A dielectric gas, or insulating gas, is a dielectric

Dielectric

A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

 material in gaseous state. Its main purpose is to prevent or rapidly quench electric discharge

Electric discharge in gases

Electric discharge in gases occurs when electric current flows through a gaseous medium. Depending on several factors, the discharge may radiate visible light.-Discharge types:...

s. Dielectric gases are used as electrical insulators in high voltage

High voltage

The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...

 applications, e.g. transformer

Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

s, circuit breaker

Circuit breaker

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...

s (namely sulfur hexafluoride circuit breaker

Sulfur hexafluoride circuit breaker

High-voltage circuit-breakers have greatly changed since they were first introduced in the mid-1950s, and several interrupting principles have been developed that have contributed successively to a large reduction of the operating energy...

s), switchgear

Switchgear

The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults...

 (namely high voltage switchgear), radar

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 waveguide

Waveguide

A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...

s,

A good dielectric gas should have high dielectric strength

Dielectric strength

In physics, the term dielectric strength has the following meanings:*Of an insulating material, the maximum electric field strength that it can withstand intrinsically without breaking down, i.e., without experiencing failure of its insulating properties....

, high thermal stability and chemical inertness against the construction materials used, non-flammability and low toxicity

Toxicity

Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...

, low boiling point

Boiling point

The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....

, good heat transfer

Heat transfer

Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...

 properties, and low cost.

The most common dielectric gas is air, due to its ubiquity and low cost. Another commonly used gas is a dry nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

.

In special cases, e.g., high voltage switches, gases with good dielectric properties and very high breakdown voltage

Breakdown voltage

The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive.The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse...

s are needed. Highly electronegative elements, e.g., halogen

Halogen

The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...

s, are favored as they rapidly recombine

Carrier generation and recombination

In the solid state physics of semiconductors, carrier generation and recombination are processes by which mobile charge carriers are created and eliminated. Carrier generation and recombination processes are fundamental to the operation of many optoelectronic semiconductor devices, such as...

 with the ion

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s present in the discharge channel. The halogen gases are highly corrosive

Corrosive

A corrosive substance is one that will destroy or irreversibly damage another surface or substance with which it comes into contact. The main hazards to people include damage to the eyes, the skin, and the tissue under the skin; inhalation or ingestion of a corrosive substance can damage the...

. Other compounds, which dissociate only in the discharge pathway, are therefore preferred; sulfur hexafluoride

Sulfur hexafluoride

Sulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...

, organofluorides (especially perfluorocarbons) and chlorofluorocarbon

Chlorofluorocarbon

A chlorofluorocarbon is an organic compound that contains carbon, chlorine, and fluorine, produced as a volatile derivative of methane and ethane. A common subclass are the hydrochlorofluorocarbons , which contain hydrogen, as well. They are also commonly known by the DuPont trade name Freon...

s are the most common.

The breakdown voltage of gases is roughly proportional to their density

Density

The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

. Breakdown voltages also increase with the gas pressure; many gases however have limited upper pressure due to their liquefaction

Liquefaction

Liquefaction may refer to:* Liquefaction, the general process of becoming liquid* Soil liquefaction, the process by which sediments become suspended* Liquefaction of gases in physics, chemistry, and thermal engineering* Liquefactive necrosis in pathology...

.

The decomposition products of halogenated compounds are highly corrosive, the occurrence of corona discharge

Corona discharge

In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...

 should therefore be prevented.

Build-up of moisture

Moisture

Humidity is the amount of moisture the air can hold before it rains. Moisture refers to the presence of a liquid, especially water, often in trace amounts...

 can degrade dielectric properties of the gas. Moisture analysis

Moisture analysis

Moisture analysis covers a variety of methods for measuring moisture content in both high level and trace amounts in solids, liquids, or gases. Moisture in percentage amounts is monitored as a specification in commercial food production. There are many applications where trace moisture...

 is used for early detection of this.

Dielectric gases can also serve as coolant

Coolant

A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...

s.

Vacuum

Vacuum

In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

 is an alternative for gas in some applications.

Mixtures of gases can be used where appropriate. Addition of sulfur hexafluoride can dramatically improve the dielectric properties of poorer insulators, e.g. helium or nitrogen. Multicomponent gas mixtures can offer superior dielectric properties; the optimum mixtures combine the electron attaching gases (sulfur hexafluoride

Sulfur hexafluoride

Sulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...

, octafluorocyclobutane

Octafluorocyclobutane

Octafluorocyclobutane, or perfluorocyclobutane, C4F8, is an organofluorine compound which enjoys several niche applications. It is related to cyclobutane by replacement of all C-H bonds with C-F bonds...

