Electrical Conductivity, Electromagnetic Interference Shielding and Dielectric Properties of Multi-walled Carbon Nanotube/Polymer Composites
Creators: name: Arjmand, Mohammad
Publisher: name: Graduate Studies
Year: 2014
Subjects: subject: Chemistry--Polymer
subject: Electricity and Magnetism
subject: Engineering--Chemical
subject: Polymer Molding
subject: Electrical Conductivity
subject: Electromagnetic Interference Shielding
subject: Polymeric Nanocomposite
subject: Carbon Nanotube

description: Driven by the ever-growing demand for versatile electronics with increased functionality, high performance, light weight, low cost and improved design options, conductive filler/polymer composites (CPCs) have emerged as a distinctive solution. Manipulating the conductive network formation in CPCs allows them to be employed in a wide range of applications, such as charge storage, electrostatic discharge dissipation and electromagnetic interference (EMI) shielding. In this dissertation, controlling the conductive network formation was the key aspect in designing the morphology of CPCs for electrical applications. Multi-walled carbon nanotube (MWCNT) was chosen as conductive filler due to its surprising electronic structure and growing industrial usage. We employed two distinct techniques to improve or deteriorate conductive network formation to improve the electrical properties in MWCNT/polymer composites, i.e. electrical conductivity, EMI shielding and dielectric properties. These techniques comprise (1) aligning MWCNTs using an injection molding machine, and (2) replacing MWCNTs with copper nanowires (CuNWs). Prior to exploring the influence of the above-mentioned techniques on the electrical properties of CPCs, a series of studies were implemented on MWCNT/polymer composites to obtain a general understanding from the electrical behaviors of CPCs as a function of MWCNT content. The results over the X-band (8.2 – 12.4 GHz) showed that the electrical conductivity, EMI shielding and dielectric properties rose with MWCNT content. The increase in electrical conductivity with MWCNT loading was attributed to the formation of conductive paths across the composite. Increase in EMI shielding with MWCNT content was related to a greater number of interacting nomadic charges and also higher real permittivity (polarization loss) and imaginary permittivity (Ohmic loss). Moreover, the broadband dielectric spectroscopy (10-1 – 10+6 Hz) showed that both real permittivity and imaginary permittivity increased drastically as the MWCNT concentration approached the percolation threshold. Increase in real permittivity was related to the formation of a large number of nanocapacitor structures, MWCNTs as electrodes and polymer matrix as dielectric material, and increase in imaginary permittivity was ascribed to greater number of dissipating charges, enhanced conductive network formation and boosted polarization loss arising from interfacial polarization. MWCNT alignment, induced by an injection molding machine, was observed to deteriorate the conductive network formation. As inferior conductive network formation reduces imaginary permittivity, this technique was introduced as an innovative technique to improve the dielectric properties of MWCNT/polymer composites. Nonetheless, MWCNT alignment indicated an adverse influence on the percolation threshold, electrical conductivity and EMI shielding due to its negative influence on conductive network formation. In brief, unavoidable flow-induced alignment of MWCNTs in injection molding process was presented as an opportunity to improve the dielectric properties for charge storage or as a challenge to be avoided for producing conductive CPCs. CuNWs were creatively displayed to be competent substitutions for MWCNTs for charge storage applications. Unavoidable oxide layer formation on the surface of CuNWs, which has always been a disadvantage for electronics applications, was employed as a benefit to decay the conductive network formation and reduce the imaginary permittivity. Moreover, higher conductivity of fresh core of CuNWs relative to MWCNTs provided the composites with more free charges contributing to real permittivity. In conclusion, high conductivity of fresh core of CuNWs combined with the presence of the oxide layer on CuNW surfaces depict a promising future for CuNW/polymer composites as charge storage materials.