The Inherent Thermal Effect of Substrates on the Growth of Ultralong Carbon Nanotubes

Ultralong carbon nanotubes (CNTs) are believed to be ideal candidates for various high-end applications because of their macroscale lengths, perfect structures, and excellent mechanical and electrical properties. The key to the wide application of ultralong CNTs is their controlled synthesis and mass production. Ultralong CNTs usually follow a flying kite-like growth mechanism, during which process there exists a thermal buoyancy that keeps ultralong CNTs floating in the gas flow. However, it remains vague for a long time about the origin of this thermal buoyancy. Herein, a simple and quantitative heat balance model is proposed to describe the inherent thermal effect of substrates, which explains the origin of the temperature difference between the substrate and the gas flow. The inherent thermal effect is found to be positively correlated with the emissivity of the substrates. Then, the local temperature gradient induced by the inherent thermal effect is found to result in both natural convection and thermophoresis. Thermophoretic force is proven to be the dominant driving force for lifting the ultralong CNTs up from the substrates. By utilizing the inherent thermal effect and designing the local temperature distribution, the areal density and orientation of ultralong CNT arrays are modulated.