Stegen J
Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands and Institute for Theoretical Physics, Utrecht University, Leuvenlaan 4, 3584 CE Utrecht, The Netherlands.
J Chem Phys. 2014 Jun 28;140(24):244908. doi: 10.1063/1.4884823.
As-produced carbon nanotubes come in bundles that must be exfoliated for practical applications in nanocomposites. Sonication not only causes the exfoliation of nanotube bundles but also unwanted scission. An understanding of how precisely sonication induces the scission and exfoliation of nanotubes will help maximising the degree of exfoliation while minimising scission. We present a theoretical study of the mechanics of carbon nanotube scission under sonicaton, based on the accepted view that it is caused by strong gradients in the fluid velocity near a transiently collapsing bubble. We calculate the length-dependent scission rate by taking the actual movement of the nanotube during the collapse of a bubble into account, allowing for the prediction of the temporal evolution of the length distribution of the nanotubes. We show that the dependence of the scission rate on the sonication settings and the nanotube properties results in non-universal, experiment-dependent scission kinetics potentially explaining the variety in experimentally observed scission kinetics. The non-universality arises from the dependence of the maximum strain rate of the fluid experienced by a nanotube on its length. The maximum strain rate that a nanotube experiences increases with decreasing distance to the bubble. As short nanotubes are dragged along more easily by the fluid flow they experience a higher maximum strain rate than longer nanotubes. This dependence of the maximum strain rate on nanotube length affects the scaling of tensile strength with terminal length. We find that the terminal length scales with tensile strength to the power of 1/1.16 instead of with an exponent of 1/2 as found when nanotube motion is neglected. Finally, we show that the mechanism we propose responsible for scission can also explain the exfoliation of carbon nanotube bundles.
刚制备出的碳纳米管呈束状，在纳米复合材料的实际应用中必须将其剥离。超声处理不仅会导致纳米管束的剥离，还会造成不必要的断裂。了解超声处理如何精确地诱导纳米管的断裂和剥离，将有助于在使断裂最小化的同时最大化剥离程度。我们基于一种被广泛接受的观点，即超声空化过程中靠近瞬间塌缩气泡处流体速度的强梯度导致碳纳米管断裂，对超声作用下碳纳米管的断裂力学进行了理论研究。我们通过考虑气泡塌缩过程中纳米管的实际运动来计算长度依赖的断裂速率，从而能够预测纳米管长度分布的时间演化。我们表明，断裂速率对超声处理参数和纳米管特性的依赖性导致了非通用的、依赖于实验的断裂动力学，这可能解释了实验观察到的断裂动力学的多样性。这种非通用性源于纳米管所经历的流体最大应变率对其长度的依赖性。纳米管所经历的最大应变率随着与气泡距离的减小而增加。由于短纳米管更容易被流体流动拖动，它们比长纳米管经历更高的最大应变率。最大应变率对纳米管长度的这种依赖性影响了拉伸强度与最终长度的标度关系。我们发现，最终长度与拉伸强度的标度关系为拉伸强度的1/1.16次方，而不是在忽略纳米管运动时所发现的1/2次方。最后，我们表明我们提出的负责断裂的机制也可以解释碳纳米管束的剥离现象。
