Synthesis of Carbon Nanotube Fibers from Carbon Nanotube Aerogel

Abstract

Carbon nanotube (CNT) fiber become the global issue as the next generation material with high tensile strength surpassing the carbon fiber. Based on such high expectations, studies on the synthesis of CNT fiber and post treatment are actively executed throughout the world. For the applying the high tensile strength of individual CNT to the CNT fiber, indepth understanding of synthesis mechanism is essential. In this study, different kinds of CNT fibers were synthesized and analyzed using vertical reactor, and from these experiments, the synthesis mechanism of CNT fiber was proposed and verified. Also, high quality and high strength were achieved from the success in synthesing the CNT fibers containing Single-Walled CNTs (SWCNTs). For the continuous synthesis of CNT fiber from the CNT aerogel in the vertical reactor, the optimization of experimental conditions is the essential process. Design of experiment (DOE) was performed for effective optimization of each conditions. Three major factors are distinguished among six experimental factors and the optimization process was performed based on the major factors Two kinds of CNT fibers were synthesized using liquid source (acetone, ferrocene, thiophene) and gas source (methane, ferrocene, sulfur). In case of synthesized CNTs using liquid source, diameter was large (5~10 nm) and iron particles surrounded with surfur were founded. On the other hand, synthesized CNTs using gas source had small diameter (1~1.5 nm) and small iron sulfide catalysts were founded. From the comparative analysis of two different CNT fibers, mechanism is proposed which is based on the decomposition temperature of reactants. When using the liquid source, the sulfur was present on the surface of the large size of iron catalyst due to the difference of decomposition temperature between the ferrocene and thiophene. When using the gas source, the contact with iron particle and sulfur became faster. In this case, the sulfur prevented the aggregation of iron particles so that the small size of catalysts were synthesized. As a result, SWCNTs were synthesized from small catalysts. To verify the proposed mechanism, CNT fibers are synthesized with the different length of sulfur injection tubes and fixed amount of reactants. Each feed material was separately injected from the top of the reactor. As the length of the sulfur injection tube increased, the contact time between the iron particles and the sulfur atoms decreased. When the contact time between iron particle and sulfur is decreased, large diameter catalysts were synthesized (average diameter of catalyat is ~12 nm). Whereas the contact time is increased, small diameter catalyst were synthesized (average diameter of catalyat is ~5 nm). This simple experimental modification was used to control the majority of CNTs in the CNT fibers synthesized from SWCNTs, through Double-Walled CNTs (DWCNTs), and finally to Multi-Walled CNTs (MWCNTs), with corresponding IG/ID ratios that varied from 26.9 to 1.5. Additionally, the SWCNT fiber which has a small diameter of CNTs showed the high strength (average strength of CNT fiber is ~1.0 N/tex). This study will provide the guideline to researchers on the CNT fibers, and also, it will contribute to the research for the improvement of mechanical properties of CNT fibers.