Date of Award

2025

Document Type

Thesis (Ph.D.)

Department or Program

Engineering Sciences

First Advisor

Jifeng Liu

Second Advisor

Geoffroy Hautier

Third Advisor

Ian Baker

Abstract

Thermoelectric materials can convert a thermal gradient to electricity. Currently, commercialized Bi2Te3 is brittle and expensive, which restricts the usage of this material. Compared to other thermoelectric materials, Fe2VAl is cost-effective, stable and more robust mechanically. However, the figure of merit (zT) had been much lower than Bi2Te3. In this thesis, our main goal is to improve the figure of merit in Fe2VAl system guided by computational modeling. We will start the discussion by presenting the Ge doped Fe2VAl sample with record thermoelectric figure of merit value. Then, we analyze the system via computational modeling and explore the underlying mechanism for high zT in Ge doped Fe2VAl. The theoretical analysis has three parts. In the first part, we discuss the impact of substitutional defects (or doping) on the phase stability. We compare several possible dopants and find that Ge defect helps to stabilize the metastable B2* and L21* phases, which have a flatter band and provides more available electrons in the thermal excitation process. The trade-off in carrier mobility is compensated by a significant increase in free carrier density such that the overall electrical conductivity still increases by 3-4 times compared to that in the base alloy. We also discuss the Fe vacancies, which will work as phonon scattering centers and reduce the lattice thermal conductivity. In the second part, we use cluster expansion to study the phase transition temperature and meta-stable phases in Fe2VAl, including the effect of Ge dopants. For n-type alloys, modeling shows that Ge-doping would reduce the order-disorder transition temperature. Furthermore, calculations show that the Al-rich and V-rich samples both have a lower order-disorder phase transition temperature, so we expect a better performance in these samples than the base alloy by enhancing the atomic disorder. In the third part, we explore the bandgap engineering in Fe2VAl. We present the previous bandgap engineering work first (especially Garmroudi’s work), focusing on opening the effective bandgap to reduce the undesirable bipolar effect at >400 K. Currently, most experimental and theoretical results support that Fe2VAl is a semimetal at 0 K, while the < 0.1 eV bandgap observed at room temperature can be attributed to three factors: finite temperature disordering, off-stoichiometry compositional fluctuations of Al and V, and Burstein-Moss shift. Corresponding to these three factors, we propose three strategies to open the bandgap in Fe2VAl: incorporating meta-stable phases, creating disorder and involving external doping.

The above computational analysis serves as insightful guidance for experiments. Based on these results, we fabricated Al-rich p-type Fe2VAl sample, which is confirmed to be the best p-type sample with record figure of merit values. The theoretical prediction coincides with the experiments quite well in this off-stoichiometric system.

In summary, this dissertation provides a systematic computational analysis of Fe2VAl material system. The theoretical modeling provides insight to understand strongly enhanced thermoelectric performance in Ge-doped n-type alloys and directly inspired the development of Al-rich p-type alloys, both breaking the record of figure of merit in Fe2VAl material system. The new record for n-type alloys is now comparable to that in Bi2Te3. These results demonstrate the importance of computational modeling in guiding the design of new thermoelectric materials.

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