School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China.
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China.
Ultrason Sonochem. 2018 Mar;41:181-188. doi: 10.1016/j.ultsonch.2017.09.007. Epub 2017 Sep 7.
Ultrasound coupled with supercritical CO has become an important method for exfoliation of graphene, but behind which a peeling mechanism is unclear. In this work, CFD simulation and experiment were both investigated to elucidate the mechanism and the effects of the process parameters on the exfoliation yield. The experiments and the CFD simulation were conducted under pressure ranging from 8MPa to 16MPa, the ultrasonic power ranging from 12W to 240W and the frequency of 20kHz. The numerical analysis of fluid flow patterns and pressure distributions revealed that the fluid shear stress and the periodical pressure fluctuation generated by ultrasound were primary factors in exfoliating graphene. The distribution of the fluid shear stress decided the effective exfoliation area, which, in turn, affected the yield. The effective area increased from 5.339cm to 8.074cm with increasing ultrasonic power from 12W to 240W, corresponding to the yield increasing from 5.2% to 21.5%. The pressure fluctuation would cause the expansion of the interlayers of graphite. The degree of the expansion increased with the increase of the operating pressure but decreased beyond 12MPa. Thus, the maximum yield was obtained at 12MPa. The cavitation might be generated by ultrasound in supercritical CO. But it is too weak to exfoliate graphite into graphene. These results provide a strategy in optimizing and scaling up the ultrasound-assisted supercritical CO technique for producing graphene.
超声结合超临界 CO 已成为剥离石墨烯的重要方法，但背后的剥离机制尚不清楚。在这项工作中，通过计算流体动力学（CFD）模拟和实验研究，阐明了该过程的机制以及工艺参数对剥离产率的影响。实验和 CFD 模拟在压力为 8MPa 至 16MPa、超声功率为 12W 至 240W、频率为 20kHz 的条件下进行。对流体流动模式和压力分布的数值分析表明，流体剪切应力和超声产生的周期性压力波动是剥离石墨烯的主要因素。流体剪切应力的分布决定了有效剥离面积，进而影响产率。随着超声功率从 12W 增加到 240W，有效面积从 5.339cm 增加到 8.074cm，相应的产率从 5.2%增加到 21.5%。压力波动会导致石墨层间的膨胀。随着操作压力的增加，膨胀程度增加，但超过 12MPa 后会减小。因此，在 12MPa 时获得最大产率。超临界 CO 中的超声可能会产生空化现象，但它太弱，无法将石墨剥离成石墨烯。这些结果为优化和扩大超声辅助超临界 CO 技术生产石墨烯提供了策略。
