Comparative Study of Condensation Routes for Formation of Nano- and Microsized Carbon Forms in Hydrocarbon, Fluorocarbon, and Fluoro-Hydrocarbon Systems at High Pressures and Temperatures

Thermal transformations of binary mixtures of hydrocarbons and fluorocarbons under high pressures demonstrate significantly lower initiation temperature thresholds for all major transformation stages, including carbonization, graphitization, and diamond formation, than the transformations of pure hydrocarbon and fluorocarbon components. In addition, along with the formation of micrometer-size diamond fraction, typical for transformation products of pure hydrocarbons, massive formation of nanosize diamond fraction has been observed in the products of conversion of mixtures. In order to gain understanding of the nature of simultaneous formation of micro- and nanosize fractions of diamond in the binary mixtures of hydrocarbon and fluorocarbon compounds, a comparative studies of the attributes of thermal transformations of pure hydrocarbon, fluorocarbons and their homogeneous and heterogeneous mixtures has been carried out under pressure of 8 GPa and temperatures up to 1500 °C. Naphthalene (C 10 H 8 ) and octafluoronaphthalene (C 10 F 8 ), two hydrocarbon and fluorocarbon structural analogues, have been used as model compounds. The massive formation of micrometer- and nano-size fractions of diamond in the products of high pressure-high temperature treatment of homogeneous C 10 H 8 –C 10 F 8 mixtures has been explained by lowering of the initiation temperature threshold for diamond formation due to synergistic effect of fluorine and hydrogen on the transformations of C 10 H 8 and C 10 F 8 in the mixture, which resulted in conservation of significant amounts of onion-like nanoparticles of 5–15 nm size along with submicrometer-size particles of graphitic carbon in the carbonization products of C 10 F 8 at temperatures ∼900–1100 °C. These two size types of carbon particles act as precursors for formation of micro- and nanosize diamond fractions. A substantial effect of gas-phase transport of carbon on the formation of solid products under high pressure in the studied systems has been considered in the discussion of mechanisms of formation of different diamond fractions. The reported results potentially open a new direction for metal catalyst-free synthesis of nano/microsize fractions of pure and doped diamonds.