Question

7. (10pt) Explain why electronegative gases have higher dielectric strength? What is the mechanism. Give two examples of electronegative gas.

Answer 1

One process that gives High Dielectric strength to a gas is the electron attachment in which free electrons get attached to neutral atoms or molecules to form negative ions.

Since negative ions like positive ions are too massive to produce ionization due to collisions,attachment represents an effective way of removing electrons which otherwise would have led to current growth and breakdown at low voltages. The gases in which attachment plays an active role are called electronegative gases. Two types of attachment are encountered in gases as;

a) Direct attachment: An electron directly attaches to form a negative ion.

AB + e- --------> AB-

b) Dissociative attachment: The gas molecules split into their constituent atoms and the electronegative atom forms a negative ion.

  AB + e- -----------> A + B-

A simple gas for this type is the oxygen and others are sulphur hexafluoride, Freon, carbon dioxide and fluorocarbons. In these gases, ‘A’ is usually sulphur or carbon atom and ‘B’ is oxygen atom or one of the halogen atoms or molecules.

The Townsend current growth equation is modified to include ionization and attachment with such gases.