Grain growth in alumina-silica fibers

Alumina-silica fibers, produced by air blowing a melt of 47 wt% alumina and 53 wt% silica, are initially amorphous, but may be heat treated to convert them into mullite and increase crystallite size. On heating to near 1000 °C, an exothermic phase transition abruptly produces mullite X-ray peaks. Further 1 hour heat treatments gradually sharpen these peaks until a minimum peak width is reached near 1600 °C. Scherrer calculations of crystallite size show that the mullite crystallites formed near 1000 °C are about 13 nm in diameter; these regions grow to about 100 nm after 1 hour at 1600 °C.