Polycrystalline samples of Ir4LaGe3Sb9, Ir4NdGe3Sb9, and Ir4SmGe3Sb9 have been made by hot isostatic pressing of powders. The lattice thermal conductivity of these filled skutterudites is markedly smaller than that of IrSb3; 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 4f electron configurations in the case of Nd3+ and Sm3+. Magnetic susceptibility and Hall-effect measurements are also presented.
We have measured the electrical resistivity, ρ, thermoelectric power, α, and thermal conductivity, κ, of the skutterudite material IrSb3 in a temperature range from 300 down to 4 K. It is found that the electrical resistivity and thermopower decrease monotonically as the temperature is reduced to 50–60 K. Below approximately 60 K the resistivity rises in a semiconducting manner. It appears the thermopower exhibits a large phonon drag peak at around 20 K and then falls towards zero. The thermal conductivity increases rapidly as the temperature is decreased with a maximum at around 20 K, corresponding to the peak in the thermopower. We will discuss these results and compare them to higher temperature data from G. A. Slack and V. G. Tsoukala [(IrSb3) J. Appl. Phys. 76, 1635 (1994)]. We have also measured some of the so-called ‘‘filled skutterudites,’’ Ir4LaGe3Sb9, Ir4NdGe3Sb9 and Ir4SaGe3Sb9. The thermoelectric properties of these materials are considerably different than those of the unfilled sample. The thermopower is considerably lower and the resistivity is a factor of 2–4 times higher than the unfilled sample at room temperature. The thermal conductivity is markedly reduced by the filling, as much as a factor of 20 reduction for some of the systems.
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