Intrinsic conducting polymer current limiting device

This thesis investigates the development of novel intrinsic conducting polymer current limiting devices (CLDs), which are small self-resettable components used to protect circuitry in portable electronic appliances during situations of overcurrent. There are many existing solutions that enable the interruption and limitation of current surges in devices. A simple and common device is the use of a hybrid system of small metallic/semiconducting particles embedded in a thermally responsive polymeric material. During an overcurrent situation the positive temperature coefficient resistance (PTCR) of the polymeric material expands due to joule heating, which causes the percolating current pathways in the device to break which in turn causes the device to become temporarily non conducting. Although commonly used in the electronics industry today there are some limitations in the conductive fillers such as fretting and degradation that cause unwanted shifts in the percolation threshold, which can result in faulty operation. There is also increasing demand for faster operating devices as well as cheaper and more convenient methods of fabrication. One possibility that has emerged is the use of intrinsic conducting polymers as current limiting materials. There are several benefits of this for commercialization, such as cheap starting materials, simple chemical or electrochemical solution-based methods for device fabrication and potentially very fast device operation if the mechanism is electronic rather than mechanical in nature. Although preliminary measurements in these materials have shown evidence of switching to a non-conducting state at high currents, which is resettable upon cooling, the mechanism that causes the switching has not been established and no studies on this effect have been reported in the literature.This motivates further work into developing current limiting devices based on these materials. The intention of this research is (1) to develop current limiting devices based on the intrinsic conducting polymer polyaniline (PANI), (2) to improve the electronic properties of the device, ideally we desire to have low resistance at low current level (normal operating conditions) and saturated or limited conductivity at high current level (overcurrent/short-circuit conditions), (3) elucidate the switching mechanism of the devices and (4) investigate the devices stability and resetability characteristics in different environmental conditions.The results of this thesis show that the presence of moisture in the conducting polymer plays an important role in the current limiting performance. Our results indicate that the electronic switching of the devices is a result of a partial de-doping/re-doping effect caused by water diffusing into and out of the polymer film during joule heating/ cooling of the devices. We speculate that the polymer/metal interface region plays an important role in this and as such significant improvement in the electrical properties were made using self-assembled monolayer (SAM) modified electrodes.Studies were also performed to understand the temperature dependent electrical resistivity of the polymer material, shown to be variable range hopping in three dimensions, and as well, investigations were made to understand device performance in different environmental conditions (varying from vacuum to 100% relative humidity). Stability and degradation testing was performed in order to find the best operational and stable conditions for the current limiting devices. Results showed that devices were very stable in low to medium voltage values. In an effort to understand the effect of dopant type on the moisture content in the polymer film, thermal analyses (thermogravimetric and differential scanning calorimetry) of samples with different dopants were investigated. Polyaniline doped with methanesulfonic acid was found to be among the samples with the highest moisture content