Inter-tube bonding, graphene formation and anisotropic transport properties in spark plasma sintered multi-wall carbon nanotube arrays

Millimeter long, vertically oriented, multi-walled carbon nanotube (MWCNT) arrays were pre-aligned and densified using spark plasma sintering (SPS) technique to form aligned MWCNT bulk samples. The combined results of X-ray powder diffraction, Raman spectroscopy, scanning electron microscopy and high-resolution transmission electron microscopy show that the MWCNTs largely retain their orientation and individual tubular morphology in the aligned MWCNT bulk samples, and that the SPS process induces inter- and intra-tubular bonding as well as graphene formation. In view of the one-dimensional nature of individual MWCNTs, it is particularly noteworthy that the transverse electrical resistivity ρT is slightly lower than the longitudinal resistivity ρL, whereas the transverse thermal conductivity κT is ∼50% of κL. The room temperature κL is ∼31 W/(m K), one of the highest reported in MWCNT bulk samples. In addition, the thermopower measurements show anisotropy and features of phonon drag.