Time-resolved powder neutron diffraction study of the phase transformation sequence of kaolinite to mullite

Mullite formation from kaolinite was studied by means of high-temperature in situ powder neutron diffraction by heating from room temperature up to 1370 degrees C. Neutron diffractometry under this non-isothermal conditions is suitable for studying high-temperature reaction kinetics and to identify short-lived species which otherwise might escape detection. Data collected from dynamic techniques (neutron diffraction, DTA, TGA and constant-heating rate sintering) were consistent with data gathered in static mode (conventional X-ray diffraction and TEM). The full process occurs in successive stages: (a) kaolinite dehydroxylation yielding metakaolinite in the similar to 400-650 degrees C temperature range, (b) nucleation of mullite in the temperature range similar to 980-992 to similar to 1121 degrees C (primary mullite) side by side with a crystalline cubic phase (Si-Al spinel) detected in the similar to 983-1030 degrees C temperature interval; (c) growth of mullite crystals from similar to 1136 degrees C, (d) high (or beta) cristobalite crystallization at T>similar to 1200 degrees C and (e) secondary mullite crystallization at T>similar to 1300 degrees C. The calculated activation energy for the kaolinite dehydration was 115 kJ/mol; for the mullite nucleation was 278 kJ/mol and for the growth of mullite process was 87 kJ/mol; finally for cristobalite nucleation the calculated apparent activation energy was 481 kJ/mol. (C) 2013 Elsevier Ltd. All rights reserved.