Luis A. Navarro

Superhydrophobic SAM Modified Electrodes for Enhanced Current Limiting Properties in Intrinsic Conducting Polymer Surge Protection Devices

Surface interface engineering using superhydrophobic gold electrodes made with 1-dodecanethiol self-assembled monolayer (SAM) has been used to enhance the current limiting properties of novel surge protection devices based on the intrinsic conducting polymer, polyaniline doped with methanesulfonic acid. The resulting devices show significantly enhanced current limiting characteristics, including current saturation, foldback, and negative differential effects. We show how SAM modification changes the morphology of the polymer film directly adjacent to the electrodes, leading to the formation of an interfacial compact thin film that lowers the contact resistance at the Au-polymer interface. We attribute the enhanced current limiting properties of the devices to a combination of lower contact resistance and increased Joule heating within this interface region which during a current surge produces a current blocking resistive barrier due to a thermally induced dedoping effect caused by the rapid diffusion of moisture away from this region. The effect is exacerbated at higher applied voltages as the higher temperature leads to stronger depletion of charge carriers in this region, resulting in a negative differential resistance effect.

Novel conducting polymer current limiting devices for low cost surge protection applications

We report on the development of novel intrinsic conducting polymer two terminal surge protection devices. These resettable current limiting devices consist of polyaniline nanofibres doped with methane sulphonic acid electrochemically deposited between two 55 μm spaced gold electrodes. At normal applied voltages, the low resistance devices act as passive circuit elements, not affecting the current flow. However during a current surge the devices switch from ohmic to non-ohmic behaviour, limiting current through the device. After the current surge has passed, the devices reset back to their original state. Our studies show that a partial de-doping/re-doping process caused by the rapid diffusion of moisture out of or into the polymer film during joule heating/cooling is the underlying mechanism responsible.

Large dopant dependence of the current limiting properties of intrinsic conducting polymer surge protection devices

New two terminal surge protection devices based on intrinsic conducting polymers are demonstrated to be strongly affected by the dopant molecule type. Thermogravimetric analysis combined with current–voltage studies show a causal link between the dopant molecule, moisture content and the current limiting capability of the devices. Polyaniline thin-films with high moisture content produce devices with current saturation and foldback effects at high applied voltages while low moisture content films exhibit no current rectification and instead demonstrate decreasing resistivity with increasing voltage. Polyaniline doped with sulfuric acid (H2SO4) exhibited the largest moisture content and surge protection devices built with this material produced for the first time negative differential resistance under ambient conditions. A further improvement was made upon this through surface engineering of the interface between the polymer and electrodes using self-assembled monolayers.