Unfilled Skutterudite Research Articles

Structure and thermoelectric properties of Se- and Se/Te-doped CoSb3 skutterudites synthesized by high-pressure technique

Se- and Te/Se-doped n-type CoSb3 skutterudites were synthesized by high-pressure synthesis (HPS) followed by spark plasma sintering. Se and Te substitutions on the Sb sites were verified by X-ray powder diffraction, energy dispersive spectrometry, and Raman spectroscopy. The experimental determined solubility of Se in CoSb3, was significantly enhanced by HPS technique, and reached a large value of 3%. Se doping shows a greater impact on the Seebeck coefficient than on the electrical resistivity because of the combined effects of the carrier transport performance, grain size, and pores. Meanwhile, the thermal conductivity was significantly suppressed after Se doping. The thermal conductivity of CoSb2.7Se0.3 is below 1.83 W/mK and is the lowest value for unfilled skutterudites to date. Extra doping of Te significantly increased the concentration and weighted mobility of the charge carrier, leading to an enhanced power factor. Moreover, the thermal conductivity was further reduced by Te co-doping due to the strong distortion of Sb4-ring in the framework of skutterudite. The highest ZT value of 1.29 is achieved at 780 K for CoSb2.8Te0.15Se0.05 with an appropriate Te/Se co-doping, which is the best ZT value for unfilled n-type skutterudites to the best of our knowledge. HPS is therefore a good choice to synthesize elemental filled and substituted (for Sb sites) skutterudites with increasing filling and doping levels for improving TE performance.

Electronic transport properties of pnictogen-substituted skutterudites with alkaline-earth fillers

ABSTRACTThe materials class of skutterudites is one of the promising thermoelectric materials due to its decent electronic properties and cage-like structural feature that can be filled with guest atoms. First principles calculations have been performed in order to investigate electronic band structures and related transport properties of pnictogen-substitued skutterudites filled with alkaline-earth elements (MxCo4A6Te6 where M=Ca, Sr, or Ba, A=Ge or Sn, and x=0.5 or 1). The Seebeck coefficient and the power factor, which are electronic transport properties related to thermoelectricity, are computed by using the Boltzmann transport formalism within the constant-relaxation-time-approximation. The results are compared against the corresponding properties of the unfilled pnictogen-substitued ternary skutterudites (CoA1.5Te1.5) to identify the effects of filling, based on which the potential of filled pnictogen-substituted skutterudites for thermoelectric applications is evaluated. The possible changes in the ionic character of the interatomic bonding, which was suspected to be an important scattering source, are probed by analyzing the projected density of states.

Realization of high thermoelectric performance in p-type unfilled ternary skutterudites FeSb2+xTe1−x via band structure modification and significant point defect scattering

FeSb2Te, a ternary derivative of binary CoSb3, displays anomalous electrical and thermal transport properties because of considerable modifications in the band structure induced by Fe and significant mixed valence state (namely Fe2+ and Fe3+) scattering of phonons. The substitution of Te for Sb generates more holes without notably affecting the band structure, while markedly improving the electrical conductivity and retaining a high Seebeck coefficient due to the enhanced density of states, thereby leading to dramatically increased power factors. Furthermore, the heat carrying phonons are strongly scattered with increasing x value because of the formation of solid solutions between two end members: □FeSb2Te and □FeSb3 (where □ can be viewed as a vacancy). Consequently, high thermoelectric figures of merit were achieved in the FeSb2+xTe1−x compounds, with the largest ZT value reaching ∼0.65 for the sample with x=0.2. This is the highest value among all p-type unfilled skutterudites and is comparable with some filled compositions. Prospects for further improving the performance of p-type FeSb2Te-based skutterudites are discussed.

Structural Instability of Unfilled Skutterudite CompoundsMX3(M= Co, Rh, and Ir;X= As and Sb) under High Pressure

Powder X-ray diffraction patterns of the binary unfilled skutterudite compounds MX 3 ( M = Co, Rh, and Ir; X = As and Sb) have been studied under high pressures up to 60 GPa, using synchrotron radiation and a diamond anvil cell. The X-ray diffraction patterns of MX 3 above a critical pressure show anomalous reductions in the intensities of several Bragg peaks. Furthermore, the pressure-volume curves of MX 3 reveal a saturation behavior of the volume reduction with pressure. Upon pressure release, MX 3 exhibit a volume greater than those of pristine compounds. This anomalous behavior could be interpreted as a pressure-induced self-insertion reaction. Furthermore, the high-temperature and high-pressure phase diagram of RhSb 3 was obtained. The structural instability of MX 3 under high pressures is discussed.

