Fatigue resistance is a key property of the service lifetime of structural materials. Carbon nanotubes (CNTs) are one of the strongest materials ever discovered, but measuring their fatigue resistance is a challenge because of their size and the lack of effective measurement methods for such small samples. We developed a noncontact acoustic resonance test system for investigating the fatigue behavior of centimeter-long individual CNTs. We found that CNTs have excellent fatigue resistance, which is dependent on temperature, and that the time to fatigue fracture of CNTs is dominated by the time to creation of the first defect.
Metrics & MoreArticle Recommendations CONSPECTUS: Superstrong materials can be utilized in many fields such as bulletproof vests, airframes, suspension bridges, flywheel energy storage, etc. As one of the strongest materials, carbon nanotubes (CNTs) can potentially be used to fabricate superstrong fibers. However, the tensile strength of CNTs is impaired a lot by defects, and the CNT fibers prepared so far have strengths much lower than that of a single CNT, showing a "size effect". The conventional study of solid mechanics is usually based on the assumption that materials contain defects. Defect-free ultralong CNTs can hopefully help us avoid the "size effect" of nanomaterials and produce superstrong CNT fibers. They would also provide a system for the study of the mechanical behavior of ideal solids with a nonlocalized "quantum stress singularity".In this Account, we discuss our recent studies on the mechanical behavior of defect-free single CNTs and CNT bundles. First, we introduce the defect-free structure of ultralong CNTs, which is one type of ideal solid. Second, we review the investigation of the static tensile properties and dynamic fatigue resistance and their temperature-dependence of single centimeters long defect-free CNTs. The results showed that defect-free CNTs have superior comprehensive mechanical properties, including superstrength, -toughness, and -durability. Different from traditional materials, the fatigue lifetime and fracture of CNTs are dominated by the first single-bond-sized defect (quantum stress singularity), showing "superbrittleness". Third, by using a gas flow focusing in situ synthesis method as well as a synchronous tightening and relaxing strengthening strategy, we successfully fabricated CNT bundles with tensile strengths approaching that of single CNTs and showed that the "size effect" can be avoided. In addition, the advantages and promising future of using CNTs in flywheel energy storage are discussed. Finally, we provide our perspectives on the challenges and future directions in this field.
Silicon-based anodes are considered ideal candidate materials for nextgeneration lithium-ion batteries due to their high capacity. However, the low conductivity and large volume variations during cycling inevitably result in inferior cyclic stability. Herein, a dry method without binders is designed to fabricate Si-based electrodes with single-walled carbon nanotubes (SWCNTs) network and to explore the different mechanisms between SWCNT and multiwalled carbon nanotubes (MWCNTs) as a conductive network. As expected, higher initial discharge capacity (1785 mAh g −1 ), higher initial Coulombic efficiency (ICE, 81.52%) and outstanding cyclic stability are obtained from the SiO x @C|SWCNT anodes. Furthermore, its lithium-ion diffusion coefficient (D Li+ ) is 3-4 orders of magnitude higher than that of SiO x @C|MWCNT. The underlying mechanism is clarified by in situ Raman spectroscopy and theoretical analysis. It is found that the SWCNTs can maintain good contact with SiO x @C even under tensile stresses up to 6.2 GPa, while the MWCNTs lose electrical contact due to alternating compressive stress up to 8.9 GPa and tensile stress up to 2.5 GPa during long-term cycling. Under such very large stresses, the more flexible SWCNTs and their stronger van der Waals forces ensure that SiO x @C still has good contact with SWCNTs.
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