Metals the most prevalent choice of vacuum chamber materials, with stainless steel (SS) far ahead of other metals such as mild steel. For example, deciding to use SS doesn’t mean any and all SS alloys. Free-machining alloys such as 303 SS contain sulfur (S), but the vapor pressure of the sulfur (S) is too high for high vacuum systems. 304 SS, though, is the most common choice. This helps narrow things down, but for ultrahigh vacuum (UHV) the low-carbon 304L alloy is recommended.
Historically, the following three factors have made stainless steel the preferred material for the manufacture of vacuum chambers.
- Ease of achieving vacuum leak-tight welds.
- Ease of cleaning and bake out.
- Availability of standard sealing systems.
Meanwhile, aluminum has advanced offering these advantages not possible with stainless steel.
- Weight – aluminum is lighter than stainless steel
- Machining – aluminum is easier to machine than stainless steel
- Price – aluminum costs less than stainless steel
Today’s advances in aluminum processes have erased the advantages stainless steel once enjoyed as the fabrication material of choice for constructing vacuum chambers.
Welded Aluminum vessels now enjoy the same three factors:
- Experience in achieving vacuum leak tight welds. — Experienced companies, understand how to properly prepare and weld aluminum to achieve vacuum tight welds.
- Advances in cleaning techniques. — Projects like the Advanced Photon Source at Argonne National Laboratory and the National Ignition Facility at Lawrence Livermore Laboratory have transferred technology to industry that allows the cleaning of aluminum for high vacuum or ultra-high vacuum levels.
- Sealing systems that achieve better than 1×10-8 std cc/sec. — Elastomer sealing systems allow aluminum chambers to operate to 1×10-8 std cc/sec levels. Methods such as Titanium Nitrating of aluminum knife-edge type flanges and the use of explosion bonded transition pieces to allow the use of standard stainless steel knife-edge flanges on aluminum vacuum vessels allow the use of chambers regimes <1×10-9 std cc/sec.