The Mechanical Properties of Carbon Nanotube Yarns

Abstract

Considerable efforts have been made to realize excellent material properties of carbon nanotubes(CNTs). CNT yarns have been recognized as the most attractable products derived from CNTs for their macroscopic applications. For applications to be successful, it is essential to understand the geometrical arrangement and orientation of individual CNT within CNT yarns during their deformation and the effect of such geometrical parameters on the mechanical behavior of CNT yarns. Above all, this thesis aims to predict the strength of CNT yarns.

At first, strength of CNT yarns was investigated from previous staple yarn theories based on the similarity of the geometry, i.e. individual CNTs were regarded as short fibers. From the difference between experimental and theoretical values predicted by yarn theories, it was figured out that requirement to design the strength prediction model.

The mechanical behavior of CNT yarns was investigated along with their internal changes using in-situ polarized Raman spectroscopy. Firstly, a fiber orientation function for each CNT within the CNT yarn was determined using the relationship between polarized intensities. Strain induced Raman band shifts which are related to the mechanical deformation of individual CNT within CNT yarns were investigated at concurrence. The tensile and torsional behavior of CNT yarns were then studied focusing on the motion of individual CNTs using in situ Raman spectroscopy and the existing staple yarn theory.

A new model for predicting the tensile strength of CNT yarns is developed based on their inner structure changes. To develop the model, deformation of CNT yarns was observed by using in-situ tensile test and the changes in inner structures were investigated by focused ion beam milling process. Based on the experimental results, failure mechanism was defined and theoretical model was built up in respect of interfacial shear stress originated from Van der Waals forces and frictional stresses due to the lateral pressure generated from inherent yarn structure. A new concept, CNT clusters which divides CNT bundles like as unit cell and have hexagonal packed structure was introduced to define the model. With the CNT cluster concept accepted, the strength of CNT yarns was predicted by combine interfacial shear stress and frictional stress. The prediction is then compared with experiments to validate that the current model incorporating the CNT cluster is highly suitable for predicting the tensile strength of CNT yarns.

Finally, strategy for enhancing the strength of CNT yarns was suggested based on the results of this study.