Current Happenings

Projects: Skutterudites

The field of Thermoelectrics has been advanced by the testing of skutterudite compounds, such as XCoSb3, X being a rare earth and transition metal element.  These skutterudite materials have the greatest potential of giving a desired ZT, or figure of merit.  , where α is thermopower, T is absolute temperature, ρ is electrical resistivity, and κ is thermal conductivity.1,2  In order to achieve the highest desired ZT, efforts have been made to increase the power factor, or , as well as lowering the thermal conductivity, κ. 
To decrease the thermal conductivity while maintaining the high power factor, a hydrothermal treatment technique is applied to the skutterudite bulk matrix material.  The hydrothermal treatment technique resulted in a surface layer on the bulk matrix of the material which serves to scatter phonons; therefore, lowering thermal conductivity, and to compensate the decreasing mobility by introducing extra carriers into the system.
The process of hydrothermal treatment begins with preparing the material through solid state reactions. Then, crush the bulk material, XCoSb3, into a powder.  The powder is then exposed to the hydrothermal environment in a Teflon-lined autoclave at ~ 200 °C for 2-3 days (Figure 2).  The newly treated material is then hot pressed into a disk shape and cut into a rectangular shape for measurements of resistivity, thermopower, and thermal conductivity.  Potentially, this revolutionary process provides a more convenient way to improve the ZT.

Jennifer W Hubbard


Click to Enlarge the Graph

Skutterudite material has long been drawing much attention as promising thermoelectric materials owing to its high carrier mobilities, good Seebeck coefficients and a complex crystal structure. Though its intrinsic lattice thermal conductivity is somewhat high due to the strong covalent bonding in the lattice, the naturally formed voids (or cages) in the structure can be filled with loosely bound guest ions, and the low frequency localized phonon modes of these guest ions, namely, “rattlers”, can strongly scatter the heat-carrying acoustic phonons and thereby decrease the lattice thermal conductivity, making it a promising “Phonon Glass Electron Crystal” (PGEC) material.
Our project is focused on multi-filled skutterudites. In filled skutterudites the interaction between the localized modes of the fillers and the heat carrying acoustic phonon modes is strong when their frequency is close. In the multi-filled skutterudites, when the frequencies of different filler are sufficiently different from one another, the localized modes would scatter a broader spectrum of heat-carrying phonons. Also, bear in mind that each filler element is being effective with regards to its constructive contribution to the thermoelectric properties, in a limited temperature regime, so the multiple-filling approach would serve for a broader temperature regime. We have carried out systematic research on synthesis and thermoelectric properties of InxYbyCo4Sb12 (x = 0.0 – 0.20, y = 0.0 – 0.20), and a state-of-the-art ZT value of 0.97 is attained in In0.1Yb0.1Co4Sb12 at 750 K, with ZT > 0.85 from 570 K to 900 K.

Jiangying Peng

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