Phase evolution of SiOC-based ceramic nanocomposites derived from a polymethylsiloxane modified by Hf- and Ti-alkoxides

SiOC/HfO₂-based ceramic nanocomposites with in situ formed HfO₂ nanoparticles were prepared via a single-source precursor (SSP) approach starting from a polymethylsilsesquioxane (PMS) modified by Hf- and Ti-alkoxides. By varying the alkyl-group of the employed Hf-alkoxides, SiOC/HfO₂-based ceramic nanocomposites with different HfO₂ polymorphs formed via thermal decomposition of the SSP under the same heat-treatment conditions. Using PMS chemically modified by Hf(OⁿBu)₄, tetragonal HfO₂ phase was formed after the synthesis at 1100°C in Ar, whereas both, tetragonal and monoclinic HfO₂ nanocrystals, were analyzed when replacing Hf(OⁿBu)₄ by Hf(OⁱPr)₄. After oxidation of the synthesized nanocomposites in air at 1500°C, a facile formation of oxidation-resistant HfSiO₄ (hafnon) phase occurred by the reaction of HfO₂ nanocrystals with silica present in the SiOC nanocomposite matrix derived from Hf(OⁱPr)₄-modified SSPs. Moreover the amount of hafnon is dramatically increased by the additional modification of the polysiloxane with Ti-alkoxides. In contrast, ceramic nanocomposites derived from Hf(OⁿBu)₄-modified SSPs, almost no HfSiO₄ is detected after oxidation at 1500°C even though in the case of Ti-alkoxide-modified single-source precursor.