Hu Wei-Chieh, Sari Shanti Kartika, Hou Shuhn-Shyurng, Lin Ta-Hui
Department of Mechanical Engineering, National Cheng Kung University, Tainan 70101, Taiwan.
Department of Mechanical Engineering, Kun Shan University, Tainan 71070, Taiwan.
Materials (Basel). 2016 Nov 18;9(11):939. doi: 10.3390/ma9110939.
In this study, methane-ethylene jet diffusion flames modulated by acoustic excitation in an atmospheric environment were used to investigate the effects of acoustic excitation frequency and mixed fuel on nanomaterial formation. Acoustic output power was maintained at a constant value of 10 W, while the acoustic excitation frequency was varied ( = 0-90 Hz). The results show that the flame could not be stabilized on the port when the ethylene volume concentration (Ω) was less than 40% at = 10 Hz, or when Ω = 0% (i.e., pure methane) at = 90 Hz. The reason for this is that the flame had a low intensity and was extinguished by the entrained air due to acoustic modulation. Without acoustic excitation ( = 0 Hz), the flame was comprised of a single-layer structure for all values of Ω, and almost no carbon nanomaterials were synthesized. However, with acoustic excitation, a double-layer flame structure was generated for frequencies close to both the natural flickering frequency and the acoustically resonant frequency. This double-layer flame structure provided a favorable flame environment for the fabrication of carbon nanomaterials. Consequently, the synthesis of carbon nano-onions was significantly enhanced by acoustic excitation near both the natural flickering frequency and the acoustically resonant frequency. At = 20 Hz (near the natural flickering frequency) for 0% ≤ Ω ≤ 100%, a quantity of carbon nano-onions (CNOs) piled like bunches of grapes was obtained as a result of improved mixing of the fuel with ambient air. High-density CNOs were also produced at = 70 Hz (close to the acoustically resonant frequency) for 40% ≤ Ω ≤ 100%. Furthermore, carbon nanotubes (CNTs) were synthesized only at 80 Hz for Ω = 0%. The suitable temperature range for the synthesis of CNTs was slightly higher than that for the formation of CNOs (about 600 °C for CNTs; 510-600 °C for CNOs).
在本研究中，利用大气环境中受声激励调制的甲烷 - 乙烯射流扩散火焰来研究声激励频率和混合燃料对纳米材料形成的影响。声输出功率保持在10 W的恒定值，而声激励频率则变化（f = 0 - 90 Hz）。结果表明，当乙烯体积浓度（Ω）在f = 10 Hz时小于40%，或在f = 90 Hz时Ω = 0%（即纯甲烷）时，火焰无法在喷口处稳定。其原因是火焰强度较低，由于声调制被夹带的空气熄灭。在没有声激励（f = 0 Hz）的情况下，对于所有Ω值，火焰均由单层结构组成，几乎没有合成碳纳米材料。然而，在有声激励的情况下，对于接近自然闪烁频率和声学共振频率的频率，会产生双层火焰结构。这种双层火焰结构为碳纳米材料的制备提供了有利的火焰环境。因此，在自然闪烁频率和声学共振频率附近的声激励显著增强了碳纳米洋葱的合成。在f = 20 Hz（接近自然闪烁频率）且0%≤Ω≤100%时，由于燃料与周围空气的混合改善，获得了大量像葡萄串一样堆积的碳纳米洋葱（CNOs）。在f = 70 Hz（接近声学共振频率）且40%≤Ω≤100%时也产生了高密度的CNOs。此外，仅在f = 80 Hz且Ω = 0%时合成了碳纳米管（CNTs）。合成CNTs的合适温度范围略高于形成CNOs的温度范围（CNTs约为600℃；CNOs为510 - 600℃）。
