cluster approximation кластерне наближення density functional theory method Trilon B X-ray diffraction activation energy of degradation aluminum carbide aluminum nanoclusters ammonium bifluoride catalysis coronene density functional theory electrochemical oxidation epoxy resin expanded graphite flotation-enriched graphite fullerenes graphene oxide graphene-like plane graphite intercalation compounds hydrogen peroxide nitrogen-containing carbon nanotubes silicon carbide structure surface properties thermal decomposition thermal stability thermally expanded graphite thermogravimetry азотовмісні вуглецеві нанотрубки