| Summary: | 碩士 === 國立臺灣大學 === 化學工程學研究所 === 90 === Cerium oxide (CeO2) and cerium hydroxide carbonate (CeOHCO3) were synthesized at low temperatures via hydrothermal process. The crystalline phase and microstructures of the synthesized powders were investigated while the ratio of the concentration of ammonia to urea was varied. The decomposition kinetic of cerium hydroxide carbonate at high temperatures was also investigated via kinetic analysis.
The amount of added ammonia significantly effected on the morphology and the crystalline phase of synthesized powders. With increasing ammonia concentration, the main structure of hydrothermally obtained powder was changed from orthorhombic cerium hydroxide carbonate to cubic cerium oxide. The structure of hydrothermally obtained powder was identified as pure phase cubic cerium oxide at high concentrations of ammonia. According to TGA analysis, the phase transformation temperature of pure cerium hydroxide carbonate to cerium oxide was 280℃. The phase transformation temperature decreased with increasing the content of cerium oxide formed in the powders. It implies that the existence of cerium oxide enhanced the decomposition reaction of cerium hydroxide carbonate. The change in the ratio of ammonia concentration to urea concentration effectively influenced the structure of obtained powders. However, the morphology of obtained powders was not changed after high-temperature quenching treatment. During the heat treatment, cerium oxide nuclei were formed and gradually grew in cerium hydroxide carbonate. As a result, the grain size of the cerium oxide obtained from the decomposition of cerium hydroxide carbonate was smaller than that of the powders obtained directly from hydrothermal reaction.
The decomposition kinetic mechanism of cerium hydroxide carbonate at high temperatures was identified to be two-dimensional nuclear and growth mechanism. The activation energy was calculated as 84.06 kJ/mol. In comparison with the decomposition activation energy of other carbonates, the activation energy obtained via this research was lower than others. It was because that the existence of cerium oxide would decrease the nucleation energy.
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