When quartz pinch seals are made, the seal is typically made with molybdenum foils going through the quartz. I have the following questions about this process:

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1) Why Molybdenum?
Quartz has a CTE of around .55e-6 which is very very low. So you would want to use the metal with the lowest CTE that you can find, right? Well (according to Google AI overview) molybdenum has a CTE of 5.2e-6 while tungsten has a CTE of 4.8e-6. Why wouldn't they choose tungsten for this application?

2) Definition of "Foil"...
So they use molybdenum foil for these seals, but what exactly do they mean by foil? Are we talking about .1mm or something like .01mm? I would assume the thinner the better, but obviously there is a practical limit to the thinness of foil.

3) "Feathered Edges"?
I always hear descriptions of Mo foil seals mentioning that the edges of the foil are "feathered". What is that supposed to mean? When I look at a halogen capsule all I see is foil, I don't see any visible edge treatment done to the foil.

4) Vycor Glass?
I just learned that there is something called Vycor, which is 96% quartz, but technically a type of borosilicate glass. It has a CTE of ~.75e-6. Was this ever used in lamps? I feel like straight up quartz might be overkill for something like a small halogen capsule.

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Thanks!

You need something that has a reasonable adhesion to the quartz/glass in order to form a good seal, it needs to be workable into rather thin foil, it should be weldable, withstand working temperatures when forming the seal and withstand temperatures needed to form the seals. Plus it should not be prohibitively expensive material. Plus it should not be that hard, it should allow (preferably elastic) deformation without needing that high pressures

The cross section needs to have high aspect ratio between width to thickness. Because the trick is to redirect the expansion into the foil thickening, instead of widening, without creating excessive forces to crack the quartz. So then the quartz walls just have to slightly flex out, to create the room for the material expansion. And the thinner the foil, the less the quartz has to flex, so the tougher seal you can get.

"Feathered edges" are there to hold it in place, so the forces related to expansion won't move it and by that lose the adhesion and form possible leak paths. Plus because the thickness is never exactly even, any shift would mean stress concentrations on some spots, sp making it more likely to crack. usually normal burrs from the foil cutting are enough, it means the tooling is explicitely designed to keep them in place and not eliminate them (as is the aim for any normal sheet/foil metal processing).

Gradual seals (so gradually transitioning from one expansion coefficient to another to distribute the expansion) is a technique that was (and I guess sometimes even is) used in lamp making. But it is quite complex process, a lot of steps, so not that much compatible with high volume production. So generally it is avoided whenever possible. And with quartz it is possible...

I believe also that feathered ends serve to discourage hairline bubbles forming along the sides of the foil, which cause slow leaks, a nightmare for the manufacturer.

@Medved
So tungsten could be used, but it is just impractical?

So "feathered" just means it has really sharp cut edges, right?

I was also aware of those graded seals to tungsten wire, but like you said they are barely ever used anymore.

Tungsten is very hard metal and that is what makes it not practical I guess...

Feathered means sharp edges, as well with metal spikes drawn from those edges. The kind of edge you get if you want to cut a sheet metal with shears but the shears are very of bad quality.

@Medved
Ahh I see, good to know. I guess maybe tungsten would be too fragile in foil state.