Thermoelectric Property Studies on Lead Chalcogenides, Double-filled Cobalt Tri-Antimonide and Rare Earth-Ruthenium-Germanium.

Abstract

Motivated by the energy applications of thermoelectrics (TE) such as power generation and refrigeration, my research goal is to develop novel materials with high dimensionless figure-of-merit ZT. Thermoelectric materials are characterized over a broad temperature range from 2 K to 800 K. According to the definition of ZT (=S2σΤ/κ where S is the Seebeck coefficient, σ the electrical conductivity, κ the thermal conductivity, and T the absolute temperature), reducing thermal conductivity κ increases ZT. However, it is challenging to tailor material structures to enhance acoustic phonon scattering without impeding charge transport. We investigated three types of solid-state compounds: lead chalcogenides, filled skutterudites and ternary germanides, for their structural, electronic, thermal, and magnetic properties. For lead chalcogenides, nanoparticles embedded in the lattice enhance the mid- to long-wavelength phonon scattering, significantly reducing lattice thermal conductivity κL (~ 0.4 Wm-1K-1 at 300 K for PbTe with 8% PbS). These nanostructured PbTe-based alloys exhibit the highest ZT of approximately 1.5. For n-type filled skutterudites, ZT close to 1.4 is achieved by placing Ba and Yb into the interstitial void of CoSb3. Rare-earth ruthenium germanium compounds of the composition R3Ru4Ge13 have the cage-like structure similar to filled skutterudites, resulting in relatively low κL (~ 2 Wm-1K-1 at 300 K). They could potentially be developed into a new class of prospective TE materials.