|dc.description.abstract||It has been demonstrated that palladium can be an exceptional catalyst toward the deposition of solid carbon from ethylene in two distinct forms: nanofibers and thin films. Four forms of palladium were tested: sputtered film, foil, sub-micron powder, and nanopowder. The deposition of carbon can be achieved by a very simple method. In this method ethylene and oxygen or hydrogen are flowed through a single-zone, horizontal tube furnace at atmospheric pressure and temperatures typically from 550-700°C. The addition of a secondary gas such as oxygen or hydrogen is vital in driving the deposition. Although both gases improve deposition, the manner in which they do differs. Ethylene-oxygen mixtures are preferred at lower temperatures (i.e. 550°C) than ethylene-hydrogen mixtures (i.e. 700°C). Pd sub-micron was the most prolific form of palladium at producing solid carbon in a combustion environment, whereas nanopowder was in ethylene-hydrogen mixtures. Palladium, of any form, did not catalyze appreciable carbon deposition at any temperature in ethylene alone. These findings suggest that radical species may be imperative to inciting carbon deposition. Independent of the previous finding, it is suggested different mechanisms of growth exist for fibers and thin films. This difference in mechanism is attributed to carbon acting to self-catalyze further deposition.
The resulting carbon deposition rate and morphology were found to be a function of temperature, position in the reactor, duration of the reaction, gaseous environment, and form of palladium. These factors were all interconnected, and had to be considered collectively to predict the efficacy of the reaction toward solid carbon production. Crystallinity was found to increase with temperature, and ethylene-hydrogen mixtures produced more crystalline structures than were formed in a combustion environment, however the carbon produced under any conditions tested here was never fully graphitic, and instead was turbostratic or nearly amorphous.
Based on the findings of the general catalysis study, the promise of application for the carbon nanofibers was anticipated and demonstrated through the formation of fibrous carbon foams. These foams can be generated using a small quantity of palladium (<5% carbon output by mass), and both the macro- and microscale properties will define the overall properties, and therefore the projected use. These fibrous carbon foams can be combined with other materials to form composites which can be integrated during the formation of the foam. Because the foam process does not require high temperatures, a variety of materials with low melting temperatures can be safely incorporated. Also discussed is the potential of carbon nanofibers as an improved method of polymer reinforcement by tailoring morphology through reaction parameters.||en_US