We prepared binder-free monoliths of carbon nanohorns（CNHs）, hole-opened carbon nanohorns （CNHoxs）, multi-walled carbon nanotube（MWCNT）, and single-walled carbon nanotube（SWCNT）using spark plasma sintering（SPS）method at 1800℃ and 80 MPa in vacuum. The density of the SPS-treated SWCNT is 1.8 g/cm3, which is close to that of graphite, while those of CNHs, CNHoxs, and MWCNT remained 1.1-1.3 g/cm3 even after SPS treatment. We evaluated the monoliths using Raman spectroscopy and scanning electron microscopy observation, which showed a significant defect formation and graphitization of SWCNTs. Moreover, the increase of defects density in CNHs, CNHoxs, and MWCNT was moderate, and sub-micron size structures remained. We observed that the monoliths of CNHs and CNHoxs were highly conductive with a Hall mobility of positive holes of ～50 cm2/Vs and electrical conductivity of ～300 S/cm. These experimental results indicated that the SPS treatment under appropriate conditions could provide a CNHs monolith with nanostructure and good electrical conductivity.
