PhD Thesis Defense: Cory Cline

To attend via videoconference, please email cory.t.cline.th@dartmouth.edu.

"Improvements in the thermoelectric performance Fe2VAl Heusler alloy for low temperature operations"

Abstract

Fe2VAl shows great promise as an ecofriendly and low cost replacement to conventional low temperature (250-500 K) thermoelectric materials. Current thermoelectric materials use toxic and expensive elements like Te and Sb, whereas Fe2VAl offers a larger power factor at a lower cost and a reduced risk to environmental pollution. The key issue with Fe2VAl is the alloy’s relatively large thermal conductivity compared to its semiconductor competitors.

The aim of this study is to investigate the best hierarchical approach to introducing various dimensional defects to scatter a wide range of phonon wavelengths. Point defects from intrinsic antisite defects and coherent interfaces, in the form of antiphase boundaries (APBs), are promoted through rapid quenching in the material which allows for large reduction in the base Fe2VAl alloy. As furnace temperature prior to quenching is increased, the APB density and the number of Fe antisite defects both increase to produce even more decrease in the lattice thermal conductivity. This drop in lattice thermal conductivity is further enhanced with extrinsic doping using Ge and Ti to produce an n-type and p-type thermoelectric material, respectively. The thermal conductivity of both Fe2V0.8Ti0.2Al and Fe2VAl0.9Ge0.1 decreased over 50% of its original value to 10.6 W m-1 K-1 and 4.8 W m-1 K-1, respectively, due the coupled effect of coherent interfaces and heavy element substitution reducing the transport of heat-carrying phonons. The increase in Fe antisite defects also produce magnetic clusters within the bulk of the material increase the effective mass of carriers and the Seebeck coefficient in the alloy. Overall, the thermoelectric performance factor of the n-type Fe2VAl0.9Ge0.1 water quenched alloy reached a max of 0.75 at 400 K presenting one of the highest observed performance factors in this material system.

Thesis Committee

• Jifeng Liu (Chair)
• Ian Baker
• William Scheideler
• Pascal Jacques

Contact

For more information, contact Theresa Fuller at theresa.d.fuller@dartmouth.edu.