Date of Award

3-2023

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Engineering Physics

First Advisor

Adib Samin, PhD

Abstract

Materials that can withstand higher temperatures are paramount for next-generation aircraft design. Hypersonic capabilities and jet-turbine engines operate in extreme environments. Choosing materials that have high thermal stability and oxidation resistance for these applications can increase engine efficiency, reduce size, weight and power (SWaP), and increase the maneuverability of the aircraft. The mixed oxide CrTaO4 has been experimentally observed to significantly contribute to oxidation resistance at high temperatures. However, and despite its significance, its properties remain largely unknown. This work explores the thermal properties of this material from a multi-scale approach, by obtaining an accurate description of the thermodynamics via computing the Gibbs free energy. Furthermore, The coefficient of thermal expansion (CTE) is shown to generally increase with temperature but decrease with pressure. There is a close match in the CTE of CrTaO4 and the underlying experimentally observed oxide. The reported heat capacity increases with temperature but generally does not change with pressure. This result, paired with the low thermal conductivity of 0.28 W/ m-K calculated for CrTaO4, gives insight as to why this continuous oxide layer provides promising oxidation resistance. This work accounts for the ground state, electronic, quasi-harmonic, and anharmonic contributions to the free energy when calculating thermal properties. When including the anharmonic contributions, the CTE and heat capacity generally increase, especially at temperatures above 900K. The addition of the anharmonic contributions result in a CTE that is even closer to the CTE of the underlying experimentally observed oxide. In addition, this work provides a flexible code for calculating anharmonic contributions to the free energy by performing thermodynamic integration via Langevin dynamics for sampling configurations. The anharmonic contribution, which is typically ignored, is especially important when considering elevated temperatures. For the anharmonic contribution, a machine learning interatomic potential was trained.

AFIT Designator

AFIT-ENP-MS-23-M-086

Comments

A 12-month embargo was observed.

Approved for public release. PA case number on file.

Plain-text title form: Towards an Understanding of the Thermodynamic Properties of CrTaO4: A Computational Perspective

Additional Files
4. SF 298 - Gordon.pdf (39 kB)
SF298 form for AFIT-ENP-MS-23-M-086
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