Fabrication and Characterization of Carbon Nanotube Grafted Carbon Fibers and Their Composites

Abstract

Hierarchical structuring of the reinforcement such as nano-structural add-ons on micron-scale substrates is an effective method for improving the mechanical, electrical, and optical properties of polymer composites. In this study, the hierarchical nano-tethering of carbon nanotubes (CNT) on micron-scale carbon fiber (CF) via chemical vapor deposition (CVD) is studied to prepare multifunctional reinforcements suitable for advanced polymer composites and to evaluate the enhanced performance of the composites reinforced with such hierarchical reinforcements. The effect of processing conditions for catalytically growing CNTs on CF on the mechanical properties of the substrate CF is firstly investigated by tracing the tensile strength and the microstructure of CFs at each process. The main process and mechanism for degrading the substrate CF are revealed, based on which the CNT grafted CFs with even increased tensile strength are fabricated. The interface and adhesion between CNTs and CF are investigated quantitatively. After analyzing electron microscopy, it is concluded that under specific conditions, the foot-carbons of CNT form carbon-carbon bonds with the graphitic carbons of CF due to the catalyst nanoparticles wrapped by the graphitic layers before CVD, which can be observed only for the carbon substrate. The bonding energy of single CNT on CF is measured using an experimental method developed in this study and also compared with simulation results obtained by molecular mechanics. As the bonding energy is about 7 pJ, it is not comparable to other substrate cases (Si or Cu), supporting the carbon-carbon bonds formed at between CNTs and CF surface. Interfacial shear strength (IFSS) of hierarchical reinforcements developed in this study is evaluated by fabricating single-fiber epoxy composites and carrying out fragmentation test. CNT-grafted CFs, prepared using a semi-optimized process established throughout this study, demonstrate that the tensile strength of CF and IFSS of the composite are concurrently improved as much as about 14% and 474%, respectively. Lastly, the mechanical properties of CNT grafted CF reinforced composite are predicted using finite element method, showing that the tensile modulus and strength of the composites can be increased by ten and several hundred percentage in the longitudinal and transverse directions, respectively.