Accounts of Materials Research
Mechanical Behavior of Single and Bundled Defect-Free Carbon Nanotubes
Yunxiang Bai, Hongjie Yue, Rufan Zhang, Weizhong Qian, Zhong Zhang, and Fei Wei
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.