Growth of Carbon Nanotube Forests for Fiber Application

Abstract

Carbon nanotube (CNT) fiber has gained attention as a new material to replace commercial carbon fibers. However, the research level is primitive, so a thorough basic studies are necessary to surpass the properties of carbon fibers. As a model system for such academic studies, CNT forest is appropriate. In this study, CNT forests were synthesized necessary for CNT fiber applications.

In the first part, we studied the growth of spinnable CNT forests. For the production a strong CNT yarn from a CNT forest, it is desirable to spin the CNT yarn from a tall CNT forest. Unfortunately, the waviness in the lower region of CNT forests that appears after prolonged growth times was revealed to decrease the spinnability. Raman analysis indicated that the wavy morphology is an intrinsic characteristic of individual CNTs. Thus, to grow taller spinnable CNT forests, optimum growth conditions should be maintained for as long as possible, to avoid producing wavy region. Another important aspect in growing spinnable CNT forests is the growth termination process. If the termination process is slow, a wavy region is produced at the bottom of the CNT forest, which dramatically decreases its spinnability. It is therefore desirable to blow off the residual C2H2 gas with an excessive Ar flow, to prevent the production of a wavy bottom region.

In the second part, we studied the growth kinetics of tall CNT forests. The growth of CNT forests has been limited to the centimeter scale due to insufficient understanding of their growth kinetics. To investigate the growth kinetics of CNT forests, we characterized the mass transport phenomena arising during CNT forest. We formulated the hypothesis that such growth is mass transport limited and proposed a model describing this mass transport. According to our model, the effects of diffusion boundary layers on the growth rate are significant. The initial growth rate is expected to increase with the velocity of the bulk gas flow as the boundary layer thickness decreases. To test this prediction, CNT forests were grown at various total gas flow rates in the range 170 sccm to 1700 sccm, which correspond to flow velocities in the range 0.79 cm/s to 7.9 cm/s. The initial growth rate was found to increase from 1.4 mm/h to 3.5 mm/h as the total flow rate increases from 170 sccm to 1700 sccm. Thus there is a clear inverse proportionality between the initial growth rate and the thickness of the diffusion boundary layer, which confirms that the growth of CNT forests is mass transport limited. These results provide new insight into the growth kinetics of CNT forests.

The precise control over the growth of CNT forest was required as the CNT forest becomes important as a model system for CNT fiber studies. The study provides a better understanding on the growth of CNT forests. Spinnable CNT forest and tall CNT forest as tall as 1.8 cm were successfully grown. The research is expected to contribute to the research for the improvement of mechanical properties of CNT fibers.