PhD Thesis Defense: Xiaoxue Gao

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"Cyclic Thermal Stability of Native Oxide Solar Thermal Absorbers on FeMnNiAlCr High Entropy Alloys for Energy Applications"

Abstract

High-entropy alloys (HEAs) are promising candidates for cost-effective high-temperature materials in high-efficiency power cycles. A potential application is in concentrated solar power (CSP) systems, where native oxides of cost-effective FeMnNiAlCr HEAs can also synergistically serve as a high-efficiency solar thermal absorber. However, the huge and repetitive temperature change of the CSP system in day-night cycles greatly challenges its thermal stability.

In this thesis, we investigated the cyclic oxidation behavior of FeMnNiAlCr HEAs to improve the optical performance and thermal stability of their native oxides as a solar absorber for next-generation CSP systems. We found that the duplex oxide scale on the two-phase FeMnNiAlCr HEAs consists of Mn2O3 on the surface contributing to solar absorption, and Al2O3 along the oxide/HEA interface serving as a protective layer to prevent excessive oxidation. To enhance the spallation resistance during thermal cycles, a steady flux of Al diffusion to the oxide/alloy interface is crucial not only to ensure the establishment of the protective layer at the early-stage oxidation but also to sustain its continuity during the thermal cycles. An initial attempt to increase the Al content by replacing Mn helped the establishment of the protective scale Al2O3 but prevented the diffusion of Mn to the surface to form a continuous solar absorbing layer.

We developed a more effective approach to maintain the integrity of the interfacial Al2O3 layer by engineering the HEA’s microstructures through cold-working and annealing, which results in finer microstructures and new phase boundaries to enhance Al supply through short-cut diffusion. Using this approach, the native oxides sustained >20 simulated day-night thermal cycles with ~87% optical-to-thermal energy conversion efficiency. Additionally, we examined the effect of reactive elements on the spallation resistance. Y doping shows promising improvement in the strength of the oxide/alloy interface

Thesis Committee

• Jifeng Liu (chair)
• Ian Baker
• Geoffroy Hautier
• Yanfei Gao (University of Tennessee)

Contact

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