Thermoelectric properties of YbxCo4Sb12 system

YbxCo4Sb12 polycrystals were fabricated by vacuum melting combined with hot-press sintering. The effect of Yb-filling on thermoelectric property of unfilled skutterudite CoSb3 was investigated, which indicated the enhancement of the power factor of the material. Transport properties of materials changed from semi-conductor to semi-metal during the measurement of electrical conductivity, which indicated the change of electronic band structure. The maximum value of electrical conductivity was about 190000 S/m at 300 K for all samples. On the basis of Yb-filling, power factor of Yb0.2Co4Sb12 reached 5–6 mW/(mK) during the measurement temperature. Thermal conductivity decreased with increase of Yb content, and the thermal conductivity of Yb0.2Co4Sb12 reached 3.2 W/(mK) at 600 K. The ZT value of Yb0.2Co4Sb12 reached 1.16 at 700 K due to positive contribution from high power factor and low thermal conductivity.

Enhanced thermoelectric properties of Co1−x−y Ni x+y Sb3−x Sn x materials

Co1−x−y Nix+y Sb3−x Sn x polycrystals were fabricated by vacuum melting combined with hot-press sintering. The effect of alloying on the thermoelectric properties of unfilled skutterudite Co1−x Ni x Sb3−x Sn x was investigated. A leap of electrical conductivity from the Co0.93Ni0.07Sb2.93Sn0.07 sample to the Co0.88Ni0.12Sb2.88Sn0.12 sample occurs during the measurement of electrical conductivity, indicating the adjustment of band structure by proper alloying. The results show that alloying enhances the power factor of the materials. On the basis of alloying, the thermoelectric properties of Co0.88Ni0.12Sb2.88Sn0.12 are improved by Ni-doping. The thermal conductivities of Ni-doping samples have no reduction, but their power factors have obvious enhancement. The power factor of Co0.81Ni0.19Sb2.88Sn0.12 reaches 3.0 mW·m−1·K−2 by Ni doping. The dimensionless thermoelectric figure of merit reaches 0.55 at 773 K for the unfilled Co0.81Ni0.19 Sb2.88Sn0.12.

Thermoelectric and structural properties of high-performance In-based skutterudites for high-temperature energy recovery

Abstract

Physical Properties of Unfilled Skutterudite NiP3

We succeeded to prepare large single crystals. We present the electrical resistivity, the magnetic susceptibility and the low-temperature specific heat measurements of unfilled skutterudite NiP 3 . The temperature dependence of the electrical resistivity of NiP 3 is metallic and the residual resistivity ratio, RRR, is estimated to be about 30. No superconductivity was found down to 0.078 K. The magnetic susceptibility shows weakly temperature dependent Pauli paramagnetism. The electronic specific-heat coefficient was determined to be 8.0 mJ/mole·K 2 .

High-Pressure X-ray Diffraction Study of Unfilled Skutterudite Compound RhAs3

We have studied the synchrotron powder x-ray diffraction of the unfilled skutterudite compound RhAs 3 under high pressure up to around 55 GPa. The x-ray diffraction pattern of RhAs 3 above 35 GPa shows anomalous reduction of the intensity for several Bragg peaks. Further, the pressure–volume curve of RhAs 3 reveals a saturation behavior of the volume reduction with pressure. Similar phenomena have been observed for M Sb 3 ( M = Co, Rh, and Ir). This anomalous behavior could be interpreted as a pressure-induced self-insertion reaction.

Structural instability of unfilled skutterudite compounds TSb3 (T=Co, Rh and Ir) under high pressure

Filled skutterudite compounds attracted many researchers' interest as strongly correlated electron systems. The skutterudite family includes binary, unfilled skutterudite compounds. We have studied the powder x-ray diffraction of unfilled skutterudite compounds TSb3 (T=Rh and Ir) under high pressure up to around 40 GPa, using synchrotron radiation. Pressure-induced irreversible structural changes of TSb3 (T=Rh and Ir) have been observed. Structural instability of the unfilled skutterudite compounds TSb3 (T=Co, Rh and Ir) under high pressure is discussed.

