Mullite single crystal fibres produced by the internal crystallization method (ICM)

Optically clear, inclusion-free mullite fibers up to 80 mm in length and 80×70 μm2 in cross-section were grown from an alumino silicate melt by the internal crystallization method (ICM). Microprobe analysis reveals a high chemical homogeneity at 76.5±0.5 wt.% Al2O3 and 23.5±0.5 wt.% SiO2, which is close to the 2/1-mullite composition. Areas of slightly decreased Al2O3 content (≈74.5 wt.%) occur rarely, whereas areas of increased Al2O3 content were not observed. Polarized infrared-reflection micro-spectroscopy using spot sizes of 60 μm diameter on sections cut parallel and perpendicular to the fiber axis show the single crystal character of the fibers. Optical microscopy shows that the fibers consist of a mosaic of single crystal areas up to about 5 mm in length. The c axis of the single crystal individuals are misalligned up to θ ≈±3° with respect to the fiber axis. These orientational misfits are believed to be the reason for the development of the mosaic type microstructure during the growth process.