Abstract
The present study is concerned with the formation and decomposition of 9 Al20, 2 B2Oupon heating the mixed powder; it was prepared by the simultaneous hydrolysis of aluminum and boron alkoxides. Techniques employed were TG, DTA, electron microscopy, and X-ray analysis.9 Al203- 2 B20, was found to form via following steps: 133+-substituted A10(OH) gel > Amorphous material> Al-B spinel > 9 Al203.2 B203 The reaction process from the mixed powder to 9 Al203 2 B203 was similar to that of mullite (3 Al203- 2 SiO2), formed by heating a clay mineral, kaolinite (Al203 2 Si02 2 H20). The fraction of formation of 9 Al203.2 B20, from Al-B spinel was determined by the lapse of time, as it was a function of time at different temperatures. The data were interpreted in terms of the second-order equation, as a special case of Avrami's equation. On the basis of the Arrhenius plot of log k vs.1/ T the apparent activation energy was found to be 113 kcal/mol. This activation energy corresponded to the energy to establish the growth of active centers. The kinetics of the decomposition of 9 Al20, 2 B20, was determined. The decomposition reaction was of the first-order and the activation energy was 81 kcal/mol. The rate-determining step of the reaction is considered to be the nucleation process.
