Filled Skutterudite System Research Articles

Unraveling the structural and thermoelectric properties of the Sn-doped filled skutterudite Smy(FexNi1-x)Sb11.5Sn0.5 (y = 0.17–0.34, x = 0.43–0.64)

A systematic study of the filled skutterudite system Smy(FexNi1-x)4Sb11.5Sn0.5 was carried out with the aim of investigating the effect of the partial substitution of Sb by Sn on the structural and thermoelectric properties of the material. The presence of Sn induces a shift of the p/n crossover toward lower values of x compared to the corresponding Sn-free system, as a consequence of the smaller number of electrons supplied. Moreover, a discontinuity at the p/n crossover is observed in the cell parameter and related structural features. The thermoelectric properties suggest lower thermal conductivity values in comparison to similar Sn-free skutterudite systems, resulting in higher ZT. This result highlights the significant role of Sn in creating new scattering centers able to affect the phonon transmission through the crystal lattice.

Effect of the annealing treatment on structural and transport properties of thermoelectric Sm y (Fe x Ni1−x )4Sb12 thin films

The crystallographic and transport properties of thin films fabricated by pulsed laser deposition and belonging to the Sm y (Fe x Ni1-x )4Sb12 filled skutterudite system were studied with the aim to unveil the effect exerted by temperature and duration of thermal treatments on structural and thermoelectric features. The importance of annealing treatments in Ar atmosphere up to 523 K was recognized, and the thermal treatment performed at 473 K for 3 h was selected as the most effective in improving the material properties. With respect to the corresponding bulk compositions, a significant enhancement in phase purity, as well as an increase in electrical conductivity and a drop in room temperature thermal conductivity, were observed in annealed films. The low thermal conductivity, in particular, can be deemed as deriving from the reduced dimensionality and the consequent substrate/film interfacial stress, coupled with the nanometric grain size.

Compositional Optimization and Structural Properties of the Filled Skutterudite Smy(FexNi1−x)4Sb11.5Sn0.5

A compositional and crystallographic study was carried out on the Smy(FexNi1−x)4Sb11.5Sn0.5 filled skutterudite system (0.40 ≤ x ≤ 0.80) with the aim to determine the equilibrium Sm filling fraction (y) within the considered x range. The relevance of the material lies in its potential thermoelectric properties: in analogy with similar skutterudites systems, these features should in fact result as being improved with respect to the ones of the corresponding Sn-free system thanks to the partial substitution of Sn for Sb, which is expected to lower the phonon thermal conductivity. The results of Rietveld refinements allowed us to study the skutterudite structural properties and to discuss them, adopting a comparative approach with respect to the ones of the Sn-free system Smy(FexNi1−x)4Sb12. Relying on the refined Sm occupancy factors, the p/n crossover is shown to be located at x ~ 0.53, meaning that the introduction of Sn induces an enlargement of the p-region; moreover, at variance with the Sn-free system, the coefficient of thermal expansion does not show any significant mismatch between n- and p-compositions, which should ensure a prolonged lifetime of a device made of n- and p-legs that both derive from the studied system.

Multiband superconductivity in the correlated electron filled skutterudite systemPr1−xCexPt4Ge12

Studies of superconductivity in multiband correlated electronic systems has become one of the central topics in condensed matter/materials physics. In this paper, we present the results of thermodynamic measurements on the superconducting filled skutterudite system Pr$_{1-x}$Ce$_x$Pt$_4$Ge$_{12}$ ($ 0 \leq x \leq 0.2$) to investigate how substitution of Ce at Pr sites affects superconductivity. We find that an increase in Ce concentration leads to a suppression of the superconducting transition temperature from $T_{c}\sim 7.9$ K for $x=0$ to $T_c\sim 0.6$ K for $x=0.14$. Our analysis of the specific heat data for $x\leq 0.07$ reveals that superconductivity must develop in at least two bands: the superconducting order parameter has nodes on one Fermi pocket and remains fully gapped on the other. Both the nodal and nodeless gap values decrease, with the nodal gap being suppressed more strongly, with Ce substitution. Ultimately, the higher Ce concentration samples ($x>0.07$) display a nodeless gap only.

An Optimization of Composition Ratio among Triple‐Filled Atoms in In0.3-x-yBaxCeyCo4Sb12 System

Bulk nanostructured materials are important as energy materials. Among thermoelectric materials, the skutterudite system of CoSb3 is a representative material of bulk nanostructured materials. Filling a skutterudite structure with atoms that have different localized frequencies (also known as triple filling) was reported to be effective for lowering thermal conductivity. Among studies representing superior power factors, In‐filled skutterudite systems showed higher Seebeck coefficients. This study sought to optimize the composition ratio among the triple‐filled atoms in an In0.3-x-yBaxCeyCo4Sb12 system. The composition dependence of the thermoelectric properties was investigated for specimens with different ratios among the three kinds of filler atoms in the In0.3-x-yBaxCeyCo4Sb12 system. In addition, the process variables were carefully optimized for filled skutterudite systems to obtain a maximum ZT value.

