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Gustavo Costa

Publications and source records attributed to Gustavo Costa.

10 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2026

Thermochemical Stability of Ca2Yb8(SiO4)6O2 Apatite in Presence of Molten Calcium-Magnesium-Aluminosilicate (CMAS)

Thermochemical stability of ytterbium silicon oxyapatite Ca 2 Yb 8 (SiO 4 ) 6 O 2 (CYbS) in the presence of molten calcium-magnesium aluminosilicate (CMAS) has been investigated at elevated temperatures for consideration as a thermal and environmental barrier coating (T/EBC) material. CYbS apatite powder was synthesized from the constituent oxides via a solid-state reaction method. Hot-pressed apatite substrates were exposed to molten CMAS at 1200, 1300, and 1400 °C for 1, 10, and 50 h. Development of phases in the interaction region of the heat-treated specimens was monitored using scanning electron microscopy, transmission electron microscopy, high-angle annular dark-field imaging, selected area electron diffraction, and energy dispersive x-ray spectroscopy. Monoclinic cyclosilicate Ca 3 Yb 2 (Si 3 O 9 ) 2 formed from interaction of CYbS apatite with CaO in the CMAS melt at the apatite-CMAS reaction front and continued to nucleate and grow within the residual CMAS in diffusion couples annealed for 1 to 50 h at 1200 °C as well as in those heat treated at 1300 °C for 1 h. Residual CMAS was depleted of Ca when cyclosilicate was present. Dendritic wollastonite (CaSiO3) was observed within the residual CMAS in couples annealed at 1200 and 1300 °C. Ingress of molten CMAS, because of its exponential decrease in viscosity, occurred through open pores and along the grain boundaries of the apatite substrates without any detectable chemical reaction at 1300 and 1400 °C. Results of this study indicate that Ca 2 Yb 8 (SiO 4 ) 6 O 2 apatite has the potential to mitigate the CMAS corrosion up to about 1200 °C but not at higher temperatures.

Narottam P Bansal↗
NASA NTRS2026

Thermodynamic Properties of Uranium-Zirconium Carbide Solid Solutions with Oxygen

Uranium-zirconium carbide solid solutions are potential candidates for fuel in space nuclear reactors. To support development of advanced robust fuels for space applications, (U y Zr 1-y )C x O z solid solutions were synthesized via carbothermic reduction with uranium, carbon, and oxygen contents raging from y=0.05 – 0.3, x=0.95 – 0.98, and z = 0.02 – 0.07. These solid solutions were subsequently characterized by X-ray diffraction and elemental combustion analysis. High-temperature oxidative drop solution calorimetry in molten sodium molybdate solvent at 1073 K was employed to determine the formation, oxidation, and mixing enthalpies of (U y Zr 1-y )C x O z solid solutions, and thereby for evaluating their thermodynamic stability. The solid solutions are enthalpically less stable than the UC and ZrC end members. They also become less stable and more reactive towards oxidation with increasing uranium content. Incorporation of oxygen into solid solutions increases their solid solution-stability and improves their oxidation resistance.

Calorimetry↗
NASA NTRS2025

Thermochemical Stability of Ca2Yb8(SiO4)6O2 Apatite in Presence of Molten Calcium-Magnesium-AluminoSilicate (CMAS)

Thermochemical stability of ytterbium silicon oxyapatite Ca 2 Yb 8 (SiO 4 ) 6 O 2 (CYbS) in the presence of molten calcium-magnesium aluminosilicate (CMAS) has been investigated at elevated temperatures. CYbS apatite powder was synthesized from the constituent oxides via solid state reaction method. Hot pressed apatite substrates were exposed to molten CMAS at 1200, 1300, and 1400 °C for 1, 10, and 50 h. Development of phases in the interaction region of the heat-treated specimens was monitored using scanning electron microscopy, transmission electron microscopy, high angle annular dark field imaging, selected area electron diffraction and energy dispersive X-ray spectroscopy. Monoclinic cyclosilicate Ca 3 Yb 2 (Si 3 O 9 ) 2 formed from interaction of CYbS apatite with CaO in the CMAS melt at the apatite-CMAS reaction front and continued to nucleate and grow within the residual CMAS in diffusion couples annealed for 1-50 h at 1200 °C and those heat treated at 1300 °C for 1 h. Residual CMAS was Ca-depleted when cyclosilicate was present. Dendritic wollastonite CaSiO 3 was observed within the residual CMAS in couples annealed at 1200 and 1300 °C. Ingress of molten CMAS, because of its exponential decrease in viscosity, occurred through open pores and along the grain boundaries of the apatite substrates without any detectable chemical reaction at 1300 and 1400 °C. Results of this study indicate that Ca 2 Yb 8 (SiO 4 ) 6 O 2 apatite has the potential to mitigate the CMAS corrosion up to about 1200 °C but not at higher temperatures.

