As the most studied SF

6

substitute gas

CF

3

I and c-C

4

F

8

need to be mixed with buffer gases to reduce liq-uefaction temperature. For the purpose

to assess the possibility for air to be buffer gas in SF

6

substitute gas

electronswarm parameters of CF

3

I/air and c-C

4

F

8

/air are calculated through two-term approximated Boltzmann equation.First

the reduced ionization coefficient

reduced attachment coefficient

reduced effective ionization coefficient

limiting reduced electric strength and c value（the slope around zero point of reduced effective ionization coefficientas a function of reduced electric field strength）which reflects the sensitivity to non-uniform electric field of CF

3

I/airand c-C

4

F

8

/air are calculated. Then

the limiting reduced electric strengths and c values are compared to the mixturewith the same electronegative gases but different buffer gases（CO

2

and N

2

）. Assessment is made from two aspects:in-sulation strength and sensitivity to non-uniform electric field. The consequence is that CF

3

I/air or c-C

4

F

8

/air with themix ratio 30%/70% possesses similar insulation property to SF

6

. According to the result

among air

CO

2

and N

2

air isthe best choice to mix with CF

3

I

and air and N

2

possess similar properties to mix with c-C

4

F

8

both better than CO

2

.