I know that a capacitor with a dielectric can operate normally up till a certain voltage (AFAIK called breakdown voltage) which depends on the strength of the dielectric placed between the plates. After this voltage, the circuit becomes short and current flows between the plates and thus the capacitor breaks down. But i want to know what is exactly happening when we say a dielectric "breaks down" ? What I know about a dielectric is that due to the electric field (because of the plates of the capacitor) the molecules of the dielectric align themselves accordingly and set up an electric field in the opposite direction, thus decreasing the net electric field. So, please can anyone tell me what happens during breakdown?
Answer
Breakdowns are electron cascades. There are different kinds:
1) Intrinsic breakdown of the material occurs when the electric field is sufficiently strong to ionize an atom of the dielectric (or accelerate a stray electron sufficiently to do the same), with the resultant new free electrons then being accelerated by the field to repeat the process with another atom. If more free electrons are produced than reattached, the process grows exponentially and breakdown results.
That said, intrinsic breakdown is rare, because other types of breakdown occur at much lower field strengths:
2) If there is a void in the dielectric material (a bubble) the residual gas in the void will break down at some (lower) electric field strength (again an electron cascade), and the freed electrons will strike the sides of the void, heating the dielectric and eroding it. This type of discharge is small and perhaps unnoticeable, but given enough time, the void will grow and eventually destroy the dielectric.
3) Surfaces between different materials (say the boundary between the dielectric and the material encasing the capacitor) can delaminate, again creating an empty space in which gas breakdown can occur.
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