GITAM, Department of Engineering Physics

Dielectric Breakdown

As you know, the first law of Materials science is "Everything can be broken". Dielectrics are no exception to this rule. If you increase the voltage applied to a capacitor, eventually you will produce a big bang and a lot of smoke - the dielectric material inside the capacitor will have experienced "electrical breakdown" or electrical break-through, an irreversible and practically always destructive sudden flow of current.

The critical parameter is the field strength E in the dielectric. If it is too large, breakdown occurs. The (DC) current vs. field strength characteristic of a dielectric therefore may may look look this:

After reaching Ecrit, a sudden flow of current may, within very short times (10–8 s) completely destroys the dielectric to a smoking hot mass of undefinable structure.

Unfortunately, Ecrit is not a well defined material property, It depends on many parameters, the most notable (besides the basic material itself) being the production process, the thickness, the temperature, the internal structure (defects and the like), the age, the environment where it is used (especially humidity) and the time it experienced field stress.

Important types of electrical breakdown are

The following table gives a rough idea of critical field strengths for certain dielectric materials

Material Critical Field Strength
(kV/cm)
Oil 200
Glass, ceramics 200...400
Mica 200...700
Oiled paper 1800