Decomposition-Coarsening Model of SiOC/HfO_2 Ceramic Nanocomposites Upon Isothermal Anneal at 1300℃

摘要

Pronounced variations of the crystallite size of hafnia (HfO_2) precipitates in silicon oxycarbide (SiOC)-HfO_2 ceramic nanocomposites were studied by TEM upon annealing bulk samples at 1300℃ 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℃. 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-Wag-ner (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.