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To transfer the incredible properties, including ultrahigh tensile strength, Young's modulus, and electrical conductivity of an individual carbon nanotube (CNT) into composite applications, the constituent nanotubes need to possess adequate alignment, interfacial bonding and a high CNT volume fraction. Direct incorporation of the CNT films, or buckypaper, materials into carbon fiber laminated structures to manufacture hybrid composites is an effective approach to utilize the lightweight, conductive and nanostructured nature of dense CNT networks for multifunctional applications of structural carbon fiber composites. This work studied the microstructure-property relationships of CNT networks when orientation is induced. The mechanical stretching method is shown to be scalable and effective for ultra-high alignment. A manufacturing technique of applying a viscous resin treatment before the stretching procedure is shown to allow up to 80% stretching strain and a resultant alignment fraction of 0.93. The resin acts as an effective load transfer media to substantially enhance the ductility for high stretching strain. The alignment characterization is carried out through Raman spectroscopy and X-ray diffraction methods that reveal the graphitic crystal structure of the film. The load transfer mechanisms and failure modes of aligned CNT composites are explored through high concentration CNT reinforced nanocomposites. Atomic resolution transmission electron microscopy (TEM) analysis reveals unusual CNT crystal packing and permit the observation of interesting structural features of the CNTs and their assemblages, including collapse, flattened packing, preferred stacking, folding and twisting phenomena, as well as CNT pullouts from bundles and the resin matrix. The intimate surface-to-surface contact areas between aligned and flattened nanotubes, driven by van der Waals interactions, give rise to a high density packing of the flattened CNTs in the nanocomposite, resembling a graphitic crystal material. Molecular dynamics (MD) simulations were performed through collaboration to model the packing structure and understand the dependence of density on the relative content of flattened nanotube and void space. Macroscopic modeling predictions illustrate how the alignment and volume fraction of the encompassed CNTs affect the stiffness of the overall composite. CNT thin films were integrated into carbon fiber (CF) prepreg composites to create hybrid composite materials with high CNT content through industry standard autoclave fabrication processing. Resin bleeding along the through-thickness direction was inhibited due to extra-low permeability, nano/micro dual-scale flow characteristics and high resin absorbing capacity of the CNT thin film in hybrid composites. CNT swelling effects and resin starvation phenomena are studied in relation to the amount and orientation of the CNT laminates. The flexural three-point bending results of the random and aligned CNT/CF hybrids exhibit an increased resistance to catastrophic failure even under repeated loading parameters as compared to the CF control samples. The dramatic improvements in both in-plane and through-thickness electrical conductivities demonstrate potential for both structural and multifunctional applications of the resultant hybrid composites.
A Dissertation submitted to the Department of Industrial and Manufacturing Engineering in partial fulfillment of the Doctor of Philosophy.
Includes bibliographical references.
Zhiyong Liang, Professor Co-Directing Dissertation; Arda Vanli, Professor Co-Directing Dissertation; Lisa Spainhour, University Representative; Okenwa Okoli, Committee Member; Robin Maskell, Committee Member.
Florida State University
Downes, R. (2015). Scalable Carbon Nanotube (CNT) Alignment: Process Development, Alignment Mechanisms and CNT/Carbon Fiber Hybrid Composite Applications. Retrieved from http://purl.flvc.org/fsu/fd/FSU_2015fall_Downes_fsu_0071E_12844