8Sb21Si2Te1, and Co8Sb21Si1Te2. The applied pressure P values are 0 (ambient), 10, and 100 GPa. The densities of states (DOSs) of Si- and Te-doped CoSb3 compounds under P = 0, 10, and 100 GPa are investigated to compare them with each other. Their DOS shapes and Fermi level positions vary under pressure. In particular, the variations in DOS shape between P = 10 and 100 GPa in Co8Sb21Si2Te1 and Co8Sb21Si1Te2 are markedly large, although they are relatively small between P = 0 and 10 GPa. The gap states of Co8Sb21Si2Te1 and Co8Sb21Si1Te2 around the Fermi level disappear under P = 100 GPa. Seebeck coefficients do not increase under high pressures.

Multi-localization transport behaviour in bulk thermoelectric materials.

Simultaneously optimizing electrical and thermal transport properties of bulk thermoelectric materials remains a key challenge due to the conflicting combination of material traits. Here, we have explored the electrical and thermal transport features of In-filled CoSb3 through X-ray absorption fine structure, X-ray photoemission spectra, transport measurement and theoretical calculation. The results provide evidence of three types of coexisting multi-localization transport behaviours in the material; these are heat-carrying phonon-localized resonant scattering, accelerated electron movement and increase in density of states near the Fermi level. The 5p-orbital hybridization between In and Sb is discovered in the In-filled CoSb3 compound, which results in a charge transfer from Sb to In and the enhancement of p–d orbital hybridization between Co and Sb. Our work demonstrates that the electrical and thermal properties of filled skutterudite bulk thermoelectric materials can be simultaneously optimized through the three types of coexisting multi-localization transport behaviours in an independent way.

Fabrication and characterization of nanostructured thermoelectric FexCo1-xSb3

Abstract A novel synthesis route for the fabrication of p-type nanostructured skutterudite, FexCo1-xSb3 in large quantity is reported. This scalable synthesis route provides nano-engineered material with less impact on the environment compared to conventional synthesis procedures. Several Fe substituted compositions have been synthesized to confirm the feasibility of the process. The process consists of a nano-sized precursor fabrication of iron and cobalt oxalate, and antimony oxides by chemical co-precipitation. Further thermochemical processes result in the formation of iron substituted skutterudites. The nanopowders are compacted by Spark Plasma Sintering (SPS) technique in order to maintain nanostructure. Detailed physicochemical as well as thermoelectric transport properties are evaluated. Results reveal strongly reduced thermal conductivity values compared to conventionally prepared counterparts, due to nanostructuring. P-type characteristic was observed from the Seebeck measurements while electrical conductivity is high and shows metallic behavior. The highest TE figure of merit of 0.25 at 800 K has been achieved, which is strongly enhanced with respect to the mother compound CoSb3. This suggests the promise of the utilized method of fabrication and processing for TE applications with improved performance.

Fabrication, spark plasma consolidation, and thermoelectric evaluation of nanostructured CoSb3

Nanostructured powders of thermoelectric (TE) CoSb3 compounds were synthesized using a chemical alloying method. This method involved co-precipitation of oxalate precursors in aqueous solution with controlled pH, followed by thermochemical treatments including calcination and reduction to produce stoichiometric nanostructured CoSb3. Moreover, CoSb3 nanoparticles were consolidated by spark plasma sintering (SPS) with a very brief processing time. Very high compaction densities (>95%) were achieved and the grain growth was almost negligible during consolidation. An iterative procedure was developed to maintain pre-consolidation particle size and to compensate Sb evaporation during reduction. Significant changes in particle size and morphology were observed, and the post-reduction cooling was found to be an important stage in the process. The spark plasma sintering (SPS) parameters were optimized to minimize the grain growth while achieving sufficient densification. Grain sizes in the range of 500nm to 1μm, with compaction density of 95–98% were obtained. Preliminary measurements of thermal diffusivity and conductivity showed the dependence on grain size as well as on porosity. TE transport properties were measured in the temperature range of 300–650K. Sample showed p-type behavior with a positive Seebeck coefficient, which increases with increasing temperature. Electrical conductivity measurements indicate metallic behavior and it decreases with increasing temperature. Thermal conductivity also decreases with increasing temperature and major contribution is due to the lattice component. A TE figure of merit of 0.15 was achieved for high purity CoSb3 nanostructured TE material at 650K and these results are comparable with the values reported for the best unfilled/undoped CoSb3 in the literature.

