High-Temperature Hydroxylation and Surface Corrosion of 2/1-Mullite Single Crystals in Water Vapor Environments
Abstract:2/1-mullite single crystal (001) plates with thicknesses between 0.9 and 1.9 mm were exposed for 1.5, 3, 6, and 12 h at 1670°C to a slowly flowing (100 mL/min) water-rich gas mixture (O2/H2O 80/20). Under the given experimental conditions, 2/1-mullite yielded significant amounts of structurally bound OH groups across the bulk and decomposition of the crystal surface on a micrometer scale. Decomposition products are (i) sodium-containing silicon-rich alumino silicate glass formed from melt and (ii) α-alumina, which crystallizes within melt cavities. The crystal plates that are free of any OH absorption before the corrosion experiments show a steep increase in OH absorption intensity up to 3 h of corrosion and a flattening toward longer times of exposure. The evaluation of OH intensity profiles implies an effective diffusion coefficient DH in the range between 1.5 and 2.5 × 10−7 cm2/s.
Document Type: Research Article
Affiliations: 1: Institute for Mineralogy and ZFM, Hannover University, Welfengarten 1, 30167 Hannover, Germany 2: Laboratory of Solid State Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060, Japan 3: Institute of Materials Research, German Aerospace Center, 51147 Köln, Germany
Publication date: September 1, 2006