Synchrotron X-ray study of filled skutterudites CeFe 4Sb 12 and Ce 0.8Fe 3CoSb 12

In situ synchrotron X-ray diffraction measurements are carried out on filled skutterudites CeFe 4Sb 12 and Ce 0.8Fe 3CoSb 12 up to 32 and 20 GPa, respectively, at room temperature. No phase transformation was observed for both samples in the pressure range. Fitting the pressure–volume data (up to 10 GPa) to the third-order Birch–Murnaghan equation of state, the bulk modulus B 0 is determined to be 74(4) GPa, with the pressure derivative B′ 0=7(2) for CeFe 4Sb 12, and B 0=71(2) GPa and B′ 0=8(2) for Ce 0.8Fe 3CoSb 12. The bulk moduli of filled skutterudites CeFe 4Sb 12 and Ce 0.8Fe 3CoSb 12 in our study are smaller than those from previous studies on unfilled skutterudite CoSb 3. The P–V curves of the unfilled skutterudite CoSb 3 and filled skutterudites Ce y Fe 4− x Co x Sb 12 showed good agreement, indicating that the Ce filling fraction and the replacement of Fe with Co have little effect on their compression behaviors.

ChemInform Abstract: High-Pressure Stability, Pressure-Volume Equation of State, and Crystal Structure under Pressure of the Thermoelectric Material IrSb3.

There is considerable current interest in filled and unfilled skutterudites because of their potential applications as thermoelectric materials. We have compressed the unfilled skutterudite IrSb3 to pressures of 42 GPa and find that it is surprisingly stable in view of the presence of large cavities in the structure. This has implications for the both the implantation of atomic or molecular species within the cavities by means of pressure and the stability of IrSb3 under hot isostatic pressing conditions. From a fit to the pressure−volume equation of state, values were obtained for the bulk modulus and pressure derivative of the bulk modulus of 136 ± 5 GPa and 4.8 ± 0.5, respectively. Rietveld refinement of the crystal structure at high-pressure further demonstrates the stability of the cavities under compression.

Lattice thermal conductivity of skutterudite compounds

A generalized expression is used on the basis of relaxation time approximation to facilitate calculation of lattice thermal conductivity of dielectric materials as well as skutterudite family consists of compounds of the form AB3. It is assumed that phonon scattering processes are independent and is represented by frequency dependent relaxation times. The contributions of normal three phonon scattering processes are included explicitly as redistribution of phonon momentum between two oscillation branches is considered. Magnitudes of relaxation times are estimated from the experimental data. The result for CoSb3 is in reasonably good agreement with the experimental result in the temperature range 1–1000°K. It is observed that redistribution of phonon momentum between two oscillation branches leads to a significant suppression of thermal conductivity maximum and it is observed that for unfilled skutterudite the main dominant mechanism at the thermal conductivity maximum is three phonon normal scattering process.

ON THE SKUTTERUDITE Pt4Sn4.4Sb7.6

Pt 4 Sn 4.4 Sb 7.6 ( a = 9.3304(2) Å, space group [Formula: see text] ) is an unfilled skutterudite member of the solid solution Sn x Pt 4 Sn y Sb 12- x . In contrast to Sn x Ni 4 Sn y Sb 12- y and Sn x Pt 4 Sn y Sb 12- x no Sn could be found in the 2 a sites of the Sn / Sb framework. Physical properties have been data for Ni 4 Sn 3 Sb 9 and Ni 4 Sn 3.5 Sb 8.5. Resistivity measurements for Pt 4 Sn 4.4 Sb 7.6 reveal a temperature dependent crossover from metallic to semiconducting behaviour connected with a corresponding change from negative to positive thermopower at 30 K. Vicker's hardness, thermal expansion and elastic moduli compare well with values of other Sb - investigated in the temperature range from 4.2 K to 800 K and have been compared with based skutterudites.

Systematic Study of Lattice Specific Heat of Filled Skutterudites

The lattice specific heat C lat of La-based filled skutterudites La T 4 X 12 ( T = Fe, Ru and Os; X = P, As, and Sb) has been systematically studied, and both the Debye temperature Θ D and the Einstein temperature Θ E of La T 4 X 12 were carefully estimated. We confirmed that a correlation exists between Θ D and the reciprocal of the square root of average atomic mass for La T 4 P 12 , La T 4 As 12 , and La T 4 Sb 12 . The Θ D of filled skutterudites was found to depend mainly on the nature of the species X forming the cage. The temperature dependence of C lat / T 3 for La T 4 X 12 exhibited a large broad maximum at low temperatures (10–30 K), which suggests a nearly dispersionless low-energy optical mode characterized by Einstein specific heat. Since no such broad maximum exists for the unfilled skutterudite RhP 3 , the low-energy optical modes are associated with vibration involving La ions in the X 12 cage (the so-called “guest ion modes”). The Θ E of filled skutterudites was found to roughly correspon...

