Indium seal

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Indium wire as used in indium seals

An indium seal is a gas-tight , metal seal made of indium wire, which is preferably used in the construction of cryostats .

Advantages and disadvantages

Elastomer seals cannot be used at very low temperatures because they become brittle and lose their elasticity . Some metals do not lose this property even at low temperatures, but the different thermal expansion can be problematic. The soft indium is characterized primarily by its easy plastic deformability. The excellent tightness of indium, which is pressed between two harder metals, is due to its ability to flow into the smallest bumps and thus glue the two flanges together.

The disadvantage of an indium seal is that, unlike an elastomer seal, it cannot be reused. However, compared to other metal seals, for example copper , it is much easier to form a wire from a pinched seal. Another disadvantage is that the components to be sealed are literally glued together. As a rule, you need additional threads or the like in order to be able to press off a flange .

Manufacturing

Indium wire is intended to be used as a sealing material between two flanges. To do this, the wire is placed in the groove , with the two ends overlapping, if necessary beveled. The groove is dimensioned in such a way that the indium does not fully fit into it in terms of volume. The other flange with a smooth sealing surface is evenly pressed on with screws. After some time, when the indium has flowed a little, the screws must be tightened repeatedly. Since the restoring forces of the indium are very low, it is recommended that instead of retightening the thread, a permanent preload, for example with disc springs , is installed. The seal produced in this way is suitable for ultra-high vacuum (UHV) and does not lose this property when it cools down anywhere near absolute zero. The advantage of this connection is that it is detachable. However, one of the two flanges should have a continuous thread so that the other flange can be pressed off with screws. The deformed indium can be taken out of the groove and must be replaced with new wire.

When using an indium seal for the first time on a new component, it is advisable to grease the area very thinly with vacuum grease, as otherwise the indium will be very difficult or impossible to remove.

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