Fermion's 100 Questions, Issue 6 | Accessories in a Vacuum

16. How to choose vacuum screws?

Depending on the material, there are generally the following types of screws:
Stainless steel screws are the most common, but because many of them are magnetic, especially at low temperatures, the magnetism will be significantly enhanced, affecting the test experiment, so our low-temperature products cannot use this type of screws.

Mo screws, Mo is generally used as a high-temperature material, and is also used on our sample rack. The advantages are that it is non-magnetic and has a high thermal conductivity (138W/m•K); the disadvantages are that Mo is brittle and easy to break. In addition, Mo is not suitable for oxidizing high-temperature environments. If there is O above 500 ℃, Mo will oxidize to molybdenum oxide and then continue to sublimate.

Ta screws, Ta is generally used as a high-temperature material, and its processing performance is similar to that of stainless steel, and it is corrosion-resistant. The disadvantage is that it is more expensive (the price of high-purity Ta is close to 10,000 yuan per kilogram), and the thermal conductivity is not very good (57.5 W/m•K).

Copper screws, please note that this is definitely not pure copper, because the basic requirement for screw material is hardness, so oxygen-free copper is not feasible for screws, and it will break after a few uses. The same is true for pure aluminum.
In addition, the screws we encountered are:

PEEK screws, which are used for insulation, are slightly weaker than metals, but are more ductile than metals.

Titanium screws, which are also common vacuum screws, but the thermal conductivity of Ti is the worst among metals (21.9 W/m•K). If used in a low-temperature environment, the cooling of titanium screws will be relatively slow, which may cause a slight lag in the overall heating and cooling.

In addition, for special occasions, it is best to have different materials for screws and nuts, because our use environment is a vacuum. In a vacuum, because the interface is very simple and there is no lubrication, there is a possibility of "cold welding" of the same material, that is, the same materials will stick together. If the materials are different, it will be much better.
For this problem, we must be clear that the choice of screws is limited according to the application. For example, in a low-temperature environment, stainless steel screws cannot be used. The poor thermal conductivity of Ta/Ti and PEEK may affect the extremely low temperature index. If it is a non-replaceable screw, the uncertainty of the brittleness of the thread is very large if Mo is used. Therefore, beryllium copper or phosphor bronze screws are currently the best choice. If you want to avoid cold welding, you can add solid lubrication at the interface, such as MoS2, to prevent the two copper interfaces from overlapping.

17. Why do screws used in blind holes need to have vent holes?

Some air will remain between the screw and the bottom of the blind hole. Under vacuum conditions, the venting speed through the screw seam is very slow. If the vent hole is not drilled, these gases will be difficult to exhaust and will affect the system vacuum.

18. Can pure aluminum be used for metal sealing gaskets?

In fact, pure aluminum gaskets exist and are used in large quantities (mainly in the industrial field), please refer to Lesker's website: (https://www.lesker.com/newweb/flanges/hardware_cf_gaskets.cfm?pgid=aluminum)

Al Gasket products can also be seen in the product catalogs of many other vacuum suppliers.
As a sealing material, the hardness of pure Al is a little softer than that of oxygen-free copper. From this point of view alone, pure aluminum can be used as a sealing gasket material. However, compared with oxygen-free copper washers, aluminum has obvious disadvantages:
It is more troublesome to clean pure aluminum than oxygen-free copper, because ultrasonic cleaning will produce voids on the aluminum surface and affect the vacuum. Therefore, ultrasonic cleaning of pure aluminum requires a more complicated process than oxygen-free copper, which limits the use of pure aluminum washers.
The melting point of aluminum is around 600℃, which is lower than that of pure copper. Moreover, as the temperature rises, the strength of aluminum decreases significantly, so the flange with aluminum gaskets generally requires that the baking temperature should not exceed 200℃, while the maximum baking temperature of normal CF flanges can reach 400℃. The normal baking temperature of many standard vacuum components (such as ion pumps) is 200℃ and above, which limits the scope of use of aluminum gaskets.
In addition, in addition to oxygen-free copper, there are some materials that can also be used as sealing gaskets for CF flanges, such as aluminum, as described above.
Viton (fluororubber) is generally used in high vacuum fields, but the deviation in the permeability resistance of fluororubber causes a high leakage rate. However, as a reusable sealing ring, fluororubber gaskets are used in large quantities in the high vacuum field.
Nickel, nickel can also be used as a sealing gasket. The hardness of Ni is about HV274, which is harder than copper and aluminum, but lower than 304 and 316 stainless steel. Nickel washers are mainly used in corrosive environments. Because of nickel's corrosion resistance, they can be used as sealing washers in corrosive environments.