The influence of the mass transport regime on laser chemical vapor deposited carbon fibers

Abstract

Laser Chemical Vapor Deposition (LCVD) is a process through which fibers are additively deposited by the dissociation and deposition of a precursor gas into the solid phase under a translating laser focal point. In the hyperbaric pressure conditions, LCVD has two identified reaction regimes. The first being limited by surface reaction kinetics (SRK) and the second being limited by mass transport (MT). This article examines the effects of mass transport (MT) limitations on carbon fiber growth using either ethane (C2H6) or ethylene (C2H4). Both hydrocarbon precursors reveal a growth rate reduction when transitioning from SRK-to-MT to meet the no-slip condition in fluid dynamics. This transition correlates to a fiber microstructure change from a core shell morphology with a graphitic structure (SRK) to a nodular morphology with an amorphous structure (MT). The fiber fracture stress is generally found to be higher in the MT than SRK regime because of the reduction of easily cleaved graphitic planes in the MT fibers. Collectively, these results demonstrate the importance of the MT limited growth regime and how fiber characteristics can be manipulated when grown in this regime.