Mechanical behaviour of carbon nanotube fibers spun from vertically aligned arrays
Date of Issue2013
School of Mechanical and Aerospace Engineering
Due to their unique structure, carbon nanotubes (CNTs) have excellent properties including high thermal conductivity, high current capacity, and superior mechanical and chemical properties, which make them excellent one-dimensional materials with potentially wide applications. In order to utilize these properties, nano-scale CNTs must be prepared into macro-scale assemblies. CNT fibers, notably the fibers spun from vertically aligned CNT arrays, have therefore attracted great attention in recent years. Although these fibers have the inherent potential of leveraging on the excellence of CNTs, the individual fiber’s properties strongly depend on their structures and morphologies. The strength of the best fibers is still much lower than the theoretical and experimental results from individual CNTs or small CNT bundles. Moreover, up to now, no attention has been paid to the influence of test condition, specifically strain rate effects on mechanical performances of CNT fibers. The goal of this research is to investigate the structural properties of CNT fibers, and study the strain rate dependent failure mechanisms and its relation to the fibers’ structure and morphology of failure. CNT fibers were directly spun from vertically aligned CNT arrays. The mechanical behaviors of CNT fibers were investigated for a wide range of tensile strain rates employing several characterization techniques, including static/dynamic tensile tests, polarized Raman spectroscopy, and fiber fracture surface observations. From these systematic studies, the failure mechanism of CNT fibers was analyzed and their relation to fibers’ structures was deduced. The results could be divided into the following sections: Firstly, the general properties of CNT fibers that were spun from CNT arrays were provided. The mechanical strength and electrical conductivity of CNT fibers were measured statistically. The structural properties of CNT fibers were characterized by Raman spectroscopy and non-uniform twisting was found inside the fiber. It was found that once the spinning process was fixed, the uniformity of fibers’ mechanical strength and electrical conductivity was fixed. Based on these systematic experiments, a fixed fiber spinning process was determined for the follow-on studies. Then, CNT fibers were tensile tested in a wide range of strain rates. It was found that the mechanical response of CNT fibers exhibited a strain rate strengthening effect, and two different failure mechanisms dominated at high and low tensile strain rates, respectively. The key factors, inter-tube slippage and CNT alignment, that limited the mechanical properties of current CNT fibers were then discussed and possible failure mechanisms were proposed based on fibers’ mechanical behavior and the observations of fracture surfaces by scanning electron microscopy (SEM). Mechanical behavior and reliability of CNT fibers were then studied in detail under low strain rates. Static mechanical test and electrical measurement, combined with an in situ Raman spectroscopy, were used to monitor the load transfer and failure process. The morphology of CNTs in the fibers was characterized by transmission electron microscopy (TEM). The results further confirmed that the performance of CNT fibers at low strain rates was determined by slippage and breakage between nanotubes or small tube bundles. An improvement approach was proposed and studied by introducing a second element with a 3-D network structured polymer to improve the load transfer efficiency between CNTs (or small CNT bundles) inside the fibers. It was demonstrated that the CNTs in the fiber could be effectively constrained, resulting in more effective load transfer. Comparatively, at high strain rates, the structure dependent mechanical performance of the CNT fibers was investigated. A facile approach was applied to improve the alignment of CNTs by re-wetting, swelling, and re-drying the CNT fibers in a dilute HCl solution. The alignment of CNTs and its influence on fibers were examined. The mechanical behaviors of CNT fibers before and after post-treatment were compared as well. It was found that the alignment of CNTs showed significant influence on fibers’ mechanical strength. Finally, statistical methods employing modified Weibull models was developed to analyze the distribution of mechanical strength and failure mechanisms of fibril materials, was introduced to examine the breaking mechanisms under different strain rates. Variation of fibers’ strength and its dependence on fibers’ diameters and strain rates were investigated. Based on statistical analysis of our experimental data, further mathematical evidence could be deduced to support the failure mechanisms proposed for low and high strain rates.