Mechanical and thermal expansion properties of aligned carbon nanotube reinforced epoxy composites

Abstract

Aligned MWCNT/epoxy composites made with the three types of the multi-walled carbon nanotubes (MWCNTs) with different thermal annealing temperature have been prepared, and the tensile tests and thermal expansion tests of the composites in the MWCNT alignment direction were conducted to evaluate the effects of crystallinity of MWCNTs on the mechanical and thermal expansion properties of aligned MWCNT reinforced epoxy composites. Additionally, the coefficients of thermal expansion (CTEs) of the MWCNTs in the axial direction were computed by using the Young's modulus of the MWCNTs and the CTE of the composites in Turner's model. The annealing temperatures of the MWCNTs were 2400°C and 2900°C. It was shown that the thermal annealing of MWCNTs was effective for improving the crystallinity of the MWCNTs, while it had no major effects on both tensile strength and Young's modulus of the composites. This is mainly because the thermally annealed MWCNTs themselves possessed almost comparable nominal tensile strength (~4-5 GPa) and Young's modulus (~200 GPa) compared with the as-grown MWCNTs. On the other hand, we demonstrated by thermal expansion testing that the thermal contraction of the composites in the nanotube alignment direction was observed by the addition of MWCNTs. All the MWCNTs possessed negative CTEs and the CTEs tend to become more negative with increasing annealing temperature. This is probably because of a decrease in the number of small defects (vacancies and Stone-Wales defects) and intershell cross-linking defects associated with sp³ carbons, and an increase in the sp² carbons owing to the high-temperature thermal annealing.