Effect of Hafnium content on structural evolution of SiHfOC ceramics at high temperatures

SiHfOC ceramics with different Hf contents were prepared by the sol-gel method using polymethylsilsesquioxane and HfOCl₂·8H₂O as the raw materials. The high-temperature structural evolution of the SiHfOC ceramics with different Hf contents was investigated under an inert atmosphere. The results showed that Hf existed in the free state in the SiHfOC precursors with Hf:Si atomic ratios of 0.05 and 0.1. When the Hf:Si ratio was increased to 0.2, Hf was bridged in the SiOC network to form Si–O–Hf bonds, which changed the pyrolysis behavior of the ceramics and improved the ceramic yield. Further, the formation of Si–O–Hf bonds promoted the uniform dispersion of HfO₂ nanocrystals inside the ceramic, which further inhibited carbothermal reduction and increased the thermal stability of the SiHfOC ceramics.