SiOC-based ceramics

Recently, SiOC-based ceramic nanocomposites (SiOC, SiZrOC, SiHfOC) were investigated with respect to their hydrothermal corrosion behavior (Figme 13). Dense SiOC-based samples were corroded in water in subcritical conditions (sealed containers temperatures from 100 to 250 °C) (Linck, 2012). 

Interestingly, the SiOC matrix was found to be effective in suppressing the corrosion induced phase transformation of the tetragonal oxide phase into monoclinic ZrOj/HfOj, which is a well known problem in the case of zirconia and hafnia materials exposed to hydrothermal conditions. Thus, the excellent behavior of the SiZrOC/SiHfOC-based ceramic nanocomposites relies on a unique synergistic effect related to the reinforcing ZrO /HfO phase and the SiOC matrix protecting against corrosion induced phase transformation of the oxide nanoparticles (Linck, 2012). 

Porous materials can also be coated with zeolite films by direct synthesis. For example, microcellular SiOC ceramic foams in the form of monoliths were coated on their cell walls with thin films of silicalite-1 and ZSM-5 using a concentrated precursor solution for in situ hydrothermal growth (Fig. 9).[62] The zeolite-coated monoliths show a bimodal pore system and are thermally stable to at least 600 °C. A related strategy is based on the conversion of macroporous Vycor borosilicate glass beads, having pores of about 100 nm, to MFI-type zeolite-containing beads retaining the same macroscopic shape.[63] This conversion was achieved by hydrothermal treatment with an aluminium source and a template such as TPABr. 

Despite the fully amorphous nature of the PDCs, the SAXS method has been able to demonstrate the heterogeneons strnctnre of these ceramics and thus helps to elaborate nanodomain models for SiOC- and SiCN-based PDCs (Mera, 2010b Saha, 2006). 

Calorimetry studies on SiOC- and SiCN-based PDCs showed that there is a strong correlation between the composition of the samples as well as the temperature of annealing and the final enthalpy of formation. In the case of SiOC ceramics, a less exothermic enthalpy was measured as the amount of free carbon increases. Moreover, the enthalpy becomes more exothermic as the amount of oxygen increases. 

Polymer derived ceramics have been known for the last four decades and are prepared via solid-state thermolysis of preceramic polymers. They exhibit a unique combination of remarkable properties due to their covalent bonding and amorphous nature. Thus, silicon oxycarbide (SiOC) and silicon carbonitride (SiCN) based ternary PDCs have been shown to possess outstanding high-temperature properties such as stability with respect to crystallization and decomposition, oxidation and corrosion resistance as well as excellent thermomechanical properties (e.g., near zero steady state creep resistance up to temperatures far beyond 1000 °C). Their properties are directly influenced by the chemistry and the architecture of the preceramic precursors, thus there is an enormous potential in tuning the microstructure and properties of the PDCs by using tailored polymers. Furthermore, suitable chemical modification of the preceramic precursors leads to quaternary and multinary ceramics, as it has been shown for instance for silicon boron carbonitride ceramics in the last 25 years, which in some cases exhibit improved properties as compared to those of the ternary materials. 

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