REACTIVITY OF CARBIDES WITH FLUORINE OR HOW TO GET CARBON NANOSTRUCTURES?

摘要

Introduction Since a decade, a new way to produce new nanoporous carbons is to use chlorination on carbide in order to etch metallic atoms and to leave the carbon matrix [1,2]. Gaseous chlorine reacts with metallic atoms at high temperature (600 -900°C) to form most of the time gaseous chloride which is easily removed and carbon called carbide derived carbon (CDC). Many studies have been devoted to the chlorination of silicon and titanium carbide because of the multiplicity of carbon structures found after chlorination : from amophous carbon to carbon onions and diamond [3]. However, in order to thermodynamically favor the formation of SiCl_4 or TiCl_4 rather than CCl_4, a chlorination temperature of about 900°C is required [4]. Another way of halogenation can be studied in order to decrease the synthesis temperature: fluorination [5,6,7]. In the same way than chlorination, fluorination of silicon or titanium carbide can form fluoride and carbon. For SiC, only one type fluoride can be obtained: SiF_4 which is gaseous at room temperature. For TiC, two types of fluorides could be formed: TiF_3 and TiF_4 which are solids at room temperature. Such differences of fluoride nature combined to the different carbides morphology (highly cristallized powder or thin solid films) imply to modulate the applied fluorination method. Dynamic and static fluorination methods applied on metallic titanium carbide powder demonstrate the ability of fluorine to etch titanium atoms from the carbide at low temperatures (<150°C) and to form a microporous carbon with a monodisperse pore size distribution. Fluorination of covalent silicon carbide has also been conducted to evaluate effects of carbide nature. An accurate control of the fluorine / carbide stoichiometry is needed to avoid carbon fluorination. Two fluorination processes can be used: dynamic fluorination assisted by quantitative in situ gas phase FTIR or controlled fluorination by the decomposition of a fluorinating agent such as xenon difluoride. The first process applied on powder allows to determine kinetics of gaseous SiF_4 and fluorocarbons formation during the fluorination . The second one has been successfully applied on SiC thin films in spite of the low level of fluorine needed for the selective etching.