July 28, 2005
FRONTIERS IN MATERIALS SCIENCE
VINSE COLLOQUIUM SERIES
Dr. John Dismukes
Professor of Chemical and Environmental Engineering
Director Manufacturing Value Chain Innovation (MVCI) Center
The University of Toledo
"Growth and Structure of Conical Carbon NanoFiber"\
Abstract. Conical carbon nanofiber (CNF) possesses many of the outstanding electrical, thermal, and mechanical properties of carbon nanotubes, making them excellent candidates as additives and reinforcing agents for polymeric and metal matrix nanocomposites. Conical CNF is currently commercially available from Applied Sciences, Inc. at $100/pound, and plans for a production scale plant (10 million pounds/year) have been announced to make the material widely available at $3-5/pound.However, conical CNF is not as well understood scientifically as SWNT and MWNT. From a chemical engineering viewpoint, there is a need to understand the CNF surface and bulk structure and the surface stoichiometry of oxygen used to bind the fibers to polymer oligomers used to facilitate dispersion and bonding in polymeric matrices.
This presentation summarizes a phenomenological, hierarchical model for the growth and structure of conical CNF, by a cooperative, self-assembly mechanism involving graphene plane nucleation and VLS growth from a liquid Fe-S nanoparticle. The unique growth mechanism incorporates the following steps in the CVD synthesis environment at 1000-1100oC: 1) initial formation of a spherical, liquid Fe-S nanoparticle, 2) decomposition of CH4 at the nanoparticle surface and diffusion through the particle to the particle sides, 3) cooperative nucleation of the truncated graphene cones or graphene plane segments that facilitate shape conversion of the initially spherical nanoparticle to the final conical shape during growth, and 4) growth of the conical CNF by vertical motion of the nanoparticle, reaching a steady state velocity where extrusive forces originating from the nucleation of graphene planes are balanced by viscous forces of motion of the Fe-S nanoparticle through the CVD synthesis gas, and 5) termination of CNF growth either through exhaustion of the CH4 gas source or by poisoning of the surface of the Fe-S nanoparticle.
This unique model provides a self-consistent description of the fiber growth and structure, thereby enabling quantitative calculation of surface carbon atom concentration, apparent fiber density, and concentration of carboxylic acid groups on the surface of oxidized fiber. The predicted surface carboxylic acid concentrations were in reasonable agreement with titration data, thereby providing a sound basis for calculating the reaction stoichiometry of polymer oligomers (in this case a di-epoxide) used to functionalize the surface of oxidized fibers.* First presented at the 206th ECS Meeting, Honolulu, HI, October 2004; To be submitted to Journal of The Electrochemical Society.