Thermoelectric properties of europium and ytterbium doped cobalt antimonide skutterudites and novel thermal transport in type-II silicon clathrates

The work contained herein is centered around the measurement of physical properties of the skutterudite and clathrate material systems. The initial interest in each of these cage-like material systems is for thermoelectric applications. The skutterudite system research resulted in potentially device-ready bulk materials. Though the clathrate system does show some potential for thermoelectric applications, measurement of select clathrate materials shifted the focus of this research as the unfilled type-I clathrates presented exhibit novel and curious transport contrary to current theory.; Two families of skutterudite materials were investigated in order to ascertain the viability of these materials for thermoelectric applications. The electrical resistivity, thermopower (or Seebeck coefficient), and the thermal conductivity were all measured as a function of temperature in order to evaluate ZT. Results indicate that both Eu-filled and Yb-filled skutterudite compounds may be useful as a bulk thermoelectric material for high temperature applications. Additional analysis is presented in order to show trends in the Yb-filled skutterudites that suggest where new research should be focused.; Measurements carried out on the clathrate systems focused primarily on their thermal transport. The inherently low thermal conductivity in these compounds is the primary reason these materials were suggested as potential thermoelectric materials. The thermal conductivity measurements presented reveal a behavior inconsistent with current theories of phonon propagation in cage-like materials. The 'rattling' ion in cage-like structures has been identified as a phonon scattering mechanism used to reduce the lattice thermal conductivity in potential thermoelectric materials. In the case of the clathrates measured for this dissertation the 'rattling' ion does not appear to act as a phonon scattering mechanism, calling to question the accepted role of these void-filling ions.; The results of the two families of cage-like materials studied yielded significant impact into the research of thermoelectric materials. The skutterudite family provides more inspiration to continue the search for an increased figure of merit in a bulk material. The clathrate family, though not yet applicable for thermoelectric applications, provides a unique opportunity to address the role of 'rattling' ions in cage-like crystal structures and further the quest for the ideal thermoelectric material.