19-Fold thermal conductivity increase of carbon nanotube bundles toward high-end thermal design applications

In high temperature annealing of carbon nanotube (CNTs) bundles for structure and thermal conductivity (κ) improvement, the statistical errors from sample-wide structure variation and sample transfer/preparation significantly overshadows the understanding of structure-κ correlation and change. In this work, the sequential process of current-induced thermal annealing (CITA) on improving the structure, electrical and thermal conductivities of chemical vapor deposition grown CNT bundles is studied for the first time. Our in-situ κ measurement using the transient electro-thermal technique uncovers the conjugated dynamic electrical, thermal, and structural properties. The electrical resistance and thermal diffusivity evolution of CNT bundles during CITA is studied. The thermal diffusivity and κ before and after CITA are measured from room temperature down to 10 K to uncover the reduction of defect density and enhancement of inter-tube connection strength after CITA. Our micro-Raman spectroscopy study from the most annealed region to the non-annealed region reveals significantly improved order in sp² bonding carbon structure and reduced defects along the sample length. The resulting κ has 5–19 times increase at the most annealed region of CNT bundles. The intrinsic κ of CNT walls against the annealing temperature is also determined, which reaches a level as high as 754 W/m·K after CITA.