Identification of Collapsed Carbon Nanotubes in High-Strength Fibers Spun from Compositionally Polydisperse Aerogels

Carbon nanotubes (CNTs) of a sufficiently large diameter and a few layers self-collapse into flat ribbons at atmospheric pressure, forming bundles of stacked CNTs that maximize packing, and thus CNT interaction. Their improved stress transfer by shear makes collapsed CNTs ideal building blocks in macroscopic fibers of CNTs with high-performance longitudinal properties, particularly high tensile properties as reinforcing fibers. This work introduces cross-sectional transmission electron microscopy of focused ion beam-milled samples as a method to univocally identify collapsed CNTs and to determine the full population of different CNTs in macroscopic fibers produced by spinning from floating catalyst chemical vapor deposition. We show that close proximity in bundles is a major driver for collapse and that CNT “stoutness” (the number of layers/diameter), which dominates the collapse onset, is controlled by the growth promotor. Despite differences in the decomposition route, different carbon precursors lead to similar distributions of the ratio layers/diameter. The synthesis conditions in this study give a maximum fraction of collapsed CNTs of 70% when using selenium as the promotor, corresponding to an average of 0.25 layer/nm.