107 Silicon Volatility and Transport in Aluminum Silicate Refractory for SOFC Applications

Wednesday, November 4, 2009: 12:00 PM
Charolais (Camino Real Hotel)
Paul S. Gentile, Ph.D. Candidate , Department of Mechanical Engineering, Montana State University, Bozeman, MT
Dr. Stephen Sofie , Department of Mechanical Engineering, Montana State University, Bozeman, MT
Dr. Richard Smith , Department of Physics, Montana State University, Bozeman, MT
Camas Key , Department of Physics, Montana State University, Bozeman, MT
Aluminum silicate is a key material for use as a low cost refractory in SOFC insulation and gas delivery components.  Silicon has been shown to diffuse to the triple phase boundary (TPB) where it acts as an ionic and electronic insulator, degrading SOFC performance.  Therefore, the transport of silicon from the bulk aluminum silicate to the surface where chemical reactions can cause the excess silicon to vaporize and then be transported to the TPB is of importance.  Chemical and microstructure analysis of standard high purity alumina and replacement low purity aluminum silicates was conducted to evaluate the rate, mechanism, and species which contribute to potential poisoning of the SOFC.
EDS conducted on aluminum silicate cross-sectioned samples exposed to humidified air, forming gas and humidified forming gas for 100 hours shows approximately double the quantity of silicon near the surface of the specimens relative to the bulk material, samples exposed to dry air and untreated samples.  XRD results indicate that mullite is a potential source of the volatile gas phase silicon species.  Rutherford Backscattering (RBS) shows high silicon volatility release rates at 800 C in humidified air and forming gas that significantly decrease, reaching a plateau after 100 hours and remaining steady up to 800 hours.    Additionally XPS has been performed to determine a depth sensitive profile of the aluminum silicate's chemistry, including oxidation states of aluminum and silicon compounds upon thermal treatment at 1000 C in air and forming gas, both humidified and dry.