Thermal Conductivity Reduction by Fluctuation of the Filling Fraction in Filled Cobalt Antimonide Skutterudite Thermoelectrics

Abstract

The reduction of the lattice thermal conductivity is one of the crucial steps in improving thermoelectric materials. In skutterudites, a well-known approach is to reduce the thermal conductivity by filling the structural cage with rare-earth atoms. In this work, we show that it is not just the amount of such filling itself but its nanoscale structuration that lowers the thermal conductivity. A straightforward synthesis procedure under high pressure yields Ce-and Yb-filled CoSb 3 skutterudites, with and without an inhomogeneous distribution of the filler atoms. The composition of the phases is evaluated from synchrotron Xray diffraction (SXRD) data; the highly nanostructured morphology is verified by high-resolution transmission electron microscopy (TEM). The filling fluctuation, i.e., the uneven distribution of filling atoms in the sample originating a phase segregation, brings about low lattice thermal conductivity, as a strong source of phonon scattering. This effect is prominent in the Ce-filled compound, where Ce is segregated into Ce-rich and Ce-poor regions, and the lattice contribution of the thermal conductivity κ L shows a concomitant reduction, approaching values as low as 1.6 W m −1 K −1 at 800 K. Although the level of filling is much higher in Yb x CoSb 3 , its lattice thermal conductivity remains larger. Overall, though, its power factor is enhanced due to charge transfer from the Yb-filler. We thus define a new paradigm for the design of filled skutterudites with exceptionally low thermal conductivities, based on the nanoscale mixing of two phases with different filling factors, spontaneously induced by high-pressure synthesis conditions, which can be considered as pseudoamorphous structures with significant reduction in κ L .

Substantial thermal conductivity reduction in mischmetal skutterudites MmxCo4Sb12 prepared under high-pressure conditions, due to uneven distribution of the rare-earth elements

“…These values ( Fig. 6b) are improved (lower) in comparison to that measured for the pure, conventional CoSb 3 16 and are also lower than those corresponding to CoSb 3Àd , synthesized under high pressure. 23 The comparison with other filled skutterudites and filling fraction indicates that this reduction cannot be exclusively attributed to the filler resonant scattering.…”

“…The annular 15 On the other hand, the concentration of Ce, in addition to being homogeneous and higher than the concentration of La, is also higher than that obtained in a pure Ce x Co 4 Sb 12 specimen. 16 This suggests that the Ce filling factor can be increased when La is added to Ce as a doping element for CoSb 3 . Examination of other six regions (over ten grains) is consistent with these results, as shown in Fig.…”

“…5) displays an n-type behavior, with values around À100 mV K À1 at room temperature for every composition, as observed for other rare-earth-filled skutterudites. 7,8,16 The absolute Seebeck coefficient slightly increases with temperature, up to a maximum of S = À155 mV K À1 at 700 K for Mm 0.2 Co 4 Sb 12 . Above that temperature the contribution of minority carriers, cause a decreasing trend.…”

“…4 Recently, we studied the structure and thermoelectric transport properties of R x CoSb 3 (R = La, Ce, or Yb) prepared under high pressure. 15,16 A significant reduction of the thermal conductivity was found as a consequence of the uneven filler distribution in R-poor and R-rich phases at a nanometric scale. 15 This type of reduction was previously attributed to filling fraction fluctuation and consequent strain-field and long-range acoustic phonon scattering by Wei et al in La-filled Fe 4 Co 12 skutterudite.…”

“…This segregation of different filler elements contributes to further reduce the lattice thermal conductivity, from those measured for nominal La 0.5 Co 4 Sb 12 , Yb 0.5 Co 4 Sb 12 and Ce 0.5 Co 4 Sb 12 prepared under high-pressure. 15,16 This could be a good starting point to enhance even more the thermoelectric properties in filled skutterudites.…”