Negative axial thermal expansion coefficient of carbon nanotubes: Experimental determination based on measurements of coefficient of thermal expansion for aligned carbon nanotube reinforced epoxy composites

Due to their superior material properties, carbon nanotubes (CNTs) have many potential applications such as actuators, composites and electronic components. These CNT-based devices may experience high temperature during manufacture and operation. This leads to thermal expansion and residual stress in devices, and affects the device reliability. Therefore, the coefficient of thermal expansion (CTE) of CNTs is an important property for CNT-based devices. Although the CTE in radial direction of CNTs have been investigated using X-ray diffraction, there is no experimental determination for the axial CTE of CNTs and only numerical and theoretical predictions exit. Here, we investigated the CTE of aligned multi-walled CNT (MWCNT) reinforced epoxy composites in the MWCNT alignment direction in the temperature range of 30-60 °C. We demonstrated that the CTE of the composites in the MWCNT alignment direction became negative by addition of more than 10.4 vol.% MWCNTs, and the axial CTE of MWCNTs was deduced to be -1.2 × 10^-5 K^-1 using the rule of mixtures. In addition, the thermal contraction phenomenon was observed in thin sheets of MWCNTs alone. The negative CTE may be explained by out-of-plane atomic vibrations predicted by theoretical and numerical studies.