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MM2.09 - Force-Modulated Growth of Carbon Nanotubes 
December 1, 2014   4:45pm - 5:00pm

Literature abounds with examples of mechanochemistry, in which chemical reactions are shown to be influenced by mechanical loads acting on the reacting surfaces/volumes. These mechanochemical effects promise to direct the reaction pathways of chemical synthesis processes, enabling unprecedented control of the produced structures. We study the effect of externally applied mechanical forces on the collective growth of aligned carbon nanotube (CNT) "forests" by chemical vapor deposition (CVD). Previous work on CNT growth under mechanical pressure showed that the final forest height decreased with applying compressive stresses, but did not measure growth kinetics. Motivated by these results, we use a custom-built cold-wall CVD reactor to apply measurable axial loads in situ during forest growth, while recording the growth kinetics in real-time. Compressive forces in the range of 0.2 to 20 grams are applied by a probe pushing on millimeter-size CNT forests growing on a resistively heated substrate from a mixture of ethylene, acetylene, hydrogen and helium at atmospheric pressure. The real-time growth kinetics are determined by measuring the forest height increase using a feedback loop controlled actuator that moves the probe vertically upwards to maintain a constant force during growth. Results show that a mechanical force influences both the growth kinetics and the resulting forest morphology. The maximum growth rate is found to be inversely related to the applied compressive force. These findings highlight the importance of studying mechanochemistry of CVD, in which the process activation energy at the catalyst is modulated by the mechanical stresses. The exact influence of mechanical stresses on the kinetics and energetics of CNT growth provides insights into the internal mechanical feedback within a growing forest, comprising billions of CNTs per square centimeter each having diameter-dependent growth rates. If the kinetics of CNTs subjected to compressive forces is slowed down, while the kinetics of those CNTs subjected to tensile loads is accelerated, a homogenizing effect ensues, which eventually leads to reducing the growth rate mismatch and contributes towards the coordinated forest growth at a single collective growth rate. Hence, understanding these collective mechanochemical effects is key to engineering the morphology of functional aligned CNT ensembles.

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