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[2.4.1 Properties of Sulphur hexafluoride 'SF6' Gas
Sulphurhexafluoride was first produced in 1900 by French scientists Moissan and Lebeau by direct fluoronisation of sulphur. In
the beginning it was mainly used as a dielectric in atomic physics. During late 1950s, it found application in high voltage circuit
breakers. Ever since, its application in power systems has been continuously increasing.
Physical Properties:
In a SF6 molecule, six fluorine atoms arrange themselves uniformly like an octahedron on a central sulphur atom. An excited
sulphur atom can therefore form six stable covalence bonds with the strongly electronegative fluorine atoms by sharing the pair
of electrons. Amongst halogens the fluorine element, and the sulphur atom both have very high coeffi~ients of electronega.
tivity, of the order of 4 and 2.5, respectively. This coefficient is a measure of the tendency to attract electrons of other atoms to
form dipole bondage.
The rigid symmetrical structure, small binding distance and high binding energy between atoms of a SF6 molecule provide
it a high stability. With the result SF6 gas properties near that of rare ga')es at relatively low temperatures. Thermal dissociation
in highly purified SF6 gas begins at extremely high temperatures (above 1000 K). Such high temperatures in power sytems
occur only in electrical arcs. Even at continuous temperatures upto about 500 K, neither thermal decomposition of SF6 nor its
chemical reaction with other materials have been reponed. Further, SF6 is a nontoxic, colourless and odourless gas. Some
important physical properties of SF6, other than molecular properties, are brought together in Table 2.9.
Table 2.9 Physical properties of SF6 [2.74]
Property
Value
Relative Permittivity
Dielectric loss tangent
Critical temperature
Tripple point
Sublimation point
Specific heat capacity
Heat conductivity Heat transition No.
Physical
Conditions
0.1 MPa, 25°C
Symbol
€r
- 51°C (liquid)
0.1 MPa
-
tan b
51 °C(liquid)
-
6cr
p=0.22 MPa
eT
es
at 10°C and
Cp
constant
p = 0.1 MPa
at 10°C and
Cv
const. volume
30°C
Unit
1.002
1.81 :i: 0.02
< 5.10-6 < 1.10,3
°C
45.5
°C
-50.8
°C
-63.8
J molK
5.13
J molK
4.06
J cm sK
0.82.10,5
J cm2sK
0.44.10-5 .
The densities of gases increase with the ralatlve molecular mass. As the molecular mass of SF6 is quite high (146), it has a
high density. Because of high density the charge carriers have a short mean free path. This property, alongwith the properties of
electron attachment, that is, electronegativity and
high ionization energy result in high dielectric strength of SF6, as shown in Table 2.8.
Density of a gas is actually a function of external pressure p and the temperature 8. In Fig. 2.$>5 the two states of SF6,
liquid or gaseous, are shown at varying pressures and -temperatures for different constant densities of the gas. This diagram is
important from the point of view of practical application of SF6 in GIS. Within the range of working pressure and temperature
of the gas, it should not liquefy as its electrical insulation properties are affected.
"oJuu
0.81
55
,,,,. .A""
50
45
---- I
kg/m3
I
40
lral"qUld V--=.J35
06
Pressure
p
o.41
125
20
75
f-ti
:r
-
4D
-2D
D
2D
4D
j
50 O[ 80
7emperature -8"
Fig. 2.65 States of SF6 for different constant gas densities and varying temperature and pressure within practical application range [2.74].
Property of Electron Attachment:
Some atoms or molecules in their neutral gaseous state have an affinity to
acquire a free electron and form a stable negative ion. This property of
negative ion formation is known as 'electron attachment' or 'electron affinity'. It has been proved that the atomic or molecular
gases having electron affinity property lack one or two electrons in their outer shell and hence are known as electronegative
gases. For example, the halogens (F, CI, Br, I and At) have one electron missing in their outer shells, whereas 0, Sand Se have
two electrons less in their outer shells.
For the attachment of an electron to a neutral molecule, the electron must have a certain minimum kinetic energy. This
energy is of the order of 1 e'Y, very low compared to the energy required for impact ionization. For a negative ion to remain
stable for some time, its total energy must be lower than that of a neutral molecule in the ground state. This difference in energy
levels is known as 'electron affinity' of the molecule. It is released as a quantum of kinetic energy on attachment.
There are several physical processes of negative ion formation; for example, 'radiative attachment process' where excess
energy is released on attachment in the form of a light quantum; the 'collision attachment process' where the excess energy
released on attachment is acquired as kinetic energy by a third body on collision. The third process is known as 'dissociative
attachment process' where the excess energy is used to separate the molecule into a neutral particle and a negative ion. This
latter process is predominant iQ molecular gases. Other processes of negative ion formation are (a) splitting of a molecule into
negative and positive ions by impact of an electron without attaching the electron, and (b) the charge transfer process following
heavy particle collision, giving rise to a pair of positive and negative ions. However, it must be mentioned that all the above
electron attachment processes are reversible, which may lead to an electron detachment.
The attachment process in SF6 gas is characterised with 'dissociative attachment'. It is caused by an electron impact,
separating the moleculC-into a neutral particle and a negative ion having a similar mass as the neutral gas
moleculeJ