The g -value ofEr3+doped unfilled skutteruditeCoSb3(Th) reveals the existence of an additional sixth order term in the crystal field Hamiltonian

Abstract Electron spin resonance (ESR) experiments have been carried out in single crystals of the unfilled skutterudite CoSb 3 doped with Er ions. The X- (9.5 GHz) and Q- (34.4 GHz) band spectra obtained at low temperature (4–20 K) shown a temperature independent g-value of 6.21(5). This g-value can only be explained with the addition of a second sixth order B 6 t ( O 6 2 - O 6 6 ) term to the usual cubic crystal field Hamiltonian. The ESR of Er 3 + show the typical temperature dependence of the line-shape and line-width expected for insulating host.

MmFe 4Sb 12- and CoSb 3-based nano-skutterudites prepared by ball milling: Kinetics of formation and transport properties

Experiments of mechanical alloying/milling of filled and unfilled skutterudites have been performed under various ball milling conditions in order to (a) optimise the preparation of nano-sized skutterudites and (b) to elucidate formation and/or decomposition of unfilled (CoSb 3) and filled skutterudites Mm y Fe 4Sb 12 (Mm means ‘mischmetal’). State-of-the-art X-ray techniques have been used to evaluate the size distribution of the smallest crystallographically undisturbed regions (coherent-scattering-domains), which in many cases are significantly smaller than the physical size of grains. The influence of oxides in in situ precipitations on the stability of the nanostructures at 600 °C was investigated and compared with oxide-free nano-sized skutterudites. It was shown that at 600 °C the crystallites of nano-sized CoSb 3 grow rapidly reaching micro-size after 90 h, whilst in situ oxide stabilized nanostructures MmFe 4Sb 12 with crystallite size below 200 nm do not show coagulation even after 600 h of heat treatment at 600 °C. The nano-sized oxide composite has a significantly lower lattice thermal conductivity resulting in an improvement of the thermoelectric figure of merit ZT 740 K = 0.52 being about 20% higher than for oxide-free macro-crystalline Mm y Fe 4Sb 12 as reference. The composition of nano-composite was optimised for large-scale production in oxidizing atmosphere. Attempts to evaluate the dislocation density for ball-milled samples indicate that it is only slightly above the lower limit of resolution of the method of about 10 12 m −2.

New filled P-based skutterudites—promising materials for thermoelectricity?

The crystal structure, thermodynamic stability and electronic structure of the 75 filled and unfilled skutterudite compounds MxCo4- yFeyP12, with M being La, Y and Sc, x = {0, 0.125, 0.25, 0.50, 1} and y={0, 1, 2, 3, 4} are predicted from periodic density functional calculations. The rattling amplitude of the filling atom M, which is important for the thermal conductivity of the compounds, is found to depend mostly on the type of filling element and only to a small extent on the filling fraction, or the fraction of Fe in the structure. The calculated ground state stabilities show that none of the Sc-filled skutterudites are thermodynamically stable at 0 K, while the only stable combination of x and y among the studied Y-filled ones is YCoFe3P12. The La-filled skutterudites, on the other hand, are thermodynamically stable in a variety of combinations, with maximum stability when y=3x, that is three Fe atoms per filling atom. Based on the calculated electronic structure we also expect filled skutterudites with y=3x to be most interesting for thermoelectric applications.

Heavy Fermion Behaviors of Tm2Rh12P7

We present the magnetic susceptibility, the electrical resistivity and the low-temperature specific heat of Tm 2 Rh 12 P 7 . This compound crystallizes in the hexagonal Ho 2 Rh 12 As 7 -type structure. Rare-earth ions in this structure occupy voids of binary compound Rh 12 As 7 . This situation has a great resemblance to the relation between filled skutterudites and unfilled skutterudites. The magnetic susceptibility of Tm 2 Rh 12 P 7 follows a Curie–Weiss law down to 2 K, but there is a very small kink at 3.1 K. The specific heat shows a peak at 3.1 K, indicating a phase transition. C/T extrapolating to 0 K has a very large value of about 0.4 J/mol-Tm·K 2 . his result suggests that Tm 2 Rh 12 P 7 is the first Tm-based heavy fermion compound.

Study on rattling of filling atom in Sm filled-skutterudite compounds

The single-phase polycrystalline Sm-filled skutterudites SmyFexCo4-xSb12 were investigated by Rietveld refinement and Raman scattering spectral analysis. The results of Rietveld refinement clarify that the SmyFexCo4-xSb12 compound has a filled skutterudite structure. The thermal parameter of Sm is larger than that of Sb, Fe and Co, and the Sb-Sb bond length of SmyFexCo4-xSb12 is longer than that of unfilled skutterudites. The results of Raman scattering study show a shift and broadening of the Sb4 ring breathing modes. These results indicate that Sm atoms fill the Sb-icosahedron voids of the skutterudites and rattle in them.