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 h while under vacuum. These latter two chambers are designated as 316L-XHV and 316LN-XHV. Because all the chambers were of identical geometry and made by the same manufacturer, 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 316L, 316LN, 316L-XHV, and 316LN-XHV were all similar but lower than those for 304L by a factor of 3–5 lower at 104 s. The water outgassing results for the Ti and Al chambers were closer 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 125–150 °C for a minimum of 72 h. The Ti, Al, 316L-XHV, and 316LN-XHV chambers all have ultralow specific outgassing rates below 1 × 10−11 Pa l s−1 cm−2 and are a factor of 100 or better than the 304L chamber. The 304L, 316L, and 316LN chambers with no vacuum-fire heat treatment have larger hydrogen outgassing rates than the other chambers, with determined specific outgassing rates ranging between 4.0 and 8.0 × 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 ultrahigh vacuum and extreme-high vacuum applications, the choice depending on cost and other material properties.