Electrical trees in silicone gel: A combination of liquid and solid behaviour patterns

Electrical trees in liquids (streamers) occur on the nano-second timescale and produce structures that dissipate on voltage removal and sometimes even during voltage application. In AC fields the structures are a combination of fine filaments from the positive half-cycle and spheroidal cavities from the negative half-cycle. In contrast electrical trees in solids are permanent filamentary structures with a fractal geometry that grow on timescales of hours or longer at typical field values. Here we present the results of tree formation in a silicone gel under an AC applied electric field. These grow on timescales of minutes and possess a fine branched filamentary structure as well as spheroidal cavities. As in liquids, the cavities can collapse during tree growth. In contrast the filamentary structure is permanent as in solids. However the whole tree contracts following the removal of the applied voltage. The observed stability of the partial self-healing tree features are discussed in terms of the mixed liquid and solid features of the gel structure.