Determination of the Thermoelectric Properties in Filled-Skutterudite Systems by Controlling the Process Variables

This study presents the optimization of the process conditions for filled-skutterudites (with a higher melting temperature and a higher aging temperature than the original skutterudites), while many studies have focused only on introducing new filler-atom compositions. For the In-filled In0.3Co4Sb12 system, the maximum value of the thermoelectric figure-of-merit, ZT, 1.14 was obtained owing to the low thermal conductivity and high power factors. Moreover, this high ZT value is considered to result from the optimized process variables (i.e., a higher melting temperature and a higher aging temperature than the original CoSb3). For the double-filled In0.2Yb0.1Co4Sb12 system, the maximum ZT value of 1.26 was obtained owing to the low thermal conductivity and the high electrical conductivity. This high ZT value was also thought to result from the optimized process variables.

Determination of the Thermoelectric Properties in Filled-Skutterudite Systems by Controlling the Process Variables

Determination of the Thermoelectric Properties in Filled-Skutterudite Systems by Controlling the Process Variables

Thermoelectric properties of Spark Plasma Sintered In xYb yLa 0.3- x- yCo 4Sb 12 skutterudite system

A thermoelectric material with sufficient performance at high temperatures is a key factor for thermoelectric generation from solar-thermal energy (I. R. radiation). In this study, we analyzed CoSb 3 skutterudite compounds as a thermoelectric material at high temperatures and synthesized filled skutterudites with two types of filler atoms: 1) Yb- and La-fillers with a different electronegativity ( X) from the Sb atoms and 2) the In-filler with nearly the same electronegativity ( X) as the Sb atoms. All the samples were sintered with the spark plasma sintering method, and their thermoelectric properties were investigated. From the results, the Yb atoms were the most effective in decreasing the thermal conductivity of this system due to their very different electronegativity ( X) value from the Sb atoms and lower melting temperature than that of the La-filler. In addition, In-filled compositions had high power factor values while the In-free composition had the lowest power factor value due to its lower melting temperature. The addition of La atoms for filling was not effective in reducing the thermal conductivity of this filled skutterudite system due to its higher melting point. A maximum ZT value of 0.9 at 782 K was obtained in the In 0.1Yb 0.2Co 4Sb 12 specimen.

Superconductivity, magnetic order, and quadrupolar order in the filled skutterudite system Pr1−xNdxOs4Sb12

Superconductivity, magnetic order, and the high field ordered phase have been investigated in the filled skutterudite system Pr${}_{1\ensuremath{-}x}$Nd${}_{x}$Os${}_{4}$Sb${}_{12}$ as a function of composition $x$ in magnetic fields up to 9 T and at temperatures between 50 mK and 10 K. Electrical resistivity measurements indicate that the high field ordered phase, which has been identified with antiferroquadruoplar order, persists to $x$ $~$ 0.5. The superconducting critical temperature ${T}_{c}$ of PrOs${}_{4}$Sb${}_{12}$ is depressed linearly with Nd concentration to $x$ $~$ 0.55, whereas the Curie temperature ${T}_{\mathrm{FM}}$ of NdOs${}_{4}$Sb${}_{12}$ is depressed linearly with Pr composition to ($1\ensuremath{-}x$) $~$ 0.45. In the superconducting region, the upper critical field ${H}_{c2}(x,0)$ is depressed quadratically with $x$ in the range 0 $<$ $x$ $\ensuremath{\lesssim}$ 0.3, exhibits a kink at $x$ $\ensuremath{\approx}$ 0.3, and then decreases linearly with $x$ in the range 0.3 $\ensuremath{\lesssim}$ $x$ $\ensuremath{\lesssim}$ 0.6. The behavior of ${H}_{c2}(x,0)$ appears to be a result of pair breaking caused by the applied magnetic field and the exchange field associated with the polarization of the Nd magnetic moments in the superconducting state. From magnetic susceptibility measurements, the correlations between the Nd moments in the superconducting state appear to change from ferromagnetic in the range 0.3 $\ensuremath{\lesssim}$ $x$ $\ensuremath{\lesssim}$ 0.6 to antiferromagnetic in the range 0 $<$ $x$ $\ensuremath{\lesssim}$ 0.3. Specific-heat measurements on a sample with $x$ $=$ 0.45 indicate that magnetic order may occur in the superconducting state, as is also inferred from the depression of ${H}_{c2}(x,0)$ with $x$.

