Abstract
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.
Highlights
We reported a new type of low-cost two terminal resistive current limiting device for use in low and medium power surge protection.[1]
In this paper we demonstrate that surface interface engineering using superhydrophobic self-assembled monolayers (SAM) leads to current foldback and negative differential resistance effects in our surge protection devices
This was found in the case of higher DDT concentrations where 15 mM produced the best current limiting properties (Fig. 6b) Interestingly, the stronger concentrations and longer immersion times produced the best current limiting properties this was not reflected in the contact angle measurements which did not show improvement above 24 hrs immersion times or above 5 mM concentrations. This indicates that higher concentrations and longer immersion times do not increase the hydrophobicity of the surface once it has reached a maximum value but are necessary for fixing minor defects in the SAM, such as pinholes, which are more likely to affect the electronic properties of the device to a greater extent since even small pinholes would permit a significant amount of current to bypass the interface region when the polyaniline material is present
Summary
We reported a new type of low-cost two terminal resistive current limiting device for use in low and medium power surge protection.[1] The device, which is based upon a single material consisting of a thin-film of an intrinsic conducting polymer, is distinctly different from conventional surge protection devices, which are instead based upon a composite matrix material of semiconductor (carbon, borides, silicides) or metallic particles embedded within an insulating polymer of high thermal expansion coefficient (polyethylene, epoxy).[2,3] These polymeric positive temperature coefficient (PPTC) devices operate by having many conduction pathways[4,5] that break during a current surge due to joule heating and subsequent thermal expansion of the insulating polymer.[6,7] The resulting high resistance state reduces the current to safe levels, providing protection for components in the circuit and other attached electrical equipment. The single component system has the advantages of low cost starting materials and inexpensive method of fabrication
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