Time-resolved powder neutron diffraction study of the phase transformation sequence of kaolinite to mullite |
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| Affiliation: | 1. Instituto de Cerámica y Vidrio (ICV), CSIC, C/Kelsen 5, 28049-Cantoblanco Madrid, Spain;2. Instituto de Ciencia de Materiales de Barcelona (ICMAB), CSIC, UAB Campus, 08193 Bellaterra, Spain;1. School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing, China;2. National Key Laboratory of Biochemical Engineering, Institute of Process Engineering Chinese Academy of Sciences, Beijing, China;1. Dipartimento di Ingegneria Industriale, Università degli Studi di Padova, Via Marzolo 9, 35131 Padova, Italy;2. Ceramics Department, National Research Centre, El-Bohous Street, 12622 Cairo, Egypt;1. University of Chinese Academy of Sciences, Beijing 100049, China;2. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, Guizhou, China;3. Bijie Highland Porcelain Insulator Limited Liability Company, Bijie 551700, Guizhou, China |
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| Abstract: | Mullite formation from kaolinite was studied by means of high-temperature in situ powder neutron diffraction by heating from room temperature up to 1370 °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 ∼400–650 °C temperature range, (b) nucleation of mullite in the temperature range ∼980–992 to ∼1121 °C (primary mullite) side by side with a crystalline cubic phase (Si-Al spinel) detected in the ∼983–1030 °C temperature interval; (c) growth of mullite crystals from ∼1136 °C, (d) high (or β) cristobalite crystallization at T > ∼1200 °C and (e) secondary mullite crystallization at T > ∼1300 °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. |
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| Keywords: | Neutron diffraction Kaolin Mullite Cristobalite Powder–solid state reaction |
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