Friday, August 28, 2009

MECHANISM OF BREAKDOWN OF GASES

At normal temperature and pressure, the gases are excellent insulators. The current conduction is of the
order of 10–10 A/cm2. This current conduction results from the ionisation of air by the cosmic radiation
and the radioactive substances present in the atmosphere and the earth. At higher fields, charged parti-
cles may gain sufficient energy between collision to cause ionisation on impact with neutral molecules.
It is known that during an elastic collision, an electron loses little energy and rapidly builds up its
kinetic energy which is supplied by an external electric field. On the other hand, during elastic colli-
sion, a large part of the kinetic energy is transformed into potential energy by ionising the molecule
struck by the electron. Ionisation by electron impact under strong electric field is the most important
process leading to breakdown of gases.
This ionisation by radiation or photons involves the interaction of radiation with matter.
Photoionisation occurs when the amount of radiation energy absorbed by an atom or molecule exceeds
its ionisation energy and is represented as A + hν → A+ + e where A represents a neutral atom or molecule in the gas and hν the photon energy. Photoionization is a secondary ionization process and is
essential in the streamer breakdown mechanism and in some corona discharges. If the photon energy is
less than the ionization energy, it may still be absorbed thus raising the atom to a higher energy level.
This is known as photoexcitation.
The life time of certain elements in some of the excited electronic states extends to seconds.
These are known as metastable states and these atoms are known as metastables. Metastables have a
relatively high potential energy and are, therefore, able to ionize neutral particles. Let A be the atom to
be ionized and Bm the metastable, when Bm collides with A, ionization may take place according to the
reaction.
A + Bm → A+ + B + e
Ionization by metastable interactions comes into operation long after excitation and it has been
shown that these reactions are responsible for long-time lags observed in some gases.
Thermal Ionisation: The term thermal ionisation in general applies to the ionizing actions of
molecular collisions, radiation and electron collisions occurring in gases at high temperatures. When a
gas is heated to high temperature, some of the gas molecules acquire high kinetic energy and these
particles after collision with neutral particles ionize them and release electrons. These electrons and
other high-velocity molecules in turn collide with other particles and release more electrons. Thus, the
gas gets ionized. In this process, some of the electrons may recombine with positive ions resulting into
neutral molecule. Therefore, a situation is reached when under thermodynamic equilibrium condition
the rate of new ion formation must be equal to the rate of recombination. Using this assumption, Saha
derived an expression for the degree of ionization β in terms of the gas pressure and absolute tempera-
ture as follows:
[β2/1 − β2]=[1 ( 2πm(e )^ 3/ 2 ( KT ) 5 / 2 e ^− W / KT]/ph


where p is the pressure in Torr, Wi the ionization energy of the gas, K the Boltzmann’s constant, β the
ratio ni/n and ni the number of ionized particles of total n particles. Since β depends upon the tempera-
ture it is clear that the degree of ionization is negligible at room temperature. Also, if we substitute the
values of p, Wi, K and T, it can be shown that thermal ionization of gas becomes significant only if
temperature exceeds 1000° K.

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