Purification, Length Characterization and Quality Assessment of Carbon Nanotubes: A Roadmap to Spinning Fibers with Superior Electrical Conductivity and Strength
Tsentalovich, Dmitri Evgenevich
Doctor of Philosophy
The performance of carbon nanotube (CNT) fibers has been limited by an inability to accurately measure CNT length and by an inadequate understanding of how CNT fiber properties depend on intrinsic CNT properties. This dissertation describes a new method for evaluating carbon nanotube length distributions, advances in CNT purification, and a comprehensive analysis of how CNT fiber performance is influenced by properties of the starting CNT material. We determine length distributions for CNT samples from a combination of extensional viscosity measurements and isotropic cloud point measurements of semidilute CNT solutions in chlorosulfonic acid. The scaling of isotropic cloud point concentration with average CNT aspect ratio determined from extensional viscosity measurements closely matches theoretical predictions. Unlike length measurement techniques that rely on CNT sonication or functionalization, the extensional viscosity method is the only bulk measurement method that can probe CNT samples with average lengths greater than several microns. We apply this length measurement technique, along with purity measurements via thermal gravimetric analysis (TGA), and graphitic character measurements by Raman spectroscopy to optimize hydrogen peroxide based purification of CNTs. The optimal purification conditions minimize cutting of CNTs, while maximizing CNT purity, purification yield, and graphitic character. Despite appreciable shortening of CNTs by purification, transparent and conductive thin film properties improve after purification because of improved CNT purity and graphitic character. High CNT purity, excellent graphitic character, as well as high aspect ratio are all critical for producing high-performance, multifunctional CNT fibers by wet spinning from acid solutions. Contrary to extant work, number of CNT walls does not appear to considerably affect fiber properties. High purity levels are primarily important for efficiently mixing and processing CNTs, whereas, graphitic character and high aspect ratio both strongly influence fiber strength and electrical conductivity. Using the highest quality, available CNT material, we spin continuous, high-strength fibers with the highest electrical conductivity reported in the literature. The fiber properties reported here represent significant improvements over past acid-spun and solid-state spun CNT fibers. The advances described in this thesis will improve CNT manufacturers’ ability to assess CNT quality; enabling researchers to continually improve the strength and conductivity of CNT fibers.
carbon nanotube fibers; extensional rheology; purification