Due to their unique geometry and the chirality, carbon coils have been attracted as high potential materials for the electromagnetic wave absorbers with low reflection coefficient. Carbon coils have been usually found as the low-content by products of multiwalled carbon nanotubes in microwave plasma-enhanced chemical vapor deposition (MW-PECVD) or in the thermal chemical vapor deposition (TCVD) . For the practical application of carbon coils, therefore, it would be essential to achieve the large quantity production of carbon coils. Although microwave method has been reported to produce carbon coils, catalytic chemical vapor deposition (CCVD) method has been more noticed for the large quantity production of carbon coils because of its relative inexpensive and applicable feature. Besides the system parameters for CCVD such as substrate temperature and gas flow rate, the metal catalyst and incorporated additives are the important factors for the large quantity production of carbon coils.
For the metal catalyst, iron family (Fe, Co, Ni), especially Ni, were known as an effective catalyst for the formation of carbon coils. For the incorporated additives, meanwhile, a trace of the sulfur-related species was regarded as the promising additives for the formation of carbon coils.
This work presents the enhancement of carbon coils production yield on Al2O3 ceramic boat using Ni catalyst. SF6 was also added in the source gases to take the advantage for the sulfur and the fluorine species role regarding the enhancement of the nucleation sites of carbon coils. To reduce the highly hazardous problem to the health and environment, SF6 incorporation time was minimized to a few min during the initial reaction stage. The characteristics of the as-grown carbon coils, namely the formation density and the geometry were examined and discussed. The sulfur role for the formation of carbon coils and the enhanced etching ability by the fluorine-related species during the initial reaction stage were discussed.
Fig. 1 As-grown carbon coils on Al2O3 ceramic boat Fig. 2 Carbon coils gathered from as-grown one Fig. 3 FESEM images of as-grown carbon coils
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