Page 258 - Electrical Properties of Materials

240 Dielectric materials

10.12 Dielectric breakdown
There are three main mechan- Electric breakdown is a subject to which it is difficult to apply our usual sci-
isms that are usually blamed for entific rigour. A well-designed insulator (in the laboratory) breaks down in
dielectric breakdown: (1) intrinsic, service if the wind changes direction or if a fog descends. An oil-filled high-
(2) thermal, and (3) discharge voltage condenser will have bad performance, irrespective of the oil used, if
breakdown. there is 0.01% of water present. The presence of grease, dirt, and moisture
is the dominant factor in most insulator design. The flowing shapes of high-
voltage transmission line insulators are not entirely due to the fact that the
ceramic insulator firms previously made chamberpots; the shapes also reduce
the probability of surface tracking. The onset of dielectric breakdown is an im-
portant economic as well as technical limit in capacitor design. Generally, one
wishes to make capacitors with a maximum amount of stored energy. Since
1
2
the energy stored per unit volume is E , the capacitor designers value high
2
breakdown strength even more than high dielectric constant.
In general, breakdown is manifested by a sudden increase in current when
the voltage exceeds a critical value U b , as shown in Fig. 10.13. Below U b there
is a small current due to the few free electrons that must be in the conduction
band at finite temperature. When breakdown occurs it does so very quickly,
–8
I typically in 10 sinasolid.

10.12.1 Intrinsic breakdown
When the few electrons present are sufficiently accelerated (and lattice col-
U U
b lisions are unable to absorb the energy) by the electric field, they can ionize
lattice atoms. The minimum requirement for this is that they give to the bound
Fig. 10.13
(valence) electron enough energy to excite it across the energy gap of the ma-
Current voltage characteristics for
terial. This is, in fact, the same effect that we mentioned before in connection
an insulator. The current increases
with avalanche diodes.
very rapidly at the breakdown
voltage, U b .
10.12.2 Thermal breakdown
This occurs when the operating or test conditions heat the lattice. For example,

an a.c. test on a material in the region of its relaxation frequency, where is
large would cause heating by the lossy dipole interaction rather than by accel-
erating free electrons. The heated lattice ions could then be more easily ionized
by free electrons, and hence the breakdown field could be less than the intrinsic
breakdown field measured with d.c. voltages. The typical polymer, polyethyl-
8
ene, has a breakdown field of 3 to 5 × 10 Vm –1 for very low frequencies,
6
6
but this falls to about 5 × 10 Vm –1 around 10 Hz, where a molecular relax-
ation frequency occurs. Ceramics such as steatite and alumina exhibit similar
effects.
If it were not for dielectric heating effects, breakdown fields would be lower
at high frequencies simply because the free electrons have only half a period
to be accelerated in one direction. I mentioned that a typical breakdown time
8
–8
was 10 s; so we might suppose that at frequencies above 10 Hz breakdown
would be somewhat inhibited. This is true, but a fast electron striking a lattice
ion still has a greater speed after collision than a slow one, and some of these
fast electrons will be further accelerated by the field. Thus, quite spectacular
10
breakdown may sometimes occur at microwave frequencies (10 Hz) when