Dielectric ageing: materials, measurements and modelling of nano to meso scale processes

This paper considers ageing processes in polymeric dielectrics under the action of applied electric fields. Ageing can be thought of as occurring in two stages, initiation of damage and its subsequent evolution. Initially, charge carriers move through the material perturbing the local field (space charge) and inducing excited states. Such processes can result in the production of photons (electroluminescence) through carrier recombination processes or the relaxation of excited molecular states and consequent permanent chemical changes. Once local molecular damage has been initiated, growth commonly occurs through the development of fractal structures (electrical trees). All the above processes can be dependent upon both the molecular composition and microstructure of the dielectric concerned and we demonstrate the value of using material design, novel experimental techniques and modelling in concert to understand the underlying physics and chemistry of ageing