The photon energy and the energy state of the molecular or ionic species involved in
the reactions given above will determine which of the reactions will occur.
Particle impact has been attributed variously as the mechanism of partial discharge
degradation [2, 3]. Interestingly, in polyethylene, Mayoux [4] has shown that degradation
due to ion bombardment occurs at a significant rate only if the charge density
of ions exceeds ≈1.5 × 102 cm−2. Further, he has demonstrated that although, in
theory, electrons may produce degradation, electron energies in excess of 500 eV are
required to cause substantial damage. However, once again, the synergetic nature of
discharge degradation mechanisms cannot be overstressed; that one form of stressing
does not in itself result in degradation does not mean that it can be dismissed as a
contributory mechanism to degradation.
In respect of the potential for particle impact damage at a given insulating
surface, it is useful to consult one of the many excellent texts on gas breakdown,
e.g. Reference 5, to determine, for a given gaseous situation, the nature of the particles
involved and for a given set of conditions (partial pressure etc.) the statistical
distribution of energies associated with those particles. Having obtained this information,
consultation of materials texts will provide details on molecular structure
and bond energies. Comparison of the two sets of information should provide some
guidance of the likelihood of bond scission etc. and where it might occur. An example
of the latter form of data is provided in Table 4.1.