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Encyclopedia of Physical Science and Technology En004F-171 June 8, 2001 17:11

Dielectric Gases

L. G. Christophorou
S. J. Dale
Oak Ridge National Laboratory

I. Introduction
II. Basic Physical Processes and Properties
III. Breakdown Strength
IV. Uses of Dielectric Gases
V. Concluding Remarks

GLOSSARY A DIELECTRIC GAS IS a gaseous medium consisting
of one or more components with a high breakdown voltage
Breakdown voltage Critical voltage under which a gas (i.e., a gaseous medium that is a relatively poor conductor
makes the transition from an insulator to a conductor. or a nonconductor of electricity to high applied electrical
Dielectric gas Relatively poor conductor or nonconduc- strength). This article outlines the basic physical processes
tor of electricity to high applied electrical stress; gas that determine the dielectric properties of gases and dis-
with high breakdown voltage. cusses the breakdown strengths of dielectric gases and the
Electricalbreakdown(orelectricaldischargeorspark) main uses of gaseous dielectrics.
ofagas Dramaticeventwherebytheelectricalconduc-
tivity of an electrically stressed gas increases by many
orders of magnitude in times ranging from nanosec- I. INTRODUCTION
onds to milliseconds.
Electronegative gas Electron-attaching gas in which A gas in its normal state is a perfect insulator. If, however,
stable negative ions (parent and/or fragment) are the gas is electrically stressed, the free electrons present in
produced. itgainenergyfromtheappliedelectricfield;whenthelevel
Synergism of gas mixtures Property of gas mix- of the applied field is such that an appreciable number of
tures whereby they have dielectric strengths exceed- these free electrons possess kinetic energies high enough
ing the partial-pressure-weighted dielectric strengths to ionize the gas atoms or molecules, the gas “breaks
of the individual components making up the gas down” (i.e., it makes the transition from an insulator to
mixture. a conductor). The minimum critical voltage under which
Tailoring gas dielectrics Combination, on the basis the electrical conductivity of the gas increases by many or-
of fundamental physicochemical knowledge, of two ders of magnitude is known as the breakdown voltage and
or more gases to optimize their overall dielectric the phenomenon itself as electrical breakdown, electrical
properties. discharge, or spark.

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