Date of Award

Winter 12-19-2022

Document Type

Thesis (Ph.D.)

Department or Program

Engineering Sciences

First Advisor

Jifeng Liu

Second Advisor

Ian Baker

Abstract

Fe2VAl shows great promise as an eco-friendly 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 of environmental pollution. The key issue with Fe2VAl is the alloy’s relatively large thermal conductivity compared to its semiconductor competitors. This thesis aims to investigate a hierarchical approach to reduce lattice thermal conductivity through a preliminary exploration of heavy element substitutions and mechanical deformation, then through probing order-disorder transitions for increased defects and antiphase boundaries (APBs), and lastly through extending the quenching temperature through novel rapid cooling techniques to observe if any of the previous effects to decrease thermal conductivity can be enhanced. As furnace temperature prior to quenching is increased, the APB density and the number of Fe antisite defects both increase to produce an even greater decrease in the lattice thermal conductivity. This drop in lattice thermal conductivity is further enhanced by 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 by 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 adding coherent interfaces and heavy element substitution reducing the transport of heat-carrying phonons. The increase in Fe antisite defects also produces magnetic clusters within the bulk of the material which increases 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 maximum of 0.75 at 400 K presenting one of the highest observed performance factors in this material system.

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