Continuous Synthesis of Double-Walled Carbon Nanotubes with Water-Assisted Floating Catalyst Chemical Vapor Deposition.

Double-walled carbon nanotubes (DWCNTs) were synthesized and continuously collected using a water-assisted floating catalyst chemical vapor deposition (FCCVD) method. Differing from the conventional water-assisted synthesis in which water vapor is one part of the carrier gas mixture, we included de-ionized water in the catalyst system, which achieved a more uniform and controlled distribution for efficient DWCNT production. Using a water-assisted FCCVD process with optimized conditions, a transition from multi- to double-walled CNTs was observed with a decrease in diameters from 19-23 nm to 10-15 nm in tandem with an elevated Raman I/I ratio up to 10.23, and corroborated from the decomposition peak shifts in thermogravimetric data. To characterize the mechanical and electrical improvements, the FCCVD-CNT/bismaleimide (BMI) composites with different water concentrations were manufactured, revealing high electrical conductivity of 1720 S/cm along the bundle alignment (collection) direction, and the nano-indentation tests showed an axial reduced modulus at 65 GPa. A consistent value of the anisotropic ratio at ~3 was observed comparing the longitudinal and transverse properties. The continuous capability of the presented method while maintaining high quality is expected to result in an improved DWCNT mass production process and potentially enhance the structural and electrical applications of CNT nanocomposites.