Thermomechanical Properties of Medium- and High-Entropy Oxides and Their Interaction with Molten Salts

Thermal/environmental barrier coatings are a gradually important field of research arising from the growing desire to increase the temperature, and hence energy, of processes such as gas-turbine engines and solar energy storage tanks. These materials need to not only be thermally insulating but also inert in various harsh environments. This challenge is approached by using a relatively new class of materials, high entropy oxides (circa 2015), which typically have five cations in equimolar proportions randomly distributed within a lattice site. High entropy oxides inherently have low thermal conductivities compared to their constituents and compositionally simpler compounds due to the increased disorder. We explore for the first time in high-entropy ceramics into vast compositional space of non-equimolar ratios (medium-entropy). To tackle this near limitless and complex space, we screened numerous novel compositions (> 100) across multiple different (yet related) families. Their thermomechanical properties are measured, and trends are fit to the data to predict, provide guidance, and reduce effort on searching for future/promising materials in this vast compositional space. Key descriptors such as size disorder, oxygen vacancy concentration, and short-range ordering will be discussed. A similar approach is pursued to examine the stability of the oxides in contact with molten silicate and chloride salts. The difference in optical basicity (proportional to the tendency of oxygen to donate electrons) between the oxide and corrosive medium was found to be a reasonable predictor of performance. Additionally, the high-entropy oxides proved to be more stable in chloride salts than a Ni-superalloy reference above 100 h due to a passivating layer that forms. This likely represents the largest dataset provided to the high-entropy ceramic field so far and is to serve as a stepping-stone for future experimental and modeling efforts. Overall, high entropy oxides prove to be a promising route for innovating thermal/environmental barrier coatings by providing improved thermomechanical properties and a vast compositional space poised for further tuning to provide superior performance in various areas