Thermoelectric materials have received rejuvenated interest for the past two decades due to the theoretical predictions that a high dimensionless thermoelectric figure of merit ZT > 1 can be obtained in materials with complex structures and reduced dimensions, termed "nanostructured materials". The underlying benefit of nanostructuring is the possibility of at least partially decoupling the interdependent relation between the Seebeck coefficient and electrical conductivity so that one has the flexibility to tune them individually. The thermal conductivity is expected to be reduced at the same time due to the phonon scattering from the nanoscopic interfaces. In this work, we investigate the thermoelectric properties of bulk nanocomposite materials, which boast both ease of synthesis and enhanced thermoelectric performance originating from the reduced dimension. Two material systems are of our interest: bismuth telluride and p-type skutterudite. For bismuth telluride and its alloys based nanocomposites, we select Bi2Te2.85Se0.15 as the n-type matrix and Bi0.4Sb1.6Te3 as the p-type matrix respectively. The nanocomposite materials were prepared by a solution based incipient wetness impregnation method. Adding PbTe nanoparticles can effectively reduce the lattice thermal conductivity at low temperature. But the doping effect from the excess Pb ion plays a dominant role compared to nanostructuring. This results in creating a two carrier system for the n-type nanocomposites, and decreased power factors for p-type nanocomposites. For skutterudite based nanocomposites, we focus our attention on the under-developed p-type skutterudites. We start from the primitive Fe-doped binary skutterudite nanocomposite Co0.9Fe0.1Sb3 with in-situ formed FeSb2 as nanoparticles and demonstrate 100% enhancement in the overall thermoelectric performance. The success encouraged us to re-develop an Yb-filled skutterudite as the matrix material and explore the thermoelectric properties over a wide range of Yb filling fraction with various amount of antimonide based impurities (presumably FeSb2). We achieve an enhanced thermoelectric performance up to 23% in optimized nanocomposites compared to the control sample. Last but not least, we investigated the double filled p-type skutterudite Yb0.6GazFe2Co2Sb12. The unique role of Ga inducing a deep defect level in band structure enhances the Seebeck coefficient without affecting the electrical resistivity. The best performing sample demonstrates 46% enhancement of thermoelectric performance compared to Yb0.6Fe2Co2Sb12 with only Yb filler. Overall, our study indicates that nanostructuring does provide significant benefits when applied to thermoelectrics under certain conditions. Time and more research will tell if this approach is ultimately a viable one.
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