Decomposition‐Coarsening Model of <scp> <scp>SiOC</scp> </scp> / <scp> <scp>HfO</scp> </scp> ₂ Ceramic Nanocomposites Upon Isothermal Anneal at 1300°C

Abstract

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°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 m 2 /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.