The study of mechanical properties and structures of carbon fibers and carbon nanotube yarns

Abstract

This thesis intends to achieve higher performance of carbon fibers (CFs) and twisted carbon nanotube (CNT) yarns through the investigations of the compression failure mechanism of CFs and the structure changes of CNT yarns during tensile deformation. In order to measure the single fiber axial compression strength of CFs accurately, a procedure and a machine for compression tests were developed. Using this method, compression strength of single CFs was measured. By comparing the average values and the coefficients of variation for the tensile and the compression strengths, it was concluded that the govern factors were different between the tensile and the compression failure. In order to validate the compression failure mechanism, small angle X-ray scattering (SAXS) measurements during compression tests were conducted and the changes in the microvoid structure were analyzed. Based on the compression failure mechanism due to the buckling of carbon layers over the unsupported region, the length of the unsupported region of the carbon layers was calculated and compared with the microvoid length determined with SAXS. Although the length of the unsupported region was slightly smaller than the microvoid length, these lengths showed good correlation. It was concluded that compression failure of CF is initiated from the buckling of the carbon layers adjoining the microvoid. Tensile tests were conducted on twisted CNT yarns with various twist angles. Although the tensile modulus and Poisson's ratio of the twisted CNT yarns decreased with increasing twist angle, the tensile strength, the tensile strain at failure and the contraction in the transverse direction showed maximum at the twist angle of 25°. It is concluded that efficiency of the stress transfer was increased by the increase in the contraction stress in the transverse direction. The structure changes of twisted CNT yarns were measured by conducting SAXS measurements during tensile tests. The inner and outer diameters of the CNTs and the orientation distribution of the CNTs were determined from the SAXS patterns. Based on the analysis using a twisted yarn model, it was found out that the rotation of the CNTs contributes to the deformation of the twisted CNT yarns. The effectiveness of the SAXS measurements was shown for analyzing the structure of the twisted CNT yarns.