Hydrogen bonding and diffusion in mullite

Heating experiments for 12 h at 1200 °C, 6 h at 1300 °C and 4 h at 1400 °C reveal a significant decrease of OH concentration on ppm level. OH absorption profiles measured on cross-sections by infrared microscope technique yielded for example at 1300 °C diffusion coefficients of Da ≈ Db ≈ 8 × 10−9 (parallel to the a and b axis) and Dc ≈ 1.5 × 10−8 cm2/s (parallel to the c axis). The observation Dc > Da ≈ Db corresponds to a preferred diffusion parallel to the structural channels in c direction. The temperature dependence of the diffusion coefficient Dc of hydrogen outward diffusion is described with an Arrhenius activated behavior (190 kJ/mol). It includes the diffusion coefficient of hydrogen inward diffusion, which was obtained by submitting originally H-free mullite at 1670 °C to a water-rich atmosphere.