PhD Thesis Defense: Russell Taylor

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"Engineering Disorder, Defects, and Electrical Transport for High-Performance Thermoelectric Fe2VAl Heusler Alloys"

Abstract

Fe2VAl is a promising low-temperature thermoelectric material for waste-heat energy conversion because it is easily processed and comprised of earth-abundant, non-toxic, and low-cost elements. However, viability for applications faces two major challenges: high thermal conductivity and low p-type Seebeck coefficients. This primarily experimental work aims to address these issues through extrinsic and self-doping, point-defect and disorder generation through thermal and electrical processing, disordered and metastable phase composition quantification, and comparison of transport measurements to modeled electronic features.

An accessible phase quantification method was developed for bulk Fe2VAl using laboratory X-ray diffraction equipment to compare thermoelectric properties to phase compositions, and showed increased disorder improves thermoelectric performance. Because Ge-doped alloys indicated a complex phase composition, ALCHEMI was used to assess dopant site occupancy statistics of Si and Ge and revealed different occupancy behaviors.

Far off-stoichiometric Al-rich alloys were investigated, and a high p-type ZT of ~0.25 was achieved for quenched Fe2V0.7Al1.3 from a large disordered phase fraction, but the Seebeck coefficient remained lower than desired from high carrier density. Therefore, a compensation-doping strategy with Ge, our best n-type dopant, was investigated. While the strategy did not work to reduce carrier densities, it showed a strong correlation between carrier density and Seebeck performance in highly doped Fe2VAl. Further investigations into highly doped alloys compared modeled electronic structures to measured electrical transport properties. The results indicated that a simple rigid-shift doping approximation is insufficient to describe transport. Instead, Γ-point and X-point relative positioning, effective transport gaps, band anisotropy, carrier scattering, and multiband transport were determined to all contribute to the measured electrical behavior, with different contributions from different dopants.

A non-conventional electrical processing method using pulsed DC was developed to increase defect densities. Compared to quenching, this method increased disordering by ~50% and vacancy density by ~300%, which resulted in a ~3-times larger thermal diffusivity reduction. This was achieved with minimal effect on electrical resistivity, thereby partially decoupling thermal and electrical transport behavior.

Overall, this work contributes an accessible phase quantification method, identifies methods for tuning p-type electronic behavior in indirect semimetal Fe2VAl, and demonstrates increased point-defect phonon scattering through electrical processing.

Thesis Committee

• Ian Baker (Chair)
• Jifeng Liu
• Yan Li
• Tianshu Li (The George Washington University)

Contact

For more information, contact Thayer Registrar at thayer.registrar@dartmouth.edu.