Polymer-Derived Silicon Oxycarbide/Hafnia Ceramic Nanocomposites. Part I: Phase and Microstructure Evolution During the Ceramization Process

Polymer-derived SiOC/HfO2 ceramic nanocomposites were prepared via chemical modification of a commercially available polysilsesquioxane by hafnium tetra (n-butoxide). The ceramization process of the starting materials was investigated using thermal analysis and in situ Fourier-transformed infrared spectroscopy and mass spectrometry. Furthermore, solid-state NMR, elemental analysis, powder X-ray diffraction, and electron microscopy investigations were performed on ceramic materials pyrolyzed at different temperatures ranging from 800 degrees to 1300 degrees C, in order to obtain information about the structural changes and phase evolution thereof. The hafnium alkoxide-modified precursor was shown to convert into an amorphous single-phase SixHfyOzCw ceramic at temperatures up to 800 degrees C. By increasing the temperature to 1000 degrees C, amorphous hafnia begins to precipitate throughout the silicon oxycarbide matrix; thus, monodisperse hafnia particles with a diameter of < 5 nm are present in the ceramic, indicating a homogeneous nucleation of HfO2. At temperatures ranging from 1100 degrees to 1300 degrees C, crystallization of the hafnia nanoprecipitates as well as phase separation of the SiOC matrix occur. The chemical modification of the preceramic precursor with hafnium alkoxide can be considered as a promising method for the preparation of SiOC/HfO2 nanocomposites with well-dispersed hafnia nanoparticles.