PROCESSING OF MULTIWALLED CARBON NANOTUBES AS MAGNETIC ADDITIVES FOR POLYMER NANOCOMPOSITES

Open Access
Author:
Haibat, Jatin Pramod
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 13, 2016
Committee Members:
  • Namiko Yamamoto, Thesis Advisor
  • George A Lesieutre, Committee Member
  • Suzanne E Mohney, Committee Member
Keywords:
  • carbon nanotubes
  • manufacturing
  • nanocomposites
  • magnetic assembly
Abstract:
Magnetic nano-additives for polymer nanocomposites are achieved using multi-walled carbon nanotubes (MWCNTs) by functionalizing with oxygen plasma and by coating with ferromagnetic nickel (Ni) layers. CNT-polymer nanocomposites are highly sought multi-functional materials for their tailorable properties. If integrated properly, CNT-polymer nanocomposites can deliver improved interlayer mechanical strength and fracture toughness, high electrical conductivity and current carrying capacity, anisotropic thermal management, and smart functions like actuation and sensing. Currently, the most commonly used commercial aerospace structural material, carbon fiber reinforced plastic (CFRP), has high mass-specific mechanical strength, but is prone to delamination due to weak interlaminar strength and require additional heavy, metal mesh layers due to low electrical and thermal conductivities. Thus many studies have been conducted to integrate light-weight CNT-polymer nanocomposites with high performance in order to improve properties of CFRPs. However, bulk application of CNT-polymer nanocomposites is currently limited due to lack of scalable manufacturing while maintaining organization of CNTs. Dispersion and organization of CNTs are difficult as CNTs tend to agglomerate due van der Waals forces and often entangle due to their high aspect ratios (>~100). These CNT agglomerates in polymer matrices cause voids and behave as defects rather than reinforcement, resulting in property degradation. Interfaces and interphases formed between CNTs and polymer matrices also need to be tuned to avoid functioning as defects or boundary layers. Thus, it is critical to develop a method to effectively disperse and organize CNTs within polymer matrices to improve and tailor the nanocomposite's multi-functional properties. Here, application of magnetic fields is a promising, scalable method to deliver bulk amount of nanocomposites while maintaining organized nanoparticle assembly throughout the polymer matrix before the curing step. Previous studies showed effective alignment of nanoparticles with the small magnetic field (~10 G, an order of magnitude above the earth's natural magnetic field) for reasonable time (~1 hour). In addition, nanoparticle alignment spacing was successfully controlled by varying field oscillation frequency, enabling particle patterning and interface tuning. In this work, MWCNTs (~35 nm diameter and ~200 um length) were processed to be magnetic, so that they can function as additives that can be effectively organized with magnetic fields. MWCNTs, being diamagnetic as fabricated, are coated with a thin layer of Ni with high aspect ratio to exhibit anisotropic ferromagnetism. The process consisted of three steps. First, MWCNTs were synthesized with chemical vapor deposition (CVD). Second, these MWCNTs were treated with low temperature air plasma; the amorphous carbon present on the side walls and entangled CNT layers on the top surface were eliminated. The CNT surfaces were also functionalized, in order to increase adhesion with Ni coating and to improve dispersion and suspension within matrix solutions. The air plasma conditions (power and treatment time) were varied to ensure adequate cleaning, and to maximize functionalization of CNTs without overly damaging the CNT crystal structure. Treated CNTs and their dispersion degree were inspected visually with scanning electron microscopy (SEM), and functional group attachment was quantitatively evaluated using X-ray photoelectron spectroscopy (XPS). From these preliminary results, better CNT dispersion and suspension in isopropyl alcohol (IPA) was observed with the plasma treatment with low (~6.8 W) power setting for moderate duration (~4 min), which can be attributed to most attachment of ether (C-O) functional groups. Third, thin Ni layers, with varying thickness from 20 nm to 100 nm, were e-beam evaporated on the functionalized CNTs. Anisotropic ferromagnetic properties, such as hysteresis, coercivity, and remanence were measured using a vibrating sample magnetometer (VSM). With increasing thickness, magnetization increased but magnetic anisotropy decreased. Finally, preliminary study confirmed effective magnetic alignment of MWCNTs with ~100 nm thick Ni coating dispersed in IPA when applied with weak magnetic fields of ~100 G for 15 min.