Kevin M. Kohlhaas

Graphene-based composite materials

Graphene sheets—one-atom-thick two-dimensional layers of sp2-bonded carbon—are predicted to have a range of unusual properties. Their thermal conductivity and mechanical stiffness may rival the remarkable in-plane values for graphite (∼3,000 W m-1 K-1 and 1,060 GPa, respectively); their fracture strength should be comparable to that of carbon nanotubes for similar types of defects; and recent studies have shown that individual graphene sheets have extraordinary electronic transport properties. One possible route to harnessing these properties for applications would be to incorporate graphene sheets in a composite material. The manufacturing of such composites requires not only that graphene sheets be produced on a sufficient scale but that they also be incorporated, and homogeneously distributed, into various matrices. Graphite, inexpensive and available in large quantity, unfortunately does not readily exfoliate to yield individual graphene sheets. Here we present a general approach for the preparation of graphene-polymer composites via complete exfoliation of graphite and molecular-level dispersion of individual, chemically modified graphene sheets within polymer hosts. A polystyrene–graphene composite formed by this route exhibits a percolation threshold of ∼0.1 volume per cent for room-temperature electrical conductivity, the lowest reported value for any carbon-based composite except for those involving carbon nanotubes; at only 1 volume per cent, this composite has a conductivity of ∼0.1 S m-1, sufficient for many electrical applications. Our bottom-up chemical approach of tuning the graphene sheet properties provides a path to a broad new class of graphene-based materials and their use in a variety of applications.

Fracture Mechanics of One-Dimensional Nanostructures

One-dimensional (1D) nanostructures such as nanotubes and nanowires have attracted considerable attention in recent years due to their promise of applications in sensing and materials reinforcement. Over the past decade various 1D nanostructures has been synthesized. To develop applications with these nanostructures, it is important to first understand their fundamental properties. Our work focused on characterizing the mechanical properties of these novel 1D nanostructures.

Nanocrack Detection in Vibrating Nanowires

Crystalline boron (B) nanowires (NWs) have been synthesized by the CVD method with preformed metal catalyst particles. We have experimentally investigated their dynamical resonance (i) and mechanical strength (ii). Both the two independent methods suggest the possible presence of nanocracks in the tested B NWs. Nanocrack detection can in principle be achieved by analytical calculations quantifying crack position and depth.