The complex nanodomain structure and energetic features of polymer-derived SiCN and SiOC ceramics have been investigated by multinuclear 1D and 2D solid-state NMR, micro-Raman, SAXS, XRD, HRTEM and oxidative high temperature oxide melt solution calorimetry, respectively. Structural models consistent with all these lines of evidence put emphasis on interconnected fractal domains stabilized by the mixed-bond-structure and interface bonding between nanodomains. The structural and thermodynamic role of hydrogen in the stabilization of interfacial bonding between the nanodomains in polysilylcarbodiimide-derived SiCN is reported as well.

The present Review addresses current developments related to polymer-derived ceramic nanocomposites (PDC-NCs). Different classes of preceramic polymers are briefly introduced and their conversion into ceramic materials with adjustable phase compositions and microstructures is presented. Emphasis is set on discussing the intimate relationship between the chemistry and structural architecture of the precursor and the structural features and properties of the resulting ceramic nanocomposites. Various structural and functional properties of silicon-containing ceramic nanocomposites as well as different preparative strategies to achieve nano-scaled PDC-NC-based ordered structures are highlighted, based on selected ceramic nanocomposite systems. Furthermore, prospective applications of the PDC-NCs such as high-temperature stable materials for thermal protection systems, membranes for hot gas separation purposes, materials for heterogeneous catalysis, nano-confinement materials for hydrogen storage applications as well as anode materials for secondary ion batteries are introduced and discussed in detail.

Silicon oxycarbides synthesized through a conventional polymeric route show characteristic nanodomains that consist of sp² hybridized carbon, tetrahedrally coordinated SiO_4, and tetrahedrally coordinated silicon with carbon substitution for oxygen, called “mixed bonds.” Here we synthesize two preceramic polymers possessing both phenyl substituents as unique organic groups. In one precursor, the phenyl group is directly bonded to silicon, resulting in a SiOC polymer-derived ceramic (PDC) with mixed bonding. In the other precursor, the phenyl group is bonded to the silicon through Si-O-C bridges, which results in a SiOC PDC without mixed bonding. Radial breathing-like mode bands in the Raman spectra reveal that SiOC PDCs contain carbon nanoscrolls with spiral-like rolled-up geometry and open edges at the ends of their structure. Calorimetric measurements of the heat of dissolution in a molten salt solvent show that the SiOC PDCs with mixed bonding have negative enthalpies of formation with respect to crystalline components (silicon carbide, cristobalite, and graphite) and are more thermodynamically stable than those without. The heats of formation from crystalline SiO₂, SiC, and C of SiOC PDCs without mixed bonding are close to zero and depend on the pyrolysis temperature. Solid state MAS NMR confirms the presence or absence of mixed bonding and further shows that, without mixed bonding, terminal hydroxyls are bound to some of the Si-O tetrahedra. This study indicates that mixed bonding, along with additional factors, such as the presence of terminal hydroxyl groups, contributes to the thermodynamic stability of SiOC PDCs.

The present study introduces a facile single-source precursor preparative access to bamboo-like multiwalled carbon nanotubes (MWCNTs) highly dispersed within a mesoporous silica-rich matrix. The metal-free single-source precursor was synthesized via a one-pot sol–gel process using tetramethyl orthosilicate (TMOS) and 4,4′-dihydroxybiphenyl (DHBP) and converted subsequently via pyrolysis under an argon atmosphere into MWCNT/silica nanocomposites. The in situ segregation of the highly defective bamboo-like MWCNTs was carefully investigated and has been shown to occur within the mesopores of the silica-rich matrix at relatively low temperatures and without the use of a metal catalyst. The experimental results have been supported by extensive computational simulations, which correlate the molecular architecture of the single-source precursor with the structural features of the carbon phase segregating from the silica matrix. Furthermore, the role of hydrogen in the stability of the prepared nanocomposites as well as in the high-temperature evolution and morphology of the segregated MWCNTs has been discussed based on vibrational spectroscopy, calorimetric studies and empirical potential calculations. The results obtained within the present study may allow for designing highly-defined nanocarbon-containing composites with tailored structural features and property profiles.