Dielectric
breakdown
Breakdown
depends on many factors, especially thermal ones, and is a function of the time
of application of the p.d.
A dielectric
material must possess:
(a) a high
insulation resistivity to avoid leakage conduction, which dissipates the
capacitor energy in heat;
(b) a
permittivity suitable for the purpose - high for capacitors and low for
insulation
generally;
and
(c) a high
electric strength to withstand large voltage gradients, so that only thin
material is required. It is rarely possible to secure optimum properties in one
and the same material.
A practical
dielectric will break down (i.e. fail to insulate) when the voltage gradient
exceeds the value that the material can withstand. The breakdown mechanism is
complex.
Gases
With gaseous
dielectrics (e.g. air and hydrogen), ions are always present, on account of
light, heat, sparking, etc. These are set in motion, making additional
ionisation, which may be cumulative, causing glow discharge, sparking or arcing
unless the field strength is below a critical value.
Field
strength of the order of 3MV/m is a limiting value for gases at normal
temperature and pressure. The dielectric strength increases with the gas
pressure.
The
polarisation in gases is small, on account of the comparatively large distances
between molecules. Consequently, the relative permittivity is not very
different from unity.
Liquids
When very
pure, liquids may behave like gases. Usually, however, impurities are present.
A small proportion of the molecules forms positive or negative ions, and
foreign particles in suspension (fibres, dust, water, droplets) are prone to
align themselves into semiconducting filaments: heating produces vapour, and
gaseous breakdown may be initiated. Water, because of its exceptionally high
permittivity, is especially deleterious in liquids such as oil.
Solids
Solid
dielectrics are rarely homogeneous, and are often hygroscopic. Local space
charges may appear, producing absorption effects; filament conducting paths may
be present; and local heating (with consequent deterioration) may occur.
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