Thermodynamic and Kinetic Modeling of the Hf-Ta-C-O System for the Design of Oxidation Resistant HfC-TaC Ultra-High Temperature Ceramics 46 views

The hafnium-tantalum-carbon-oxygen system is modeled to develop novel mechanistic descriptions of oxidation processes of hafnium carbide-tantalum carbide (HfC-TaC) ultra-high temperature ceramics (UHTCs). Applications include aerospace thermal protection systems, leading edges and nose tips for hypersonic vehicles, and solar receivers. HfC and TaC have melting temperatures of 3959 and 3768 °C, respectively, the highest of binary UHTCs. However, their oxides, HfO2 and Ta2O5, have inadequate protective capabilities that cause the carbides to severely oxidize within minutes of oxygen exposure, restricting their applications. (Hf,Ta)C solution phases, ideally the optimally oxidation resistant 3 HfC : 1 TaC composition, form a thermally grown ternary oxide, Hf6Ta2O17, which is part of the Hf(n-5)/2Ta2On series and has been proposed to have slower oxygen transport. Yet, the stability of Hf(n-5)/2Ta2On and its degree and mechanisms of oxygen transport relative to those of HfO2 and Ta2O5 remain unclear. To investigate these knowledge gaps, present research objectives are (1) CALculation of PHAse Diagrams (CALPHAD) model development of the Hf-Ta-C-O system and HfO2-Ta2O5 isoplethal section supported by first-principles calculations, (2) Performance of first-principles calculations of oxygen vacancy formation and diffusion in thermally grown HfO2, Ta2O5, and Hf6Ta2O17, and (3) Development of mechanistic descriptions and continuum models using thermodynamic and kinetic data from Objectives 1 and 2 for the oxidation of 3 HfC : 1 TaC ceramics. Results support the hypotheses: (1) Hf(n-5)/2Ta2On is entropically stabilized with larger structure size, (2) Hf(n-5)/2Ta2On has slower oxygen vacancy transport than HfO2 and Ta2O5 due to its dense atomic packing, despite forming oxygen vacancies more readily, and (3) Oxidation of 3 HfC : 1 TaC ceramics is rate-limited by oxygen transport through a carbon-containing Hf6Ta2O17 layer up to a mechanistic transition temperature and an amorphous oxycarbide layer above that temperature. Improving the oxidation resistance of HfC and TaC will significantly benefit aerospace and clean energy initiatives and contribute to fundamental knowledge regarding materials for extreme chemical and thermal conditions.

PHD (Doctor of Philosophy)

Ultra-High Temperature Ceramics; High Temperature Oxidation; HfC-TaC; Density Functional Theory; CALPHAD; Kinetic Monte Carlo; Continuum Modeling; Oxygen Diffusion

English

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2025-12-09