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Decomposition‐Coarsening Model of SiOC/HfO2 Ceramic Nanocomposites Upon Isothermal Anneal at 1300°C

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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°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°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 m2/s, whereas D Hf of the SiOC bulk was three orders of magnitude lower with 10−21 m²/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.
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Document Type: Research Article

Publication date: July 1, 2012

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