Abstract
Studies of thermal transformations of naphthalene (С10Н8), fluorographite (CF1.1) and octafluoronaphthalene (С10F8) and their binary mixtures (С10Н8 – CF1.1, С10Н8 – C10F8) under pressure of 8 GPa have been undertaken as models for gaining understanding of processes of carbonization, graphitization and diamond formation in pure hydrocarbon, fluorocarbon and carbon-hydrogen-fluorine-containing systems under high pressures. The studies found a significant reduction in the initiation temperature thresholds for all major thermal transformation processes in case of binary mixtures with respect to thresholds for pure hydrocarbon and fluorocarbon compounds. Another distinctive feature of the transformations of binary mixtures with respect to diamond formation stage of the transformations of pure hydrocarbons, has been the presence of massive quantity of nanosize (10–60 nm) diamond fraction in the products from binary mixtures along with the micron-size (5–20 μm) diamond fraction, typically observed in the transformations of pure hydrocarbons. The origin of nanodiamond was related to the specifics of carbonization of fluorocarbon compounds under pressure, which at 800–1000 °С produces, along with submicron particles of graphite-like material, a significant amount of closed shell 2–5 layer carbon nanoparticles of 5–15 nm size. These onion-like carbon nanoparticles act as precursors for formation of nano size diamond fractions in the transformations of binary mixtures of hydrocarbon and fluorocarbon compounds. These results potentially open a new direction for metal catalyst-free synthesis of pure and doped diamonds for broad applications. The present article gives an overview of this emerging area of rese arch.
Highlights
The industrial applications of diamond are enabled by its unique material properties, the highest hardness and thermal conductivity of any known bulk material, the lowest coefficient of thermal expansion, chemical inertness and wear resistance, low friction, electrically insulating and optically transparent from UV to far infrared region [1]
The main manufacturing technologies for synthetic diamond currently involve (i) high pressure-high temperature (HPHT) process which recreates conditions of natural growth deep inside the Earth, (ii) low pressure chemical vapor deposition (CVD) from carbon plasma directed over a substrate at 700‒900 °C, using methane/hydrogen mixture as feedstock, mostly for polycrystalline coatings, and single crystals of diamond a few millimeters in size, (iii) explosive synthesis, developed in USSR in 1960s and commercially used since late 1990s for producing nanodiamonds (~5 nm diameter) through detonation of carbon-containing explosives, e.g., TNT
Thermal decomposition of solid hydrocarbons with different molecular structures and bonding types of carbon occurring under a pressure of 8 GPa and temperature of 1500 °C was studied in details in a series of works [6,7,8,9,10,11]
Summary
The industrial applications of diamond are enabled by its unique material properties, the highest hardness and thermal conductivity of any known bulk material, the lowest coefficient of thermal expansion, chemical inertness and wear resistance, low friction, electrically insulating and optically transparent from UV to far infrared region [1]. The main manufacturing technologies for synthetic diamond currently involve (i) high pressure-high temperature (HPHT) process which recreates conditions of natural growth deep inside the Earth, (ii) low pressure chemical vapor deposition (CVD) from carbon plasma directed over a substrate at 700‒900 °C, using methane/hydrogen mixture as feedstock, mostly for polycrystalline coatings, and single crystals of diamond a few millimeters in size, (iii) explosive synthesis, developed in USSR in 1960s and commercially used since late 1990s for producing nanodiamonds (~5 nm diameter) through detonation of carbon-containing explosives, e.g., TNT. The discovery of diamondoids, the smallest nanodiamond structures found in mature high-temperature hydrocarbon oil [5], suggests a possible existence of chemical pathway to diamond from hydrocarbons, serving as carbon source
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