Polymer derived silicon oxycarbide ceramics (SiOC-PDCs) with widely different carbon contents have been synthesized, and their structures have been studied at different length scales using high-resolution 13 C and 29 Si magic-angle-spinning (MAS) NMR spectroscopic techniques. The data suggest that the structure of these PDCs consists of a continuous mass fractal backbone of corner-shared SiC x O 4-x tetrahedral units with "voids" occupied by sp 2 -hybridized graphitic carbon. The oxygen-rich SiC x O 4-x units are located at the interior of this backbone with a mass fractal dimension of ∼2.5 while the carbonrich units display a slightly lower dimensionality and occupy the interface between the backbone and the free carbon nanodomains.

To provide a complete picture of the energy landscape of Al 2 O 3 at the nanoscale, we directed this study toward understanding the energetics of amorphous alumina (a-Al 2 O 3 ). a-Al 2 O 3 nanoparticles were obtained by condensation from gas phase generated through laser evaporation of α-Al 2 O 3 targets in pure oxygen at25 Pa. As-deposited nanopowders were heat-treated at different temperatures up to 600 °C to provide powders with surface areas of 670−340 m 2 /g. The structure of the samples was characterized by powder X-ray diffraction, transmission electron microscopy, and solid-state nuclear magnetic resonance spectroscopy. The results indicate that the microstructure consists of aggregated 3−5 nm nanoparticles that remain amorphous to temperatures as high as 600 °C. The structure consists of a network of AlO 4 , AlO 5 , and AlO 6 polyhedra, with AlO 5 being the most abundant species. The presence of water molecules on the surfaces was confirmed by mass spectrometry of the gases evolved on heating the samples under vacuum. A combination of BET surface-area measurements, water adsorption calorimetry, and high-temperature oxide melt solution calorimetry was employed for thermodynamic analysis. By linear fit of the measured excess enthalpy of the nanoparticles as a function of surface area, the surface energy of a-Al 2 O 3 was determined to be 0.97 ± 0.04 J/m 2 . We conclude that the lower surface energy of a-Al 2 O 3 compared with crystalline polymorphs γand α-Al 2 O 3 makes this phase the most energetically stable phase at surface areas greater than 370 m 2 /g.

SiCN polymer-derived ceramics (PDCs) with different carbon contents have been synthesized by pyrolysis of poly(phenylvinylsilylcarbodiimide) and of poly(phenylsilsesquicarbodiimide), and their structure and energetics have been studied using 29 Si, 13 C, 15 N, and 1 H solid state nuclear magnetic resonance (NMR) spectroscopy and oxide melt solution calorimetry. The structure of these PDCs at lower carbon content (35−40 wt %) and pyrolysis temperatures (800 °C) consists primarily of amorphous nanodomains of sp 2 carbon and silicon nitride with an interfacial region characterized by mixed bonding between N, C, and Si atoms that is likely stabilized by the presence of hydrogen. The average size of the carbon domains increases with increasing carbon content, and a continuously connected amorphous carbon matrix is formed in PDCs with 55−60 wt % C. The interfacial silicon−carbon and nitrogen− carbon bonds are destroyed with concomitant hydrogen loss upon increasing the pyrolysis temperature to 1100 °C. Calorimetry results demonstrate that the mixed bonding between C, N, and Si atoms in the interfacial regions play a key role in the thermodynamic stabilization of these PDC. They become energetically less stable with increasing annealing temperature and concomitant decrease of mixed bonds and hydrogen loss.

Amorphous silicon oxycarbide polymer-derived ceramics (PDCs), synthesized from organometallic precursors, contain carbon-and silica-rich nanodomains, the latter with extensive substitution of carbon for oxygen, linking Si-centered SiO x C 4-x tetrahedra. Calorimetric studies demonstrated these PDCs to be thermodynamically more stable than a mixture of SiO 2 , C, and silicon carbide. Here, we show by multinuclear NMR spectroscopy that substitution of C for O is also attained in PDCs with depolymerized silica-rich domains containing lithium, associated with SiO x C 4-x tetrahedra with nonbridging oxygen. We suggest that significant (several percent) substitution of C for O could occur in more complex geological silicate melts/glasses in contact with graphite at moderate pressure and high temperature and may be thermodynamically far more accessible than C for Si substitution. Carbon incorporation will change the local structure and may affect physical properties, such as viscosity. Analogous carbon substitution at grain boundaries, at defect sites, or as equilibrium states in nominally acarbonaceous crystalline silicates, even if present at levels at 10-100 ppm, might form an extensive and hitherto hidden reservoir of carbon in the lower crust and mantle.T he carbon cycle is one of the most important components in the sustenance of life and the evolution of the environment on our planet. The exchange of carbon between the atmosphere, biosphere, and oceans primarily controls the near-surface carbon cycle in the short term (decades to millennia) (1, 2). However, a large fraction of the carbon in the planet is stored at a greater depth (i.e., in the crust, mantle, core), and it exchanges with the surficial carbon only on much longer time scales (millions of years) through processes involving subduction and volcanic activity (3, 4). The form of stored carbon in these deep reservoirs and the amounts of carbon in various reservoirs, as well as the balance between the inward and the outward fluxes of carbon from them, are poorly constrained (3, 4). In the crust and upper mantle under relatively oxidizing conditions, CO 3 2− provides the familiar trigonal (sp 2 ) coordination environment for carbon in carbonates, mixed CO 2 -H 2 O fluid phases and carbonatiteforming melts. Under more reducing conditions, graphite, diamond, and methane become important and carbonate becomes less important or absent. The substitution of tetrahedral (sp 3 ) carbon for silicon in silicate minerals and melts has often been suggested as a possibility at high pressure (5-13), but there was little experimental evidence for it until recent phase equilibrium studies indicated that CO 2 can undergo transformation from a molecular gas to an extended solid at very high pressure and temperature to form a cristobalite-like structure consisting of a network of corner-sharing CO 4 tetrahedra (13,14). Recent studies have also indicated the possibility of a chemical reaction between SiO 2 and CO 2 to form a silicon carbonate phase under pressure (12) and the...