X-ray diffraction↗
NASA NTRS2024

Theoretical Prediction of Thermal Expansion Anisotropy for Y 2 Si 2 O 7 Environmental Barrier Coatings Using a Deep Neural Network Potential and Comparison to Experiment

Environmental barrier coatings (EBCs) are an enabling technology for silicon carbide (SiC)-based ceramic matrix composites (CMCs) in extreme environments such as gas turbine engines. However, development of new coating systems is hindered by the large design space and difficulty in predicting properties for these materials. Density Functional Theory (DFT) has successfully been used to model and predict some thermodynamic and thermo-mechanical properties of high-temperature ceramics for EBCs, although these calculations are challenging due to their high computational costs. In this work, we use machine learning to train a deep neural network potential (DNP) for Y 2 Si 2 O 7 , which is then applied to calculate thermodynamic and thermo-mechanical properties at near-DFT accuracy much faster and using less computational resources than DFT. We use this DNP to predict phonon-based thermodynamic properties of Y 2 Si 2 O 7 with good agreement to DFT and experiments. We also utilize the DNP to calculate the anisotropic, lattice direction-dependent coefficients of thermal expansion (CTEs) for Y 2 Si 2 O 7 . Molecular dynamics trajectories using the DNP correctly demonstrate accurate prediction of the anisotropy of the CTE in good agreement with diffraction experiments. In the future, this DNP could be applied to accelerate additional property calculations for Y 2 Si 2 O 7 compared to DFT or experiments.

rare earth silicates↗
NASA NTRS2022

Energetics of Reactions between Ceramic Coating Materials and their Binary Oxide Components with Silicate Melts

This paper summarizes our previous and current studies of using high-temperature calorimetry to investigate the energetics of reactions of ceramic coating materials (e.g., yttrium disilicate and 7-wt% yttria-stabilized zirconia) and their binary oxide components with silicate melts in the CaO–MgO–Al 2 O 3 –SiO 2 (CMAS) system. Such interactions are found to become stronger (more exothermic) with increasing difference in acid–base character between these materials and the melt. Our results suggest that the reactivity between the coating materials and the melt increases with decreasing thermodynamic stability (less exothermic enthalpy of formation from oxide components) of the coating material. They also suggest that ceramic coating materials made from binary oxides that have less exothermic enthalpies of solution and mixing are less susceptible to CMAS melt corrosion when in contact with an acidic, relatively polymerized, melt rich in SiO 2 . Thus, we propose that new coating material formulation and CMAS melt corrosion mitigation strategies should be optimized based on the energetic contributions of their binary oxide components.

CMAS↗
NASA NTRS2021

Thermodynamics of the Lu2O3 – SiO2 system and comparison to other Rare Earth Silicates

Environmental barrier coatings are necessary to protect SiC based ceramics and composites from water vapor degradation in harsh engine environments. Currently, rare earth (RE) silicates are the most promising systems to protect SiC based ceramics and composites. This protection is largely based on reduced silica activity in these rare earth silicates which results in a lowered reactivity with water vapor. To that end, previous Knudsen effusion mass spectrometry (KEMS) studies have explored RE = Yb, Y silicates to measure the reduced silica activity and subsequent water vapor reactivity. Similarly, this work employs the KEMS technique to measure the SiO(g) vapor pressure in Lu containing RE silicates to calculate the activity of silica within the monosilicate [log(a(SiO2)) = -2351.1*1/T-1.6731] and the disilicate (log(a(SiO2)) = -4884.0*1/T + 2.2208) as a function of temperature. The enthalpies of formation for Lu monosilicate from the oxides and the elements were calculated to be -45±3 kJ/mol at 1550 K and -2831.1±12 kJ/mol at 298 K, respectively. The measured enthalpy of formation and those found in literature are compared to modeled values from density functional theory and those estimated using electronegativity.

Knudesen Effusion Mass Spectrometry↗
NASA NTRS2020

Corrosion Behavior of Stainless Steel 304 and Nickel 625 Under Iodine Vapor at 300 °C

The chemical, structural, and microstructural behavior of stainless steel 304 (SS 304) and nickel 625 (Ni 625) were probed after exposure to iodine vapor laminar flow at 300 °C. This work was conducted in support of the development of in-space propulsion utilizing iodine as a propellant. The kinetics of corrosion was measured in a custom-built iodine-compatible thermogravimetric analyzer rig at 300 °C in which samples were exposed to an iodine laminar vapor flow of 1 mg/min (carried by 20 mL/min argon gas) for up to 31 days. Samples were characterized after the experiment by X-ray diffraction, electron microscopy, energy dispersive X-ray spectroscopy, and dynamic secondary ion mass spectrometry. All samples formed scales consisting mainly of metal oxyiodides showing different chemistry, microstructure, and crystalline phases. The paralinear rate law best describes the kinetics of oxidation of the samples. SS 304 exhibited slightly slower kinetics of oxidation (106±3·10(exp –5) mg/sq.cm/day) than Ni 625 (122±3·10(exp –5) mg/sq.cm/day) in the low-oxygen and low-water environment.

Chemistry and Materials↗
NASA NTRSDate not supplied

Thermochemistry of Aerospace Materials

The reliability and development of aerospace materials in extreme environments relies on a comprehensive understanding of their thermochemical properties. Such properties are commonly used for equilibrium phase stability calculations of interactions with corrosive environments at very high temperatures. These calculations can lead to in-depth understanding of degradation mechanisms in both coatings and components for applications such as nuclear thermal propulsion and gas turbine engines, both of which are part of NASA’s current R&D focus for improving propulsion systems for aerospace missions. The first part of this talk summarizes and discuss our current thermochemical calculations of the behavior of ceramic-ceramic (cercer) UN-based fuels and their coating systems during nuclear thermal rocket operation. The second part of the talk summarizes and discuss the energetics of reactions of ceramic coating materials and their binary oxide components with silicate melts measured by high temperature reaction calorimetry.

Nuclear Thermal propulsion↗