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John O | March 2018

Researchers create predictable patterns from unpredictable carbon nanotubes

By Josh Perry, Editor


Scientists at the Massachusetts Institute of Technology (MIT) in Cambridge, Mass. have developed a systematic method for predicting the two-dimensional patterns that carbon nanotubes will form when packed together by evaporating drops of acetone or ethanol.


A recolored optical image obtained by MIT researchers shows a heart-shaped carbon nanotube cell.  (Ashley Kaiser and Itai Stein/MIT)


According to a report from MIT, the researchers have demonstrated that carbon nanotube cell size and wall stiffness are proportional to cell height.


With the increasing study and use of carbon nanotubes in a variety of applications, including coatings and thermal solutions such as heat sinks, being able to produce them in high volume is critical to commercialization.


Chemical vapor deposition (CVD) has been used to manufacture them but the results are too sparse for most applications. Scientists have applied drops of liquids like acetone to more densely pack the carbon nanotubes (CNT) but were unable, until now, to predict the geometry of the arrays.


“One way to think of this CNT behavior is to imagine how entangled fibers such as wet hair or spaghetti collectively reinforce each other,” the report explained. “The larger this entangled region is, the higher its resistance to bending will be. Similarly, longer CNTs can better reinforce one another in a cell wall. The researchers also find that CNT binding strength to the base on which they are produced, in this case, silicon, makes an important contribution to predicting the cellular patterns that these CNTs will form.”


The researchers found that first wetting and then evaporating the carbon nanotubes produced material that was “hundreds to thousands of times less stiff than expected by theoretical values.” This was caused by elastocapillarity, which is also how a sponge dries into a more compact shape than prior to being wetted.


Adding heat when placing the CNT on a substrate increased the hardness by as much as four times, which has led to another line of research to try and tune that property. After analyzing the CNT with a scanning electron microscope, researchers saw, “While gently applying liquid to the CNT arrays in this study caused them to densify into predictable cells, vigorously immersing the CNTs in liquid imparts much stronger forces to them, forming randomly shaped CNT networks.”


The research was recently published in Physical Chemistry Chemical Physics. The abstract stated:


“Capillary-mediated densification is an inexpensive and versatile approach to tune the application-specific properties and packing morphology of bulk nanofiber (NF) arrays, such as aligned carbon nanotubes.


“While NF length governs elasto-capillary self-assembly, the geometry of cellular patterns formed by capillary densified NFs cannot be precisely predicted by existing theories. This originates from the recently quantified orders of magnitude lower than expected NF array effective axial elastic modulus (E), and here we show via parametric experimentation and modeling that E determines the width, area, and wall thickness of the resulting cellular pattern.


“Both experiments and models show that further tuning of the cellular pattern is possible by altering the NF–substrate adhesion strength, which could enable the broad use of this facile approach to predictably pattern NF arrays for high value applications.”

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