INVESTIGATION OF THE MULTIFUNCTIONAL CARBON NANOTUBES-GLASS FIBER-EPOXY COMPOSITES: MICROWAVE ABSORPTION AND MECHANICAL PROPERTIES
Authors: ZHAO RONG , ZHAO GUANG-LIN 
Published: October 2022

With the rapid arising of wireless technology and other microwave applications towards gigahertz frequency in modern communications, microwave absorbing materials (MAM) are playing an increasingly significant role in healthcare, electronic reliability, and defense security. In this work, we aim to explore multifunctional polymer nanocomposites based on carbon nanotubes (CNTs) –epoxy with glass fiber reinforcement, which can be used for microwave absorption and also as structural materials. We fabricated and systematically investigated the mechanical and microwave absorption properties of multi-walled carbon nanotube (MWCNT) - glass fiber (GF) - epoxy composites. Various MWCNT concentrations (1, 3, 5, 7, 9, and 11 wt%) in epoxy resin have been synthesized by shear-mixing method and the reinforcing GF contents (55, 67, and 74 wt%) were controlled by the number of GF plies in the composites. We used low mechanical stirring to avoid introducing air bubbles in MWCNTs - epoxy solution and implemented a press molding (press forces up to 700 N∙cm-2) during the curing process of the composites to reduce voids in the composites. We studied the mechanical properties, electrical conductivity, and microwave absorption properties of the composites, in addition to the morphology and dispersion properties of MWCNTs and GFs in the composites. The scanning electron microscope (SEM) images indicated a more uniform distribution of MWCNTs in epoxy resin than that in the previous work. The microwave absorption measurements of the composites show excellent electromagnetic (EM) wave absorption performance and high reflection loss (RL) at particular frequency range. The maximum RL of the composites can reach to -48 dB at 24.5 GHz for 3.1 wt% MWCNT loadings and 55 wt% GFs in the composites, with an effective bandwidth (i.e., the frequency range for RL below -10 dB) of about ~6 GHz. The EM wave absorption of the composites strongly depends on MWCNT and GF contents and can reach to ~70% at 26.5 GHz with 4.1 wt% MWCNTs in the composite; further, such dependence is peculiar, contradicting to the conventional wisdom, due to the high density interfaces in the materials, which lead to multiple scatterings and multiple absorptions of EM waves. In addition, the tensile strength of the composites was enhanced to ~427 MPa with ~74 wt% GFs, that is comparable to that of commercial Al alloy 6061 (~300 MPa, but not much EM wave absorption). Our results showed that the tensile strength of the MWCNT-GF-epoxy composites was dominated by the contribution of the GF content, while the EM properties are highly impacted by MWCNTs and the interfaces between MWCNT-epoxy matrices and GFs. The mass densities of the composites were about 1.55, 1.73, 1.88 g∙cm-3 for 55, 67, 74 wt% GF reinforced composites, respectively, which are about 30% ~ 43% lighter than the commercial aluminum alloy 6061 (mass density of 2.7 g∙cm-3). The results suggest that the MWCNT-GF-epoxy composites have the potential as multifunctional microwave absorption and low-weight structural materials without the need of additional coating.