Filled Skutterudites: from Single to Multiple Filling

This paper shortly reviews our recent work on filled skutterudites, which are considered to be one of the most promising thermoelectric (TE) materials due to their excellent power factors and relatively low thermal conductivities. The filled skutterudite system also provides a platform for studying void filling physics/chemistry in compounds with intrinsic lattice voids. By using ab initio calculations and thermodynamic analysis, our group has made progresses in understanding the filling fraction limit (FFL) for single fillers in CoSb₃, and ultra-high FFLs in a few alkali-metal-filled CoSb₃ have been predicted and then been confirmed experimentally. FFLs in multiple-element-filled CoSb₃ are also investigated and anonymous filling behavior is found in a few specific systems. The calculated and measured FFLs, in both single and multiple-filled CoSb₃ systems, show good accordance so far. The thermal transport properties can be understood qualitatively by a phonon resonance scattering model, and it seems that a scaling rule may exist between the lattice thermal resistivity and the resonance frequency of filler atoms in filled system. Even though a few things become clear now, there are still many unsolved issues that call for further work.

Coexisting on-center and off-centerYb3+sites inCe1−xYbxFe4P12skutterudites

Electron-spin-resonance (ESR) measurements performed on the filled skutterudite system Ce1-x Ybx Fe4 P12 (x≲0.003) unequivocally reveal the coexistence of two Yb3+ resonances associated with sites of considerably different occupations and temperature behaviors. Detailed analysis of the ESR data suggests a scenario where the fraction of oversized (Fe2 P3) 4 cages that host Yb ions are filled with a low occupation of on-center Yb3+ sites and a highly occupied T -dependent distribution of off-center Yb3+ sites. Analysis of the Y 171 b3+ (I=1/2) isotope hyperfine splittings reveal that these two sites are associated with a low (∼1 GHz) and a high (≲ 15 GHz) rattling frequency, respectively. Our findings introduce Yb3+ in Th symmetry systems and use the Yb3+ ESR as a sensitive microscopic probe to investigate the Yb3+ ions dynamics. © 2009 The American Physical Society.

Novel Features Realized in the Filled Skutterudite Structure

Full varieties of strongly correlated electron phenomena have been found in the filled skutterudite systems, by changing the constituent elements. The unique crystal structure leads to novel features even in the systems containing rare earth elements with plural number of 4 f -electrons. According to the intensive competitions and cooperation among experimental and theoretical research works, remarkable progress has been made in sorting out several basic origins that bring about the variety of this system, such as the strong c – f hybridization, the small CEF level splitting, the orbital degree of freedom (multipoles), and the positional degree of freedom within a pnictogen cage. However, there still remain many attractive questions to be answered.

31P-NMR and µSR Studies of Filled Skutterudite Compound SmFe4P12: Evidence for Heavy Fermion Behavior with Ferromagnetic Ground State

The 31P-NMR (nuclear magnetic resonance) and muSR (muon spin relaxation) measurements on the filled skutterudite system SmFe4P12 have been carried out. The temperature T dependence of the 31P-NMR spectra indicates the existence of the crystalline electric field effect splitting of the Sm3+$ (J = 5/2) multiplet into a ground state and an excited state of about 70 K. The spin-lattice relaxation rate 1/T1 shows the typical behavior of the Kondo system, i.e., 1/T1 is nearly T independent above 30 K, and varies in proportion to T (the Korringa behavior, 1/T1 \propto T) between 7.5 K and 30 K. The T dependence deviated from the Korringa behavior below 7 K, which is independent of T in the applied magnetic field of 1 kOe, and suppressed strongly in higher fields. The behavior is explained as 1/T1is determined by ferromagnetic fluctuations of the uncovered Sm3+ magnetic moments by conduction electrons. The muSR measurements in zero field show the appearance of a static internal field associated with the ferromagnetic order below 1.6 K.

Synthesis and thermoelectric properties of Ce(Ru0.67Rh0.33)4Sb12.

ABSTRACTExotic filled skutterudite compositions show promise for thermoelectric applications. Current work was undertaken with a nominal composition of Ce(Ru0.67Rh0.33)4Sb12 to experimentally verify its potential as an n-type thermoelectric material. Nominal electroneutrality was expected at 0.89 cerium filling and fully filled materials were expected to be strongly n-type. Filled precursors of the nominal composition were synthesized using straightforward solid state reaction techniques, but standard synthesis routes failed to produce a fully-filled homogenous phase. Instead, the filled thermoelectric Ce(Ru0.67Rh0.33)4Sb12 was synthesized using a combination of solid state reaction of elemental constituents and high pressure hot pressing. A range of pressure-temperature conditions was explored; the upper temperature limit of filled skutterudite in this system decreases with increasing pressure and disappears by 12 GPa. The optimal synthesis was performed in multi-anvil devices at 4–6 GPa pressure and dwell temperatures of 350–700 °C. rutheniumThe result of this work, a Ce(Ru0.67Rh0.33)4Sb12 fully filled skutterudite material, exhibited unexpected p-type conductivity and an electrical resistance of 1.755 mΩ-cm that increased with temperature. Thermal conductivity, Seebeck coefficient, and resistivity were measured on single phase samples. In this paper, we report the details of the synthesis routeand measured thermoelectric properties, speculate on the deviation from expected carrier charge balance, and discuss implications for other filled skutterudite systems.