Thermoelectric Performance of Multiple-Doped Co4Sb12−x−y−z Ge x Te y S z Skutterudite Compounds

CoSb3 skutterudites multiply doped with Ge, Te, and S were synthesized by solid-state reaction and spark plasma sintering. x-Ray diffraction studies revealed that Ge, Te, and S entered the lattice of the CoSb3 compounds, and while Te increased the lattice volume, Ge and S decreased it. Compared with the undoped and single-doped CoSb3 compounds, the thermal conductivity and lattice thermal conductivity are significantly suppressed due to greatly increased point defect scattering. It is found that S is more effective for decreasing the lattice thermal conductivity than Te and Ge. The highest thermoelectric figure of merit, ZT, exceeds 1.1 for the Co4Sb11.25Ge0.05Te0.63S0.07 compound at 800 K.

Ab initio calculations of structural, optical and thermoelectric properties for CoSb3 and ACo4Sb12 (A = La, Tl and Y) compounds

We present the structural, optical and thermoelectric properties of unfilled and filled CoSb3. The full potential linearized augmented plane wave (FP-LAPW) method within the generalized gradient approximation (GGA) is used to investigate the optoelectronic properties and for thermoelectric properties we have used the semi-classical Boltzmann transport theory. The calculated fundamental band gap of unfilled CoSb3 at Γ symmetry point is 0.036eV. Furthermore, filling of CoSb3 by A (A=La, Tl and Y) strongly affects the optoelectronic properties and carrier concentration of this compound. The bulk modulus of the unfilled CoSb3 is smaller than that of filled CoSb3 with Tl and Y. The results of study predict that the filling of Tl effectively enhances the efficiency of the thermoelectric properties of CoSb3.

Microwave-assisted synthesis and thermoelelectric properties of CoSb3 compounds

Using CoCl2 and SbCl3 as precursors and NaBH4 as reducing agent, we obtained nano CoSb3 particles of about 10 nm with the assistance of microwave radiation. Different Sb/Co ratio and different microwave radiation time were used to prepare CoSb3. The results show that single CoSb3 phase was synthesized successfully under the condition of Sb/Co ratio with 5:1 and microwave radiation time with 5 min. The samples with microstructure and nanostructure were synthesized respectively by cold isostatic pressing and sintering method, and their thermoelectric properties were studied. Due to its powder preparation method and its small grain size, the sample with nanostructure has lower electrical resistivity and thermal conductivity than the sample with microstructure. The maximum ZT value was found to be 0.11 at 650 K in the sample with nanostructure, which is about 10 times higher than that of the sample with microstructure.

Realization of high thermoelectric performance in n-type partially filled skutterudites

Abstract

Thermoelectric Properties of Carbon Nanotubes Added n-type CoSb3 Compound

ABSTRACTCobalt triantimonide compounds are well known as materials with good thermoelectric properties over temperature range of 550-900 K. For further improving thermoelectric performance, reduction of thermal conductivity is required. In this study, we attempted to disperse carbon nanotubes (CNTs) homogeneously into the n-type CoSb3 compound for lowering lattice thermal conductivity by the phonon scattering. Powders of Co, Ni, Sb and Te were blended with molar ratios of n-type Co0.92Ni0.08Sb2.96Te0.04 compound, and the compound was synthesized through a pulse discharge sintering (PDS) process. After coarsely grinding the synthesized compound, CNTs were mixed with the compound powder at different mass% (0, 0.01, 0.05 and 0.1 mass%). Then, the mixture was mechanically ground with a planetary ball milling equipment. The ground composite powder was compacted and sintered by PDS. Thermoelectric properties (Seebeck coefficient, electrical resistivity and thermal conductivity) of the sintered samples were measured. It was confirmed that the fibrous CNTs existed homogeneously in the compound matrix. The absolute value of Seebeck coefficient slightly decreased with increase of CNT content. The minimum thermal conductivity was obtained at addition of 0.01mass%CNT, and the electrical resistivity was a little increased with CNT content. The maximum ZT of 0.98 was achieved at 853 K in the 0.01mass%CNT-added sample.

