Carbon-carbon materials composed of carbon nanofibers grown inside the macro-/mesoporous matrix of carbon xerogels

Abstract

This paper reports the synthesis and characterization of carbon-carbon composites based on carbon xerogels with different porosity and carbon nanofibers. Initial carbon xerogels were prepared by pyrolysis in an inert atmosphere of organic xerogels synthesized via a resorcinol-formaldehyde polycondensation route. Carbon nanofibers were grown inside the xerogel matrix by means of catalytic chemical vapor deposition of ethylene on dispersed nickel particles. Two approaches, such as an incipient wetness impregnation and a one-pot synthesis, were used for doping carbon xerogels with nickel. The kinetics of the carbon accumulation were studied in a reactor equipped with McBain balances. The activation energy values estimated for two regions with different carbon accumulation rates were found to be 80 and 100 kJ/mol. In the second region, the higher activation barrier is explained by the deactivation of nickel particles due to their partial blockage by amorphous carbon deposits. The porous structure of initial carbon xerogels was found to affect both the carbon accumulation rate and the carbon yield. Thus, the carbon yield values of 47.1–58.5 g/g_Ni at 450–470 °C were obtained in the case of macroporous xerogel. The content of CNFs in the composites influences their textural characteristics. The optimal CNF loadings were found to be 25 wt% for CX-meso and 45 wt% for CX-macro. In these cases, the carbon deposition procedure should last 3 and 7 min, respectively. Since the grown carbon filaments are characterized by a coaxial-conical morphology, the carbon-carbon composite materials possess a more ordered structure than as-prepared xerogels.