The polymer‐based synthesis of ceramics such as SiCO (and SiCN) leads to the incorporation of significant amounts of carbon into their molecular structure. A key feature of the nanostructure of these polymer‐derived ceramics is the revelation of persistent, 1–5 nm size domains by small‐angle X‐ray scattering. Here we present a model for these nanodomains, which is consistent with the nuclear magnetic resonance (NMR) data and with the phenomenological properties of SiCO (high resistance to creep and viscoelastic behavior). The model consists of clusters of silica tetrahedra encased within an interdomain wall constituted from mixed bonds of SiCO, and from a network of sp2 carbon. The model predicts the domain size as a function of the carbon content. These predictions are in reasonable agreement with the measurements of the nanodomains in SiCO synthesized with varying carbon contents (the domain size decreases with higher carbon). Simple maps are developed for easy reading of the domain size and the width of the interdomain boundary in the composition diagrams.

Carbon can be used to create unusual nanostructures of Si–C–O by controlled pyrolysis of silsesquioxane organics. Unlike silica, these ceramics resist crystallization at ultrahigh temperatures. Their structure has been compared with that of polymers, where crosslinked chains of polymers in organics are replaced by crosslinked networks of graphene in the ceramics. The network sequesters nanoscale domains of SiO4 tetrahedra. The resistance to crystallization of these nanodomain networks has been attributed to kinetic factors, namely obstruction of long‐range diffusion of silica. In this work, we identify a thermodynamic hindrance to crystallization. Calorimetric measurements of heats of dissolution in a molten oxide solvent show that these ceramics possess a negative enthalpy relative to their crystalline constituents (silicon carbide, cristobalite, and graphite). The thermodynamic stability of the nanodomain structure is explained by a low free energy of the graphene–silica interfaces, perhaps related to the presence of mixed bonds of silicon bonded to both carbon and oxygen.

Amorphous silicon-oxycarbide (SiCO) can retain large mole fractions of carbon when it is made from controlled pyrolysis of silicon-based polymers. The crystallization resistance of these ceramics, which is quite remarkable, varies with the carbon content. In high-carbon materials, crystallization is usually accompanied by weight loss (resulting from the carbothermal reduction of silica), whereas phase separation can lead to crystallization without significant weight loss in the low-carbon materials. A simple graphical method is developed to estimate the composition of the crystallized material, and the concomitant weight loss. The results are presented as maps for a quick estimate of crystallization and weight loss for any composition. Experiments with a medium-high carbon SiCO are used to quantify the degree of crystallization and the associated weight loss at 13001C and at 13501C; these results show that, in the case of medium high carbon content, crystallization begins with phase separation but becomes quickly dominated by weight loss.R. Riedel-contributing editor

A combination of methods, Bragg diffraction, small-angle X-ray scattering (SAXS), and transmission electron microscopy, is applied to the characterization of nanodomains and nanocrystals in polymer-derived ceramics (PDCs). Detailed study of two materials, silicon carbonitride (SiCN) and a SiCN-zirconia nanocomposite, is presented. The first contains domains which can be measured only by SAXS. However, the nanocrystallites of zirconia in the second material can be quantatively studied by all three techniques. In both instances, we find the SAXS to be particularly useful because these data provide detailed information regarding the size distribution of the domains and the crystallites. This information can be valuable in understanding the materials science of PDCs: e,g., the change in the distribution and the average size of the nanoclusters can be modeled to understand the kinetic mechanisms of coarsening at high temperatures. J ournal