Development of a spray process for manufacturing carbon nanotube films

This dissertation describes the development of a processing route for fabricating conventional and doped multi-wall carbon nanotube (MWCNT)/polymer composite films for dielectric applications. Previous research has shown that such composites are promising dielectric materials, but their commercial development has been impeded by the nanotube agglomeration in the polymer matrix and the inefficiency in forming uniform films. Moreover, the harsh fabrication treatments often disrupt the structure of the nanotubes, hence damaging their inherent electrical properties. This work presents safer routes for forming non-aqueous, surfactant free dispersions of conventional and doped MWCNTs, which can be readily mixed with polymers and formed into films through aerosol spraying. Dispersibility behaviour of in-house synthesised conventional, nitrogen doped (N-MWCNTs), and boron doped (B-MWCNTs) MWCNTs was studied in 22 organic solvents. Based on thermodynamic theories it was suggested that doping, in particular nitrogen doping, significantly reduced the surface energy of the nanotubes. This aspect was crucial to understand the dispersibility of N-MWCNTs in low surface energy solvents and to achieve dispersions with high nanotube concentrations (0.82 mg/ml). Also, a "destruction reduced sonication protocol" involving mild sonication was suggested for forming MWCNT dispersions in organic solvents. Dispersions formed using this protocol were homogeneous and showed high stability of at least 2.5 years. Furthermore, the effect of ultrasonic probes on MWCNT lengths was studied and a decrease of 96–99% for MWCNTs and 85–95% for N-MWCNTs was observed. A numerical value for the nanotube length decrease during sonication has been reported for the first time. Preliminary studies to generate dielectric films of MWCNT/perfluoro alkoxy polymer were performed using aerosol spraying. An improvement in the dielectric constant (3.56) with a low dissipation factor (0.003) was observed in 0.3 wt.% B- MWCNT/PFA composite films. Consistency in the test results from various parts of the films confirmed the uniformity of the nanotube distribution within the composite. Future work should concentrate on the effects of B-MWCNTs and N-MWCNTs at the percolation threshold due to their inherent electric properties.