NIST and the Applied Vacuum Division of Anderson Dahlen Complete CRADA for the Study of XHV-Capable Chamber Materials
After extensive collaboration efforts, the Applied Vacuum Division of Anderson Dahlen has completed a Cooperative Research and Development Agreement (CRADA) with NIST to study the effectiveness of various material and pre-fabrication processing techniques for achieving extremely low Hydrogen outgassing rates in vacuum chambers. The requirement for extreme-high vacuum (XHV) systems is increasingly common in advanced research and manufacturing. The key to creating XHV (< 10-12 torr) systems is to produce a chamber with extremely low H2 outgassing rates. To achieve the desired outgassing rates, interstitial H2 in the bulk chamber material must be removed or prevented from leaving the material surface thus increasing the pressure within the chamber.
NIST Researcher (Julia Scherschligt) working on the XHV chamber outgassing test stand
What material and processing options yield the lowest H2 outgassing rates?XHV chambers are typically constructed of 300-series stainless steel, 6061-T aluminum or titanium. Various heat treatments and surface treatments are typically employed to achieve the necessary H2 outgassing rates. However, commercial vendors of vacuum chambers rarely report or specify outgassing rates of chambers, mostly because of the difficulty and expertise required to perform such measurements. Furthermore, it is often difficult to compare published outgassing results using various techniques because different studies use different chamber geometries, environments, and poorly defined or implemented measurement techniques. To date, there has been no consensus on the best, most efficient way of producing vacuum chambers with ultra-low outgassing rates. With this groundbreaking study, we can now quantitatively state which materials are ideal for XHV applications based on their project requirements – budget, leadtime, and magnetic permeability.
(2) examples of the chambers with the exact same geometry used for the XHV outgassing study.
NIST and Applied Vacuum Shared Expertise to Develop a Test Matrix for XHV Chamber MaterialsThe objective was to test an array of vacuum chambers with identical geometries, but produced using different materials and material treatments. NIST is capable of producing high-quality traceable measurements of outgassing rates. NIST and Applied Vacuum agreed on a test matrix defining the material composition and treatment of each vacuum chamber.
|Ti||Titanium Grade 2||None|
|Al||6061 Aluminum Alloy||None|
|304L||304L Stainless Steel||None|
|316L||316L Stainless Steel||None|
|316L-XHV||316L Stainless Steel||Vacuum Fired, 950 °C|
|316LN-ESR||316LN Electro-slag Re-melt Stainless Steel||None|
|316LN-ESR-XHV||316LN Electro-slag Re-melt Stainless Steel||Vacuum Fired, 950 °C|
Preliminary Results and Commercial ImplicationsNIST and Applied Vacuum are in the process of publishing a “white paper” of the final results and the official data is currently under peer review and not able to be made public – yet. However, we can share some preliminary conclusions:
- Vacuum-firing of 316L SS reduces H2 outgassing by a factor of 150 over untreated 316L SS
- 316L SS (vacuum-fired), 6061 Aluminum, and Grade 2 Titanium offer XHV-capable H2 outgassing
- 316LN ESR (vacuum-fired) surprisingly offers no advantage over 316L SS for XHV
- 316L Stainless Steel is easily acquired in all sizes of sheet metal, plate and bar. Most manufacturing companies are tooled to machine 316L, which is also a very “weldable” material for UHV/XHV applications. Price-wise, 316L is on a par with aluminum and less expensive versus titanium.
- Aluminum is difficult to weld for UHV/XHV applications owing to the large heat zone during welding. Aluminum also requires the use of bimetal flanges (i.e. explosion-bonded aluminum to stainless-steel or titanium), mainly because aluminum knife-edges are soft and will deform or fail to seal properly as users open and close the vacuum chamber. However, aluminum is a magnetically inert material – a critical feature for some big-science experiments.
- Titanium is significantly more expensive when compared with 316L and aluminum. It’s also harder to machine and not as easy to acquire in the same assortment of material sizes. Furthermore, welding titanium for UHV/XHV applications requires a completely oxygen-free environment – which means that the manufacturer either needs a glovebox or heavy inert-gas purge of the material while welding. An added complication is titanium’s coefficient of thermal expansion, which is nearly half that of stainless steel. This means there are potential sealing issues when instruments are attached to the vacuum chamber with stainless-steel flanges – such as when the chamber is baked during operation. As per aluminum, titanium is better than stainless steel if users need a magnetically inert material.