Scalable fabrication and characterization of magnetically aligned and diazotized CNT-epoxy composites for interlaminar reinforcement of aerospace composites
Open Access
- Author:
- Braga Nogueira Branco, Ricardo
- Graduate Program:
- Aerospace Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 07, 2021
- Committee Members:
- Namiko Yamamoto, Thesis Advisor/Co-Advisor
Amy Ruth Pritchett, Program Head/Chair
Charles E Bakis, Committee Member - Keywords:
- aerospace
materials
composites
carbon nanotubes
polymer nanocomposites
nanocomposites
nanotubes
functionalized
covalent functionalization
diazotization
three-point bending
fracture toughness
magnetic assembly - Abstract:
- Interlaminar integration of carbon nanotubes (CNTs) of high aspect ratio and high specific properties is a promising option to improve through-thickness mechanical properties, such as fracture toughness and fatigue life, of fiber-reinforced plastics (FRPs) without the adverse effects of degrading in-plane properties which results from stitching, z-pinning, and 3D weaving. The current challenges of CNT integration into FRPs include CNT dispersion and organization within matrices, and scalable process to infiltrate CNTs and carbon preforms. In this work, these challenges were addressed with the following two objectives, focusing on nanocomposites consisting of an aero-grade epoxy and CNTs for simplicity. The first objective was to develop a process to disperse CNTs and align CNTs in an epoxy matrix in a scalable manner, and the second objective was to evaluate the effectiveness of the above process and resulting composites on toughness reinforcement. To achieve these objectives, two research tasks were set. First, corresponding to the first objective, the process to functionalize and magnetically assemble CNTs in an epoxy matrix was scaled up. CNTs were grown using the chemical vapor deposition (CVD) process with increased hydrogen flow to produce multi-walled CNTs of higher quality with a faster growth rate. The grown CNTs were magnetized by coating them with a thin nickel layer, and then surface functionalized via diazotization. These magnetized and functionalized CNTs were processed up to 37 mg per process. The CNTs were mixed with an epoxy matrix (Epon 862 and Epikure W hardener) to form polymer nanocomposites of varying CNT volume fraction (0.1 vol.% and 0.5 vol.%) and organization (randomly oriented, magnetically aligned with 180 G and 300 G). Second, corresponding to the second objective, the fabricated CNT-epoxy composites were characterized for their fracture toughness using three-point bending test. Fracture toughness did not increase linearly with increasing CNT volume fraction for all aligned CNT-epoxy PNCs. When aligned with 180 G, the fracture toughness was observed to increase 37.08% with 0.1 vol.% and to 71.66% with 0.5 vol.%. However, this trend of increasing toughness with higher CNT fraction was not seen with the CNT-epoxy PNCs aligned with 300 G: 47.15% improvement with 0.1 vol.%, and 20.49% with 0.5 vol.%. The higher magnetic field strength of 300 G could have caused agglomeration of CNTs with high volume fraction. Fracture toughness values did not show many differences when integrated CNTs (0.1 vol%) are randomly oriented or aligned (180 and 300 G); the highest improvement of 60.30% was observed with the randomly oriented CNT-epoxy sample. On the other hand, with 0.5 vol% of CNTs, the sample prepared with 180 G exhibited increased toughness, while the sample prepared with 300 G exhibited decreased toughness. This trend can again be attributed to CNT agglomeration with higher magnetic field of 300 G. Highest fracture toughness improvement of 71.66% was achieved with the sample of 0.5 vol.% CNTs aligned with 180 G, which is higher than that those achieved in the past (12.50% with 3 vol.%). When the fracture surfaces were inspected, CNT bridging and breakage were observed as major toughening mechanisms through SEM inspection of PNC fracture surface. Meanwhile, more detailed inspection about CNT morphology change with varying CNT fraction and magnetic field strength is necessary to further study the relationship between the process, CNT agglomeration and alignment, and toughening effectiveness. The fabrication technology to produce CNT-epoxy composites with magnetically aligned CNTs and resulting toughened CNT-epoxy will be useful when in the next step to integrate CNTs into FRPs.