Electric strength of polymers

In the theory of electric strength, usually referred to as free-volume theory, breakdown is initiated by free electrons accelerated by electric field in the largest of the holes present in the amorphous phase of all polymers. In contrast to previous publications, the breakdown is treated as the final stage of the disproportionate rise of the electron current in very high field. Breakdown starts when the energy Fx gained by free electrons in field F on free path of length x attains with probability P = 1 the barrier energy E(mu). Then all electrons with this energy hop over the barriers E(mu) separating the traps in the compact structure. In this way breakdown is induced; the breakdown field therefore depends on the same factors as the longest free path. It is shown that, besides field-exposed volume and temperature, the pre-breakdown field stress is one of these factors. The resulting dependence of the breakdown field on these factors is derived starting from the distribution of the largest hole volumes. Calculated curves of electric strength as a function of temperature are in agreement with the measurements. The consequences for application of polymers as high-voltage insulation are indicated.