Abstract

This thesis investigates the effect of pressure and temperature on electrical conductivity of CNT-PEEK composites. The nanocomposites were manufactured using a co-rotating intermeshing twin screw extruder and the samples of required size and shape were fabricated by compression molding. Electrical properties of the nanocomposite samples were measured by Dielectric Analyzer (DEA) and a detailed analysis is presented in the framework of percolation theory. It was identified that nanotube contact resistance due to the formation of a thin insulating polymer layer around carbon nanotubes plays an important role in determining the overall conductivity of the samples. Detailed analysis of this contact resistance is presented based on experimental results in combination with theoretical models.

To investigate the effect of temperature and pressure on electrical conductivity, highly conductive samples with three different nanotube weight concentrations (8%, 9% and 10%) were selected. Metallic coatings (gold/silver epoxy) are conventionally used as electrodes to measure electrical conductivity at ordinary temperature and pressure. To measure electrical conductivity of these samples at elevated pressure and temperature, a new technique was developed to measure DC electrical conductivity by introducing a conductive copper mesh. Change of electrical conductivity of the samples was investigated under application of high compression, high temperature and a combination of both. Conduction mechanisms for both pressure and temperature were discussed on the basis of experimental findings. It was found that electrical conductivity increases up to a certain level due to application of both pressure and temperature. The effect was more significant at lower pressure and temperature. In the case of repeated loading-unloading and heating-cooling cycles, hysteresis and electrical set were observed. The pressure always acts favorably in increasing electrical conductivity while the effect of temperature was found to be complex, controlled by parameters that counteract each other, especially when it was heated above the glass transition temperature and the nanotube content was high. Two possible mechanisms, namely, ‘conduction by electron transport (nanotube contact)’ and ‘conduction by electron tunneling’ were identified to explain such contradicting behavior. Sensitivity of the samples was also checked for possible application as sensor.