The formation of graphite encapsulated metal nanoparticles during mechanical activation and annealing of soot with iron and nickel

The investigation of morphological and structural changes during high energy ball milling and thermal annealing of the mixtures soot-iron and soot-nickel demonstrated that the activation is accompanied by the formation of nano-sized metal particles (10-50 nm) distributed over the amorphous carbon matrix. Prolonged mechanical activation of the amorphous soot-iron system (for more than 3-5 min) leads to the formation of nano-sized cementite Fe3C phase. Moreover. mechanical activation of the soot-metal compositions causes a substantial decrease in graphitization temperature of the amorphous carbon: for the soot-iron system, the temperature at which the amorphous carbon starts to crystallize is 250-300degreesC while for the soot-nickel system, the minimal temperature at which the crystallization of the amorphous carbon was observed exceeded 600degreesC. Morphological characteristics of the annealed, mechanically activated soot-metal samples depend on the time of preliminary mechanical activation. The annealing of soot-metal samples obtained after short-time mechanical activation ( 1-3 min) causes a crystallization of the amorphous carbon as onion-like graphite-metal structures. Annealing of the soot/metal samples after mechanical treatment for more than 5 min leads to the formation of metal nanoparticles (40-50 nm) encapsulated by graphite. The longer preliminary mechanical activation, the smaller the size of encapsulated particles. In-situ electron microscopic studies of the interaction of metal particles with amorphous carbon thin film showed that the interaction starts in these systems at temperatures about 600degreesC. The interaction in the systems iron-amorphous carbon film and nickel-amorphous carbon film proceeds via the formation of the carbide phases Fe3C and Ni3C; their decomposition results in the formation of crystal carbon and metal nanoparticles. (C) 2002 Elsevier Science B.V. All rights reserved.