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Re: Using SF6 for cap..
>Message-ID: <199607071825.MAA05875-at-poodle.pupman-dot-com>
>Date: Sun, 7 Jul 1996 12:25:01 -0600
>Subject: Re: Using SF6 for cap..
>
>From EDHARRIS-at-MPS.OHIO-STATE.EDUSun Jul 7 11:26:05 1996
>Date: Sun, 07 Jul 1996 10:26:20 -0400 (EDT)
>From: EDHARRIS-at-MPS.OHIO-STATE.EDU
>To: tesla-at-pupman.COM
>Subject: Re: Using SF6 for cap..
[ snip ]
>Last issue first:
>I use SF6 in a plasma etching system I built here at OSU. In its gaseous,
>room temperatue state SF6 is actually INERT. You can breathe the stuff if you
>want with only the fear of asphixiation - it's not toxic otherwise.
[ snip ]
I always thought it was toxic. Well, I learn something every day!
It looks like the pure gas is OK, but when you vent a chamber which
has operated at voltage with SF6 gas, there *could* be some very
nasty contaminants in the used gas! (see below)
I couldn't find an online MSDS sheet on SF6, but here is a web page
on SF6: I got this from a simple query on
http://altavista.digital-dot-com
for "sulphur hexafluoride" . I make no claim on the accuracy.
************************** Begin SF6 Web page *******************************
Electrical discharges in gases
Sulphur hexaflouride (SF6) is widely used as an insulator in high voltage systems including
switchgear. SF6 is a non-toxic, strongly electronegative (electron-attaching) gas with a high
dielectric strength, and a high thermal stability at temperatures up to around 1000 K. When an
electrical discharge such as an arc or spark occurs in high pressure SF6, the on-axis temperature
rises above 10000 K and results in complete dissociation of the SF6 molecules. In SF6 glow
discharges, where temperatures are normally below 1000 K, and coronas, where temperatures are only a
few degrees above ambient, little thermal dissociation occurs. In glows and coronas, however,
electron energies are sufficiently high to cause dissociation of SF6 molecules through the
electron-impact process of
e + SF6 --> SFx + (6 - x) F + e
Thus, in any electrical discharge through SF6, some dissociation into lower order SFx (x < 6) and
fluorine will always occur. In a clean discharge environment, these products of dissociation will
recombine completely to SF6, following cessation of the discharge and return to ambient gas
temperature. Invariably, however, the discharge environment will include contaminant gases and
vapours, in particular, oxygen and water vapour. The dissociation products may react with these
contaminants to form molecules such as the sulphur oxyfluorides; SOF2, SO2F2 and SOF4, as well as
other species such as SO2 and HF. In addition, containing-vessel, electrode and insulation surfaces
present further reaction possibilities, particularly for free fluorine, and other species produced
may include SiF4, CF4 and CO. Solid by-products include metal fluorides and sulphur.
In this project, we are examining the gaseous and solid by-products produced in glow and corona
discharges in SF6. The techniques being used for gas analysis are mass spectrometry, infra-red
spectroscopy and refractivity measurement; and for analysis of solid by-products, X-ray probing and
optical fibre chemical sensing.
_______________________________________________________________________________________________
Back to OFS group welcome
Compiled by Giuseppe Scelsi (30 March 1995)
Send comments to: <gscelsi-at-metz.une.edu.au>
************************** End of SF6 Web page *******************************
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