Abstract:
Laser Chemical Vapor Deposition (LCVD) is a promising new processing technique by which freestanding structures, such as fibers, can be deposited. The deposition of carbon fibers by use of ethylene as a precursor gas can be easily achieved and has been investigated as a way to complement or even replace current carbon fiber production techniques. The properties of carbon fibers deposited from ethylene via LCVD have been investigated for low precursor pressures thus far. However, deposition rates for low precursor pressures are limited and rates that are orders of magnitude faster can be achieved by use of higher precursor pressures. No detailed studies on properties of fibers processed at these higher pressures have been conducted. This thesis fills this knowledge gap by exploring the relationships between processing conditions, growth behavior, microstructure and mechanical properties of carbon fibers deposited from ethylene at hyperbaric pressures. It is found that the fiber growth rates are limited by surface reaction kinetics at low temperatures, while they are mass transport limited or gas phase nucleation limited at high temperatures. When grown under mass transport limited conditions, fibers exhibit drastic changes in morphology and microstructure. The tensile strengths of the carbon fibers grown by LCVD are generally found to be poor due to the nature of graphitic carbon deposits. However, the Weibull modulus among the LCVD grown carbon fibers is found to be high. Trends in mechanical properties with processing conditions and microstructure are observed.