) with molecules capable of thermalizing (slowing down) accelerated electrons (e.g. tetrafluoromethane

Tetrafluoromethane

Tetrafluoromethane, also known as carbon tetrafluoride, is the simplest fluorocarbon . It has a very high bond strength due to the nature of the carbon–fluorine bond. It can also be classified as a haloalkane or halomethane...

, fluoroform

Fluoroform

Fluoroform is the chemical compound with the formula CHF3. It is one of the "haloforms", a class of compounds with the formula CHX3 . Fluoroform is used in diverse niche applications and is produced as a by-product of the manufacture of Teflon...

. The insulator properties of the gas are controlled by the combination of electron attachment, electron scattering

Electron scattering

Electron scattering is the process whereby an electron is deflected from its original trajectory. As they are charged particles, they are subject to electromagnetic forces.-Phenomena:...

, and electron ionization

Electron ionization

Electron ionization is an ionization method in which energetic electrons interact with gas phase atoms or molecules to produce ions...

.

Atmospheric pressure

Atmospheric pressure

Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

 significantly influences the insulation properties of air. High-voltage applications, e.g. xenon flash lamps, can experience electrical breakdowns at high altitudes.

Relative spark breakdown voltages of insulating gases at 1 atm

Gas	Formula	Breakdown voltage relative to air	Molecular weight (g/mol)	Density* (g/l)	ODP Ozone depletion potential The ozone depletion potential of a chemical compound is the relative amount of degradation to the ozone layer it can cause, with trichlorofluoromethane being fixed at an ODP of 1.0. Chlorodifluoromethane , for example, has an ODP of 0.055...	GWP Global warming potential Global-warming potential is a relative measure of how much heat a greenhouse gas traps in the atmosphere. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A GWP is calculated over a specific time...	Properties
Sulfur hexafluoride Sulfur hexafluoride Sulfur hexafluoride is an inorganic, colorless, odorless, and non-flammable greenhouse gas. has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in...		3.0	146.06	6.164		22800	The most popular insulating gas. It is dense and rich in fluorine Fluorine Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic... , which is a good discharge quencher. Good cooling properties. Excellent arc quenching. Corrosive decomposition products. Its price increased and supply got limited, as many manufacturing plants switched to production of more profitable perfluorocarbons. The most potent known greenhouse gas Greenhouse gas A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...  with extremely long atmospheric lifetime. Although most of the decomposition products tend to quickly re-form , arcing or corona Corona discharge In electricity, a corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized...  can produce disulfur decafluoride Disulfur decafluoride Disulfur decafluoride is a gas discovered in 1934 by Denbigh and Whytlaw-Gray. Each S of the S2F10 molecule is octahedral, and surrounded by 5 fluorines. S2F10 is highly toxic, with toxicity similar to phosgene...  , a highly toxic gas, with toxicity similar to phosgene Phosgene Phosgene is the chemical compound with the formula COCl2. This colorless gas gained infamy as a chemical weapon during World War I. It is also a valued industrial reagent and building block in synthesis of pharmaceuticals and other organic compounds. In low concentrations, its odor resembles... . Sulfur hexafluoride in an electric arc may also react with other materials and produce toxic compounds, e.g. beryllium fluoride Beryllium fluoride Beryllium fluoride is the inorganic compound with the formula BeF2. This white solid is the principal precursor for the manufacture of beryllium metal.-Structure and bonding:...  from beryllium oxide Beryllium oxide Beryllium oxide , also known as beryllia, is an inorganic compound with the formula BeO. This colourless solid is a notable electrical insulator with a higher thermal conductivity than any other non-metal except diamond, and actually exceeds that of some metals. As an amorphous solid, beryllium...  ceramics. Frequently used in mixtures with e.g. nitrogen or air.
Nitrogen Nitrogen Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...		1.15	28	1.251	–	–	Often used at high pressure. Does not facilitate combustion. Can be used with 10–20% of SF6 as a lower-cost alternative to SF6. Can be used standalone or in combination with CO2.
