Structure and Properties of Conical Carbon Nanofibers

Structure and Properties of Conical Carbon Nanofibers

Abstract

The discovery carbon nanotubes (CNT?s) in 1991 by Ijima and others electrified the scientific world and jumpstarted the nanotechnology revolution. However, despite many years of efforts by numerous investigators a reliable method for the mass production these materials has yet to be developed, keeping their price extremely high. As a result almost none of the many remarkable applications for CNT?s have been commercialized. Carbon nanofibers (CNF?s) are another form of carbon nanostructures for which a successful method of mass production has been developed allowing them to be produced cheaply. However, their structure is not well understood and some of their properties, particularly the strength of individual nanofibers, have not been investigated because of their complexity. As a result they have not yet achieved widespread interest and use as CNT?s. In this presentation we report on three separate investigations that were undertaken to understand the structure and properties of CNF?s. The first part involved carrying out ab initio simulations to understand the structure of carbon nanocones and nanofibers. A stacked cone structure had been proposed for CNF?s and it was believed that this could accommodate the observed multiple cones angles for this structure. Our analysis showed that a stacked cone structure could only accommodate 5 cone angles, which meant that CNF?s had a different structure. Further analysis showed that this structure was a cone-helix explaining many of the observations made on CNF?s. The second part of the investigation involved confirming the structure of CNF?s using HRTEM. The structure of CNF?s are not perfect enough for their direct confirmation. However, when they are heat treated to 1,500?C and above up to 3,000?C, they were found to undergo a phase transformation to a segmented stacked cone structure with only the 5 standard cone angles. This indirectly confirmed the absence of the stacked cone structure in non-heat treated fibers and the presence of the cone-helix structure. The final part of the investigation involved the development of a new technique to measure the strength of individual CNF?s. Their structural complexity meant that any technique to measure their strength must also be capable determining its morphology accurately. The technique we developed that combines AFM, HRTEM and FIB does precisely that. As a result of these investigations the full structure and strength of CNF?s have been clarified allowing them to be used much more widely.