Thermoelectric Properties of the Co-doped n-type CoSb3 Compound

Thermoelectric Properties of the Co-doped n-type CoSb3 Compound

Thermoelectric properties of n-type Sr x M y Co4Sb12 (M=Yb, Ba) double-filled skutterudites

Alkaline-earth (AE) and rare-earth (RE) atoms are usually used as void fillers in the caged compound CoSb3 to improve the thermoelectric performance of the filled system. Polycrystalline single-filled Sr0.21Co4Sb12, double-filled SrxYbyCo4Sb12, and SrxBayCo4Sb12 skutterudites have been synthesized. Rietveld structure refinement confirms that both Sr and Yb occupy the Sb-icosaedron voids in skutterudite frame work. In this paper, we report the high-temperature thermoelectric properties including electrical conductivity, Seebeck coefficient, and thermal conductivity. Double filling of the Sr–Yb combinations shows a stronger suppression on lattice thermal conductivity than that of Sr–Ba combination. Furthermore, the double-filled SrxYbyCo4Sb12 skutterudites exhibit a much higher power factor than the Sr-filled system. The maximum power factor for Sr0.22Yb0.03Co4Sb12.12 reaches 41 μW cm−1 K−2 at room temperature and 57.5 μW cm−1 K−2 at 850 K, respectively. The enhanced thermoelectric figures of merit are 1.32 for SrxYbyCo4Sb12 and 1.22 for SrxBayCo4Sb12 at 850 K, respectively.

パルス放電焼結法によって作製したn型CoSb<SUB>3</SUB>化合物の熱電特性に及ぼすアルミナナノ粒子添加の影響

Thermoelectric power generation is a promising green energy system recovering waste heat. CoSb3 skutterudite compound is a candidate material for thermoelectric power generation. For improving thermoelectric performance of the material, it is necessary to lower the thermal conductivity. In this work, Al2O3 nanoparticles were added and dispersed by mechanical grinding (MG) into n-type CoSb3 compound prepared by pulse discharge sintering (PDS) method. Over long MG treatment brought high electrical resistivity, lowering thermoelectric performance of the Al2O3 nanoparticles added CoSb3 compound. Seebeck coefficient almost did not change regardless of additional amount of Al2O3 nanoparticles. Increasing additional amount of Al2O3 nanoparticles brought increasing electrical resistivity and decreasing thermal conductivity. It is found that there is an optimal additional amount of Al2O3 nanoparticles that leads to improvement of thermoelectric performance of the CoSb3 compound. The maximum thermoelectric performance (ZT=0.58) was achieved at 684 K in the CoSb3 compound including Al2O3 nanoparticles of 0.1 mass%.

Electronic structure and transport coefficients of binary skutterudite antimonide

Using first-principles calculations, we investigate the electronic structure of CoSb3 compound by considering the spin–orbit interaction. Within the framework of Boltzmann theory, the transport coefficient (power factor) is evaluated as a function of chemical potential assuming a rigid-band picture and constant relaxation time. It is found that appropriate n-type doping in the compound may be better than p-type doping to enhance the power factor. Our theoretical calculations give a plausible guide on how to optimize the thermoelectric performance of this compound, and the upper limit of its ZT value is estimated.

Syntheses of Ni-doped and Fe-doped CoSb3 Thermoelectric Nanoparticles through Modified Polyol Process

