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

Abstract

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]

Vacuum furnace for degassing stainless-steel vacuum components

“…To achieve lower water outgassing rates and lower base pressures, a vacuum bake for at least 1 day to 3 days in the temperature range of 120 °C to 150 °C is typically performed. This effectively removes most of the water on the surface of the stainless steel, and a total outgassing flux in the range of 10 −10 Pa L cm −2 s −1 to 10 −9 Pa L cm −2 s −1 can be expected 1 . Once reduced to this level, the predominant outgassing species is H 2 .…”

“…This technique has been used previously by the authors and is described in detail in other publications. 1,16 Briefly, the gate valve was placed between an all-metal valve and a spinning rotor gauge as shown in Figure 6(b). The GV remained in an open position and never closed during the outgassing measurement.…”

“…It has been demonstrated that a 400 °C to 450 °C heat-treatment in vacuum is an effective method for reducing the outgassing rate 1–5 . Such a medium-temperature (herein defined as 400 °C to 450 °C) vacuum bake on a stainless-steel vacuum chamber can be performed utilizing external heaters, such as heater tape, and covering the heaters and chamber with insulation.…”

A constant pressure flowmeter for extremely high vacuum

“…Such exposure is possible, for example, from contamination in the gas used to fill the flowmeter, and thus a low-outgassing flowmeter must be designed for repeated bakes. Hydrogen outgassing can be more difficult to minimize, requiring either a low hydrogen-outgassing material, like titanium or aluminum [10], or a process of baking the vacuum components at higher temperatures [11,12,13].…”

Note: A 3D-printed alkali metal dispenser

“…All stainless steel components of the vacuum chamber were baked at 450 °C for 21 days prior to assembly to reduce hydrogen outgassing 17,18 . All vacuum components, including the 3D-printed titanium AMD, were cleaned in alkaline detergent, followed by acetone, then ethanol.…”