Effect of partial void filling on the lattice thermal conductivity of skutterudites (original) (raw)

The effect of rare-earth filling on the lattice thermal conductivity of skutterudites

Journal of Applied Physics, 1996

have been made by hot isostatic pressing of powders. The lattice thermal conductivity of these filled skutterudites is markedly smaller than that of IrSb 3 ; thus, void filling shows promise as a method for improving the thermoelectric properties of these materials. We present the lattice thermal conductivity of these filled skutterudites in an effort to quantify the impact of void filling in this structure. It is believed that the atoms ''rattle'' in the voids of the structure and therefore interact with a broad spectrum of lattice phonons, reducing their mean free paths substantially below that in the ''unfilled'' skutterudites. An additional phonon scattering mechanism is caused by phonon-stimulated transitions between the low-lying energy levels of the 4 f electron configurations in the case of Nd 3ϩ and Sm 3ϩ . Magnetic susceptibility and Hall-effect measurements are also presented.

Lattice thermal conductivity of skutterudite compounds

Indian Journal of Physics, 2010

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 AB 3. 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 CoSb 3 is in reasonably good agreement with the experimental result in the temperature range 1-1000 0 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.

Thermoelectric Properties of Microstructurally Modified CoSb3 Skutterudite by Hf-Addition

Journal of Electronic Materials, 2016

A polycrystalline phase of Hf-filled CoSb 3 skutterudite was successfully prepared by the mechanical alloying technique followed by the spark plasma sintering process. X-ray diffraction and scanning electron microscopy linked with energy-dispersive x-ray spectroscopy were used to investigate the result filling the void spaces with Hf. HfCo 4 Sb 12 skutterudite possesses a very low thermal conductivity [1.8 W/(m K)], lower than that of some lanthanide-filled skutterudites, while exhibiting p-type conduction. The anharmonicity of oscillation of the loosely bond Hf atoms, the point defects on the lattice microstructure and the large area fraction of the grain boundaries were the reasons for the significant drop in the thermal conductivity of Hf-filled CoSb 3. Thus, this work is useful in deriving a pathway for improvement in thermoelectrics through the introduction of smaller rattling cations with higher mass to increase the disorder of the lattice structure.

Low temperature transport and structural properties of misch-metal-filled skutterudites

Journal of Applied Physics, 2007

Skutterudites, such as CoSb 3 , are a promising class of thermoelectric materials, particularly when the voids in the crystal structure are filled with guest atoms. We report a comprehensive study of the effects of filling skutterudites with misch-metal ͑Mm͒, a rare-earth alloy having the naturally occurring La, Ce, Pr, and Nd composition. Our power diffraction experiments show that Mm filling causes a larger expansion and an unusual distortion of the CoSb 3 lattice compared with single-element-filled skutterudites. We probed the response of crystal lattice, electronic structure, and carrier and phonon scattering mechanisms to Mm filling using neutron powder diffraction, Hall effect, electrical resistivity, thermopower, and thermal conductivity measurements between 2 and 300 K on a series of Mm y Fe 4−x Co x Sb 12 samples. The thermoelectric properties of these Mm-filled skutterudites in this low temperature range are comparable to those of pure Ce-filled skutterudites despite the anomalous lattice expansion and distortion. We expect that these materials will have high thermoelectric figures of merit at elevated temperatures.

Expanding the investigation of the thermoelectric properties of rare-earth-filled skutterudites

2002

observed, using Raman Spectroscopy.[4] There is increasing interest in the skutterudite family of compounds as potential thermoelectric materials mainly due to the marked reduction in the thermal conductivity that occurs when voids in the structure are filled with "guest" atoms. The "rattlling" of these void-fillers atoms damps the phonon propagation through the lattice. The phonon scattering mechanism(s) are not yet compoletely understood. Ongoing research focuses on compensating for the charge of the voidfilling ions and finding the optimum void filler that yields maximal reduction in the thermal conductivity while maintaining favorable electronic properties. Thus far published data on Ge and Fe as charge compensators resulting in P-type samples has been reported. In this paper we present data on skutterudites incorporating Sn for charge compensation. The defmition of a good thermoelectric magnitude of the materials' 2 value[5] z s20 e-K where S is the Seebeck coefficient, o the electrical conductivity, and K the total thermal conductivity (K=K~+K~; K~ and \ being the lattice and electronic contributions, respectively.) Since the dimensions of 2 are inverse temperature, a more convenient quantity is the dimensionless figure of merit, ZT, where T is the absolute temperature.

