Synthesis of Carbon Nano-Materials in Methane-Ethylene Jet Diffusion Flames Modulated by Acoustic Excitation

Abstract

Recently flame synthesis of carbon nano-materials has been widely explored due to its advantages over other production methods that use expensive inputs such as electricity as the heat source In this study methane-ethylene jet diffusion flames modulated by acoustic excitation in an atmospheric environment were used to synthesize carbon nanostructures on a catalytic nickel substrate Experiments were conducted to investigate the effects of acoustic excitation frequency (f = 0~90 Hz) and mixed fuel (ethylene concentration ΩE =0~100% in blends of methane-ethylene) on flame structure and nano-material formation The results show that the flame could not be stabilized on the port for ethylene concentrations (ΩE) of less than 40% at f = 10 Hz or for ΩE = 0% (i e pure methane) at f = 90 Hz because the flame had a low intensity and was extinguished by the entrained air due to acoustic modulation Without acoustic excitation f = 0 Hz the flame comprised a single-layer structure for all values of ΩE and almost no carbon nano-materials were synthesized But when frequencies were in the range of 10~90 Hz a double-layer flame structure was observed However when acoustic excitation neared the natural flickering frequency (i e at f = 20 Hz for 0% ? ΩE ? 100%) at z = 10 mm when P = 10 and 15 W good carbon synthesis occurred as a result of improved mixing of the fuel with the ambient air The synthesized products were carbon nano-onions (CNOs) piled like bunches of grapes High-density CNOs were produced at f = 70 Hz for 40% ? ΩE ? 100% and 60% ? ΩE ? 100% at 10 and 15 W respectively Further carbon nano-tubes (CNTs) were synthesized at 80 Hz for ΩE = 0% and 0% ? ΩE ? 5% at 10 and 15 W respectively For P = 15 W at 70 Hz for ΩE = 0% the CNTs also covered the Ni substrate Other than these cases almost no carbon nano-materials were formed The suitable temperature ranges for the synthesis of CNTs were slightly higher than for CNOs (580~630oC for CNTs 500~600oC for CNOs) Gas composition analysis indicated that at low CH4 and CO concentration (low carbon source) and low temperature no carbon nano-materials could be observed In the synthesis region of CNTs the concentration of CH4 was greater than 50%

Date of Award	2014 Jul 1
Original language	English
Supervisor	Ta-Hui Lin (Supervisor)