Complex half-Heusler phases as high temperature thermoelectric materials
Abstract
The potential of n-type MNiSn and p-type MCoSb (M=Ti, Zr, Hf) half-Heusler phases, as thermoelectric elements, for high temperature power generation applications is explored. Chapter 1 describes the background and theory behind the thermoelectric effects and physical characteristics common to promising thermoelectric materials. Chapter 2 is a review of half-Heusler alloys and their value as thermoelectric materials. In chapter 3, a description of material synthesis and thermoelectric properties characterization techniques is given. The effects of compositional changes on the thermoelectric properties of MNiSn alloys are studied and analyzed in chapter 4. In this system, Sb doping at the Sn site is shown to partially mitigate the compensated behavior observed in these materials, resulting in an increase of both the figure of merit, ZT, and the temperature at which ZT is maximized. The effects of alloying at the M and Ni sites, which modifies the band structure, resulting in changes in the electronic transport properties, and introduces phonon scattering centers, thereby decreasing the lattice thermal conductivity, are reported. In addition to the benefits of increased alloying at the M site, on the thermal conductivity and thermoelectric transport properties, chapter 5 also presents an examination of Sn substitution on the Sb sublattice of MCoSb. This substitution is shown to transition these materials from semimetals to robust n-type and p-type thermoelectric materials. The significant lack of p-type thermoelectric materials, which operated at temperature above 700°C, make these materials appealing for study. In chapter 6, the effect of alloy and boundary scattering on the high temperature lattice thermal conductivity is also studied and analyzed. Alloy scattering at high temperature is analyzed using the Klemens-Callaway model, while a model for boundary scattering from grains, is adapted from the work of Sharp, Poon, and Goldsmid. These models are found to be effective tools for prediction of the high temperature lattice thermal conductivity and are used to explain the anomalous upturn observed in the lattice thermal conductivity at temperatures above 900K. It is found from these models and experiment that the effectiveness of alloy and boundary scattering, for reduction of the thermal conductivity, is diminished at elevated temperatures. Thus, it is necessary to explore other phonon scattering mechanisms that can further reduce the lattice thermal conductivity while maintaining the promising electronic transport properties of these materials. The thermoelectric figure of merit for select Sb-doped MNiSn alloys are found to reach ZT≈0.8 at T≈800°C, while the not-yet-optimized MCoSb phases realize ZT≈0.5 at 1000K (measured) and ZT≈0.6 at 1100K (extrapolated). These values meet or exceed the industry benchmarks for high temperature thermoelectric materials, set by SiGe alloys.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 2007
- Bibcode:
- 2007PhDT.......266C