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Random networks comprised of millimeter-long multi-walled carbon nanotubes (CNTs) have shown unique microstructure change mechanisms under uniaxial strain. These networks can be modified into highly aligned microstructures from strain-induced plastic deformation. Applying a treatment consisting of an uncured resin as a load transfer enhancement medium leads to a dramatically increased degree of alignment and final mechanical properties of the CNT networks. The structural evolution of the CNT networks includes different modes: de-bundling, elongation to reduce waviness, sliding friction, and packing for self-assembly into large bundles. The high ductility of the treated networks, which allows the network to achieve high degrees of strain-induced alignment is mainly because the extra high aspect ratios of the individual CNTand their bundles as well as enhanced load transfer. High aspect ratio causes high degrees of entanglement and locking points between the nanotubes in the random network, which are critical to provide adequate nanotube to nanotube load transfer for ductile deformation and lead to substantially increased CNT alignment during mechanical stretching. The classical strain strengthening mechanisms used in metals and polymers such as strain hardening and crystallization of long molecular chains are discussed and compared to CNT network deformation mechanisms. The CNT network strain hardening parameter n value is as high as 0.65, over three times that of annealed low-carbon steel and more than four times of polycarbonate plastics. Strength coefficient K values for the CNT network also show high values up to roughly 450 MPa, comparable to that of annealed magnesium alloys. The results show how the high degree of alignment of CNT networks and strain strengthening can be achieved through simple uniaxial strain and load transfer medium.

This paper reports on a study of carbon nanotube (CNT) thin film, or buckypaper (BP), integrated into carbon fiber (CF) prepreg composites to create hybrid composite materials with high CNT content. The autoclave process of manufacturing hybrid composite laminates was investigated to gain an understanding of nano/micro dual-scale resin flow characteristics. The study found that resin bleeding along the through-thickness direction was inhibited due to extra-low permeability and high resin absorbing capacity of the BP. Resin matrix-impregnated BP layers were much thicker than dry pristine BP due to high resin absorbency and swelling effects. The BP/unidirectional carbon fiber (UD-CF) hybrid composites with local fiber volume fraction of 61.46 vol% in CF ply and local CNT volume fraction of 26.57 vol% in BP layer, had a tensile strength of 2519 AE 101 MPa and modulus of 149 AE 18 GPa. 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.

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