Characterization of carbon nanotube growth region in flame using wire-based macro-imaging method

Abstract

Carbon nanotube (CNT) synthesis in flame has enormous potential as an energy-efficient and economical production method compared to the conventional catalytic chemical vapor deposition (CCVD) synthesis process. However, synthesis control remains a great challenge for flame synthesis due to the limited understanding on the effect of flame inlet condition toward CNT growth region in a heterogeneous flame environment and premature catalyst surface encapsulation by the amorphous carbon layer. The present study formulates a simple, yet accurate method called wirebased macro image analysis (WMA) for thorough growth region identification. The WMA method is employed to investigate the effects of reactant composition and aerodynamics on the spatial distribution of CNT growth region. Besides that, bend wire method is developed to provide cross-sectional analysis of the CNT growth region with focus on the amorphous carbon layer thickness (ACLT) at variable reactant concentration including fuel from 50% to 100% and oxygen from 19% to 27%, with addition of water vapor up to 0.14 mg/sec mass flow rate within the fuel stream. The CNT is synthesized on a 0.4 mm diameter pure nickel wire within the methane diffusion flame with a stainless-steel wire mesh placed on top and water vapor is introduced in a fuel stream using a bubbler mechanism. The CNT growth region is confined within the flame sheet, gradually shifts from flame front to flame centreline as height above the burner increases. The growth region is more sensitive towards the change in the oxygen concentration compared to that of the fuel concentration due to the significant change of flame height caused by the former. A segregation of growth region temperature with temperature difference of 100 ? that is observed between the upstream and downstream growth region is governed by the proximity with respect to the flame sheet. The ACLT reduces in lean flame due to the reduction in excess carbon concentration and the addition of water vapor remarkably reduces ACLT by 17% on average in any combination of inlet conditions due to the water-induced etching and oxidation of amorphous carbon on the catalyst surface. Development of the WMA and bend wire method leads to deeper fundamental understanding of CNT flame synthesis and further enhance possibility of highly efficient and economical CNT production process in the future.