Epitaxial pyrolytic carbon coatings templated with defective carbon nanotube cores for structural annealing and tensile property improvement

Thermal annealing of chemical vapor deposition (CVD)-grown carbon nanotubes (CNTs) is a practical method for reducing defects in CNTs, which is essential for promoting applications of CNTs in nanoscale or microscale materials. However, the increase in annealing temperature fails to bring noticeable improvements in the tensile properties of CNTs, implying the negative influence of carbon sublimation on the defect healing process. Here we propose a two-step annealing strategy for improving the microstructure and tensile properties of CVD-grown CNTs. This has been achieved through the epitaxial growth of pyrolytic carbon thin layers at the CNT surface and then followed by graphitization treatment at 2200 °C for 1 h. Tensile performances of two-step annealed CNTs have been investigated by in situ tests in a scanning electron microscope. The results show that the average Young’s modulus and fracture strength of two-step annealed CNTs are improved approximately by 35% and 10%, respectively, compared to those of one-step annealed CNTs. Such an enhancement can be ascribed to the well-aligned CNT walls with fewer structural defects, supported by the characterization results from transmission electron microscope, X-ray diffraction and Raman spectroscopy. The two-step annealing strategy developed in this study for improving the mechanical properties of CNTs is expected to be applicable to a practical fabrication process. Graphical abstract: [Figure not available: see fulltext.]