Dipole polarizability of onion-like carbons and electromagnetic properties of their composites.

Onion-like carbons (OLC) obtained by thermal transformation of nanodiamonds are agglomerates of multi-shell fullerenes, often covered by an external graphitic mantle. For the present work, elemental OLC units were constructed on the computer by coalescence of several two-layer fullerenes, in a structure similar to carbon peapods with a corrugated external wall. The electrical polarizability of such pod-of-peas fullerenes has been computed by a classical monopole-dipole atomistic theory. The description of pod-of-peas fullerenes was further simplified by representing them as linear arrays of point-like objects, whose polarizability matches that of the starting molecules. Calculations demonstrated that the static polarizability of spherically shaped assemblies of these arrays, modeling real OLC materials, is weakly dependent on the geometry of its constituent molecules and is chiefly proportional to the volume of the whole cluster. It increases with increasing filling fraction of the pod-of-peas fullerenes in the OLC aggregate. The polarizability so obtained can be used in Maxwell-Garnett theory to predict the permittivity of OLC-based composites, at least for static excitations. Experimental results obtained at GHz frequencies reveal a weak attenuation for OLC- and nanodiamond-based polydimethylsiloxane composites. In these silicone composites, we did not find long chains of coupled OLCs. Quite separated clusters were found instead, which contribute little to the polarizability and to the dielectric properties, in good agreement with our theoretical predictions.