To improve the sustainability of aircrafts, increasing the efficiency of gas turbines – their propulsors – is essential. Since the efficiency grows with the operation temperature, reaching higher temperatures in the engine is a key point for better performances.[1] However, the limit to the operation temperature is set by the material used in the turbines: a Nickel superalloy coated with Yttria-stabilized Zirconia (YSZ), which acts as a thermal barrier coating (TBC). In this way, it is possible to reach only 1200 °C, due to the subsequent decomposition of the YSZ in its equilibrium phases. To overcome this issue, novel high-entropy oxides (HEOs) are developed.[1,2] These materials, with several cations in equimolar amounts and evenly distributed in one or more sublattices, are stabilized by their high configurational entropy. They can have low thermal conductivity, tuneable coefficient of thermal expansion, and good thermal stability,[1] hence, they are promising for TBC applications. In this work, rare-earth high-entropy zirconates, silicates, titanates, and their combinations are investigated with the aim of developing new HEOs to be applied as TBCs on novel Cr-Si alloys, studied by the partner University of Bayreuth. In particular, the use of titanium and silicon may also help in reducing the weight of the HEOs, important for their possible application on aircrafts. The HEOs of interest are obtained either in a pyrochlore and/or defect fluorite crystalline structure, as single-phase or multiphase materials. Previous work [3] showed the possibility of predicting the crystalline structure and phase purity of the HEOs with the general formula A₂Zr₂O₇, that crystallize as pyrochlores or defect fluorites. The results hereby obtained also helped broaden the knowledge in this empiric prediction by changing the cations used in the various HEOs. In addition, different synthetic approaches have been followed: a solid-state route, via ball-milling, and a wet-chemistry route, via co-precipitation. These methods are compared to understand how the synthetic strategy influences the characteristics of the final material, and how these can be exploited for improving their performances in terms of TBC applications.

References
[1] E. Bakan; \textit{Advanced Ceramics for Energy Conversion and Storage}, \textbf{2020}, 3-62.
[2] S. Akrami; \textit{Materials Science and Engineering: R: Reports}, \textbf{2021}, \textit{146}, 100644.
[3] P. Hutterer; \textit{J Am Ceram Soc.}, \textbf{2023}, \textit{106}, 1547.