“Sub-hertz” dielectric spectroscopy

Dielectric spectroscopy measurements below 1 Hz are often dominated by ldquoconduction-likerdquo effects. For this reason, they often appear to be dismissed as being of little interest. In this paper two ldquosub-hertzrdquo responses are considered that give insights into the insulating systems concerned. The first system is that of cross-linked polyethylene, taken from a power cable system. Measurements at temperatures between 60degC and close to melting at 100degC show a change in characteristic from a percolation process to a ldquotruerdquo DC conduction at close to the melting point. Using DC conductivities, it appears to be possible to show whether the cable has been subjected to thermo-electric ageing. This might give insights into where the conduction and hence the ageing in the XLPE is occurring. The second system is an epoxy composite. By considering the sub-hertz response, it is possible to demonstrate the effect of the interface between the filler and the epoxy matrix. In this system, ageing, resulting in delamination between the glass fiber filler and the epoxy, is clearly detected by sub-hertz dielectric spectroscopy. This process is likely to be facilitated by the presence of water, which is known to lead to mechanical failure in such systems, and which can also be detected by ldquosub-hertzrdquo dielectric spectroscopy. The implications for nano-dielectrics are then briefly considered.