Electrical and thermal property enhancement of fiber-reinforced polymer laminate composites through controlled implementation of multi-walled carbon nanotubes

Abstract

Aligned carbon nanotubes (CNTs) are implemented into alumina-fiber reinforced laminates, and enhanced mass-specific thermal and electrical conductivities are observed. Electrical conductivity enhancement is useful for electrostatic discharge and sensing applications, and is used here for both electromagnetic interference (EMI) shielding and deicing. CNTs were grown directly on individual fibers in woven cloth plies, and maintained their alignment during the polymer (epoxy) infiltration used to create laminates. Using multiple complementary methods, non-isotropic electrical and thermal conductivities of these hybrid composites were thoroughly characterized as a function of CNT volume/mass fraction. DC and AC electrical conductivity measurements demonstrate high electrical conductivity of >100 S/m (at 3% volume fraction, ∼1.5% weight fraction, of CNTs) that can be used for multifunctional applications such as de-icing and electromagnetic shielding. The thermal conductivity enhancement (∼1 W/m K) suggests that carbon-fiber based laminates can significantly benefit from aligned CNTs. Application of such new nano-engineered, multi-scale, multi-functional CNT composites can be extended to system health monitoring with electrical or thermal resistance change induced by damage, fire-resistant structures among other multifunctional attributes.

Introduction

Carbon nanotubes (CNTs) can be potentially incorporated into existing aerospace structural composites to enhance multiple properties. Existing aerospace structural composites are tailored with layers (laminae) of aligned advanced fibers in a polymer to achieve tailorable and high mass-specific properties such as strength and modulus. While use of such composites reduces the structural mass, additional components and thus mass are still added to the system to compensate for composites’ non-mechanical properties. For example, carbon fiber reinforced plastics (CFRPs) have relatively low electrical conductivity (∼10³–10⁴ S/m in-plane and ∼1 S/m through-thickness [1], [2]). Enhanced electrical conduction is required for airplane structures to protect against lightning and EMI from high power transmitters [3], [4]. Thus, aircraft structures made of composites are commonly installed with metal layers or metal-coated layers like copper fabric, which weigh ∼10–30 kg/m³, ∼10 times more than composites [5]. Similarly, thermal conductivity of fiber–matrix composites is low (∼10 W/m K in-plane and ∼1 W/m K through-thickness [1], [6], [7], [8], [9]). In areas of high heat buildup such as electrical systems, engines, and aerodynamically heated sections, heat dissipation needs to be enhanced by addition of thermal management layers or thermal interface materials (TIMs).

CNTs hybridized into existing advanced composites are a new solution in the area of electrical and thermal transport, in addition to enhancing mechanical properties such as interlaminar shear strength, tension-bearing strength, and toughness [10], [11], [12], [13]. The strong C–C bonding and CNTs’ oftentimes flawless molecular structure provide advantages in multiple properties, including high mechanical stiffness and strength, and thermal and electrical conductivities comparable to or higher than metals [14], [15]. In addition, CNT density (1.4 mg/mm³ for packed bulk SWNTs of ∼1-nm diameter [16]) is lower than metals, suggesting a good fit for mass-specific properties required for aerospace applications. High aspect ratios of CNTs are advantageous to form a physical network throughout the structure that works as electrical and thermal conductive pathways [11], [12].

While many applications are anticipated, little has been thoroughly investigated for nano-scale CNT implementation in micro-scale composite structures. This work is motivated by potential applications of the new nano-engineered CNT-composites with ‘multi-functionality’. When CNTs are implemented in, for example, aerospace structural CFRPs, the same CNT network could function as mechanical reinforcement, shielding against EM waves and lightning strikes, heat distributor, resistive heater for deicing and antiicing, fire retardation with high thermal stability [17], [18], health monitor through changes in electro-thermal properties due to structural failure [19], and more.