A diffusion limited mechanism to explain current pulse rate in a pre-breakdown current burst in silicone fluids

Measurements have been made of prebreakdown cavity growth and of the accompanying current pulse bursts in silicone fluids over a range of viscosities from 0.65 cSt to 1000 cSt. At high viscosities the average time between pulses (δt) is proportional to fluid viscosity, but in the low viscosity limit the dependence is nearer to square root of viscosity. To explain this viscosity dependence we make use of the fact that a pulse cannot occur until charge from the previous pulse has been effectively removed. We consider two possible mechanisms of charge dissipation, namely (i) ion detrapping and drift in the applied field, and (ii) diffusion of an electronegative impurity species to the cavity interface, charge capture to create a mobile ion, and its subsequent drift in the field. For case (i) ion drift is the rate limiting process and this leads to δt being proportional to viscosity. For case (ii), the diffusion limited case, δt is proportional to the cube root of viscosity. This and other evidence inclines us to the diffusion limited model in the low viscosity limit