The Influence of Hydrocarbon Precursors on the Deposition Characteristics of Carbon Fibers

Laser Chemical Vapor Deposition (LCVD) is an additive processing technique in which freestanding fibers are deposited by the thermal dissociation of a precursor gas under a translating laser focal point. For carbon fiber deposition, the precursor is a hydrocarbon gas. In this study the relationships between deposited carbon fiber microstructure, mechanical properties, and processing conditions were compared using four different hydrocarbon gases that span two families, i.e., the alkanes and alkenes. The alkane gases were methane (CH4), ethane (C2H6) and propane (C3H8) and the alkene gas was ethylene (C2H4). These hydrocarbons were chosen because of their differences in carbon bond structure, molecular geometry and/or the number of available carbon atoms. At lower temperatures, all the fibers deposited in a surface reaction kinetically (SRK) limited growth regime whereas only two of the hydrocarbons, C2H4, and C2H6, were able to be induce a second growth regime that was mass transport (MT) limited. These growth regimes had significant impact to the changes in the structure and mechanical strength of the fibers, and, as noted, were not always accessible based on the hydrocarbon molecule. The reason for this inaccessibility is related to the Soret effect and steric resistance connected to the geometric structure of the molecule during deposition. Collectively, the outcomes reveal the impact of precursor chemistry selection in the deposition of LCVD carbon fibers.