Dipole polarizability of onion-like carbons and electromagnetic properties of theircomposites

Abstract

Onion-like carbons (OLC) obtained by thermal transformation of nanodiamonds areagglomerates of multi-shell fullerenes, often covered by an external graphitic mantle. Forthe present work, elemental OLC units were constructed on the computer by coalescence ofseveral two-layer fullerenes, in a structure similar to carbon peapods with a corrugatedexternal wall. The electrical polarizability of such pod-of-peas fullerenes has beencomputed by a classical monopole–dipole atomistic theory. The description ofpod-of-peas fullerenes was further simplified by representing them as linear arrays ofpoint-like objects, whose polarizability matches that of the starting molecules.Calculations demonstrated that the static polarizability of spherically shapedassemblies of these arrays, modeling real OLC materials, is weakly dependent on thegeometry of its constituent molecules and is chiefly proportional to the volume ofthe whole cluster. It increases with increasing filling fraction of the pod-of-peasfullerenes in the OLC aggregate. The polarizability so obtained can be used inMaxwell–Garnett theory to predict the permittivity of OLC-based composites, atleast for static excitations. Experimental results obtained at GHz frequenciesreveal a weak attenuation for OLC- and nanodiamond-based polydimethylsiloxanecomposites. In these silicone composites, we did not find long chains of coupledOLCs. Quite separated clusters were found instead, which contribute little to thepolarizability and to the dielectric properties, in good agreement with our theoreticalpredictions.