Air	29/mixture	1		1.2	–	–	Breakdown voltage 30 kV/cm at 1 atm. Very well-researched. When subjected to an electrical discharge, forms corrosive nitrogen oxides and other compounds, especially in presence of water. Corrosive decomposition products. Can facilitate combustion, especially when compressed.
Ammonia Ammonia Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...		1	17.031	0.86
Carbon dioxide Carbon dioxide Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...		0.95	44.01	1.977	–	1
Hydrogen sulfide Hydrogen sulfide Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...		0.9	34.082	1.363
Oxygen Oxygen Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...		0.85	32.0	1.429	–	–	Very effectively facilitates combustion.
Chlorine Chlorine Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...		0.85	70.9	3.2
Hydrogen Hydrogen Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...		0.65	2.016	0.09			Low breakdown voltage but high thermal capacity and very low viscosity. Used for cooling of e.g. hydrogen-cooled turbogenerator Hydrogen-cooled turbogenerator A hydrogen-cooled turbo generator is a turbo generator with gaseous hydrogen as a coolant. Hydrogen-cooled turbo generators are designed to provide a low-drag atmosphere and cooling for single-shaft and combined-cycle applications in combination with steam turbines... s. Handling and safety problems. Very fast deexcitation, can be used in high repetition rate spark gap Spark gap A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms,... s and fast thyratron Thyratron A thyratron is a type of gas filled tube used as a high energy electrical switch and controlled rectifier. Triode, tetrode and pentode variations of the thyratron have been manufactured in the past, though most are of the triode design... s.
Sulfur dioxide Sulfur dioxide Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...		0.30	64.07	2.551
1,2-Dichlorotetrafluoroethane 1,2-Dichlorotetrafluoroethane 1,2-Dichlorotetrafluoroethane, or R-114, is a chlorofluorocarbon with the molecular formula ClF2CCF2Cl. Its primary use has been as a refrigerant. It is a non-flammable gas with a sweetish, chloroform-like odor with critical point at 145.6 °C and 3.26 MPa. When pressurized or cooled,...  (R-114)		3.2	170.92	1.455	?		Saturated pressure at 23 °C is about 2 atm, yielding breakdown voltage 5.6 times higher than nitrogen at 1 atm. Corrosive decomposition products.
Dichlorodifluoromethane Dichlorodifluoromethane Dichlorodifluoromethane , is a colorless gas, and usually sold under the brand name Freon-12, is a chlorofluorocarbon halomethane , used as a refrigerant and aerosol spray propellant. Complying with the Montreal Protocol, its manufacture was banned in the United States along with many other...  (R-12)		2.9	120.91	6	1	8100	Vapor pressure 90 psi (6.1 atm) at 23 °C, yielding breakdown voltage 17 times higher than air at 1 atm. Yet higher breakdown voltages can be achieved by increasing pressure by adding nitrogen. Corrosive decomposition products.
1,1,1,3,3,3-Hexafluoropropane 1,1,1,3,3,3-Hexafluoropropane 1,1,1,3,3,3-Hexafluoropropane is an organic chemical, an organofluoride. It is a colorless gas, usually available in the form of a liquid gas. It is used as a fire suppression agent, a foaming agent, a highly effective refrigerant, a heat transfer medium, a dielectric gas, a sterilant carrier, a...  (R-236fa)			152.05			6300	Corrosive decomposition products.
Carbon tetrafluoride (R-14)		1.01	88.0	3.72	–	6500	Poor insulator when used alone. In mixture with SF6 somewhat decrases sulfur hexafluoride's dielectric properties, but significantly lowers the mixture's boiling point and prevents condensation at extremely low temperatures. Lowers the cost, toxicity and corrosiveness of pure SF6.
Hexafluoroethane Hexafluoroethane Hexafluoroethane is a fluorocarbon counterpart to the hydrocarbon ethane. It is a non-flammable gas negligibly soluble in water and slightly soluble in alcohol.-Physical properties:...  (R-116)		2.02	138	5.734	–	9200
Perfluoropropane (R-218)		2.2	188	8.17	–	?
Octafluorocyclobutane Octafluorocyclobutane Octafluorocyclobutane, or perfluorocyclobutane, C4F8, is an organofluorine compound which enjoys several niche applications. It is related to cyclobutane by replacement of all C-H bonds with C-F bonds...  (R-C318)		3.6	200	7.33	–	?	Possible alternative of SF6.
Perfluorobutane Perfluorobutane Perfluorobutane is a colorless gas. It is a simple fluorocarbon with the n-butane skeleton, but with hydrogen atoms in n-butane are replaced with fluorine atoms. It is used as a replacement for Halon 1301 fire extinguishers, as well as being an ultrasound imaging agent. As a pharmaceutical, it is... (R-3-1-10)		2.6	238	11.21	–	?
30% /70% air		2.0

^* the density is approximate; it is normally specified at atmospheric pressure, the temperature may vary, though it is mostly 0 °C.