Effects of partial La filling and Sb vacancy defects on CoSb3 skutterudites

Physical Review B

Over the past decade, the open frame ("cagey") structure of CoSb 3 skutterudite has invited intensive filling studies with various rare-earth elements for delivering state-of-theart mid-temperature thermoelectric performance. To rationalize previously reported experimental results and provide new insight into the underexplored roles of La fillers and Sb vacancies, ab initio density functional theory studies, along with semi-classical Boltzmann transport theory calculations, are performed for pristine CoSb 3 of different lattice settings and La-filled CoSb 3 with and without Sb's mono-and di-vacancy defects. The effects of spin-orbit coupling (SOC), partial La-filling, Sb vacancy defects, and spin polarization on the electronic and thermoelectric properties are systematically examined. The SOC shows minor effects on the electronic and thermoelectric properties of CoSb 3. The peculiar quasi-Dirac band in the pristine CoSb 3 largely survives La filling but not Sb vacancies, which instead introduce dispersive bands in the band gap region. The non-spin-polarized and spin-polarized solutions of La-filled CoSb 3 are nearly degenerate. Importantly, the band structure, density of states, and Fermi surface of the latter are significantly spin polarized, giving rise to spin-dependent thermoelectric properties. Seebeck coefficients directly calculated as a function of chemical potential are interpreted in connection with the electronic structures. Temperature-dependent Seebeck coefficients derived for the experimentally studied materials agree well with available experimental data. Seebeck coefficients obtained as a function of charge carrier concentration corroborate the thermoelectrically favorable role at high filling fractions played by the Fermi electron pockets associated with the degenerate valleys in the conduction bands, and also point toward a similar role of the Fermi hole pockets associated with the degenerate hills in the valence bands. These results serve to advance the understanding of CoSb 3 skutterudite, a class of materials with important fundamental and application implications for thermoelectrics and spintronics.

Band Structure Engineering and Thermoelectric Properties of Charge-Compensated Filled Skutterudites

Scientific reports, 2015

Thermoelectric properties of semiconductors are intimately related to their electronic band structure, which can be engineered via chemical doping. Dopant Ga in the cage-structured skutterudite Co4Sb12 substitutes Sb sites while occupying the void sites. Combining quantitative scanning transmission electron microscopy and first-principles calculations, we show that Ga dual-site occupancy breaks the symmetry of the Sb-Sb network, splits the deep triply-degenerate conduction bands, and drives them downward to the band edge. The charge-compensating nature of the dual occupancy Ga increases overall filling fraction limit. By imparting this unique band structure feature, and judiciously doping the materials by increasing the Yb content, we promote the Fermi level to a point where carriers are in energetic proximity to these features. Increased participation of these heavier bands in electronic transport leads to increased thermopower and effective mass. Further, the localized distortion ...

Improved thermoelectric performance of (Fe,Co)Sb3-type skutterudites from first-principles

Journal of Applied Physics, 2016

Skutterudite materials have been considered as promising thermoelectric candidates due to intrinsically good electrical conductivity and tailorable thermal conductivity. Options for improving thermal-to-electrical conversion efficiency include identifying novel materials, adding filler atoms, and substitutional dopants. Incorporating filler or substitutional dopant atoms in the skutterudite compounds can enhance phonon scattering, resulting in reduction of thermal conductivity, as well as improving electrical conductivity. The structures, electronic properties, and thermal properties of double-filled Ca 0.5 Ce 0.5 Fe 4 Sb 12 and Co 4 Sb 12À2x Te x Ge x compounds (x ¼ 0, 0.5, 1, 2, 3, and 6) have been studied using density functional theory-based calculations. Both Ca/Ce filler atoms in FeSb 3 and Te/Ge substitution in CoSb 3 cause a decrease in lattice constant for the compounds. As Te/Ge substitution concentration increases, lattice constant decreases and structural distortion of pnictogen rings in the compounds occurs. This indicates a break in cubic symmetry of the structure. The presence of fillers and substitutions cause an increase in electrical conductivity and a gradual decrease in electronic band gap. A transition from direct to indirect band-gap semiconducting behavior is found at x ¼ 3. Phonon density of states for both compounds indicate phonon band broadening by the incorporation of fillers and substitutional atoms. Both systems are also assumed to have acoustic-modedominated lattice thermal conductivity. For the Co 4 Sb 12À2x Te x Ge x compounds, x ¼ 3 has the lowest phonon dispersion gradient and lattice thermal conductivity, agreeing well with experimental measurements. Our results exhibit the improvement of thermoelectric properties of skutterudite compounds through fillers and substitutional doping. V