Properties of Polycrystalline Thermoelectrics

Thermoelectric generators are devices that directly convert heat into electricity. While these devices have been historically used in niche applications such as satellites and nuclear reactors, potential applications  include the optimization of gas engines, solid oxide fuel cells, etc., by harvesting the heat that these devices dissipate and transforming it into useful work.  The response of thermoelectrics is characterized through ZT, the figure of merit, which is a function of several bulk material properties: the thermal conductivity, the electrical conductivity, and the seebeck coefficient.  For most materials, these properties are  a strong function of temperature, can be strongly anisotropic, and highly dependent on their underlying topology. Features such as grain size, texture and grain boundaries all impact the associated macroscopic ZT.  Some of these mesostructural features can be controlled by using novel processing techniques, leading to structures of dramatically different dimensionality and thus potentially enhanced properties.  Such structures include nanodots (zero-dimensional), nanowires (one-dimensional), films (two-dimensional), and bulk (three-dimensional). In this paper, the effect of geometrical confinement on the thermoelectric figure of merit is analyzed in an effort to understand the relations of size and dimensionality to the macroscopic response. The effect of crystallographic texture and grain size was studied.  Optimal microstructures and morphologies are proposed.