Formation of Structural Bonded OH-Groups in Fused Mullite – Influence of Grain Boundaries on the Diffusion Detected by Transmission Micro-IR Spectroscopy

Polished plane-parallel sections (1 mm in thickness) of fused mullite (2Al 2 O 3 · 1SiO 2) were exposed to a slowly flowing (≈ 100 ml/min) water vapor containing atmosphere (H 2 O/O 2 = 20/80) at a temperature of 1670 °C for 1.5, 3, 6 and 12h, respectively. The water vapor experiments on fused mullite were carried out together with experiments on mullite single crystals reported earlier [1]. All samples were investigated by means of optical microscopy and transmission micro-infrared spectroscopy (Bruker IFS 88 with an attached Bruker IRscope II) before and after the experiments. Two effects were observed: surface corrosion characterized by the formation of α-alumina on the mullite surface and the formation of structurally bonded OH groups in the bulk due to diffusion processes. The surface corrosion effects as well as diffusion profile analysis were described in detail for mullite single crystals earlier [1,2]. In this work results obtained from the fused mullite samples showing the influence of grain boundaries on the diffusion process are reported. Transmission micro-IR-spectroscopy reveals the formation of structurally bonded OH-groups in fused mullite after water vapor treatment. The IR-spectra of the treated samples show, in comparison to the IR-spectra of the as received samples, additional peaks in the range between 3200 and 3700 cm −1 which are characteristic for OH-groups. Lorentz-profile-analysis was used for deconvolution of the IR spectra. A good fit for the IR-spectra measured with the electrical field vector polarized along the crystallographic a-axis of mullite (E ǁ a) was obtained using four peaks (Fig. 1). The results of the profile-analysis are given in Table 1. The IR-spectra measured for polarizations E ǁ b und E ǁ c are significantly different [3,4]. This anisotropy shows that the OH-groups are structurally bonded in mul-lite because an orientation depending absorption could not be due to fluid inclusions. The peak intensities are correlated to the absolute OH-content in the sample and increase with the exposure time. Further IR-measurements were carried out along cross sections of the treated samples (Fig. 2). A xy-stage was used to allow the measurement of data points with constant distances to each other. The distance between two data points was set to 0.1 mm. A slit aperture of 0.1 × 0.6 mm in size was used. All spectra were measured with the electrical field vector polarized along the crystallographic a-axis. The ab