Electrical properties and vapour sensing characteristics of a novel metal-polymer composite

Abstract

Quantum Tunnelling Composite (QTC) is a metal polymer composite, commercialised and produced using a patented manufacture process. This process ensures that the metal particles within an elastomerie polymer matrix maintain a highly fractal surface morphology where nano-scale point features are retained on the particles and are coated in polymer. This structure provides unique electronic behaviour including very high resistivity, of order 10 ΜΩ, above the expected percolation threshold and an exponential increase in conductivity under all types of mechanical deformation. Increase in sample compression leads to a lower electrical resistance through the material. Current-voltage characteristics show a hysteresis effect due to current storage in the QTC material as a current is passed. The hysteresis is shown to be reduced as the applied compression on the material is increased until an Ohmic regime is reached at very high compressions, above 70% linear compression. At these high compressions Joule heating is also proven to occur as a result of the power dissipated in the sample by I-V cycling. The Joule heating is sufficient to influence the physical characteristics of the sample and expand it, creating a current limiting device. QTC samples loaded with acicular electro-conductive particles in small fractions, less than 10% by weight, showed less sensitivity to applied compression in terms of electrical response. These samples appear to exhibit less white noise characteristics and indicate a combination of field assisted quantum tunnelling and percolation mechanism, Intrinsically conductive QTC samples were developed. These were made using QTC granules mixed into a polymer solvent solution. Upon depositing onto electrodes the solvent was allowed to evaporate leaving the constituent polymer binding the QTC granules together, compressing them into a conductive state. Samples were exposed to volatile organic compound (VOC) vapours in the concentration range 10 ppm to 100,000 ppm, causing swelling and void filling in the binding polymer. Combinations of these processes caused an increase in sample resistance, from ~50 Ω to excess of 10 ΜΩ. Sample composition and physical parameters have significant effect upon the response characteristics of the sensors. A system of experiments was undertaken and optimum sample composition was determined. Response to environmental changes were investigated^ namely temperature response and response to varying concentration of exposed solvent. It was found that samples produced using Polyphenylene Oxide ( PPO) and Polyvinyl Chloride (PVC) based binding polymer were more resistant to temperature change from 30 С to 80 С due to their molecular structures. Sensor response to different vapour concentrations was found to exhibit two distinct response regimes. High concentration exposures were found to exhibit a swelling mechanism with a CASE-II diffusion model fitting the data well. Whereas at low concentrations a void-filling based change in sample dielectric constant was attributed to the electronic response to vapour exposure. These predictions were also confirmed using a Quartz Crystal Microbalance (QCM) to measure mass uptake of vapour molecules and polymer density under similar test conditions.