Role of mechanical activation of precursors in solid state processing of nano-structured mullite phase

Effects of precursor milling on phase evolution and morphology of mullite (3Al2O3·2SiO2) processed by solid-state reaction have been investigated. Alumina and silica powders were used as starting materials and milling was taken place in a medium energy conventional ball mill and a high-energy planetary ball mill. Milling in a conventional ball mill although decreases mullite formation temperature by 200 °C, but does not considerably change mullite phase morphology. Use of a planetary ball mill after 40 h of milling showed to be much more effective in activating the oxide precursors, and mullitization temperature was reduced to below 900 °C. Whisker like mullite was formed after sintering at 1450 °C for 2 h and volume fraction of this structure was increased by increasing the milling time. XRD results showed that samples mechanically activated for 20 h in the planetary ball mill were fully transformed to mullite after sintering at 1450 °C, whereas Al2O3 and SiO2 phases were still detected in the samples milled in the conventional ball mill for 20 h and then sintered at the same conditions.