Investigations on Growth of Textured and Single Crystal Oxide Fibers Using a Quadrupole Lamp Furnace

Abstract

Fine ceramic oxide fibers are widely used as reinforcements in composites for high temperature applications. The primary goal of this research effort was to investigate the growth of single crystal or textured oxide fibers by heat treatment of polycrystalline or amorphous, extruded precursor fibers. Mullite was selected for this study due to its excellent chemical stability, creep resistance and strength at high temperatures. Precursor polycrystalline fibers of mullite were prepared by (a) sol infiltration of silk or cotton threads and (b) extrusion. Green fibers of ∼10 μm and ∼150 μm could be made by sol infiltration (of single silk filament) and by extrusion, respectively. A quadrupole lamp furnace, with a small, disc-shaped, hot zone was used for the heat treatment. The effect of temperature and traverse rate through the hot zone, on the microstructure of the polycrystalline precursor fibers, was evaluated. Mullite whiskers were synthesized and used as a template for introducing texturing in extruded mullite fibers. Textured growth of mullite fiber with elongated grains, ∼400μm in length and aligned along the long-axis of the fibers, was achieved with heat treatment. The whisker templated fibers were further subjected to repeated cycles of heat treatment to form a transparent oxide. Preliminary investigations suggest that the transparent part of the heat treated fiber is a single crystal. However, rigorous optical microscopy, x-ray diffraction and TEM investigations are underway to confirm our finding.