科研之友
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摘要
Pronounced variations of the crystallite size of hafnia (HfO2) precipitates in silicon oxycarbide (SiOC)-HfO2 ceramic nanocomposites were studied by TEM upon annealing bulk samples at 1300 degrees C for times ranging from 1 to 200 h. TEM investigations revealed homogeneous nucleation and crystallization of hafnia in the amorphous, Hf-bearing SiOC matrix upon pyrolysis at 1300 degrees C. Unexpectedly, high-temperature anneal resulted in a pronounced coarsening of the hafnia crystallites at internal surfaces, due to a decrease of the C content at surface-near regions. Based on the crystallite size, the diffusion coefficient of Hf was calculated via the Lifshitz-Slyozov-Wagner (LSW) theory for Ostwald ripening for both the bulk and internal surface regions. The diffusion coefficient of hafnium, D-Hf, in the C-depleted surface areas was 10(-18) m(2)/s, whereas D-Hf of the SiOC bulk was three orders of magnitude lower with 10(-21) m(2)/s. The present work underlines that the polymer-derived ceramics are in fact prone to phase separation and local chemical changes upon high-temperature treatment and are not as stable as commonly considered.
- 出版日期2012-7
