Re: Elastic Vacuum Glue

vgpalmieri wrote:
I'm looking for a glue that would allow to seal a large glass window
(1 m2) to a metal box and to evacuate the inner volume to about 10-5
Torr. Such glue should hold against thermal expansion mismatch up to
bake-out temperature of 120 C. So far I tried TorrSeal (epoxy) that is
ok for vacuum properties but is too rigid and cracks when thermal
cycling. VacSeal (silicone resin) should be flexible enough but is too
thin to compensate for surface imperfections. Also moisture permeation
rate could be high. Thanks in advance for any other suggestion...

Cheers

Vittorio


Welcome to engineering.

Your first assignment is to calculate the physical mismatch between the glass and the metal box at 120 C.

This tells you about how much differential displacement that the design for the seal must accomodate.

If the seal is brittle and thin, you can't absorb much differential expansion in a brittle thin seal. However, a much thicker brittle seal could well accomodate the thermal expansion differential.

Thick seals can accomodate more differential expansion via shear in the seal.

The shear capability needed can be estimated from the thermal expansion mismatch, DeltaL, and the seal thickness, t. You can estimate the thermal expansion mismatch from the temperature differential and the expansion coefficients AlphaMetal and AlphaGlass and the window "diameter" Dia.

Mismatch = DeltaTemp * (AlphaGlass-AlphaMetal) * Dia

SealShearStrain = Mismatch / t.

The last equation tells you why a larger seal thickness, t, is pretty important.

People will often elect to use an elastomer gasket of some kind with metal flanges and securements rather than an adhesive.

Good luck.

I take no responsibility for glass vacuum windows shattering. Hope you have followed good guidelines on selection of glass, thickness and freedom from scratches and that sort of thing.
.

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