Experimental and theoretical investigation of conduction in polyethylene from 4 MV/m up to ‘intrinsic’ breakdown

Abstract
The conduction current in dry polyethylene inyacuo has been measured over a wide range of stress, up to ‘intrinsic’ breakdown. The rate of increase of current is much slower than is predicted by the Poole-Frenkel law, and the current at breakdown is still much too small to produce thermal instability. A much modified form of the PF theory is developed, taking account of the space charge which appears when electrons are ‘self-trapped’ at otherwise neutral sites. The calculated results agree closely with observation over the entire range. The activation energy of conduction is increased greatly(up to 2.2 eV) by prolonged evacuation at 70°C. In this state, the conductivity is sensitive to extremely small partial pressures of water vapour. It is shown that this cannot be due to ionic conduction, and it is attributed to a large change in the entropy of trapping when the trap is able to acquire both a water molecule and an electron. The traps are regarded as sites within the amorphous fraction of the material where the field either from an electron or a polar molecule can reorient the local structure. The free energy of activation is then dependent both on polarisation by an electron and on the entropy of reordering. Observed values of activation energy and entropy agree with this model.

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