Growth of textured mullite 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 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. A quadrupole lamp furnace (QLF), with a small, disc-shaped, hot zone was used for the heat treatment. Micrographic analysis and in situ synchrotron X-ray diffraction analysis have been performed on the mullite system for anisotropic grain growth of mullite. The estimated activation energies from the SEM micrographic analysis were 644.3 and 773.7 kJ/mol for the length and thickness, respectively. An in situ synchrotron X-ray diffraction microstructure analysis was done with a curved image plate (CIP) detector and the fiber was heat treated with a QLF. A Williamson–Hall analysis was carried out for the calculation of the apparent crystallite sizes. The apparent crystallite size showed anisotropy in crystallite growth. Furthermore, the growth rates in the [0 0 1] and [1 1 0] directions demonstrated elongated crystallite growth. Mullite whiskers were prepared by HF leaching out and templated into polycrystalline mullite fiber by extrusion. Textured growth of mullite fiber with elongated grains, ∼400 μm in length and aligned along the long-axis of the fibers, was achieved by heat treatment. Repeated heat treatment cycles of a whisker-templated fiber showed a bamboo-like microstructure. It was confirmed by TEM that the growth direction along the fiber length was the [0 0 1] direction of orthorhombic mullite.