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Atlas Technologies specializes in aluminum vacuum chambers. We strongly believe that aluminum is a superior vacuum material to stainless steel. By reading the following description of aluminum, it is easy to see why the high and ultra high vacuum industry is rapidly embracing this versatile material.

Atlas Aluminum vacuum chambers have excellent ultra high vacuum (UHV) properties and, for most applications, aluminum is a superior UHV vacuum chamber material. Aluminum has 10,000,000x (7 orders of magnitude) less Hydrogen permeation than stainless steel. Consequently, aluminum chambers have far less H2, H2O, & hydrocarbon vapor at high vacuum and ultra high vacuum levels. (See references.) Aluminum has less carbon contamination than stainless steel and this reduces the amount of carbon-based gas contamination in vacuum. Atlas Technologies manufactures ultra high vacuum (UHV) aluminum chambers with a thin, dense aluminum oxide coating. This serves as a resistive barrier, reducing diffusion and desorption of high vacuum & ultra high vacuum contaminates (hydrogen, oxygen & carbon). Once baked, Atlas’ aluminum vacuum chambers generally cycle to high vacuum and ultra high vacuum levels faster than stainless steel chambers and require less pumping.

The following physical properties of aluminum describe why it is an excellent vacuum material:

Outgassing Properties

One of the most important properties of a vacuum material is the outgassing rate as this determines the ultimate pressure that may be obtained in the vacuum chamber. Repeatable outgassing rates of of <10-13 Torr liter/sec cm2 are now possible in aluminum UHV systems [Citations], comparable to the best outgassing rates obtainable with stainless steel [Citations]. This improvement in outgassing performance has been one of the principal breakthroughs that has allowed aluminum to become a competent material for the construction of UHV systems. Thermal Conductivity

Aluminum’s thermal conductivity, depending on the alloy, ranges between 170 W/mK and 230 W/mK. Stainless steels, by contrast, have thermal conductivities that are between 14 W/mK and 16 W/mK,  so aluminum is roughly 10x that of stainless steel. High thermal conductivity is an advantage when designing systems that require temperature cycling. This is the case for vacuum systems that must be baked to reach UHV levels. An aluminum chamber may be baked and then cooled much more rapidly than a stainless steel chamber. Furthermore, aluminum`s high conductivity allows a complete bakeout without re-condensation of gases on local cool spots, a common problem in stainless steel systems.

Due to aluminum’s superior thermal conductivity, aluminum vacuum chambers:

  • bake more uniformly even at lower temperatures (150 C).
  • bake-out faster. Aluminum chambers need only bake at 150 C for 24hrs to obtain the same level of baking as stainless at 250C for 48 hours

Mechanical Properties

Typical elastic modulii for aluminum alloy 6061 T6 and stainless steel alloy 304 [Citations] are 7470 kgf/mm2 and 19700 kgf/mm2, respectively. If these values are used in mechanical formulae for standard geometries, the ratios of critical thickness for the two materials are [Citations]:

Here the equation for total deflection of a flat plate  [Citations]

tp = (kPrp4/(Ed))1/3

Since the Modulus of Al is 1/3 of SS: 

tAS = (ESS/EAl)1/3 = (19,000/7,474)1/3 = 1.36 x SS

Aluminum Chambers need only be ~1.4x thicker than Stainless to maintain the same or less deflection

tp = Thickness of plate

k = .0696 if Clamped, or = 0.171 of clamped

P = Atmospheric Pressure

rp = Radius of circular plate or small side of rectangular plate

E = Modulus of Elasticity: Al = 7,474 Kgf/mm2 & SS = 19,000 Kgf/mm2

d = Acceptable deflection

tAS = Acceptable deflection

Note that the ratios are closer to unity than you would expect at first glance. An aluminum vacuum chamber, therefore, will not require parts that have appreciably greater thickness than similar ones manufactured from stainless steel. A certain minimum thickness is required to resist deformation from atmospheric pressures.

Weight

Aluminum is roughly 1/3 the weight of stainless steel (2.8 g/cm3 [Al] vs. 8.0 g/cm3 [stainless steel alloys]). The cost burden associated with excess weight begins when the raw materials are handled and progresses throughout the manufacturing process. It affects all production steps, including shipping, installation, and even the architectural engineering and construction of the environment surrounding the chamber.

Magnetic Properties

Aluminum is not magnetic whereas stainless steel, being essentially an alloy of iron, exhibits residual magnetism. The absence of magnetic properties in aluminum is advantageous for applications involving charged particle beams because the vacuum chamber will not modify the fields created from the beam control magnets.

Radioactivity

Aluminum, in comparison to stainless steel, has a much more rapid decay of induced radioactivity. If both types of materials are bombarded with the same flux of charged particles, the residual radioactivity will typically be one to two orders of magnitude less for an aluminum sample than for an identically shaped stainless steel sample [Citations]. The nuclear half-life of elements that make up stainless steel suggests that alpha-particle contamination is always present in stainless steel and a possible source of circuit damage.

Corrosion

The corrosion of both aluminum and stainless steel alloys in reactive gasses is complicated. Experimental work performed on various alloys in different reactive gaseous environments shows that both aluminum and stainless steel are subject to attack by reactive gasses. Halogen containing species are typically the most damaging and the corrosion of any given compound is usually no worse than that of its halogen component alone [Citations].

Aluminum is no worse with regard to corrosion than stainless steel. It simply has different reaction dynamics that do not serve as a source of iron and nickel contamination, one of the most significant yield-limiting factors for silicon IC production.

Aluminum chambers offer superior UHV performance. The special properties of aluminum make it ideal for many applications. Aluminum chambers can be robustly sealed with conventional all-metal-seal CF flanges using Atlas Flanges, which are bimetallic flanges that have a stainless steel knife edge sealing face on an aluminum body for weld up to aluminum chambers. Please browse our Atlas CF Flange™, and Atlas ATCR™ pages for demountable aluminum vacuum chamber sealing options.

Atlas aluminum vacuum chamber production techniques, chamber configurations, or combinations thereof, employed by Atlas include:

Plate to Plate Chambers
Ideal for large aluminum chamber applications


Cylindrical Chambers
Allows for minimum wall thickness because of the inherent strength offered by tubular geometry


Extruded Chambers
Ideal for producing long, narrow and convoluted internal chamber geometries


Spherical Chambers
Offers minimum internal surface area

Monoquoque Chambers
Allows for internal and/or external machining of intricate and precise chamber features

Please contact us to start a conversation about your aluminum vacuum chamber application. We’d be delighted to assist you in any way we can. Our aluminum chambers have extraordinary UHV performance because of our deep design, manufacturing, welding and surface treatment experience. We look forward to hearing from you!

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