Breakdown mechanisms; electronic vs orientational polarization

Dielectric breakdown is the sudden, large rise in current through an insulator when the applied field exceeds a critical value (the dielectric strength), after which the material stops insulating.

Thermal breakdown

• Every real dielectric has a small conduction current and dielectric (AC) loss, which produce heat: $P = \omega \varepsilon_0 \varepsilon_r \tan\delta\, E^2$ per unit volume.
• The conductivity of insulators rises exponentially with temperature, so more heat gives more current, which gives still more heat.
• If heat produced is more than heat that can be conducted away to the surroundings, temperature runs away, the material melts, chars or burns, and a conducting path forms.
• Features: takes seconds to minutes, depends on frequency (worse at high frequency), ambient temperature, thickness and cooling; breakdown voltage falls as temperature rises.

Electromechanical breakdown

• The applied voltage puts opposite charges on the two electrodes, which attract each other with an electrostatic pressure $p = \tfrac{1}{2}\varepsilon_0\varepsilon_r E^2$.
• Soft solids (polymers, rubber, especially when warm) are compressed by this force, so the thickness $d$ decreases.
• For the same voltage, smaller $d$ means larger field $E = V/d$, more pressure and more compression.
• When the mechanical restoring stress (Young's modulus) can no longer balance the electric pressure (at about $d \approx 0.6 d_0$), the material collapses mechanically and breaks down.

Electronic vs orientational polarization