Three-dimensional alignment of multi-walled carbon nanotubes inside aqueous media using dielectrophoresis

Since their discovery by Iijima in 1991, carbon nanotubes (CNTs) have attracted great attention in the research field due to their extraordinary mechanical, electrical, optical and magnetic properties, as well as large specific surface area and high aspect ratio. These characteristics render carbon nanotube a desirable material in a wealth of applications including biosensors, field emitters, actuators, polymer nanocomposites, and fuel cells. However, the difficulty of dispersing CNTs in most common solvents is a major obstacle toward their full utilization. Because CNTs usually entangle together, which limits their properties dramatically. Therefore, to orient/align CNTs into desired structures becomes another major challenge.; This thesis work focused on: (a) the stable dispersion of multi-walled carbon nanotubes (MWNT) in common solvents and most importantly, (b) utilization of these dispersions in the examination of MWNT interactions inside an AC electric field. Specifically, the suspension of MWNT in various organic solvents, or in water with the aid of surfactants, was investigated. Next, a feasibility study was performed on whether non-uniform AC electric fields can be used as means to achieve large-scale alignment of carbon nanotubes inside their colloidal dispersions. It was shown that induced polarization effects (known as dielectrophoresis, DEP) on MWNTs by an externally applied AC electric field can cause their deterministic transport and orientation (electric field-dipole interactions) and end-to-end alignment and contact (dipole-dipole interactions). Results were demonstrated for applied voltage between 25-250 Vpp and frequency values of 5 MHz.; Among the surfactants tested for their ability to stably suspend MWNT in H2O, Pluronic F-127 (0.4 wt%) was found to be the most effective and resulted in a stable suspension for a period of several months. Using this suspension, MWNTs were successfully aligned between different design electrodes using AC electric field (dielectrophoresis). Optical microscope pictures showed well-aligned structures at 5 MHz with field strength around 800 V/cm. The effect of AC frequency, electric field strength, and concentration of CNT dispersion were also examined. Resistivity results for MWNT suspension during dielectrophoresis experiments showed that both higher frequency and electric field strength can result in better aligned structures. Additionally, the conductivity of suspension increased with the increase of nanotube concentration and reached a percolation threshold around 0.004 wt% in aqueous media with conductivity in the order of 10-2 S/m.