The objective of this project is to utilize the high piezoresistivity of single-walled semiconducting Carbon Nanotubes (CNTs) as the active material for strain sensors. CNTs can provide a sensitivity gain several orders of magnitude greater than current metal foil strain sensors, and by using inexpensive fabrication processes like inkjet printing, such sensors may be economically competitive. One of the long-standing frustrations when it comes to using carbon nanotubes is the difficulty of their placement on a substrate. The piezoresistive response of CNTs is only significant when they are strained along their length, so it follows that the CNTs composing the active film of a sensor be aligned all in the same direction.
Currently, we have developed a way to fabricate films of well aligned and mutually oriented CNTs using solely inkjet printing. We do this by exploiting the natural liquid crystallinity of highly concentrated CNT solutions. Plainly, when CNTs are suspended in solution at high concentration they spontaneously assemble such that they are oriented in parallel to one another. By inkjet printing such solutions we found it possible to fabricate CNT films where the tubes were aligned in parallel to one another and oriented in a direction that was governed by the geometrical edge of the film. Thus, if a very high aspect ratio film was printed the CNTs were oriented largely in a line, as seen in the figures below.
Journal: Langmuir, 28 (23): 8753-8759.
Date: May 9, 2012
Abstract: An inkjet printing procedure for depositing films of carbon nanotubes (CNTs) that exhibit a very high degree of long-range mutual alignment as well as a controlled orientation with respect to the printed geometry is presented. CNT self-assembly was induced by the intrinsic lyotropic liquid crystallinity of CNT suspensions. Sufficient concentrations are reached by matching the inkjet deposition rate to the numerically modeled local evaporation rate of the printed feature and enable the CNT suspension to be printed using standard inkjet printing. Surface alignment was verified using scanning electron microscopy (SEM) and polarized light microscopy. In addition, the bulk morphology was investigated and found to be composed of stacked planar layers that did not necessarily have the same long-range orientation found on the surface. The bulk morphology was characterized by removing layers through an elastomeric peeling process and by observing cross sections of the films using SEM. CNT concentration and length were spanned experimentally, and it was found that very short and very long CNTs as well as low concentration suspensions did not yield long-range alignment. Download