Abstract
Carbon nanotubes (CNTs) have attracted considerable attention in field emitter applications due to their unique structure, high aspect ratio and high emission current. The main objectives of this study are to investigate the optimal conditions of synthesis CNTs via thermal chemical vapor deposition with an applied external magnetic field under a low temperature. High-resolution TEM and SEM images of CNTs synthesized in different magnetic fields were investigated. The average tube diameters of CNTs synthesized at 450 °C under a magnetic field are approximately 20–30 nm. CNTs synthesized under a magnetic field, there is an approximately 5 nm circular-shaped catalyst appeared at the tip of each tube, however, CNTs synthesized without a magnetic field exhibited no catalyst at the tip of each tube. A magnetic field is thus proven to affect the growth of CNTs, indicating a growth mechanism that follows a tip-growth model. Alternatively, CNTs synthesized without a magnetic field exhibit growth mechanism based on a base-growth model. From X-ray powder diffractometer (XRPD) spectra of CNTs, the main peak at 2θ=26.3o showed high intensivity. It indicated that CNTs have stronger hexagonal well-graphited structures. The TGA analyses indicated that the decompostition temperatures of CNTs were around 550-600℃. When the growth conditions included a high magnetic field (13KG), the turn-on fields were approximately 0.9 V/μm and the maximum current densities was 20 mA/cm2. CNT synthesized by thermal chemical vapor deposition with different applied external magnetic fields show different field enhancement factors β. CNTs synthesized with different magnetic fields 6.8, 12 and 13 KG, which βwere 1140, 4772 and 3320, respectiviely. The carbon nano-particles of the turbostratic stacked graphite are, by virtue of the structural reconstruction, presumably driven by the high magnetic force, which explains CNTs synthesis in a high magnetic field results in a well-graphitized structure. By increasing the magnetic field, the graphitized structure of the CNTs is enhanced at low temperature. The field emission results indicate that CNT cathodes prepared under a high magnetic field obtain lower turn-on fields and higher current densities than those synthesized under a low magnetic field.