MSE PhD Final Oral Defense of Jiapeng Xu

TITLE: Stability of High Temperature Ceramics Under Corrosive Environments ABSTRACT: Currently, ceramics are being used under increasingly demanding environments. These materials have to exhibit phase stability and resist chemical attack during service. This research involves the study of the high-temperature stability of ceramic oxide materials in two diverse applications. The first application involves the use of ceramic materials in gas turbines. SiC/SiC ceramic matrix composites (CMCs) are being used increasingly in the hot-sections of gas turbines, especially for aerospace applications. These CMCs are subject to recession of their surface if exposed to a flow of high-velocity water vapor, and to hot-corrosion when exposed to molten alkali salts. This research involves developing a hybrid system containing an environmental barrier coating (EBC) for protection of the CMC from chemical attack and a thermal barrier coating (TBC) that allows a steep temperature gradient across it to lower the temperature of the CMC for increased lifetimes. The EBC coating is a functionally graded mullite (3Al2O3•2SiO2) deposited by chemical vapor deposition (CVD), the TBC layer is yttria-stabilized zirconia (YSZ) deposited by air plasma spray (APS). The stability of this system is investigated, which includes the adhesion between the two coating layers and the substrate, the physical and chemical stability of each layer at high temperature, and the performance under severe thermal shock and during exposure to hot corrosion. The second application involves the formation of solar-grade silicon by an inexpensive and environmentally friendly electrochemical process using an YSZ solid oxide membrane (SOM) at elevated temperature (~1100 °C). The SOM membrane is exposed to a complex fluoride flux with dissolved silica, which is then electrochemically separated into silicon and oxygen. Membrane stability is crucial to ensure high efficiency and long term performance of the SOM process. Degradation of the membrane was observed with this process. In this study, the focus has been on understanding and preventing the interactions between the flux and the SOM membrane. A series of systematic experiment was designed and conducted to study the degradation mechanism. Based on the result, a strategy to counteract the degradation was incorporated to improve membrane stability. COMMITTEE: Advisor: Soumendra N. Basu, MSE/ME; Uday B. Pal, MSE/ME; Vinod K. Sarin, MSE/ME; Srikanth Gopalan, MSE/ME; Chair: Xi Lin, MSE/ME