PRIMARY ELECTRONS | Dielectric Properties of gases

     This article is based on evaluating breakdown of various insulating materials including gases, liquids and Solids. Basic definitions include primary electrons, Attachment, Detachment etc. 

      PRIMARY ELECTRONS: Free electrons exist only for very short times in air that is not subjected to a high electric field; normally they are trapped, after creation by cosmic rays, background radiation and so on, to form negative ions These have a density commonly of the order of a few hundred per cubic centimeter. However, electrons can be detached again from negative ions by acceleration and resulting collisions with neutral molecules in a strong electric field.

       A dielectric gas, or insulating gas, is a dielectric material in gaseous state. Its main purpose is to prevent or rapidly quench electric discharges Dielectric gases are used as electrical insulators in high voltage applications, e.g. transformers, circuit breakers (namely sulfur hexafluoride circuit  breakers), switchgear (namely high voltage switchgear),radar waveguide

          A good dielectric gas should have high dielectric strength, high thermal stability and chemical inertness against the construction materials used, non-flammability and low toxicity, low boiling point, good heat transfer properties, and low cost

         In special cases, e.g., high voltage switches, gases with good dielectric properties and very high breakdown voltages are needed. Highly electronegative elements, e.g., halogens, are favored as they rapidly recombine with the ions present in the discharge channel. The halogen gases are highly corrosive. Other compounds, which dissociate only in the discharge pathway, are therefore preferred; sulfur hexafluoride, organofluorides (especially perfluorocarbons) and chlorofluorocarbons are the most common. The breakdown voltage of gases is roughly proportional to their density. Breakdown voltages also increase with the gas pressure; many gases however have limited upper pressure due to their liquefaction.