| Carbide derived carbon (CDC) has been produced by selective etching of metal carbides (SiC and TiC) in halogens at ambient pressure and temperatures between 450 and 1000°C. A systematic electron microscopy study of CDC has been conducted in this work. Carbon phases and all possible carbon structures in CDC have been identified. The CDC treatment enables the formation of virtually any carbon allotrope within one sample. The structure of CDC is tunable through the process parameter time, temperature and gas composition. Depending on experimental conditions, CDC may contain sp3- or sp 2-bonded carbon phases. Amorphous, porous and poorly ordered turbostratic carbon with lattice spacing exceeding d = 0.35 nm was observed in large quantities. sp3-bonded structures nucleate at the metal carbide/CDC interface and consist of mainly 2H (lonsdaleite) and cubic diamond nanocrystals with sizes between 2 to 10 nm. Some reach sizes up to 800 nm. The synthesis of 4H, 8H and n-diamond polytypes is likely. It has been shown that the presence of hydrogen in the environment is not required for diamond synthesis. However, hydrogen can stabilize sp3-hybridization leading to the growth of thick nanocrystalline diamond films. During the annealing process, hexagonal diamond transforms to spherical carbon onions in a “zipper-like” manner where three diamond layers are expected to exfoliate into two graphitic sheets. The onions are either hollow or dense. The latter show a decrease in lattice spacing compared to d/n values for graphite indicating these onions as high-pressure cells. The pressure has been estimated. Graphitic structures include the formation of multiwall nanotubes, polyhedral particles and ribbons. The graphitization process is completed when the maximum permissible diameter of curved lattices is reached and straight graphite is formed. SiC and TiC derived CDC has been analyzed and compared. The basic structure of the substrate has an influence on the formation and structure of CDC. Impurities in CDC have been found and identified. CDC has been applied to SiC fibers and dynamic seals showing excellent potential for applications. |