AbstractSkutterudite CoSb3 compounds are of increasing interest as materials with good thermoelectric performance over the temperature range of 600 to 800 K, but the thermal conductivity of the materials is relatively high. Nanostructured materials have been shown to enhance phonon scattering and lower the thermal conductivity of the thermoelectric materials. Partial substitution of Ni or Fe on the Co site of CoSb3 is a hopeful route for improving thermoelectric performance of the CoSb3 compounds. In the present work, synthesis of Ni-doped and Fe-doped CoSb3 nanoparticles through the modified polyol process was attempted and the optimum synthesizing condition was investigated. Co(OOCH3)2·4H2O, Ni(OOCH3)2·4H2O, FeCl3·6H2O and SbCl3, were prepared as precursors. The precursors were reduced by NaBH4 in tetraethyleneglycol at 513 K in an argon atmosphere, for different reaction times (holding times). The reaction products were characterized by the X-ray diffraction, the energy dispersive X-ray spectroscopy, and transmission electron microscopy. The nanoparticles with about 20 to 30 nm in size mainly existed in the reaction products regardless of the chemical composition and the reaction time. The skutterudite phase was identified as a main phase in the sample synthesized for long reaction time, but the other phases of Sb and MSb2 (M=Co, Ni, Fe) were also detected. The lattice parameter of the synthesized skutterudite phase linearly increased with increasing the doping agent concentration, following Vegard’s law.

Electrical transport and thermoelectric properties of double filled compounds Ca0.1CexCo4Sb12 at low temperatures

The electrical transport and thermoelectric properties of CoSb3 and double filled compounds Ca0.1CexCo4Sb12 (x=0.05, 0.1, and 0.15) were investigated in the temperature range from 5 to 300 K. The results indicated that double filling of Ca and Ce has a strong influence on the temperature dependence of the electrical resistivity. For samples CoSb3 and Ca0.1Ce0.05Co4Sb12, the relationship of ln ρ∝1/T existed only in the temperature regimes of 100–200 and 160–300 K, respectively, while Mott’s law ln ρ∝T−1/4 was obeyed at lower temperatures below 100 and 160 K for CoSb3 and Ca0.1Ce0.05Co4Sb12, respectively. The results suggested a localization of charge carriers. Moreover, a transition from the semiconducting state (i.e., dρ/dT<0) (x=0.05) to the metallic state (i.e., dρ/dT>0) (0.05<x≤0.15) took place owing to heavy filling. Due to the increase in carrier concentration, the electrical resistivity for Ca0.1CexCo4Sb12 (x>0.5) was observed to be smaller than that of CoSb3. However, the increase in carrier concentration did not affect the absolute Seebeck coefficient values. In fact, a large absolute Seebeck coefficient was maintained, which was likely due to the large effective mass arising from the filler atoms. Moreover, the low-temperature lattice thermal conductivity of Ca0.1CexCo4Sb12 was observed to decrease monotonously with increasing Ce content, which was attributed to the rattling motion of the Ca and Ce atoms in the Sb cages. As a result, the figure of merit, ZT, of heavily filled compounds Ca0.1CexCo4Sb12 (x=0.1 and 0.15) improved over the whole temperature range investigated. In particular, ZT of Ca0.1Ce0.15Co4Sb12 was tenfold larger than that of CoSb3 at 300 K. The current results showed that double filling Ca and Ce is an efficient way in improving the thermoelectric properties.

Modeling of skutterudite CoSb3 with molecular dynamics method

Abstract Skutterudite materials have generated considerable interest as new thermoelectric materials over the past few years. For binary skutterudite compound CoSb3 with complicated cubic crystal structure, Morse potential energy function is employed to describe atomic interactions to lay special emphasis on its mechanical properties. The parameters of Morse potential function between different species of atom pairs (Co–Co, Sb–Sb and Co–Sb) are individually determined based on known crystal structures and elastic properties from experiment or first principle calculation results. To test the accuracy of the obtained potential parameters, molecular dynamics simulation was first performed to inverse deduce and compare with preceding used values, and the results show excellent agreement. Moreover, the practicability and feasibility of the potential was verified. In terms of the obtained potential parameters, the virtual uniaxial tensile simulation was performed for single crystal CoSb3 with bulk model using the conventional molecular dynamics algorithm. The mechanical response and deformation behavior are carefully analyzed. In contrast to conventional bulk CoSb3, single crystal bulk CoSb3 exhibits much better mechanical performances. It undergoes elastic deformation before 10% strain, and achieves ultimate stress 13.3 GPa at the strain of 10.2%, subsequently sudden fracture occurs near the middle of the model, demonstrating typical brittleness.