Stability of high temperature ceramics under corrosive environments
Currently, ceramics are being used under increasingly demanding environments. This research involves the study of high-temperature stability of ceramic materials in two diverse applications. The first application involves the use of ceramic materials in gas turbines. SiC/SiC ceramic matrix composites (CMCs) are increasingly being used in the hot-sections of gas turbines; and they are subject to recession of their surface if exposed to a flow of high-velocity water vapor, and to hot-corrosion when exposed to 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 hybrid coating system demonstrated excellent physical and chemical stability under severe thermal shock and exposure to an aggressive hot-corrosion environment. Finite element modeling showed that through-thickness cracks reduce the tensile stresses in the TBC, but also reduce the beneficial compressive stresses in the EBC, and may actually lead to the propagation of the vertical cracks into the EBC. 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. A failure model of the SOM membrane by the formation of "inner cracks" was studied, and attributed to yttrium depletion in the YSZ, which leads to phase transformation from the cubic to tetragonal phase. A series of systematic experiments were designed and performed to understand the synergistic roles of silica and YF3 in the flux in membrane degradation. It was shown that silica attacks the SOM membrane, while YF3 in the flux slows down the attack. The mechanism of the yttria depleted layer (YDL) formation was attributed to grain boundary attack by the silica in the flux, which was the rate-controlling step. This led to rapid ingress of the flux into this attacked grain boundaries, and the out diffusion of Y from the cubic YSZ grains to the grain boundary. This depletion of the Y from the cubic grains transformed them into tetragonal. Once all of the cubic grains in the YDL region converted to tetragonal YSZ grains, no further diffusion occurred. Based on the stability test results, a new flux design was proposed and tested. The flux composition did not attack the SOM membrane, and successful separation of silica in the flux to phase pure Si crystals was demonstrated without apparent damage to the SOM membrane, thereby demonstrating the viability of the Si-SOM process.
Thesis (Ph.D.)--Boston University