<p dir="ltr">Thermal Barrier Coatings (TBC) represent the key technology enabling greater efficiency and performance in jet engines and gas turbines. In modern engines, TBCs allow gas temperatures to exceed 1700°C, well above the point at which the structural alloys lose their strength. By insulating the underlying nickel-alloy components from the extreme heat generated during combustion, TBCs support a larger temperature gradient. </p><p dir="ltr">As operating temperatures are further increased to improve performance, thermal radiation becomes a more substantial carrier of heat. However, conventional TBCs are designed to provide a single barrier against only the phonon-mediated conductive heat flux, leaving the photonic radiative heat transfer largely unmitigated. We propose a Thermal Dual Barrier Coating (TDBC) to simultaneously suppress the phononic and photonic heat transfer by integrating a reflective thermal metamaterial into an independent phonon-optimized TBC.</p><p dir="ltr">The main obstacle to achieving the TDBC is in the selection of adequate reflective materials in the metamaterial. Conventional refractory metals that demonstrate the greatest stability and functionality in thermal metamaterials show instability under harsher environments. In our work, we identified and studied the key ideas, metrics, and challenges in metamaterials based on alternating layers of refractory metals and oxides for TDBC applications.</p><p dir="ltr">Our work emphasizes oxidation as a crucial degradation factor that is unavoidable in our assessment of the metamaterials. In formulating this problem, we bring the concept of oxidation-resistance through passivation to the forefront of material selection. We emphasize the passivative and oxidative properties of the metallic layer as a critical determinant in overall stability. In our work, we assess the enhancements in stability brought via passivation through the Pilling-Bedworth Ratio. We then propose the use of metal silicides in metamaterials as an overlooked class of oxidation-resistant IR reflective materials that operate through a more complex passivation method. We demonstrate strong stability in the structural integrity as well as the infrared responses of the metamaterials at up to 1200°C in atmospheric and oxidative environments.</p><p dir="ltr">After establishing the viability of metal silicides in wide-area thin films, we explore their viability in more complex thermal structures. We fabricate metal silicide metasurfaces for directional thermal emission. We demonstrate a grating structure that exhibits enhanced structural stability and maintains directional modes in the mid-IR after annealing at 1000°C.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/26347096 |
| Date | 22 July 2024 |
| Creators | Ali R Jishi (19190992) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/_b_Pushing_the_Limit_of_High-Temperature_Thermal_Metamaterials_b_/26347096 |
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