Ethylene jet diffusion flames modulated by acoustic excitation in an atmospheric environment were used to synthesize carbon nano-onions (CNOs) on a catalytic nickel substrate. The formation of CNOs was significantly enhanced by acoustic excitation at frequencies near either the natural flickering frequency or the acoustically resonant frequency. The rate of yield of CNOs was high at 10 and 20 Hz (near the natural flickering frequency) for a sampling position z = 5 mm above the burner exit where the gas temperature was about 450-520 °C, or at 10, 20 and 30 Hz for z = 10 mm with the gas temperature ranging from 420 to 500 °C. Additionally, for both z = 5 and 10 mm, a quantity of CNOs can be obtained at 60-70 Hz, near the acoustically resonant frequency, where the gas temperature was between 620 and 720 °C. Almost no CNOs were produced for the other frequencies due to low temperature or lack of carbon sources. CNOs synthesized at low frequencies had a greater diameter and a higher degree of graphitization than those at high frequencies.
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