Sulfur Hexafluoride (SF6)

SF6 is one of the most popular insulating gases (next to air). It has a number of nice properties: it's not flammable, it's non-toxic, it's (moderately) inexpensive, it's a good insulator (being an electronegative gas), with a breakdown strength of about 3 times that of air. At normal temperatures, it is non-corrosive, and is fairly inert, although at temperatures above 500C, it decomposes, and the decomposition products (Fluorine(!)) react with just about anything, especially any water vapor.

Physical Properties of SF6 (from Khalifa, from Maller and Naidu (1981))

Molecular Weight 146.05
Melting point -50.8 C
Sublimation Temperature -63.9 C
Density (liquid at 50 C) 1.98 g/mL
Density (liquid at 25 C) 1.329 g/mL
Density (gas at 1 bar and 20C) 6.164 g/L
Critical temperature 45.6 C
Critical pressure 36.557 atm
Critical density 0.755 g/mL
Surface tension (at -50C) 11.63 dyne/cm
Thermal conductivity 3.36E4 cal/s/sq cm/K/cm
Viscosity (gas at 25 C) 1.61E-4 poise
Boiling point -63.0C
Specific heat (at 30C) 0.143 cal/g
Relative density (air=1) 5.10
Vapor pressure (at 20 C) 10.62 bar

It is available from most gas dealers, particularly those specializing in electronic gases. The insulating grade is usually referred to as CP (chemically pure), although there are grades with much higher purities used in the semiconductor manufacturing business.

The current price (as of February 2000) is around $10/pound for the CP grade, with better grades going for up to $20/pound. It is typically sold by weight in medium pressure tanks at 320 psi, as a liquid.

Sources of SF6:

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