Reduction of stainless-steel outgassing in ultra-high vacuum

A room-temperature ion trapping apparatus with hydrogen partial pressure below 10-11 mbar

The lifetime of trapped ion ensembles corresponds to a crucial parameter determining the potential scalability of their prospective applications and is often limited by the achievable vacuum level in the apparatus. We report on the realization of a room-temperature ⁴⁰Ca⁺ ion trapping vacuum apparatus with unprecedentedly low reaction rates of ions with a dominant vacuum contaminant: hydrogen. We present our trap assembly procedures and hydrogen pressure characterization by analysis of the CaH⁺ molecule formation rate.

Vacuum Furnace for Degassing Stainless-Steel Vacuum Components

Ultra-high vacuum systems must often be constructed of materials with ultra-low outgassing rates to achieve pressure of 10^-6 Pa and below. Any component placed into the ultra-high vacuum system must also be constructed of materials with ultra-low outgassing rates. Baking stainless steel vacuum components to a temperature range of 400 °C to 450 °C while under vacuum is an effective method to reduce the outgassing rate of vacuum components for use in ultra-high vacuum systems. The design, construction, and operation of a vacuum furnace capable of baking vacuum components to a temperature of 450° C while maintaining a pressure of 10^-3 Pa or lower is described. The furnace has been used for extended bakes at 450 °C while maintaining pressures below 10^-5 Pa. As an example, we obtained an outgassing rate of 1.2 × 10^-9 Pa L s^-1 for a gate valve baked for 20 days at a temperature of 420 °C.

Investigations of medium-temperature heat treatments to achieve low outgassing rates in stainless steel ultrahigh vacuum chambers

The authors investigated the outgassing rates and fluxes of vacuum chambers constructed from common 304L stainless steel vacuum components and subjected to heat treatments. Our goal was to obtain H₂ outgassing flux on the order of 10^-11 Pa l s^-1cm^-2 or better from standard stainless steel vacuum components readily available from a variety of manufacturers. The authors found that a medium-temperature bake in the range of 400 to 450°C, performed with the interior of the chamber under vacuum, was sufficient to produce the desired outgassing flux. The authors also found that identical vacuum components baked in air at the same temperature for the same amount of time did not produce the same low outgassing flux. In that case, the H₂ outgassing flux was lower than that of a stainless-steel chamber with no heat treatment, but was still approximately 1 order of magnitude higher than that of the medium-temperature vacuum-bake. Additionally, the authors took the chamber that was subjected to the medium-temperature vacuum heat treatment and performed a 24-h air bake at 430°C. This additional heat treatment lowered the outgassing rate by nearly a factor of two, which strongly suggests that the air-bake created an oxide layer which reduced the hydrogen recombination rate on the surface. [http://dx.doi.org/10.1116/1.4983211].

Molecular simulation of water vapor outgassing from silica nanopores

The outgassing problem is solved numerically by molecular dynamics. A slit-shaped nanopore consisting of cavity and channel is built with an implicit tabulated wall potential that describes the water-silicon/silica interaction. A flexible three-point water model is used for the simulation. The effects of varying the system temperature, outlet pressure, geometry, and materials of the nanopore on the outgassing rate are investigated. The results show that the temperature plays an important role in the outgassing rate, while the effect of the outlet pressure is negligible as long as it is in the high to medium vacuum range. The geometry of the channel also has an influence on the outgassing rate, but not as much as the surface material. Three different types of silica materials are tested: silicon, silica-cristobalite (hydrophilic material), and silica-quartz (super hydrophilic material). The fastest outgassing rate is found for a silicon nanopore. It is also found that a thin water film is formed on the surface of the silica-quartz nanopore. This material shows hardly any outgassing of water.

Atom probe, AFM, and STM studies on vacuum-fired stainless steels

The surface morphology of grades 304L and 316LN stainless steels, after low-temperature bake-out process and vacuum annealing, has been studied by atomic force microscopy (AFM) and scanning tunnelling microscopy (STM). The local elemental composition on the surface before and after thermal treatment has been investigated by atom probe (AP) depth profiling measurements. After vacuum annealing, AFM and STM show significant changes in the surface structure and topology. Recrystallization and surface reconstruction is less pronounced on the 316LN stainless steel. AP depth profiling analyses result in noticeable nickel enrichment on the surface of grade 304L samples. Since hydrogen recombination is almost controlled by surface structure and composition, a strong influence on the outgassing behaviour by the particular surface microstructure can be deduced.