Surface Integrity and Carbon Chemical Vapor Deposition on Nickel Foil: Surface Abrasive Treatment

Abstract

Carbon chemical vapor deposition on bulk nickel surfaces generated by abrasive treatment was experimentally studied. Initial stages of deposition were closely examined to elucidate possible effect on the formation and properties of carbon nanotube layers grown directly on bulk metal substrates. Two main phenomena complicate graphite-like carbon deposition on rough nickel surface in comparison with the perfect one. The first phenomenon is the rate increase of carbon formation at surface defects including small confined spaces with large surface-to-volume ratio (pits, gaps, cracks, etc.). Small cavities in subsurface layer are also places of enhanced carbon formation. The cause of this increase is the specific chemistry of hydrocarbon pyrolysis in the conditions of prolonged residence time realized in confined space. The high rate of carbon formation leads to fast growth of highly disordered (coke-like) graphite carbon inside such defects. The second phenomenon is the fast metal dusting of micron-sized surface asperities formed by plastically deformed metal and generated by abrasive treatment. Numerous micro-cracks created on surface in course of metal dusting promote the first phenomenon becomes operative. It was proposed that the roughness of catalytically active surface can be the cause of pyrolysis surface chemistry change comparing with smooth surface.Catalytic carbon deposition on bulk metal surface during hydrocarbon pyrolysis is dependent on a number of variables. Reaction temperature, gas chemical nature and pressure are known to be important factors. 1,2 The chemical nature of the catalyst (e.g., alloy) also plays a role. 3 The carbon activity in the gas phase determines whether carbide can form from a particular catalyst metal. Carbide formation in some catalyst metals is believed to have an influence on fragmentation and the creation of filaments. 4 Duration of reaction is also important, as one form of carbon can be produced at first, with another form appearing after longer reaction times. 5 Hydrogen partial pressure has been known to impact significantly the rate at which carbon forms. 6 Hydrogen has also a significant effect on the form of carbon deposited. 5 Many other factors, such as reactive gas transport conditions 7,8 and the surface state of the metal, can have a profound effect on metal ability to catalyze the formation of carbon during a hydrocarbon conversion process. The same factors determine the rate of carbon deposition and deposit morphology. Some works touched upon the effect of metal surface state and surface topographic imperfections on carbon deposition. It has been noted earlier that surface graphite nucleates preferentially at substrate edges, kink sites and surface steps. 9,10 Initial formation of carbon was shown to be larger over the unpolished foil, as would be expected if topographical imperfections are preferential sites for graphite nucleation. 11 The similar results were noticed in more recent works. Microscopic observations revealed that some of the carbons were actuall...