Experimental study of mechanical and electrical properties of carbon nanofiber/epoxy composites

Abstract

Epoxy nanocomposites of different content of carbon nanofibers up to 1 wt.% have been fabricated under room temperature and refrigerated curing conditions. The composites were studied in terms of mechanical and electrical properties. Flexural modulus and hardness were found to increase significantly in refrigerated samples due to prevention of aggregates of nanofibers during cure condition. Increase and shifting in G-band by Raman spectra of these samples confirmed stress transfer and reinforcement between epoxy matrix and carbon nanofiber. Electrical conductivity improved by 3–6 orders after infusing carbon nanofibers in insulating epoxy. Room temperature samples acquired higher conductivity that was attributed to network formation by aggregates of nanofibers along the fiber alignment direction as revealed by electron microscopic studies.

Introduction

Carbon nanofibres (CNF) are hollow cylinders with diameters typically in the range 50–500 nm and lengths of a few tens of microns giving high aspect ratios (length/diameter >100) with parallel and homogeneous alignment of nanoscopic graphene layers along the axis. They are expected to be promising nanofiller in polymers for the preparation of composites because of their mechanical and physical properties (Young’s modulus ∼500 GPa, tensile strength ∼3 GPa, electrical conductivity ∼10³ S/cm, thermal conductivity ∼1900 W m⁻¹ K⁻¹) [1]. In the polymer field, epoxy resins are well established thermosetting matrices of advanced composites, displaying a series of interesting characteristics like good stiffness and specific strength, dimensional stability, chemical resistance and also strong adhesion to the embedded reinforcement [2]. They are used as high grade synthetic resins, for example, in the electronics, aeronautics, and astronautics industries.

Studies related to the enhancement of the mechanical properties of epoxy matrix by the introduction of CNF have been conducted [3], [4], [5], [6]. To achieve maximum utilization of the properties of nanofibers, uniform dispersion and good wetting of the nanofibers within the matrix must be ensured [7], [8], [9]. It has been extensively reported that dry nanofibers often agglomerate, and thereby greatly reduce their ability to bond with the matrix. All these local interfacial properties will affect the macro-level material behavior [10], [11]. For example, distinct dispersion behavior of CNFs in polymers had a profound effect on the physical properties of the nanocomposites investigated [12]. Investigations specific to the electrical properties of CNF/epoxy composites [6], [13], [14], [15], [16], [17] and general review of the properties of CNF-based composites [18] are available in the literature. Most of the research work reported so far on the electrical properties of CNF-based composites focused on relatively high contents of CNFs, usually higher than 2 wt.%, and aimed to obtain a high conductivity without determination of the critical weight fraction at which the system becomes conductive.

The present work aims to study the influence of reinforcing strategies of CNF/epoxy composites of different wt.% (up to a maximum 1%) of CNFs in the epoxy matrix. To optimize reinforcement of these nanofillers in matrix, a new approach in the form of curing of composite specimens at refrigerated temperature was adopted. Moreover, Raman study supplementing the mechanical properties of CNF/epoxy nanocomposites is yet to be reported in the literature. Macroscopic and microscopic properties of refrigerated nanocomposites have been analyzed and compared with room temperature cured samples. Investigating dispersion strategies and final properties of these nanocomposites will thus help to elucidate the mechanism favoring or hindering synthesis of superior novel materials.