Title : Critical assessment of the multi-functionality of CNT reinforced CFRP composites
Abstract:
Carbon fiber reinforced polymer (CFRP) composites possess attractive mechanical properties that make them the material of choice for aerospace applications. In order to improve their multi-functionality, carbon nanotubes (CNT) have long been used in the hope that these reinforcements will, without compromising the excellent mechanical and fracture properties of CFRPs, lead to better thermal conductivity, electromagnetic (EMI) shielding and weathering resistance. In the present work, we critically compare four different types of composites with a view to assessing their multi-functionality. To this end, we compare, conventional CFRP composites with, i. CF/modified epoxy where the epoxy resin is reinforced with CNTs, ii. grafted CF/epoxy where CNTs are grown on the carbon fibers insidea CVD reactor and iii. grafted CF/modified epoxy where both the processes in i. and ii are used. Our results show that, in terms of both Mode I and Mode II fracture toughness, CFRP < CF/modifies epoxy <grafted CF/epoxy <grafted CF/modified epoxy, though stiffness and strength of all four types are close. Moreover, weathering resistance increases significantly with addition of CNTs to the resin. The composite retains much of its toughness after 1000 hrs of weathering. Anisotropic thermal conductivity is measured through an ingenuous technique involving thermal imaging. But in spite of the fact that CNTs are highly conductive, thermal conductivity in all directions is only marginally affected by the addition of CNTs. The EMI shielding capabilities of an unidirectional conventional CFRP is high when the incident electric field is along the direction of the fibers, which have good electrical conductivity. When the incident field is perpendicular, shielding effectiveness is poor. However, addition of CNTs on either epoxy or grafting them on the carbon fibers do not necessarily lead to higher EMI shielding characteristics though grafted carbon fibers themselves are about 5 times more conducting than conventional fibers. Unfortunately, the wettability of these grafted fibers are poorer and composites made from grafted fibers have a higher void volume fraction than conventional CFRPs. The increase in conductivity of the fibers is unfortunately offset by the presence of a large percentage of microvoids. Finally, we identify possible modifications in the synthesis procedure for obtaining a better combination of favourable properties in CNT modified CFRP composites.
