Vapour sensing applications and electrical conduction mechanisms of a novel metal-polymer composite
A novel metal-polymer composite is presented, comprised of a micron-sized nickel powder dispersed within a silicone polymer matrix. The composite is intrinsically electrically insulating, but displays a dramatic increase in conductivity under compression, tension and torsion. The electrical response to applied compression is characterised. Combined with electron microscopy, the large sensitivity to compression is shown to be due to the uniquely spiky morphology of the filler particles. Low mechanical energy mixing techniques are essential for retaining these sharp tips. In addition, wetting of the nickel particles by the silicone polymer is highly effective, resulting in negligible inter-particle contact between metallic grains even at very high loadings and compressions. Current-voltage characteristics are highly non-linear, displaying peaks, hysteresis, negative differential resistance, trap-filling and radio frequency emission. Evidence points towards an inter-particle conduction mechanism dominated by field emission and Fowler-Nordheim tunnelling, made possible by localised field enhancements at the sharp tips. A novel mechanism of grain charging and the 'pinching-off of conduction pathways is also suggested. Granular forms of the composite display dramatic increases in resistance when exposed to organic solvent vapours, transduced by a polymer swelling mechanism. Responses are dependent upon vapour concentration, and differential responses are obtained with other polymers, indicating excellent potential for applications in artificial olfactory devices (electronic noses). Polymer-solvent interactions follow both Fickian and anomalous diffusion characteristics, and follow basic trends predicted by solubility parameters.