Outgassing rate comparison of seven geometrically similar vacuum chambers of different materials and heat treatments

Abstract

We have measured the water and hydrogen outgassing rates of seven vacuum chambers of identical geometry but constructed of different materials and heat treatments. Chambers of five different materials were tested: 304L, 316L, and 316LN stainless steels; titanium (ASTM grade 2); and 6061 aluminum. In addition, chambers constructed of 316L and 316LN stainless steel were subjected to a vacuum-fire process, where they were heated to approximately 950 °C for 24 hours while under vacuum; these two chambers are designated as 316L-XHV and 316LN-XHV. All chambers were of identical geometry and made by the same manufacturer, thus a relative comparison of the outgassing rates among these chambers can be made. Water outgassing rates were measured as a function of time using the throughput technique. The water outgassing results for the 316L, 316LN, 316L-XHV, 316LN-XHV were all similar, but lower than those of 304L by a factor of 3 to 5 lower at 10^4 s. The water outgassing results for Ti and Al chambers were close to that of 304L, Ti being slightly lower. Hydrogen outgassing rates were measured using the rate-of-rise method and performed after a low-temperature bake of 125 °C to 150 °C for a minimum of 72 hours. The Ti, Al, 316L-XHV, and 316LN-XHV chambers all have specific outgassing rates below 1 X 10^-11 Pa L s^-1 cm^-2 and are at least a factor of 100 or better than the 304L chamber. The 304L, 316L, and 316LN chambers without vacuum-fire heat treatment have larger hydrogen outgassing rates than the other chambers, with specific outgassing rates ranging between 4.0 X 10^-11 Pa L s^-1 cm^-2 and 8.0 X 10^-11 Pa L s^-1 cm^-2. We conclude that Ti, Al, 316L-XHV, and 316LN-XHV have hydrogen outgassing rates that make them excellent choices for ultra-high vacuum (UHV) and extreme-high vacuum (XHV) applications, the choice depending on cost and other material properties.