Carbon content and pyrolysis atmosphere effects on phase development in SiOC systems

Abstract

In this study, bulk silicon oxycarbides (SiOCs) were fabricated from base polysiloxane (PSO) systems with different carbon content by using Ar or Ar + H2O pyrolysis atmosphere. Compared to the Ar pyrolysis condition, the SiOC samples pyrolyzed with water vapor plus Ar generally show lower ceramic yield except for the Tospearl (polymethylsilsesquioxane) sample at 1400 °C. The SiOC ceramics contain significantly less SiC and carbon after pyrolysis under Ar + H2O atmosphere compared to pure Ar atmosphere. The carbon-poor Tospearl sample shows a crystalline SiO2 structure (cristobalite) after pyrolysis at 1400 °C in Ar + H2O, which is also confirmed using TEM diffraction pattern analysis. TEM microstructures indicate little change in microstructures for the carbon-rich samples. The fundamentals, such as total Gibbs free energy, the driving force for crystallization, and phase contents at different pyrolysis temperatures can be calculated based on a Gibbs free energy minimization method. The phase content calculations predict considerable decrease in the amounts of SiC and C and significant increase in the percent of SiO2 after pyrolysis in Ar + H2O compared to Ar. The thermodynamic calculation results match with our experimental observations. This work provides a guided method to synthesize high temperature SiOCs with desired phases.